Showing 1–44 of 44 results for author: Smits, J

A source-term interpretation of turbulent rough wall-pressure spectra

Authors: J. M. O. Massey, A. J. Smits, B. J. McKeon

Abstract: Wall-pressure fluctuations beneath a turbulent boundary layer drive the flow-induced noise and structural loading of rough surfaces. On a smooth wall their variance grows with the logarithm of the Reynolds number, a growth carried, in a source-term reading, by the nonlinear turbulence--turbulence part of the pressure source acting across the logarithmic layer. We ask what sets the same growth once… ▽ More Wall-pressure fluctuations beneath a turbulent boundary layer drive the flow-induced noise and structural loading of rough surfaces. On a smooth wall their variance grows with the logarithm of the Reynolds number, a growth carried, in a source-term reading, by the nonlinear turbulence--turbulence part of the pressure source acting across the logarithmic layer. We ask what sets the same growth once the wall is fully rough. Standard rough-wall phenomenology answers it: above the roughness the mean flow keeps its smooth-wall logarithmic form, so the active source range is cut off at the roughness height rather than the viscous length, and the variance grows on the logarithmic span between the roughness height and the layer thickness in place of the Reynolds number. Splitting the pressure source into its two physical parts then distinguishes separate contributions to the energy: a roughness-local one from the mean-shear source, fixed at high frequency on the roughness scale, and an energetic one from the nonlinear source at the outer scale, which alone carries the growth. Calibrated against rough-wall cases, the model collapses the spectral shape onto this energetic peak and holds it Reynolds-independent at fixed geometry. The canopy contribution decays with distance from the wall and contributes a finite offset. The growth coefficient is predicted to be the smooth-wall one. The present range is too narrow to discriminate that rate from twice or half it, so the contribution is a parameter-free prediction and the identification of a suitable roughness-size sweep that would test it. △ Less

Submitted 17 August, 2026; originally announced August 2026.

Optically detected nuclear magnetic resonance of carbon-13 in bulk diamond

Authors: Maxwell D. Aiello, Janis Smits, Yaser Silani, Andris Berzins, David Lidsky, Bryan A. Richards, Amilcar Jeronimo Perez, Chandrasekhar Ramanathan, Sebastián C. Carrasco, Jabir Chathanathil, Michael Goerz, Vladimir Malinovsky, Dmitry Budker, Sean Lourette, Andrey Jarmola, Victor M. Acosta

Abstract: Precision measurements based on optically detected nuclear magnetic resonance offer exquisite sensitivity to absolute shifts in spin transition frequencies, with potential applications in fundamental physics experiments and inertial sensing. We investigate 13C nuclear spins in diamond as a candidate system for solid-state implementations, which hold the promise for high-fidelity readout of large n… ▽ More Precision measurements based on optically detected nuclear magnetic resonance offer exquisite sensitivity to absolute shifts in spin transition frequencies, with potential applications in fundamental physics experiments and inertial sensing. We investigate 13C nuclear spins in diamond as a candidate system for solid-state implementations, which hold the promise for high-fidelity readout of large numbers of coherent nuclear spins in millitesla or lower magnetic fields. We demonstrate a technique that allows for both optical polarization and readout of large ensembles of ~10^{16} polarized nuclear spins. Our method takes advantage of state-selective Landau-Zener transitions under microwave frequency sweeping, which bidirectionally transfer spin polarization between Nitrogen-Vacancy (NV) electron spins and remote 13C nuclear spins. Using natural isotopic abundance diamonds with nitrogen densities of ~0.5-10 ppm, we perform optically-detected 13C Ramsey spectroscopy and realize a nuclear-spin-dependent fluorescence contrast exceeding 0.5% peak-to-peak. We observe nuclear spin dephasing times T2*~2 ms that only modestly improve with homonuclear dipolar decoupling, indicating that they are limited by the longitudinal spin relaxation of nearby NV electron spins. We study the magnetic field dependence of the optical readout and find comparable contrast and dephasing times for magnetic fields in the range 8-20 mT. Our method can be interpreted as a type of repetitive readout, where each NV center optically reads out the spin state of ~100 nuclei before nuclear spins depolarize. △ Less

Submitted 29 May, 2026; v1 submitted 30 April, 2026; originally announced May 2026.

Comments: 25 pages, 17 figures, 7 tables, 70 refs. v2 fixed two typos

Application of riblets to separating turbulent boundary layers

Authors: Amirreza Rouhi, Vishal Kumar, Oriol Lehmkuhl, Wen Wu, Melissa Kozul, Alexander J. Smits

Abstract: We conduct direct numerical simulations of separating turbulent boundary layers (TBLs) over triangular riblets with tip angles $90^o$ (T9) and $60^o$ (T6). Our setup follows the separating TBL study of Wu et al.\ ({\it J. Fluid Mech.}, vol.\ 883, 2020, p.\ A45). An equilibrium zero pressure-gradient (ZPG) TBL is generated at a reference location, followed by imposition of a Gaussian suction profil… ▽ More We conduct direct numerical simulations of separating turbulent boundary layers (TBLs) over triangular riblets with tip angles $90^o$ (T9) and $60^o$ (T6). Our setup follows the separating TBL study of Wu et al.\ ({\it J. Fluid Mech.}, vol.\ 883, 2020, p.\ A45). An equilibrium zero pressure-gradient (ZPG) TBL is generated at a reference location, followed by imposition of a Gaussian suction profile to create a separation bubble. The ZPG TBLs over the riblets and the benchmark smooth case have matched momentum thickness Reynolds number $Re_{θ_0} = 583$ (friction Reynolds number 224). We employ a well-validated spectral-element solver, and leverage its unstructured-grid nature to generate an optimal grid, based on the size of turbulent scales across the TBL. At the reference location, the T9 and T6 riblets respectively increase and reduce drag, with viscous-scaled spacings $52$ and $13$. We discover that for both riblet cases, the mean separation location occurs at a distance of $140θ_0$ downstream of the reference location, $18\%$ shorter than the mean separation distance for the smooth case ($170θ_0$). This outcome is related to the progressive enhancement of the Kelvin-Helmholtz (KH) rollers over the riblets, owing to the continuous rise in the adverse pressure-gradient. The KH rollers penetrate into the turbulent separation bubble, with significantly larger size and coherence compared to their counterparts upstream of the mean separation location. △ Less

Submitted 20 November, 2025; originally announced December 2025.

Journal ref: Proceedings of the 15th ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements (ETMM15); 2025-09-22; 396-401

3D simulations of negative streamers in CO$_2$ with admixtures of C$_4$F$_7$N

Authors: Thomas J. G. Smits, Jannis Teunissen, Ute Ebert

Abstract: CO$_2$ with an admixture of C$_4$F$_7$N could serve as an eco-friendly alternative to the extreme greenhouse gas SF$_6$ in high-voltage insulation. Streamer discharges in such gases are different from those in air due to the rapid conductivity decay in the streamer channels. Furthermore, since no effective photoionisation mechanism is known, we expect discharge growth to be more stochastic than in… ▽ More CO$_2$ with an admixture of C$_4$F$_7$N could serve as an eco-friendly alternative to the extreme greenhouse gas SF$_6$ in high-voltage insulation. Streamer discharges in such gases are different from those in air due to the rapid conductivity decay in the streamer channels. Furthermore, since no effective photoionisation mechanism is known, we expect discharge growth to be more stochastic than in air. In this paper we investigate whether conventional fluid models can be used to simulate streamers in CO$_2$ with admixtures of C$_4$F$_7$N of 1 or 10%. We focus on 3D simulations of negative streamers. First we review cross section databases for C$_4$F$_7$N and CO$_2$. Then we compare a two-term Boltzmann solver with a Monte Carlo method to compute reaction and transport coefficients from the cross sections. Afterwards we compare 3D fluid simulations with the local field (LFA) or local energy approximation (LEA) against particle simulations. In general, we find that the results of particle and fluid models are quite similar. One difference we observe is that particle simulations are intrinsically stochastic, leading to more branching. Furthermore, the LEA model does not show better agreement with the particle simulations than the LFA model. We also discuss the effect and choice of different boundary conditions on the negative rod electrode. △ Less

Submitted 8 October, 2025; originally announced October 2025.

Comments: 16 pages, 14 figures, 3 tables

Two-component inner--outer scaling model for the wall-pressure spectrum at high Reynolds number

Authors: Jonathan M. O. Massey, Alexander J. Smits, Beverley J. McKeon

Abstract: Wall-pressure fluctuations beneath turbulent boundary layers drive noise and structural fatigue through interactions between fluid and structural modes. Conventional predictive models for the spectrum--such as the widely accepted Goody model (\textit{AIAA Journal} 42 (9), 2004, 1788--1794)--fail to capture the energetic growth in the {low-frequency range} that occurs at high Reynolds number, while… ▽ More Wall-pressure fluctuations beneath turbulent boundary layers drive noise and structural fatigue through interactions between fluid and structural modes. Conventional predictive models for the spectrum--such as the widely accepted Goody model (\textit{AIAA Journal} 42 (9), 2004, 1788--1794)--fail to capture the energetic growth in the {low-frequency range} that occurs at high Reynolds number, while at the same time over-predicting the variance. To address these shortcomings, two semi-empirical models are proposed for the wall-pressure spectrum in canonical turbulent boundary layers, pipes and channels for friction Reynolds numbers $δ^+$ ranging from 180 to 47 000. The models are based on consideration of two {spectral components} that represent the contributions to the wall pressure fluctuations from inner-scale motions and outer-scale motions. The first model expresses the pre-multiplied spectrum as the sum of two overlapping log-normal {components}: an inner-scaled term that is $δ^+$-invariant and an outer-scaled term whose amplitude broadens smoothly with $δ^+$. Calibrated against large-eddy simulations, direct numerical simulations, and recent high-$δ^+$ pipe data, it reproduces the {inner-scaled peak} and the emergence of an outer-scaled peak at large $δ^+$. The second model, developed around newly available pipe data, uses theoretical arguments to prescribe the spectral shapes of the inner and outer {components}. Embedding the $δ^+$-dependence in smooth asymptotic functions yields a formulation that varies continuously with $δ^+$ {and generalises beyond the calibration range}. Both models capture the full spectrum and {recover} the observed logarithmic growth of its variance, {providing a compact, physics-informed empirical representation} for more accurate engineering predictions of wall-pressure fluctuations. △ Less

Submitted 15 April, 2026; v1 submitted 30 July, 2025; originally announced July 2025.

Earth's field diamond vector magnetometry with isotropic magnetic flux concentrators

Authors: Maziar Saleh Ziabari, Nazanin Mosavian, Ilja Fescenko, Yaser Silani, Bryan A. Richards, Andris Berzins, Maxwell D. Aiello, Keith A. Lidke, Andrey Jarmola, Janis Smits, Victor M. Acosta

Abstract: Vector magnetometers based on the optically detected magnetic resonance (ODMR) of nitrogen-vacancy centers in diamond are being developed for applications such as navigation and geomagnetism. However, at low magnetic fields, such as that on Earth (~50 μT), diamond magnetometers suffer from spectral congestion whereby ODMR peaks are not easily resolved. Here, we experimentally investigate a potenti… ▽ More Vector magnetometers based on the optically detected magnetic resonance (ODMR) of nitrogen-vacancy centers in diamond are being developed for applications such as navigation and geomagnetism. However, at low magnetic fields, such as that on Earth (~50 μT), diamond magnetometers suffer from spectral congestion whereby ODMR peaks are not easily resolved. Here, we experimentally investigate a potential solution of using an isotropic, three-dimensional magnetic flux concentrator to amplify Earth's field without altering its direction. The concentrator consists of six ferrite cones, in a face-centered cubic arrangement, centered about a diamond. We vary the direction of a 50 μT applied field and record and fit the resulting ODMR spectra. By comparing the fitted fields to those of a reference fluxgate magnetometer, we characterize the angular response of the diamond magnetometer and quantify absolute errors in the field magnitude and angle. We find that the enhancement factor is nearly isotropic, with a mean of 19.05 and a standard deviation of 0.16, when weighted by solid angle coverage. Gradient broadening of the ODMR lines is sufficiently small that the spectra are well resolved for nearly all field directions, alleviating spectral congestion. For ~98% of the total 4π solid angle, Cramér-Rao lower bounds for magnetic field estimation uncertainty are within a factor of 2 of those of the fully-resolved case, indicating minimal deadzones. We track the stability of the magnetometer over six hours and observe variations less than or approximately 40 nT/hour, limited by temperature drift. Our study presents a new route for diamond vector magnetometry at Earth's field, with potential applications in geomagnetic surveys, anomaly detection, and navigation. △ Less

Submitted 30 June, 2025; originally announced July 2025.

Comments: Main text: 8 pages, 5 figures; Total including appendices: 22 pages, 19 figures, 3 tables, 62 references

Diagnosing syndromes of biosphere-atmosphere-socioeconomic change

Authors: Wantong Li, Gregory Duveiller, Fabian Gans, Jeroen Smits, Guido Kraemer, Dorothea Frank, Miguel D. Mahecha, Ulrich Weber, Mirco Migliavacca, Andrej Ceglar, Trevor F. Keenan, Markus Reichstein

Abstract: It is increasingly recognized that the multiple and systemic impacts of Earth system change threaten the prosperity of society through altered land carbon dynamics, freshwater variability, biodiversity loss, and climate extremes. For example, in 2022, there are about 400 climate extremes and natural hazards worldwide, resulting in significant losses of lives and economic damage. Beyond these losse… ▽ More It is increasingly recognized that the multiple and systemic impacts of Earth system change threaten the prosperity of society through altered land carbon dynamics, freshwater variability, biodiversity loss, and climate extremes. For example, in 2022, there are about 400 climate extremes and natural hazards worldwide, resulting in significant losses of lives and economic damage. Beyond these losses, comprehensive assessment on societal well-being, ecosystem services, and carbon dynamics are often understudied. The rapid expansion of geospatial, atmospheric, and socioeconomic data provides an unprecedented opportunity to develop systemic indices to account for a more comprehensive spectrum of Earth system change risks and to assess their socioeconomic impacts. We propose a novel approach based on the concept of syndromes that can integrate synchronized changes in biosphere, atmosphere, and socioeconomic trajectories into distinct co-evolving phenomena. While the syndrome concept was applied in policy related to environmental conservation, it has not been deciphered from systematic data-driven approaches capable of providing a more comprehensive diagnosis of anthropogenic impacts. By advocating interactive dimensionality reduction approaches, we can identify key interconnected socio-environmental changes as syndromes from big data. We recommend future research tailoring syndromes by incorporating granular data, particularly socio-economic, into dimensionality reduction at different spatio-temporal scales to better diagnose regional-to-global atmospheric and environmental changes that are relevant for socioeconomic changes. △ Less

Submitted 12 March, 2025; v1 submitted 11 March, 2025; originally announced March 2025.

Chemically resolved nuclear magnetic resonance spectroscopy by longitudinal magnetization detection with a diamond magnetometer

Authors: Janis Smits, Yaser Silani, Zaili Peng, Bryan A. Richards, Andrew F. McDowell, Joshua T. Damron, Maxwell D. Aiello, Maziar Saleh Ziabari, Andrey Jarmola, Victor M. Acosta

Abstract: Non-inductive magnetometers based on solid-state spins offer a promising solution for small-volume nuclear magnetic resonance (NMR) detection. A remaining challenge is to operate at a sufficiently high magnetic field to resolve chemical shifts at the part-per-billion level. Here, we demonstrate a Ramsey-M_z protocol that uses Ramsey interferometry to convert an analyte's transverse spin precession… ▽ More Non-inductive magnetometers based on solid-state spins offer a promising solution for small-volume nuclear magnetic resonance (NMR) detection. A remaining challenge is to operate at a sufficiently high magnetic field to resolve chemical shifts at the part-per-billion level. Here, we demonstrate a Ramsey-M_z protocol that uses Ramsey interferometry to convert an analyte's transverse spin precession into a longitudinal magnetization (M_z), which is subsequently modulated and detected with a diamond magnetometer. We record NMR spectra at B0=0.32 T with a fractional spectral resolution of ~350 ppb, limited by the stability of the electromagnet bias field. We perform NMR spectroscopy on a ~1 nL detection volume of ethanol and resolve the chemical shift structure with negligible distortion. Through simulation, we show that the protocol can be extended to fields up to B0=3 T, with minimal spectral distortion, using composite nuclear-spin inversion pulses. For sub-nanoliter analyte volumes, we estimate a resolution of ~1 ppb and concentration sensitivity of ~40 mM s^{1/2} is feasible with improvements to the sensor design. Our results establish diamond magnetometers as high-resolution NMR detectors in the moderate magnetic field regime, with potential applications in metabolomics and pharmaceutical research. △ Less

Submitted 3 March, 2025; originally announced March 2025.

Comments: Main text: 5 figures, 7 pages; Total including Appendices: 20 pages, 13 figures, 2 tables, 62 references

Time-resolved diamond magnetic microscopy of superparamagnetic iron-oxide nanoparticles

Authors: B. A. Richards, N. Ristoff, J. Smits, A. Jeronimo Perez, I. Fescenko, M. D. Aiello, F. Hubert, Y. Silani, N. Mosavian, M. Saleh Ziabari, A. Berzins, J. T. Damron, P. Kehayias, D. Egbebunmi, J. E. Shield, D. L. Huber, A. M. Mounce, M. P. Lilly, T. Karaulanov, A. Jarmola, A. Laraoui, V. M. Acosta

Abstract: Superparamagnetic iron-oxide nanoparticles (SPIONs) are promising probes for biomedical imaging, but the heterogeneity of their magnetic properties is difficult to characterize with existing methods. Here, we perform widefield imaging of the stray magnetic fields produced by hundreds of isolated ~30-nm SPIONs using a magnetic microscope based on nitrogen-vacancy centers in diamond. By analyzing th… ▽ More Superparamagnetic iron-oxide nanoparticles (SPIONs) are promising probes for biomedical imaging, but the heterogeneity of their magnetic properties is difficult to characterize with existing methods. Here, we perform widefield imaging of the stray magnetic fields produced by hundreds of isolated ~30-nm SPIONs using a magnetic microscope based on nitrogen-vacancy centers in diamond. By analyzing the SPION magnetic field patterns as a function of applied magnetic field, we observe substantial field-dependent transverse magnetization components that are typically obscured with ensemble characterization methods. We find negligible hysteresis in each of the three magnetization components for nearly all SPIONs in our sample. Most SPIONs exhibit a sharp Langevin saturation curve, enumerated by a characteristic polarizing applied field, B_c. The B_c distribution is highly asymmetric, with a standard deviation (1.4 mT) that is larger than the median (0.6 mT). Using time-resolved magnetic microscopy, we directly record SPION Néel relaxation, after switching off a 31 mT applied field, with a temporal resolution of ~60 ms that is limited by the ring-down time of the electromagnet coils. For small bias fields B_{hold}=1.5-3.5 mT, we observe a broad range of SPION Néel relaxation times--from milliseconds to seconds--that are consistent with an exponential dependence on B_{hold}. Our time-resolved diamond magnetic microscopy study reveals rich SPION sample heterogeneity and may be extended to other fundamental studies of nanomagnetism. △ Less

Submitted 3 February, 2025; v1 submitted 20 November, 2024; originally announced November 2024.

Comments: Main text: 9 pages, 5 figures. Entire manuscript including Appendices: 31 pages, 24 figures

Journal ref: ACS nano 19 (10), 10048-10058 (2025)

The impact of microwave phase noise on diamond quantum sensing

Authors: Andris Berzins, Maziar Saleh Ziabari, Yaser Silani, Ilja Fescenko, Joshua T. Damron, John F. Barry, Andrey Jarmola, Pauli Kehayias, Bryan A. Richards, Janis Smits, Victor M. Acosta

Abstract: Precision optical measurements of the electron-spin precession of nitrogen-vacancy (NV) centers in diamond form the basis of numerous applications. The most sensitivity-demanding applications, such as femtotesla magnetometry, require the ability to measure changes in GHz spin transition frequencies at the sub-millihertz level, corresponding to a fractional resolution of better than 10^{-12}. Here… ▽ More Precision optical measurements of the electron-spin precession of nitrogen-vacancy (NV) centers in diamond form the basis of numerous applications. The most sensitivity-demanding applications, such as femtotesla magnetometry, require the ability to measure changes in GHz spin transition frequencies at the sub-millihertz level, corresponding to a fractional resolution of better than 10^{-12}. Here we study the impact of microwave (MW) phase noise on the response of an NV sensor. Fluctuations of the phase of the MW waveform cause undesired rotations of the NV spin state. These fluctuations are imprinted in the optical readout signal and, left unmitigated, are indistinguishable from magnetic field noise. We show that the phase noise of several common commercial MW generators results in an effective pT s^{1/2}-range noise floor that varies with the MW carrier frequency and the detection frequency of the pulse sequence. The data are described by a frequency domain model incorporating the MW phase noise spectrum and the filter-function response of the sensing protocol. For controlled injection of white and random-walk phase noise, the observed NV magnetic noise floor is described by simple analytic expressions that accurately capture the scaling with pulse sequence length and the number of pi pulses. We outline several strategies to suppress the impact of MW phase noise and implement a version, based on gradiometry, that realizes a >10-fold suppression. Our study highlights an important challenge in the pursuit of sensitive diamond quantum sensors and is applicable to other qubit systems with a large transition frequency. △ Less

Submitted 7 October, 2024; v1 submitted 8 July, 2024; originally announced July 2024.

Comments: Main text: 8 pages, 5 figures. Entire manuscript including Appendices: 23 pages, 14 figures, 6 tables

Journal ref: Physical Review Research 6 (4), 043148 (2024)

Spanwise wall forcing can reduce turbulent heat transfer more than drag

Authors: Amirreza Rouhi, Marcus Hultmark, Alexander J. Smits

Abstract: Direct numerical simulations are performed of turbulent forced convection in a half channel flow with wall oscillating either as a spanwise plane oscillation or to generate a streamwise travelling wave. The friction Reynolds number is fixed at $Re_{τ_0} = 590$, but the Prandtl number $Pr$ is varied from 0.71 to 20. For $Pr>1$, the heat transfer is reduced by more than the drag, 40\% compared to 30… ▽ More Direct numerical simulations are performed of turbulent forced convection in a half channel flow with wall oscillating either as a spanwise plane oscillation or to generate a streamwise travelling wave. The friction Reynolds number is fixed at $Re_{τ_0} = 590$, but the Prandtl number $Pr$ is varied from 0.71 to 20. For $Pr>1$, the heat transfer is reduced by more than the drag, 40\% compared to 30\% at $Pr=7.5$. This outcome is related to the different responses of the velocity and thermal fields to the Stokes layer. It is shown that the Stokes layer near the wall attenuates the large-scale energy of the turbulent heat-flux and the turbulent shear-stress, but amplifies their small-scale energy. At higher Prandtl numbers, the thinning of the conductive sublayer means that the energetic scales of the turbulent heat-flux move closer to the wall, where they are exposed to a stronger Stokes layer production, increasing the contribution of the small-scale energy amplification. A predictive model is derived for the Reynolds and Prandtl number dependence of the heat-transfer reduction based on the scaling of the thermal statistics. The model agrees well with the computations for Prandtl numbers up to 20. △ Less

Submitted 22 October, 2024; v1 submitted 14 June, 2024; originally announced June 2024.

Journal ref: J. Fluid Mech. 1010 (2025) A59

Wall-wake laws for the mean velocity and the turbulence

Authors: Alexander J. Smits

Abstract: A new wall-wake law is proposed for the streamwise turbulence in the outer region of a turbulent boundary layer. The formulation pairs the logarithmic part of the profile (with a slope A_1 and additive constant B_1) to an outer linear part, and it accurately describes over 95% of the boundary layer profile at high Reynolds numbers. Once the slope A_1 is fixed, B_1 is the only free parameter determ… ▽ More A new wall-wake law is proposed for the streamwise turbulence in the outer region of a turbulent boundary layer. The formulation pairs the logarithmic part of the profile (with a slope A_1 and additive constant B_1) to an outer linear part, and it accurately describes over 95% of the boundary layer profile at high Reynolds numbers. Once the slope A_1 is fixed, B_1 is the only free parameter determining the fit. Most importantly, B_1 is shown to be proportional to the wake factor in the wall-wake law for the mean velocity, revealing a previously unsuspected connection between the turbulence and the mean flow. △ Less

Submitted 3 February, 2024; originally announced February 2024.

Journal ref: J. Fluid Mech. 989 (2024) A15

Acceleration is the Key to Drag Reduction in Turbulent Flow

Authors: Liuyang Ding, Lena Sabidussi, Brian C. Holloway, Marcus Hultmark, Alexander J. Smits

Abstract: A turbulent pipe flow experiment was conducted where the surface of the pipe was oscillated azimuthally over a wide range of frequencies, amplitudes and Reynolds number. The drag was reduced by as much as 30\%. Past work has suggested that the drag reduction scales with the velocity amplitude of the motion, its period, or the Reynolds number. Here, we find that the key parameter is simply the acce… ▽ More A turbulent pipe flow experiment was conducted where the surface of the pipe was oscillated azimuthally over a wide range of frequencies, amplitudes and Reynolds number. The drag was reduced by as much as 30\%. Past work has suggested that the drag reduction scales with the velocity amplitude of the motion, its period, or the Reynolds number. Here, we find that the key parameter is simply the acceleration, which reduces the complexity of the phenomenon by two orders of magnitude. This insight opens new potential avenues for reducing fuel consumption by large vehicles and for reducing energy costs in large piping systems. △ Less

Submitted 3 February, 2024; v1 submitted 19 December, 2023; originally announced December 2023.

Comments: 8 pages, 5 figures

Super-resolution diamond magnetic microscopy of superparamagnetic nanoparticles

Authors: Nazanin Mosavian, Forrest Hubert, Janis Smits, Pauli Kehayias, Yaser Silani, Bryan A. Richards, Victor M. Acosta

Abstract: Scanning-probe and wide-field magnetic microscopes based on Nitrogen-Vacancy (NV) centers in diamond have enabled remarkable advances in the study of biology and materials, but each method has drawbacks. Here, we implement an alternative method for nanoscale magnetic microscopy based on optical control of the charge state of NV centers in a dense layer near the diamond surface. By combining a donu… ▽ More Scanning-probe and wide-field magnetic microscopes based on Nitrogen-Vacancy (NV) centers in diamond have enabled remarkable advances in the study of biology and materials, but each method has drawbacks. Here, we implement an alternative method for nanoscale magnetic microscopy based on optical control of the charge state of NV centers in a dense layer near the diamond surface. By combining a donut-beam super-resolution technique with optically detected magnetic resonance spectroscopy, we imaged the magnetic fields produced by single 30-nm iron-oxide nanoparticles. The magnetic microscope has a lateral spatial resolution of ~100 nm, and it resolves the individual magnetic dipole features from clusters of nanoparticles with interparticle spacings down to ~190 nm. The magnetic feature amplitudes are more than an order of magnitude larger than those obtained by confocal magnetic microscopy due to the smaller characteristic NV-nanoparticle distance within nearby sensing voxels. We analyze the magnetic point-spread function and sensitivity as a function of the microscope's spatial resolution and identify sources of background fluorescence that limit the present performance, including diamond second-order Raman emission and imperfect NV charge-state control. Our method, which uses less than 10 mW laser power and can be parallelized by patterned illumination, introduces a new format for nanoscale magnetic imaging. △ Less

Submitted 9 October, 2023; originally announced October 2023.

Comments: 20 pages, 16 figures, 4 tables

Journal ref: ACS nano 18 (8), 6523-6532 (2024)

Nuclear quadrupole resonance spectroscopy with a femtotesla diamond magnetometer

Authors: Yaser Silani, Janis Smits, Ilja Fescenko, Michael W. Malone, Andrew F. McDowell, Andrey Jarmola, Pauli Kehayias, Bryan Richards, Nazanin Mosavian, Nathaniel Ristoff, Victor M. Acosta

Abstract: Sensitive Radio-Frequency (RF) magnetometers that can detect oscillating magnetic fields at the femtotesla level are needed for demanding applications such as Nuclear Quadrupole Resonance (NQR) spectroscopy. RF magnetometers based on Nitrogen-Vacancy (NV) centers in diamond have been predicted to offer femtotesla sensitivity, but published experiments have largely been limited to the picotesla lev… ▽ More Sensitive Radio-Frequency (RF) magnetometers that can detect oscillating magnetic fields at the femtotesla level are needed for demanding applications such as Nuclear Quadrupole Resonance (NQR) spectroscopy. RF magnetometers based on Nitrogen-Vacancy (NV) centers in diamond have been predicted to offer femtotesla sensitivity, but published experiments have largely been limited to the picotesla level. Here, we demonstrate a femtotesla RF magnetometer based on an NV-doped diamond membrane inserted between two ferrite flux concentrators. The device operates in bias magnetic fields of 2-10 microtesla and provides a ~300-fold amplitude enhancement within the diamond for RF magnetic fields in the 0.07-3.6 MHz range. The magnetometer's sensitivity is ~70 fT s^{1/2} at 0.35 MHz, and the noise floor decreases to below 2 fT after 1 hour of acquisition. We used this sensor to detect the 3.6 MHz NQR signal of 14N in sodium nitrite powder at room temperature. NQR signals are amplified by a resonant RF coil wrapped around the sample, allowing for higher signal-to-noise ratio detection. The diamond RF magnetometer's recovery time after a strong RF pulse is ~35 us, limited by the coil ring-down time. The sodium-nitrite NQR frequency shifts linearly with temperature as -1.00 +/- 0.02 kHz/K, the magnetization dephasing time is T2* = 887 +/- 51 us, and a spin-lock spin-echo pulse sequence extends the signal lifetime to 332 +/- 23 ms, all consistent with coil-based NQR studies. Our results expand the sensitivity frontier of diamond magnetometers to the femtotesla range, with potential applications in security, medical imaging, and materials science. △ Less

Submitted 23 February, 2023; originally announced February 2023.

Comments: Main text: 8 pages, 6 figures. Entire manuscript including Appendices: 21 pages, 15 figures, 3 tables

Journal ref: Science advances 9 (24), eadh3189 (2025)

On the relationship between manipulated inter-scale phase and energy-efficient turbulent drag reduction

Authors: Rahul Deshpande, Dileep Chandran, Alexander J. Smits, Ivan Marusic

Abstract: We investigate the role of inter-scale interactions in the high-Reynolds number skin-friction drag reduction strategy reported by Marusic et al. (Nat. Commun., vol. 12, 2021). The strategy involves imposing relatively low-frequency streamwise travelling waves of spanwise velocity at the wall to actuate the drag generating outer-scales. This approach has proven to be more energy-efficient than the… ▽ More We investigate the role of inter-scale interactions in the high-Reynolds number skin-friction drag reduction strategy reported by Marusic et al. (Nat. Commun., vol. 12, 2021). The strategy involves imposing relatively low-frequency streamwise travelling waves of spanwise velocity at the wall to actuate the drag generating outer-scales. This approach has proven to be more energy-efficient than the conventional method of directly targeting the drag producing inner-scales, which typically requires actuation at higher frequencies. Notably, it is observed that actuating the outer-scales at low frequencies leads to a substantial attenuation of the major drag producing inner-scales, suggesting that the actuations affect the non-linear inner-outer coupling inherently existing in wall-bounded flows. In the present study, we find that increased drag reduction, through imposition of spanwise wall oscillations, is always associated with an increased coupling between the inner and outer scales. This enhanced coupling emerges through manipulation of the phase relationships between these triadically linked scales, with the actuation forcing the entire range of energy-containing scales, from the inner (viscous) to the outer (inertial) scales, to be more in-phase. We also find that a similar enhancement of this non-linear coupling, via manipulation of the inter-scale phase relationships, occurs with increasing Reynolds number for canonical turbulent boundary layers. This indicates improved efficacy of the energy-efficient drag reduction strategy at very high Reynolds numbers, where the energised outer-scales are known to more strongly superimpose and modulate the inner-scales. Leveraging the inter-scale interactions, therefore, offers a plausible mechanism for achieving energy-efficient drag reduction at high Reynolds numbers. △ Less

Submitted 25 August, 2023; v1 submitted 14 November, 2022; originally announced November 2022.

Comments: Manuscript accepted in the Journal of Fluid Mechanics comprising 16 pages, 5 figures

Journal ref: Journal of Fluid Mechanics 2023, Volume 972, A12

Turbulent drag reduction by spanwise wall forcing. Part 2: High-Reynolds-number experiments

Authors: Dileep Chandran, Andrea Zampiron, Amirreza Rouhi, Matt K. Fu, David Wine, Brian Holloway, Alexander J. Smits, Ivan Marusic

Abstract: Here, we present measurements of turbulent drag reduction in boundary layers at high friction Reynolds numbers in the range of $4500 \le Re_τ\le 15000$. The efficacy of the approach, using streamwise travelling waves of spanwise wall oscillations, is studied for two actuation regimes: (i) inner-scaled actuation (ISA), as investigated in Part 1 of this study, which targets the relatively high-frequ… ▽ More Here, we present measurements of turbulent drag reduction in boundary layers at high friction Reynolds numbers in the range of $4500 \le Re_τ\le 15000$. The efficacy of the approach, using streamwise travelling waves of spanwise wall oscillations, is studied for two actuation regimes: (i) inner-scaled actuation (ISA), as investigated in Part 1 of this study, which targets the relatively high-frequency structures of the near-wall cycle, and (ii) outer-scaled actuation (OSA), which was recently presented by Marusic et al. (Nat. Commun., vol. 12, 2021) for high-$Re_τ$ flows, targeting the lower-frequency, outer-scale motions. Multiple experimental techniques were used, including a floating-element balance to directly measure the skin-friction drag force, hot-wire anemometry to acquire long-time fluctuating velocity and wall-shear stress, and stereoscopic-PIV (particle image velocimetry) to measure the turbulence statistics of all three velocity components across the boundary layer. Under the ISA pathway, drag reduction of up to 25% was achieved, but mostly with net power saving losses due to the high-input power cost associated with the high-frequency actuation. The low-frequency OSA pathway, however, with its lower input power requirements, was found to consistently result in positive net power savings of 5 - 10%, for moderate drag reductions of 5 - 15%. The results suggest that OSA is an attractive pathway for energy-efficient drag reduction in high Reynolds number applications. Both ISA and OSA strategies are found to produce complex inter-scale interactions, leading to attenuation of the turbulent fluctuations across the boundary layer for a broad range of length and time scales. △ Less

Submitted 26 March, 2023; v1 submitted 7 November, 2022; originally announced November 2022.

Turbulent drag reduction by spanwise wall forcing. Part 1: Large-eddy simulations

Authors: Amirreza Rouhi, Matt K. Fu, Dileep Chandran, Andrea Zampiron, Alexander J. Smits, Ivan Marusic

Abstract: Turbulent drag reduction through streamwise travelling waves of spanwise wall oscillation is investigated over a wide range of Reynolds numbers. Here, in Part 1, wall-resolved large-eddy simulations in a channel flow are conducted to examine how the frequency and wavenumber of the travelling wave influence the drag reduction at friction Reynolds numbers $Re_τ= 951$ and $4000$. The actuation parame… ▽ More Turbulent drag reduction through streamwise travelling waves of spanwise wall oscillation is investigated over a wide range of Reynolds numbers. Here, in Part 1, wall-resolved large-eddy simulations in a channel flow are conducted to examine how the frequency and wavenumber of the travelling wave influence the drag reduction at friction Reynolds numbers $Re_τ= 951$ and $4000$. The actuation parameter space is restricted to the inner-scaled actuation (ISA) pathway, where drag reduction is achieved through direct attenuation of the near-wall scales. The level of turbulence attenuation, hence drag reduction, is found to change with the near-wall Stokes layer protrusion height $\ell_{0.01}$. A range of frequencies is identified where the Stokes layer attenuates turbulence, lifting up the cycle of turbulence generation and thickening the viscous sublayer; in this range, the drag reduction increases as $\ell_{0.01}$ increases up to $30$ viscous units. Outside this range, the strong Stokes shear strain enhances near-wall turbulence generation leading to a drop in drag reduction with increasing $\ell_{0.01}$. We further find that, within our parameter and Reynolds number space, the ISA pathway has a power cost that always exceeds any drag reduction savings. This motivates the study of the outer-scaled actuation (OSA) pathway in Part 2, where drag reduction is achieved through actuating the outer-scaled motions. △ Less

Submitted 25 March, 2023; v1 submitted 6 November, 2022; originally announced November 2022.

Assessment of nanoparticle immersion depth at liquid interfaces from chemically equivalent macroscopic surfaces

Authors: Joeri Smits, Rajendra Prasad Giri, Chen Shen, Diogo Mendonça, Bridget Murphy, Patrick Huber, Kurosch Rezwan, Michael Maas

Abstract: Hypothesis: We test whether the wettability of nanoparticles (NPs) straddling at an air/water surface or oil/water interface can be extrapolated from sessile drop-derived macroscopic contact angles (mCAs) on planar substrates, assuming that both the nanoparticles and the macroscopic substrates are chemically equivalent and feature the same electrokinetic potential. Experiments: Pure silica (SiO2)… ▽ More Hypothesis: We test whether the wettability of nanoparticles (NPs) straddling at an air/water surface or oil/water interface can be extrapolated from sessile drop-derived macroscopic contact angles (mCAs) on planar substrates, assuming that both the nanoparticles and the macroscopic substrates are chemically equivalent and feature the same electrokinetic potential. Experiments: Pure silica (SiO2) and amino-terminated silica (APTES-SiO2) NPs are compared to macroscopic surfaces with extremely low roughness (root mean square [RMS] roughness <= 2 nm) or a roughness determined by a close-packed layer of NPs (RMS roughness about 35 nm). Equivalence of the surface chemistry is assessed by comparing the electrokinetic potentials of the NPs via electrophoretic light scattering and of the macroscopic substrates via streaming current analysis. The wettability of the macroscopic substrates is obtained from advancing (ACAs) and receding contact angles (RCAs) and in situ synchrotron X-ray reflectivity (XRR) provided by the NP wettability at the liquid interfaces. Findings: Generally, the RCA on smooth surfaces provides a good estimate of NP wetting properties. However, mCAs alone cannot predict adsorption barriers that prevent NP segregation to the interface, as is the case with the pure SiO2 nanoparticles. This strategy greatly facilitates assessing the wetting properties of NPs for applications such as emulsion formulation, flotation, or water remediation. △ Less

Submitted 9 February, 2022; v1 submitted 8 February, 2022; originally announced February 2022.

Journal ref: Journal of Colloid and Interface Science 2022, 611, 670-683

Tunable magnetic field source for magnetic field imaging microscopy

Authors: Andris Berzins, Hugo Grube, Reinis Lazda, Marc A. Hannig, Janis Smits, Ilja Fescenko

Abstract: In this work we present a novel, compact, power efficient magnetic field source design for magnetic field imaging microscopy. The device is based on a pair of diametrically magnetized permanent magnet cylinders with electro-mechanical rotation control and ferrite field concentrators. A Hall probe and NV centers in diamond are used to demonstrate a proof of concept of a proposed magnetic field setu… ▽ More In this work we present a novel, compact, power efficient magnetic field source design for magnetic field imaging microscopy. The device is based on a pair of diametrically magnetized permanent magnet cylinders with electro-mechanical rotation control and ferrite field concentrators. A Hall probe and NV centers in diamond are used to demonstrate a proof of concept of a proposed magnetic field setup and to characterise the homogeneity of the produced magnetic field on a micrometer scale. Numerical simulation results are compared with experimental results showing good agreement of the distribution of the magnetic field in the setup. As a result, a magnetic field source with a tunable field amplitude in the range from 1 mT to 222 mT is demonstrated, achieving a magnetic field homogeneity of 2 ppm/$μ$m or 0.5 $μ$T/$μ$m at 222 mT in a 25$\times$25 $μ$m field of view. △ Less

Submitted 31 December, 2021; originally announced December 2021.

Comments: 11 pages, 9 figures

Journal ref: Ultramicroscopy 2022

Proposal for the search for new spin interactions at the micrometer scale using diamond quantum sensors

Authors: P. -H. Chu, N. Ristoff, J. Smits, N. Jackson, Y. J. Kim, I. Savukov, V. M. Acosta

Abstract: For decades, searches for exotic spin interactions have used increasingly-precise laboratory measurements to test various theoretical models of particle physics. However, most searches have focused on interaction length scales greater than 1 mm, corresponding to hypothetical boson masses less than 0.2 meV. Recently, quantum sensors based on Nitrogen-Vacancy (NV) centers in diamond have emerged as… ▽ More For decades, searches for exotic spin interactions have used increasingly-precise laboratory measurements to test various theoretical models of particle physics. However, most searches have focused on interaction length scales greater than 1 mm, corresponding to hypothetical boson masses less than 0.2 meV. Recently, quantum sensors based on Nitrogen-Vacancy (NV) centers in diamond have emerged as a promising platform to probe spin interactions at the micrometer scale, opening the door to explore new physics at this length scale. Here, we propose experiments to search for several hypothetical interactions between NV electron spins and moving masses. We focus on potential interactions involving the coupling of NV spin ensembles to both spin-polarized and unpolarized masses attached to vibrating mechanical oscillators. For each interaction, we estimate the sensitivity, identify optimal experimental conditions, and analyze potential systematic errors. Using multi-pulse quantum sensing protocols with NV spin ensembles to improve sensitivity, we project new constraints that are ~5 orders-of-magnitude improvement over previous constraints at the micrometer scale. We also identify a spin-polarized test mass, based on hyperpolarized 13C nuclear spins in a thin diamond membrane, which offers a favorable combination of high spin density and low stray magnetic fields. Our analysis is timely in light of a recent preprint (arXiv:2010.15667) reporting a surprising non-zero result of micrometer-scale spin-velocity interactions. △ Less

Submitted 29 December, 2021; originally announced December 2021.

Comments: 15 pages, 14 figures, 3 tables, 18 equations

Journal ref: Physical Review Research 4, 023162 (2022)

NV microscopy of thermally controlled stresses caused by Cr$_2$O$_3$ thin films

Authors: Andris Berzins, Janis Smits, Andrejs Petruhins, Roberts Rimsa, Gatis Mozolevskis, Martins Zubkins, Ilja Fescenko

Abstract: Many modern applications, including quantum computing and quantum sensing, use substrate-film interfaces. Particularly, thin films of chromium or titanium and their oxides are commonly used to bind various structures, such as resonators, masks, or microwave antennas, to a diamond surface. Due to different thermal expansions of involved materials, such films and structures could produce significant… ▽ More Many modern applications, including quantum computing and quantum sensing, use substrate-film interfaces. Particularly, thin films of chromium or titanium and their oxides are commonly used to bind various structures, such as resonators, masks, or microwave antennas, to a diamond surface. Due to different thermal expansions of involved materials, such films and structures could produce significant stresses, which need to be measured or predicted. In this paper, we demonstrate imaging of stresses in the top layer of diamond with deposited structures of Cr$_2$O$_3$ at temperatures 19$^{\circ}$C and 37$^{\circ}$C by using stress-sensitive optically detected magnetic resonances (ODMR) in NV centers. We also calculated stresses in the diamond-film interface by using finite-element analysis and correlated them to measured ODMR frequency shifts. As predicted by the simulation, the measured high-contrast frequency-shift patterns are only due to thermal stresses, whose spin-stress coupling constant along the NV axis is 21$\pm$1 MHz/GPa, that is in agreement with constants previously obtained from single NV centers in diamond cantilever. We demonstrate that NV microscopy is a convenient platform for optically detecting and quantifying spatial distributions of stresses in diamond-based photonic devices with micrometer precision and propose thin films as a means for local application of temperature-controlled stresses. Our results also show that thin film structures produce significant stresses in diamond substrates, which should be accounted for in NV-based applications. △ Less

Submitted 9 February, 2023; v1 submitted 25 November, 2021; originally announced November 2021.

Comments: 11 pages, 6 figures

Journal ref: Optics Express 2023

Reynolds stress scaling in the near-wall region of wall-bounded flows

Authors: Alexander J. Smits, Marcus Hultmark, Myoungkyu Lee, Sergio Pirozzoli, Xiaohua Wu

Abstract: A new scaling is derived that yields a Reynolds number independent profile for all components of the Reynolds stress in the near-wall region of wall bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence beh… ▽ More A new scaling is derived that yields a Reynolds number independent profile for all components of the Reynolds stress in the near-wall region of wall bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behavior. △ Less

Submitted 9 June, 2021; originally announced June 2021.

Journal ref: Journal of Fluid Mechanics, 926, p.A31, 2021

Reynolds stress scaling in the near-wall region

Authors: Alexander J. Smits, Marcus Hultmark

Abstract: A new scaling is derived that yields a Reynolds number independent profile for all components of the Reynolds stress in the near-wall region of wall bounded flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behavior. A new scaling is derived that yields a Reynolds number independent profile for all components of the Reynolds stress in the near-wall region of wall bounded flows. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence behavior. △ Less

Submitted 12 March, 2021; originally announced March 2021.

Journal ref: Journal of Fluid Mechanics, 926, p.A31, 2021

Scaling turbulence in the near-wall region

Authors: Marcus Hultmark, Alexander J. Smits

Abstract: A new velocity scale is derived that yields a Reynolds number independent profile for the streamwise turbulent fluctuations in the near-wall region of wall bounded flows for $y^+<25$. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence distribution. A new velocity scale is derived that yields a Reynolds number independent profile for the streamwise turbulent fluctuations in the near-wall region of wall bounded flows for $y^+<25$. The scaling demonstrates the important role played by the wall shear stress fluctuations and how the large eddies determine the Reynolds number dependence of the near-wall turbulence distribution. △ Less

Submitted 12 March, 2021; v1 submitted 2 March, 2021; originally announced March 2021.

Journal ref: Journal of Fluid Mechanics, 926, p.A31, 2021

Surface magnetic structure investigation of a nanolaminated Mn$_2$GaC thin film using a magnetic field microscope based on Nitrogen-Vacancy centers

Authors: Andris Berzins, Janis Smits, Andrejs Petruhins, Hugo Grube

Abstract: This work presents a magnetic field imaging method based on color centers in diamond crystal applied to a thin film of a nanolaminated Mn$_2$GaC MAX phase. Magnetic properties of the surface related structures have been described around the first order transition at 214 K by performing measurements in the temperature range between 200 K and 235 K with the surface features fading out by increasing… ▽ More This work presents a magnetic field imaging method based on color centers in diamond crystal applied to a thin film of a nanolaminated Mn$_2$GaC MAX phase. Magnetic properties of the surface related structures have been described around the first order transition at 214 K by performing measurements in the temperature range between 200 K and 235 K with the surface features fading out by increasing temperature above the transition temperature. The results presented here demonstrate how Nitrogen-Vacancy center based magnetic microscopy can supplement the traditionally used set of experimental techniques, giving additional information of microscopic scale magnetic field features, and allowing to investigate the temperature dependent magnetic behavior. The additional information acquired in this way is relevant to applications where surface magnetic properties are of essence. △ Less

Submitted 11 November, 2020; originally announced November 2020.

Comments: 8 pages, 5 figures

Characterization of microscopic ferromagnetic defects in thin films using magnetic microscope based on Nitrogen-Vacancy centres

Authors: Andris Berzins, Janis Smits, Andrejs Petruhins

Abstract: In this work we present results acquired by applying magnetic field imaging technique based on Nitrogen-Vacancy centres in diamond crystal for characterization of magnetic thin films defects. We used the constructed wide-field magnetic microscope for measurements of two kinds of magnetic defects in thin films. One family of defects under study was a result of non-optimal thin film growth condition… ▽ More In this work we present results acquired by applying magnetic field imaging technique based on Nitrogen-Vacancy centres in diamond crystal for characterization of magnetic thin films defects. We used the constructed wide-field magnetic microscope for measurements of two kinds of magnetic defects in thin films. One family of defects under study was a result of non-optimal thin film growth conditions. The magnetic field maps of several regions of the thin films created under very similar conditions to previously published research revealed microscopic impurity islands of ferromagnetic defects, that potentially could disturb the magnetic properties of the surface. The second part of the measurements was dedicated to defects created post deposition - mechanical defects introduced in ferromagnetic thin films. In both cases, the measurements identify the magnetic field amplitude and distribution of the magnetic defects. In addition, the magnetic field maps were correlated with the corresponding optical images. As this method has great potential for quality control of different stages of magnetic thin film manufacturing process and it can rival other widely used measurement techniques, we also propose solutions for the optimization of the device in the perspective of high throughput. △ Less

Submitted 30 October, 2020; originally announced October 2020.

Comments: 10 pages, 7 figures

Investigation of temperature dependence of magnetic properties of Cr$_2$O$_3$ thin film structure using a magnetic field imaging technique based on Nitrogen-Vacancy centres in diamond crystal

Authors: Andris Berzins, Janis Smits, Andrejs Petruhins, Roberts Rimsa, Martins Zubkins, Gatis Mozolevskis

Abstract: This work presents a magnetic field imaging method based on color centres in diamond crystal applied to thin film structure. To demonstrate the capacity of our device we have used it for characterization of magnetic properties in microscopic scale of Cr$_2$O$_3$ thin film structure above and below Néel temperature. The obtained measurement results clearly identify the detection of the magnetic pha… ▽ More This work presents a magnetic field imaging method based on color centres in diamond crystal applied to thin film structure. To demonstrate the capacity of our device we have used it for characterization of magnetic properties in microscopic scale of Cr$_2$O$_3$ thin film structure above and below Néel temperature. The obtained measurement results clearly identify the detection of the magnetic phase transition of Cr$_2$O$_3$ thin film with an unexpected diamagnetic like behaviour at 19$^{\circ}$C (below the Néel temperature of Cr$_2$O$_3$). To have better insights in the magnetic fields created by the thin films we present simulations of the magnetic fields near the thin film surface. We also analysed the optically detected magnetic resonance (ODMR) profiles to be sure that the measured property is related only to the shift of the optically detected magnetic resonance. We demonstrate how Nitrogen-Vacancy centre based magnetometry can deliver a convenient platform for research of phase transition dynamics and spatial magnetic field distributions of thin films, that can rival other widely used measurement techniques. △ Less

Submitted 17 December, 2021; v1 submitted 10 August, 2020; originally announced August 2020.

Comments: The measurements and simulations presented in this manuscript are correct, however, the data interpretation (magnetic field as a main source of the signal) is incorrect. After submission to a scientific journal and discussion with referees, we understood that the main cause of the ODMR frequency shifts is strain-related. Thus the authors decided to present it as a new manuscript: arXiv:2111.12979

Complete eddy self-similarity in turbulent pipe flow

Authors: Leo Hellström, Tyler Van Buren, John Vaccaro, Alexander J. Smits

Abstract: For wall-bounded turbulent flows, Townsend's attached eddy hypothesis proposes that the logarithmic layer is populated by a set of energetic and geometrically self-similar eddies. These eddies scale with a single length scale, their distance to the wall, while their velocity scale remains constant across their size range. To investigate the existence of such structures in fully developed turbulent… ▽ More For wall-bounded turbulent flows, Townsend's attached eddy hypothesis proposes that the logarithmic layer is populated by a set of energetic and geometrically self-similar eddies. These eddies scale with a single length scale, their distance to the wall, while their velocity scale remains constant across their size range. To investigate the existence of such structures in fully developed turbulent pipe flow, stereoscopic particle image velocimetry measurements were performed in two parallel cross-sectional planes, spaced apart by a varying distance from 0 to 9.97$R$, for $Re_τ= 1310$, 2430 and 3810. The instantaneous turbulence structures are sorted by width using an azimuthal Fourier decomposition, allowing us to create a set of average eddy velocity profiles by performing an azimuthal alignment process. The resulting eddy profiles exhibit geometric self-similar behavior in the $(r,θ)$-plane for eddies with spanwise length scales ($λ_θ/R$) spanning from 1.03 to 0.175. The streamwise similarity is further investigated using two-point correlations between the two planes, where the structures exhibit a self-similar behaviour with length scales ($λ_θ/R$) ranging from approximately $0.88$ to $0.203$. The candidate structures thereby establish full three-dimensional geometrically self-similarity for structures with a volumetric ratio of $1:80$. Beside the geometric similarity, the velocity magnitude also exhibit self-similarity within these ranges. However, the velocity scale depends on eddy size, and follow the trends based on the scaling arguments proposed by \cite{Perry1986}. △ Less

Submitted 24 November, 2019; originally announced November 2019.

Imaging trapped quantum gases by off-axis holography

Authors: J. Smits, A. P. Mosk, P. van der Straten

Abstract: We present a dispersive imaging method for trapped quantum gases based on digital off-axis holography. Both phase delay and intensity of the probe field are determined from the same image. Due to the heterodyne gain inherent to the holographic method it is possible to retrieve the phase delay induced by the atoms at probe beam doses two orders of magnitude lower than phase-contrast imaging methods… ▽ More We present a dispersive imaging method for trapped quantum gases based on digital off-axis holography. Both phase delay and intensity of the probe field are determined from the same image. Due to the heterodyne gain inherent to the holographic method it is possible to retrieve the phase delay induced by the atoms at probe beam doses two orders of magnitude lower than phase-contrast imaging methods. Using the full field of the probe beam we numerically correct for image defocusing. △ Less

Submitted 21 November, 2019; originally announced November 2019.

Comments: 4 pages, 5 figures

Diamond magnetometer enhanced by ferrite flux concentrators

Authors: Ilja Fescenko, Andrey Jarmola, Igor Savukov, Pauli Kehayias, Janis Smits, Joshua Damron, Nathaniel Ristoff, Nazanin Mosavian, Victor M. Acosta

Abstract: Magnetometers based on nitrogen-vacancy (NV) centers in diamond are promising room-temperature, solid-state sensors. However, their reported sensitivity to magnetic fields at low frequencies (<1 kHz) is presently >10 pT s^{1/2}, precluding potential applications in medical imaging, geoscience, and navigation. Here we show that high-permeability magnetic flux concentrators, which collect magnetic f… ▽ More Magnetometers based on nitrogen-vacancy (NV) centers in diamond are promising room-temperature, solid-state sensors. However, their reported sensitivity to magnetic fields at low frequencies (<1 kHz) is presently >10 pT s^{1/2}, precluding potential applications in medical imaging, geoscience, and navigation. Here we show that high-permeability magnetic flux concentrators, which collect magnetic flux from a larger area and concentrate it into the diamond sensor, can be used to improve the sensitivity of diamond magnetometers. By inserting an NV-doped diamond membrane between two ferrite cones in a bowtie configuration, we realize a ~250-fold increase of the magnetic field amplitude within the diamond. We demonstrate a sensitivity of ~0.9 pT s^{1/2} to magnetic fields in the frequency range between 10 and 1000 Hz, using a dual-resonance modulation technique to suppress the effect of thermal shifts of the NV spin levels. This is accomplished using 200 mW of laser power and 20 mW of microwave power. This work introduces a new dimension for diamond quantum sensors by using micro-structured magnetic materials to manipulate magnetic fields. △ Less

Submitted 14 November, 2019; v1 submitted 12 November, 2019; originally announced November 2019.

Comments: Main text: 6 pages, 5 figures. Entire manuscript including Appendices: 18 pages, 16 figures

Journal ref: Phys. Rev. Research 2, 023394 (2020)

Large-amplitude oscillations of foils for efficient propulsion

Authors: Daniel Floryan, Tyler Van Buren, Alexander J. Smits

Abstract: Large-amplitude oscillations of foils have been observed to yield greater propulsive efficiency than small-amplitude oscillations. Using scaling relations and experiments on foils with peak-to-peak trailing edge amplitudes of up to two chord lengths, we explain why this is so. In the process, we reveal the importance of drag, specifically how it can significantly reduce the efficiency, and how thi… ▽ More Large-amplitude oscillations of foils have been observed to yield greater propulsive efficiency than small-amplitude oscillations. Using scaling relations and experiments on foils with peak-to-peak trailing edge amplitudes of up to two chord lengths, we explain why this is so. In the process, we reveal the importance of drag, specifically how it can significantly reduce the efficiency, and how this effect depends on amplitude. The scaling relations and experimental data also reveal a fundamental tradeoff between high thrust and high efficiency, where the drag also plays a crucial role. △ Less

Submitted 26 September, 2019; originally announced September 2019.

Comments: 17 pages

Journal ref: Physical Review Fluids, Vol. 4, Issue 9, pp. 093102 (2019)

Swimmers' wake structures are not reliable indicators of swimming performance

Authors: Daniel Floryan, Tyler Van Buren, Alexander J. Smits

Abstract: The structure of swimmers' wakes is often assumed to be an indicator of swimming performance, that is, how momentum is produced and energy is consumed. Here, we discuss three cases where this assumption fails. In general, great care should be taken in deriving any conclusions about swimming performance from the wake flow pattern. The structure of swimmers' wakes is often assumed to be an indicator of swimming performance, that is, how momentum is produced and energy is consumed. Here, we discuss three cases where this assumption fails. In general, great care should be taken in deriving any conclusions about swimming performance from the wake flow pattern. △ Less

Submitted 26 March, 2020; v1 submitted 25 June, 2019; originally announced June 2019.

Comments: 6 pages

Journal ref: Bioinspiration & Biomimetics, Vol. 15, Issue 2, pp. 024001 (2020)

Roughness effects in laminar pipe flow

Authors: Utku Senturk, Alexander J. Smits

Abstract: The impact of wall roughness on fully developed laminar pipe flow is investigated numerically. The roughness is comprised of square bars of varying size and pitch. Results show that the inverse relation between the friction factor and the Reynolds number in smooth pipes still persists in rough pipes, regardless of the rib height and pitch. At a given Reynolds number, the friction factor varies qua… ▽ More The impact of wall roughness on fully developed laminar pipe flow is investigated numerically. The roughness is comprised of square bars of varying size and pitch. Results show that the inverse relation between the friction factor and the Reynolds number in smooth pipes still persists in rough pipes, regardless of the rib height and pitch. At a given Reynolds number, the friction factor varies quadratically with roughness height and linearly with roughness pitch. We propose a single correlation for the friction factor that successfully collapses the data. △ Less

Submitted 29 May, 2019; originally announced May 2019.

Efficient cruising for swimming and flying animals is dictated by fluid drag

Authors: Daniel Floryan, Tyler Van Buren, Alexander J. Smits

Abstract: Many swimming and flying animals are observed to cruise in a narrow range of Strouhal numbers, where the Strouhal number ${St = 2fA/U}$ is a dimensionless parameter that relates stroke frequency $f$, amplitude $A$, and forward speed $U$. Dolphins, sharks, bony fish, birds, bats, and insects typically cruise in the range $0.2 < St < 0.4$, which coincides with the Strouhal number range for maximum e… ▽ More Many swimming and flying animals are observed to cruise in a narrow range of Strouhal numbers, where the Strouhal number ${St = 2fA/U}$ is a dimensionless parameter that relates stroke frequency $f$, amplitude $A$, and forward speed $U$. Dolphins, sharks, bony fish, birds, bats, and insects typically cruise in the range $0.2 < St < 0.4$, which coincides with the Strouhal number range for maximum efficiency as found by experiments on heaving and pitching airfoils. It has therefore been postulated that natural selection has tuned animals to use this range of Strouhal numbers because it confers high efficiency, but the reason why this is so is still unclear. Here, by using simple scaling arguments, we argue that the Strouhal number for peak efficiency is largely determined by fluid drag on the fins and wings. △ Less

Submitted 10 April, 2019; originally announced April 2019.

Comments: 10 pages

Journal ref: Proceedings of the National Academy of Sciences, Vol. 115, Issue 32, pp. 8116-8118 (2018)

Achromatic varifocal metalens for the visible spectrum

Authors: Maxwell D. Aiello, Adam S. Backer, Aryeh J. Sapon, Janis Smits, John D. Perreault, Patrick Llull, Victor M. Acosta

Abstract: Metasurface optics provide an ultra-thin alternative to conventional refractive lenses. A present challenge is in realizing metasurfaces that exhibit tunable optical properties and achromatic behavior across the visible spectrum. Here, we report the design, fabrication, and characterization of metasurface lenses ("metalenses") that use asymmetric TiO2 nanostructures to induce a polarization-depend… ▽ More Metasurface optics provide an ultra-thin alternative to conventional refractive lenses. A present challenge is in realizing metasurfaces that exhibit tunable optical properties and achromatic behavior across the visible spectrum. Here, we report the design, fabrication, and characterization of metasurface lenses ("metalenses") that use asymmetric TiO2 nanostructures to induce a polarization-dependent optical response. By rotating the polarization of linearly-polarized input light, the focal length of a 40 micrometer-diameter metalens is tuned from 220-550 micrometers with nearly diffraction-limited performance. We show that imparting a wavelength-dependent polarization rotation on incident light enables achromatic focusing over a wide band of the visible spectrum, 483-620 nm. We use this property to demonstrate varifocal color imaging with white light from a halogen source. Tunable achromatic metalenses may be useful for applications in imaging and display. △ Less

Submitted 12 March, 2019; v1 submitted 10 March, 2019; originally announced March 2019.

Comments: Main text: 9 pages, 5 figures. Supplemental Information: 12 pages, 10 figures

Journal ref: ACS Photonics 6, 2432 (2019)

Two-dimensional nuclear magnetic resonance spectroscopy with a microfluidic diamond quantum sensor

Authors: Janis Smits, Joshua Damron, Pauli Kehayias, Andrew F. McDowell, Nazanin Mosavian, Ilja Fescenko, Nathaniel Ristoff, Abdelghani Laraoui, Andrey Jarmola, Victor M. Acosta

Abstract: Quantum sensors based on nitrogen-vacancy centers in diamond have emerged as a promising detection modality for nuclear magnetic resonance (NMR) spectroscopy owing to their micron-scale detection volume and non-inductive based detection. A remaining challenge is to realize sufficiently high spectral resolution and concentration sensitivity for multidimensional NMR analysis of picoliter sample volu… ▽ More Quantum sensors based on nitrogen-vacancy centers in diamond have emerged as a promising detection modality for nuclear magnetic resonance (NMR) spectroscopy owing to their micron-scale detection volume and non-inductive based detection. A remaining challenge is to realize sufficiently high spectral resolution and concentration sensitivity for multidimensional NMR analysis of picoliter sample volumes. Here, we address this challenge by spatially separating the polarization and detection phases of the experiment in a microfluidic platform. We realize a spectral resolution of 0.65 +/- 0.05 Hz, an order-of-magnitude improvement over previous diamond NMR studies. We use the platform to perform two-dimensional correlation spectroscopy of liquid analytes within an effective ~20 picoliter detection volume. The use of diamond quantum sensors as in-line microfluidic NMR detectors is a significant step towards applications in mass-limited chemical analysis and single cell biology. △ Less

Submitted 9 January, 2019; originally announced January 2019.

Comments: Main text: 7 pages, 4 figures. Supplemental Information: 9 pages, 7 figures, 1 table

Journal ref: Science Advances 5, eaaw7895 (2019)

Diamond magnetic microscopy of malarial hemozoin nanocrystals

Authors: Ilja Fescenko, Abdelghani Laraoui, Janis Smits, Nazanin Mosavian, Pauli Kehayias, Jong Seto, Lykourgos Bougas, Andrey Jarmola, Victor M. Acosta

Abstract: Magnetic microscopy of malarial hemozoin nanocrystals was performed using optically detected magnetic resonance imaging of near-surface diamond nitrogen-vacancy centers. Hemozoin crystals were extracted from $Plasmodium$-$falciparum$-infected human blood cells and studied alongside synthetic hemozoin crystals. The stray magnetic fields produced by individual crystals were imaged at room temperatur… ▽ More Magnetic microscopy of malarial hemozoin nanocrystals was performed using optically detected magnetic resonance imaging of near-surface diamond nitrogen-vacancy centers. Hemozoin crystals were extracted from $Plasmodium$-$falciparum$-infected human blood cells and studied alongside synthetic hemozoin crystals. The stray magnetic fields produced by individual crystals were imaged at room temperature as a function of applied field up to 350 mT. More than 100 nanocrystals were analyzed, revealing the distribution of their magnetic properties. Most crystals ($96\%$) exhibit a linear dependence of stray field magnitude on applied field, confirming hemozoin's paramagnetic nature. A volume magnetic susceptibility $χ=3.4\times10^{-4}$ is inferred using a magnetostatic model informed by correlated scanning electron microscopy measurements of crystal dimensions. A small fraction of nanoparticles (4/82 for $Plasmodium$-produced and 1/41 for synthetic) exhibit a saturation behavior consistent with superparamagnetism. Translation of this platform to the study of living malaria-infected cells may shed new light on hemozoin formation dynamics and their interaction with antimalarial drugs. △ Less

Submitted 19 September, 2018; v1 submitted 10 August, 2018; originally announced August 2018.

Comments: Main text: 8 pages and 5 figures, Supplemental Information: 9 pages and 8 figures

Journal ref: Phys. Rev. Applied 11, 034029 (2019)

Scaling and performance of simultaneously heaving and pitching foils

Authors: Tyler Van Buren, Daniel Floryan, Alexander J. Smits

Abstract: We consider the propulsive performance of an unsteady heaving and pitching foil, experimentally studying an extensive parameter space of motion amplitudes, frequencies, and phase offsets between the heave and pitch motions. The phase offset $φ$ between the heaving and pitching motions proves to be a critical parameter in determining the dynamics of the foil and its propulsive performance. To maxim… ▽ More We consider the propulsive performance of an unsteady heaving and pitching foil, experimentally studying an extensive parameter space of motion amplitudes, frequencies, and phase offsets between the heave and pitch motions. The phase offset $φ$ between the heaving and pitching motions proves to be a critical parameter in determining the dynamics of the foil and its propulsive performance. To maximize thrust, the heave and pitch motions need to be nearly in phase ($φ=330^\circ$), but to maximize efficiency, the pitch motion needs to lag the heave motion ($φ=270^\circ$), corresponding to slicing motions with a minimal angle of attack. We also present scaling relations, developed from lift-based and added mass forces, which collapse our experimental data. Using the scaling relations as a guide, we find increases in performance when foil amplitudes (specifically pitch) increase while maintaining a modest angle of attack. △ Less

Submitted 23 January, 2018; originally announced January 2018.

Flow speed has little impact on propulsive characteristics of oscillating foils

Authors: Tyler Van Buren, Daniel Floryan, Nathaniel Wei, Alexander J. Smits

Abstract: Experiments are reported on the performance of a pitching and heaving two-dimensional foil in a water channel in either continuous or intermittent motion. We find that the thrust and power are independent of the mean freestream velocity for two-fold changes in the mean velocity (four-fold in the dynamic pressure), and for oscillations in the velocity up to 38\% of the mean, where the oscillations… ▽ More Experiments are reported on the performance of a pitching and heaving two-dimensional foil in a water channel in either continuous or intermittent motion. We find that the thrust and power are independent of the mean freestream velocity for two-fold changes in the mean velocity (four-fold in the dynamic pressure), and for oscillations in the velocity up to 38\% of the mean, where the oscillations are intended to mimic those of freely swimming motions where the thrust varies during the flapping cycle. We demonstrate that the correct velocity scale is not the flow velocity but the mean velocity of the trailing edge. We also find little or no impact of streamwise velocity change on the wake characteristics such as vortex organization, vortex strength, and time-averaged velocity profile development---the wake is both qualitatively and quantitatively unchanged. Our results suggest that constant velocity studies can be used to make robust conclusions about swimming performance without a need to explore the free-swimming condition. △ Less

Submitted 16 January, 2018; v1 submitted 18 July, 2017; originally announced July 2017.

Comments: A more recent version of this article has been accepted to Physical Review Fluids

Journal ref: Phys. Rev. Fluids 3, 013103 (2018)

Scaling the propulsive performance of heaving and pitching foils

Authors: Daniel Floryan, Tyler Van Buren, Clarence W. Rowley, Alexander J. Smits

Abstract: Scaling laws for the propulsive performance of rigid foils undergoing oscillatory heaving and pitching motions are presented. Water tunnel experiments on a nominally two-dimensional flow validate the scaling laws, with the scaled data for thrust, power, and efficiency all showing excellent collapse. The analysis indicates that the behaviour of the foils depends on both Strouhal number and reduced… ▽ More Scaling laws for the propulsive performance of rigid foils undergoing oscillatory heaving and pitching motions are presented. Water tunnel experiments on a nominally two-dimensional flow validate the scaling laws, with the scaled data for thrust, power, and efficiency all showing excellent collapse. The analysis indicates that the behaviour of the foils depends on both Strouhal number and reduced frequency, but for motions where the viscous drag is small the thrust closely follows a linear dependence on reduced frequency. The scaling laws are also shown to be consistent with biological data on swimming aquatic animals. △ Less

Submitted 24 April, 2017; originally announced April 2017.

Comments: 11 pages

Journal ref: Journal of Fluid Mechanics, Vol. 822, pp. 386-397 (2017)

Forces and energetics of intermittent swimming

Authors: Daniel Floryan, Tyler Van Buren, Alexander J. Smits

Abstract: Experiments are reported on intermittent swimming motions. Water tunnel experiments on a nominally two-dimensional pitching foil show that the mean thrust and power scale linearly with the duty cycle, from a value of 0.2 all the way up to continuous motions, indicating that individual bursts of activity in intermittent motions are independent of each other. This conclusion is corroborated by PIV f… ▽ More Experiments are reported on intermittent swimming motions. Water tunnel experiments on a nominally two-dimensional pitching foil show that the mean thrust and power scale linearly with the duty cycle, from a value of 0.2 all the way up to continuous motions, indicating that individual bursts of activity in intermittent motions are independent of each other. This conclusion is corroborated by PIV flow visualizations, which show that the main vortical structures in the wake do not change with duty cycle. The experimental data also demonstrate that intermittent motions are generally energetically advantageous over continuous motions. When metabolic energy losses are taken into account, this conclusion is maintained for metabolic power fractions less than 1. △ Less

Submitted 5 April, 2017; v1 submitted 2 March, 2017; originally announced March 2017.

Comments: 9 pages

Journal ref: Acta Mechanica Sinica, Vol. 33, Issue 4, pp.725-732 (2017)

Solution nuclear magnetic resonance spectroscopy on a nanostructured diamond chip

Authors: P. Kehayias, A. Jarmola, N. Mosavian, I. Fescenko, F. M. Benito, A. Laraoui, J. Smits, L. Bougas, D. Budker, A. Neumann, S. R. J. Brueck, V. M. Acosta

Abstract: We demonstrate nuclear magnetic resonance (NMR) spectroscopy of picoliter-volume solutions with a nanostructured diamond chip. Using optical interferometric lithography, diamond surfaces were nanostructured with dense, high-aspect-ratio nanogratings, enhancing the surface area by more than a factor of 15 over mm^2 regions of the chip. The nanograting sidewalls were doped with nitrogen-vacancy (NV)… ▽ More We demonstrate nuclear magnetic resonance (NMR) spectroscopy of picoliter-volume solutions with a nanostructured diamond chip. Using optical interferometric lithography, diamond surfaces were nanostructured with dense, high-aspect-ratio nanogratings, enhancing the surface area by more than a factor of 15 over mm^2 regions of the chip. The nanograting sidewalls were doped with nitrogen-vacancy (NV) centers so that more than 10 million NV centers in a (25 micrometer)^2 laser spot are located close enough to the diamond surface (5 nm) to detect the NMR spectrum of 1 pL of fluid lying within adjacent nanograting grooves. The platform was used to perform 1H and 19F NMR spectroscopy at room temperature in magnetic fields below 50 mT. Using a solution of CsF in glycerol, we demonstrate that 4 +/- 2 x 10^12 19F spins in a 1 pL volume, can be detected with a signal-to-noise ratio of 3 in 1 s integration. This represents nearly two orders of magnitude improvement in concentration sensitivity over previous NV and picoliter NMR studies. △ Less

Submitted 5 January, 2017; originally announced January 2017.

Comments: 7 pages, 6 figures (main text) + 11 pages, 7 figures, 5 tables (Supplemental)

Measurement of local dissipation scales in turbulent pipe flow

Authors: Sean C. C. Bailey, Marcus Hultmark, Joerg Schumacher, Victor Yakhot, Alexander J. Smits

Abstract: Local dissipation scales are a manifestation of the intermittent small-scale nature of turbulence. We report the first experimental evaluation of the distribution of local dissipation scales in turbulent pipe flows for a range of Reynolds numbers, 2.4x10^4<=Re_D<=7.0x10^4. Our measurements at the nearly isotropic pipe centerline and within the anisotropic logarithmic layer show excellent agreeme… ▽ More Local dissipation scales are a manifestation of the intermittent small-scale nature of turbulence. We report the first experimental evaluation of the distribution of local dissipation scales in turbulent pipe flows for a range of Reynolds numbers, 2.4x10^4<=Re_D<=7.0x10^4. Our measurements at the nearly isotropic pipe centerline and within the anisotropic logarithmic layer show excellent agreement with distributions that were previously calculated from numerical simulations of homogeneous isotropic box turbulence and with those predicted by theory. The reported results suggest a universality of the smallest-scale fluctuations around the classical Kolmogorov dissipation length. △ Less

Submitted 29 June, 2009; v1 submitted 5 June, 2009; originally announced June 2009.

Comments: 4 pages, 4 Postscript figures, Physical Review Letters, v1: accepted version, v2: published version, v3: replaced published version with authors' tex version to avoid copyright violation

Journal ref: Physical Review Letters, 103, 014502 (2009)