Showing 1–29 of 29 results for author: Khoo, B C

Drag reduction study of naturally occurring oscillating axial flow induced by helical corrugated surface in Taylor Couette flow

Authors: M. A. Razzak, B. C. Khoo, K. B. Lua, C. M. J. Tay

Abstract: This study investigates drag reduction capability of naturally-occurring-oscillating axial secondary flow(ASF) induced by helical-corrugated surface in Taylor Couette flow(TCFHelical) for three values of pitch to wavelength-ratios(P* =1,2,3) and amplitude to wavelength-ratio(A*) of 0.25. As reported in Razzak et al. (2020), emergence of naturally-occurring-oscillating ASF induced by longitudinal-c… ▽ More This study investigates drag reduction capability of naturally-occurring-oscillating axial secondary flow(ASF) induced by helical-corrugated surface in Taylor Couette flow(TCFHelical) for three values of pitch to wavelength-ratios(P* =1,2,3) and amplitude to wavelength-ratio(A*) of 0.25. As reported in Razzak et al. (2020), emergence of naturally-occurring-oscillating ASF induced by longitudinal-corrugated surface in TCF(TCFLongitudinal) and increasing trend on its magnitude with Reynolds number (Re) results in the occurrence of drag reduction. This has motivated us to study the possibility of enhancing drag reduction by maintaining a consistently increasing trend with Re in the magnitude of naturally-occurring-oscillating ASF induced by the helical-corrugated surface on the stationary outer cylinder in TCF. From flow structures, steady ASF with non-zero mean is observed at Re=60 which suppresses the strength of azimuthal vorticities for Re>85 and contributed to occurrence of drag reduction. As Re is increased to 100,90 and 85 for P* =1,2, and 3, respectively, formation of periodic oscillating ASF with non-zero mean and its increasing trend in magnitude with Re suppresses azimuthal vorticities further which contributes to the maximum drag reduction of 13%. For Re>165,145 and 140 for P*=1,2 and 3, respectively, non-periodic oscillating ASF is observed, and its magnitude remains nearly unchanged or decreases slightly with Re which results in the suppression effect of azimuthal vortices to be weaker. This results in decrease in drag reduction. Oscillating ASF observed in TCFHelical is found to occur at earlier Re and it is stronger than that of TCFLongitudinal which contributes to occurrence of higher drag reduction in TCFHelical. △ Less

Submitted 30 May, 2023; v1 submitted 12 May, 2023; originally announced May 2023.

A variational quantum algorithm-based numerical method for solving potential and Stokes flows

Authors: Yangyang Liu, Zhen Chen, Chang Shu, Patrick Rebentrost, Yaguang Liu, S. C. Chew, B. C. Khoo, Y. D. Cui

Abstract: This paper presents a numerical method based on the variational quantum algorithm to solve potential and Stokes flow problems. In this method, the governing equations for potential and Stokes flows can be respectively written in the form of Laplace's equation and Stokes equations using velocity potential, stream function and vorticity formulations. Then the finite difference method and the general… ▽ More This paper presents a numerical method based on the variational quantum algorithm to solve potential and Stokes flow problems. In this method, the governing equations for potential and Stokes flows can be respectively written in the form of Laplace's equation and Stokes equations using velocity potential, stream function and vorticity formulations. Then the finite difference method and the generalised differential quadrature (GDQ) method are applied to discretize the governing equations. For the prescribed boundary conditions, the corresponding linear systems of equations can be obtained. These linear systems are solved by using the variational quantum linear solver (VQLS), which resolves the potential and Stokes flow problems equivalently. To the best of authors' knowledge, this is the first study that incorporates the GDQ method which is inherently a high-order discretization method with the VQLS algorithm. Since the GDQ method can utilize much fewer grid points than the finite difference method to approximate derivatives with a higher order of accuracy, the size of the input matrix for the VQLS algorithm can be smaller. In this way, the computational cost may be saved. The performance of the present method is comprehensively assessed by two representative examples, namely, the potential flow around a circular cylinder and Stokes flow in a lid-driven cavity. Numerical results validate the applicability and accuracy of the present VQLS-based method. Furthermore, its time complexity is evaluated by the heuristic scaling, which demonstrates that the present method scales efficiently in the number of qubits and the precision. This work brings quantum computing to the field of computational fluid dynamics. By virtue of quantum advantage over classical methods, promising advances in solving large-scale fluid mechanics problems of engineering interest may be prompted. △ Less

Submitted 3 March, 2023; originally announced March 2023.

Accurate near wall steady flow field prediction using Physics Informed Neural Network (PINN)

Authors: Vinothkumar Sekar, Qinghua Jiang, Chang Shu, Boo Cheong Khoo

Abstract: In this paper, Physics Informed Neural Network (PINN) is explored in order to obtain flow predictions near the wall region accurately with measurements (or sampling points) away from the wall. Often, in fluid mechanics experiments, it is difficult to perform velocity measurements near the wall accurately. Therefore, the present study reveals a new and elegant approach to recover the flow solutions… ▽ More In this paper, Physics Informed Neural Network (PINN) is explored in order to obtain flow predictions near the wall region accurately with measurements (or sampling points) away from the wall. Often, in fluid mechanics experiments, it is difficult to perform velocity measurements near the wall accurately. Therefore, the present study reveals a new and elegant approach to recover the flow solutions near the wall. Laminar boundary layer flow over a flat plate case is considered for this study in order to explore the ability of PINN to accurately predict the flow field. All the required sampling data for this study is obtained from CFD simulations. A wide range of Reynolds number cases from Re=500 to 100000 has been investigated. First, using PINN, the boundary layer solution is obtained with three different types of boundary conditions. Further, the influence of the location of the sampling points on the accuracy is analysed. From the velocity profiles and the skin friction coefficient distribution, it is clear that PINN results are reasonably accurate near the wall with only a few sampling points away from the wall. This approach has potential application in experiments to obtain the near wall solutions accurately with measurements away from the wall. △ Less

Submitted 7 April, 2022; originally announced April 2022.

A high-fidelity numerical study on the propulsive performance of pitching flexible plates

Authors: Guojun Li, Gael Kemp, Rajeev Kumar Jaiman, Boo Cheong Khoo

Abstract: In this paper, we numerically investigate the propulsive performance of three-dimensional pitching flexible plates with varying flexibility and trailing edge shapes. To eliminate the effect of other geometric parameters, only the trailing edge angle is varied from 45° (concave), 90° (rectangular) to 135° (convex) while maintaining the constant area of the flexible plate. We examine the impact of t… ▽ More In this paper, we numerically investigate the propulsive performance of three-dimensional pitching flexible plates with varying flexibility and trailing edge shapes. To eliminate the effect of other geometric parameters, only the trailing edge angle is varied from 45° (concave), 90° (rectangular) to 135° (convex) while maintaining the constant area of the flexible plate. We examine the impact of the frequency ratio f* defined as the ratio of the natural frequency of the flexible plate to the actuated pitching frequency. Through our numerical simulations, we find that the global maximum mean thrust occurs near f*=1 corresponding to the resonance condition. However, the optimal propulsive efficiency is achieved around f*=1.54 instead of the resonance condition. While the convex plate with low and high bending stiffness values shows the best performance, the rectangular plate with moderate bending stiffness is the most efficient propulsion configuration. Through dynamic mode decomposition, we find that the passive deformation can help in redistributing the pressure gradient thus improving the efficiency and thrust production. A momentum-based thrust evaluation approach is adopted to link the instantaneous vortical structures with the time-dependent thrust. When the vortices detach from the trailing edge, the instantaneous thrust shows the largest values due to the strong momentum change and convection process. Moderate flexibility and convex shape help transfer momentum to the fluid, thereby improving thrust generation and promoting the transition from drag to thrust. The increase of the trailing edge angle can broaden the range of flexibility that produces positive mean thrust. △ Less

Submitted 7 October, 2021; v1 submitted 2 March, 2021; originally announced March 2021.

Flow-excited membrane instability at moderate Reynolds numbers

Authors: Guojun Li, Rajeev Kumar Jaiman, Boo Cheong Khoo

Abstract: In this paper, we study the fluid-structure interaction (FSI) of a three-dimensional (3D) flexible membrane immersed in an unsteady separated flow at moderate Reynolds numbers. We employ a body-conforming variational FSI solver based on the recently developed partitioned iterative scheme for the coupling of turbulent fluid flow with nonlinear structural dynamics. Of particular interest is to under… ▽ More In this paper, we study the fluid-structure interaction (FSI) of a three-dimensional (3D) flexible membrane immersed in an unsteady separated flow at moderate Reynolds numbers. We employ a body-conforming variational FSI solver based on the recently developed partitioned iterative scheme for the coupling of turbulent fluid flow with nonlinear structural dynamics. Of particular interest is to understand the flow-excited instability of a 3D flexible membrane as a function of the non-dimensional mass ratio, Reynolds number and aeroelastic number. For a wide range of the parameters, we examine two distinctive stability regimes of fluid-membrane interaction: deformed-steady state (DSS) and dynamic balance state (DBS). We propose stability phase diagrams to demarcate the DSS and DBS regimes for the parameter space of mass ratio vs. Reynolds number and mass ratio vs. aeroelastic number. Based on the aeroelastic mode analysis, we observe a frequency synchronization between the vortex shedding frequency and the membrane vibration frequency which leads to self-sustained vibrations in the dynamic balance state. To characterize the origin of the frequency lock-in, we derive an approximate analytical formula for the nonlinear natural frequency by considering the added mass effect and employing a large deflection theory for a simply supported rectangular membrane. Through our systematic high-fidelity numerical investigation, we find that the onset of the membrane vibration and the mode transition has a dependence on the frequency lock-in between the natural frequency of the tensioned membrane and the vortex shedding frequency or its harmonics. These findings on the fluid-elastic instability of membranes have implications for the design and development of control strategies for membrane wing-based unmanned systems and drones. △ Less

Submitted 7 October, 2021; v1 submitted 20 November, 2020; originally announced November 2020.

Comments: arXiv admin note: text overlap with arXiv:2006.13467

Journal ref: J. Fluid Mech. 929 (2021) A40

Aeroelastic mode decomposition framework and mode selection mechanism in fluid-membrane interaction

Authors: G. Li, R. K. Jaiman, B. C. Khoo

Abstract: In this study, we present a global Fourier mode decomposition framework for unsteady fluid-structure interaction. We apply the framework to isolate and extract the aeroelastic modes arising from a coupled three-dimensional fluid-membrane system. The proposed framework is employed to decompose the physical variables in the fluid and structural domains into frequency-ranked aeroelastic modes in a un… ▽ More In this study, we present a global Fourier mode decomposition framework for unsteady fluid-structure interaction. We apply the framework to isolate and extract the aeroelastic modes arising from a coupled three-dimensional fluid-membrane system. The proposed framework is employed to decompose the physical variables in the fluid and structural domains into frequency-ranked aeroelastic modes in a unified way. We observe the frequency synchronization between the vortex shedding and the structural vibration via mode decomposition analysis. We examine the role of flexibility in the aeroelastic mode selection and perform a systematic comparison of flow features among a rigid wing, a rigid cambered wing and a flexible membrane. With the aid of our mode decomposition technique, we find that the dominant structural mode exhibits a chordwise second and spanwise first mode at different angles of attack. The structural natural frequency corresponding to this mode is estimated using an approximate analytical formula. By examining the dominant frequency of the coupled system, we find that the dominant membrane vibrational mode is selected via the frequency lock-in between the dominant vortex shedding frequency and the structural natural frequency. From the fluid modes and the mode energy spectra at $α=20^\circ$ and $25^\circ$, the aeroelastic modes corresponding to the non-integer frequency components lower than the dominant frequency are found to be associated with the bluff body vortex shedding instability. The non-periodic aeroelastic response observed at higher angles of attack are related to the interaction between aeroelastic modes caused by the frequency lock-in and the bluff-body-like vortex shedding. △ Less

Submitted 12 July, 2021; v1 submitted 24 June, 2020; originally announced June 2020.

Models for Predicting Transonic Flutter of a Wing-Section with Sloshing in an Embedded Fuel Tank

Authors: Shashank Srivastava, Murali Damodaran, Boo Cheong Khoo

Abstract: The present study focuses on the development, application, and comparison of three computational frameworks of varying fidelities for assessing the effects of fuel sloshing in internal fuel tanks on the aeroelastic characteristics of a wing section. The first approach uses the coupling of compressible flow solver for external aerodynamics integrated with structural solver and incompressible multip… ▽ More The present study focuses on the development, application, and comparison of three computational frameworks of varying fidelities for assessing the effects of fuel sloshing in internal fuel tanks on the aeroelastic characteristics of a wing section. The first approach uses the coupling of compressible flow solver for external aerodynamics integrated with structural solver and incompressible multiphase flow solver for fuel sloshing in the embedded fuel tank As time-domain flutter solution of these coupled solvers is computationally expensive, two approximate surrogate models to emulate sloshing flows are considered. One surrogate model utilizes a linearised approach for sloshing load computations by creating an Equivalent Mechanical System (EMS) with its parameters derived from potential flow theory. The other surrogate model aims to efficiently describe the dominant dynamic characteristics of the underlying system by employing the Radial Basis Function Neural Networks (RBF-NN) using limited CFD-based data to calibrate this model. The flutter boundaries of a wing section with and without the effects of fuel sloshing are compared. The limitation of the EMS surrogate to represent nonlinearities are reflected in this study. The RBF-NN surrogate shows remarkable agreement with the high-fidelity solution for sloshing with significantly low computational cost, thereby motivating extension to three-dimensional problems. △ Less

Submitted 15 December, 2019; originally announced December 2019.

Machine Learning Surrogates for Predicting Response of an Aero-Structural-Sloshing System

Authors: Shashank Srivastava, Murali Damodaran, Boo Cheong Khoo

Abstract: This study demonstrates the feasibility of developing machine learning (ML) surrogates based on Recurrent Neural Networks (RNN) for predicting the unsteady aeroelastic response of transonic pitching and plunging wing-fuel tank sloshing system by considering an approximate simplified model of an airfoil in transonic flow and sloshing loads from a partially filled fuel tank rigidly embedded inside t… ▽ More This study demonstrates the feasibility of developing machine learning (ML) surrogates based on Recurrent Neural Networks (RNN) for predicting the unsteady aeroelastic response of transonic pitching and plunging wing-fuel tank sloshing system by considering an approximate simplified model of an airfoil in transonic flow and sloshing loads from a partially filled fuel tank rigidly embedded inside the airfoil and undergoing a free unsteady motion. The ML surrogates are then used to predict the aeroelastic response of the coupled system. The external aerodynamic loads on the airfoil and the two-phase sloshing loads data for training the RNN are generated using open-source computational fluid dynamics (CFD) codes. The aerodynamic force and moment coefficients are predicted from the surrogate model based on its motion history. Similarly, the lateral and vertical forces and moments from fuel sloshing in the fuel tank are predicted using the surrogate model resulting from the motion of the embedded fuel tank. Comparing the free motion of the airfoil without sloshing tank to the free motion of the airfoil with a partially-filled fuel tank shows that the effects of sloshing on the aeroelastic motion of the aero-structural system. The effectiveness of the predictions from RNN are then assessed by comparing with the results from the high-fidelity coupled aero-structural-fuel tank sloshing simulations. It is demonstrated that the surrogate models can accurately and economically predict the coupled aero-structural motion. △ Less

Submitted 18 November, 2019; originally announced November 2019.

Stokesian dynamics of pill-shaped Janus particles with stick and slip boundary conditions

Authors: Q. Sun, E. Klaseboer, B. C. Khoo, D. Y. C. Chan

Abstract: We study the forces and torques experienced by pill-shaped Janus particles of different aspect ratios where half of the surface obeys the no-slip boundary condition and the other half obeys the Navier slip condition of varying slip lengths. Using a recently developed boundary integral formulation whereby the traditional singular behaviour of this approach is removed analytically, we quantify the s… ▽ More We study the forces and torques experienced by pill-shaped Janus particles of different aspect ratios where half of the surface obeys the no-slip boundary condition and the other half obeys the Navier slip condition of varying slip lengths. Using a recently developed boundary integral formulation whereby the traditional singular behaviour of this approach is removed analytically, we quantify the strength of the forces and torques experienced by such particles in a uniform flow field in the Stokes regime. Depending on the aspect ratio and the slip length, the force transverse to the flow direction can change sign. This is a novel property unique to the Janus nature of the particles. △ Less

Submitted 10 February, 2019; originally announced February 2019.

Journal ref: Physical Review E 87, 043009 (2013)

A robust and non-singular formulation of the boundary integral method for the potential problem

Authors: Q. Sun, E. Klaseboer, B. C. Khoo, D. Y. C. Chan

Abstract: A non-singular formulation of the boundary integral method (BIM) is presented for the Laplace equation whereby the well-known singularities that arise from the fundamental solution are eliminated analytically. A key advantage of this approach is that numerical errors that arise due to the proximity of nodes located on osculating boundaries are suppressed. This is particularly relevant in multi-sca… ▽ More A non-singular formulation of the boundary integral method (BIM) is presented for the Laplace equation whereby the well-known singularities that arise from the fundamental solution are eliminated analytically. A key advantage of this approach is that numerical errors that arise due to the proximity of nodes located on osculating boundaries are suppressed. This is particularly relevant in multi-scale problems where high accuracy is required without undue increase in computational cost when the spacing between boundaries become much smaller than their characteristic dimensions. The elimination of the singularities means that standard quadrature can be used to evaluate the surface integrals and this results in about 60% savings in coding effort. The new formulation also affords a numerically robust way to calculate the potential close to the boundaries. Detailed implementations of this approach are illustrated with problems involving osculating boundaries, 2D domains with corners and a wave drag problem in a 3D semi-infinite domain. The explicit formulation of problems with axial symmetry is also given. △ Less

Submitted 7 February, 2019; originally announced February 2019.

Journal ref: Engineering Analysis with Boundary Elements 43 (2014) 117

Boundary regularised integral equation formulation of the Helmholtz equation in acoustics

Authors: Q. Sun, E. Klaseboer, B. C. Khoo, D. Y. C. Chan

Abstract: A boundary integral formulation for the solution of the Helmholtz equation is developed in which all traditional singular behaviour in the boundary integrals is removed analytically. The numerical precision of this approach is illustrated with calculation of the pressure field due to radiating bodies in acoustic wave problems. This method facilitates the use of higher order surface elements to rep… ▽ More A boundary integral formulation for the solution of the Helmholtz equation is developed in which all traditional singular behaviour in the boundary integrals is removed analytically. The numerical precision of this approach is illustrated with calculation of the pressure field due to radiating bodies in acoustic wave problems. This method facilitates the use of higher order surface elements to represent boundaries, resulting in a significant reduction in the problem size with improved precision. Problems with extreme geometric aspect ratios can also be handled without diminished precision. When combined with the CHIEF method, uniqueness of the solution of the exterior acoustic problem is assured without the need to solve hypersingular integrals. △ Less

Submitted 7 February, 2019; originally announced February 2019.

Journal ref: Royal Society Open Science 2 (2015) 140520

Boundary regularized integral equation formulation (BRIEF) of Stokes flow

Authors: Q. Sun, E. Klaseboer, B. C. Khoo, D. Y. C. Chan

Abstract: Single-phase Stokes flow problems with prescribed boundary conditions can be formulated in terms of a boundary regularized integral equation that is completely free of singularities that exist in the traditional formulation. The usual mathematical singularities that arise from using the fundamental solution in the conventional boundary integral method are removed by subtracting a related auxiliary… ▽ More Single-phase Stokes flow problems with prescribed boundary conditions can be formulated in terms of a boundary regularized integral equation that is completely free of singularities that exist in the traditional formulation. The usual mathematical singularities that arise from using the fundamental solution in the conventional boundary integral method are removed by subtracting a related auxiliary flow field, $\boldsymbol{w}$, that can be constructed from one of many known fundamental solutions of the Stokes equation. This approach is exact and does not require the introduction of additional cutoff parameters. The numerical implementation of this boundary regularized integral equation formulation affords considerable savings in coding effort with improved numerical accuracy. The high accuracy of this formulation is retained even in problems where parts of the boundaries may almost be in contact. △ Less

Submitted 7 February, 2019; originally announced February 2019.

Journal ref: PHYSICS OF FLUIDS 27, 023102 (2015)

Influence of Magnetic Force on the Flow Stability in a Rectangular duct

Authors: Rahman Anisur, Wenqia Xu, Kunhang Li, Hua-Shu Dou, Boo Cheong Khoo, Jie Mao

Abstract: The stability of the flow under the magnetic force is one of the classical problems in fluid mechanics. In this paper, the flow in a rectangular duct with different Hartmann (Ha) number is simulated. The finite volume method and the SIMPLE algorithm are used to solve a system of equations and the energy gradient theory is then used to study the (associated) stability of magnetohydrodynamics (MHD).… ▽ More The stability of the flow under the magnetic force is one of the classical problems in fluid mechanics. In this paper, the flow in a rectangular duct with different Hartmann (Ha) number is simulated. The finite volume method and the SIMPLE algorithm are used to solve a system of equations and the energy gradient theory is then used to study the (associated) stability of magnetohydrodynamics (MHD). The flow stability of MHD flow for different Hartmann (Ha) number, from Ha=1 to 40, at the fixed Reynolds number, Re=190 are investigated. The simulation is validated firstly against the simulation in literature. The results show that, with the increasing Ha number, the centerline velocity of the rectangular duct with MHD flow decreases and the absolute value of the gradient of total mechanical energy along the streamwise direction increases. The maximum of K appears near the wall in both coordinate axis of the duct. According to the energy gradient theory, this position of the maximum of K would initiate flow instability (if any) than the other positions. The higher the Hartmann number is, the smaller the K value becomes, which means that the fluid becomes more stable in the presence of higher magnetic force. As the Hartmann number increases, the K value in the parallel layer decreases more significantly than in the Hartmann layer. The most dangerous position of instability tends to migrate towards wall of the duct as the Hartmann number increases. Thus, with the energy gradient theory, the stability or instability in the rectangular duct can be controlled by modulating the magnetic force. △ Less

Submitted 6 February, 2019; v1 submitted 1 February, 2019; originally announced February 2019.

Comments: 13 pages, 9 figures

Journal ref: Advances in Appl. Math. and Mech., 11(1), Feb. 2019, 24-37

Numerical study on wide gap Taylor Couette flow with flow transition

Authors: Md Abdur Razzak, Boo Cheong Khoo, Kim Boon Lua

Abstract: This study aims to investigate the possible sources of non-axisymmetric disturbances and their propagation mechanism in Taylor Couette flow (TCF) for wide gap problems using direct numerical simulation with a radius ratio of 0.5 and Reynolds number (Re) ranging from 60 to 650. Here, attention is focused on the viscous layer (VL) thickness in near-wall regions and its spatial distribution along the… ▽ More This study aims to investigate the possible sources of non-axisymmetric disturbances and their propagation mechanism in Taylor Couette flow (TCF) for wide gap problems using direct numerical simulation with a radius ratio of 0.5 and Reynolds number (Re) ranging from 60 to 650. Here, attention is focused on the viscous layer (VL) thickness in near-wall regions and its spatial distribution along the axial direction to gain an insight into the origin and propagation of non-axisymmetric disturbances. The results show that an axisymmetric Taylor-vortex flow occurs when Re is between 68 and 425. Above Re = 425, transition from axisymmetric to non-axisymmetric flow is observed up to Re = 575 before the emergence of wavy-vortex flow. From the variation of VL thickness with Re, the VL does not experience any significant changes in the flow separation region of the inner wall, as well as jet impingement region of both the inner and outer walls. However, a sudden increase in VL thickness in the flow separation region of the outer wall reveals possible source of non-axisymmetric disturbances in the flow separation region of the outer wall. These disturbances develop into the periodic secondary flow as the axisymmetric flow transforms into non-axisymmetric flow and this leads to the emergence of azimuthal wave. The periodic secondary flow contributes to sudden increase in the natural wavelength and rapid reduction in the strength of two counter-rotating Taylor vortices. This in turn leads to a substantial reduction of torque in the transition flow vis-a-vis axisymmetric Taylor-vortex flow. △ Less

Submitted 23 August, 2019; v1 submitted 25 January, 2019; originally announced January 2019.

Stability of boundary layer flow based on energy gradient theory

Authors: Hua-Shu Dou, Wenqian Xu, Boo Cheong Khoo

Abstract: The flow of the laminar boundary layer on a flat plate is studied with simulation of Navier-Stokes equations. The mechanisms of flow instability at external edge of the boundary layer and near the wall are analyzed using the energy gradient theory. The simulation results show that there is an overshoot on the velocity profile at the external edge of the boundary layer. At this overshoot, the energ… ▽ More The flow of the laminar boundary layer on a flat plate is studied with simulation of Navier-Stokes equations. The mechanisms of flow instability at external edge of the boundary layer and near the wall are analyzed using the energy gradient theory. The simulation results show that there is an overshoot on the velocity profile at the external edge of the boundary layer. At this overshoot, the energy gradient function is very large which results in instability according to the energy gradient theory. It is found that the transverse gradient of the total mechanical energy is responsible for the instability at the external edge of the boundary layer, which induces the entrainment of external flow into the boundary layer. Within the boundary layer, there is a maximum of the energy gradient function near the wall, which leads to intensive flow instability near the wall and contributes to the generation of turbulence. △ Less

Submitted 19 June, 2018; originally announced June 2018.

Comments: 5 pages; 4 figures

Journal ref: Modern Physics Letters B, Vol. 32, Nos. 12 & 13 (2018), 1840003

Simulations of Detonation Wave Propagation in Rectangular Ducts Using a Three-Dimensional WENO Scheme

Authors: Hua-Shu Dou, Her Mann Tsai, Boo Cheong Khoo, Jianxian Qiu

Abstract: This paper reports high resolution simulations using a fifth-order weighted essentially non-oscillatory (WENO) scheme with a third order TVD Runge-Kutta time stepping method to examine the features of detonation front and physics in square ducts. The simulations suggest that two and three-dimensional detonation wave front formations are greatly enhanced by the presence of transverse waves. The mot… ▽ More This paper reports high resolution simulations using a fifth-order weighted essentially non-oscillatory (WENO) scheme with a third order TVD Runge-Kutta time stepping method to examine the features of detonation front and physics in square ducts. The simulations suggest that two and three-dimensional detonation wave front formations are greatly enhanced by the presence of transverse waves. The motion of transverse waves generates triple points (zones of high pressure and large velocity coupled together), which cause the detonation front to become locally overdriven and thus form "hot spots". The transversal motion of these hot spots maintains the detonation to continuously occur along the whole front in two and three-dimensions. The present simulations indicate that the influence of the transverse waves on detonation is more profound in three dimensions and the pattern of quasi-steady detonation fronts also depends on the duct size. For a narrow duct (4LX4L where L is the half reaction length), the detonation front displays a distinctive "spinning" motion about the axial direction with a well-defined period. For a wider duct (20LX20L), the detonation front exhibits a "rectangular mode" periodically, with the front displaying "convex" and "concave" shapes one following the other and the transverse waves on the four walls being partly out-of-phase with each other. △ Less

Submitted 7 June, 2010; originally announced June 2010.

Comments: 31 pages. 14 figuees

Journal ref: Combustion & Flame, Vol.154, 2008, pp.644-659

Effect of Initial Disturbance on The Detonation Front Structure of a Narrow Duct

Authors: Hua-Shu Dou, Boo Cheong Khoo

Abstract: The effect of an initial disturbance on the detonation front structure in a narrow duct is studied by three-dimensional numerical simulation. The numerical method used includes a high resolution fifth-order weighted essentially non-oscillatory scheme for spatial discretization, coupled with a third order total variation diminishing Runge-Kutta time stepping method. Two types of disturbances are us… ▽ More The effect of an initial disturbance on the detonation front structure in a narrow duct is studied by three-dimensional numerical simulation. The numerical method used includes a high resolution fifth-order weighted essentially non-oscillatory scheme for spatial discretization, coupled with a third order total variation diminishing Runge-Kutta time stepping method. Two types of disturbances are used for the initial perturbation. One is a random disturbance which is imposed on the whole area of the detonation front, and the other is a symmetrical disturbance imposed within a band along the diagonal direction on the front. The results show that the two types of disturbances lead to different processes. For the random disturbance, the detonation front evolves into a stable spinning detonation. For the symmetrical diagonal disturbance, the detonation front displays a diagonal pattern at an early stage, but this pattern is unstable. It breaks down after a short while and it finally evolves into a spinning detonation. The spinning detonation structure ultimately formed due to the two types of disturbances is the same. This means that spinning detonation is the most stable mode for the simulated narrow duct. Therefore, in a narrow duct, triggering a spinning detonation can be an effective way to produce a stable detonation as well as to speed up the deflagration to detonation transition process. △ Less

Submitted 7 June, 2010; originally announced June 2010.

Comments: 30 pages and 11 figures

Journal ref: Shock Waves(Germany) 20:163-173,2010

Criteria Of Turbulent Transition In Parallel Flows

Authors: Hua-Shu Dou, Boo Cheong Khoo

Abstract: Based on the energy gradient method, criteria for turbulent transition are proposed for pressure driven flow and shear driven flow, respectively. For pressure driven flow, the necessary and sufficient condition for turbulent transition is the presence of the velocity inflection point in the averaged flow. For shear driven flow, the necessary and sufficient condition for turbulent transition is t… ▽ More Based on the energy gradient method, criteria for turbulent transition are proposed for pressure driven flow and shear driven flow, respectively. For pressure driven flow, the necessary and sufficient condition for turbulent transition is the presence of the velocity inflection point in the averaged flow. For shear driven flow, the necessary and sufficient condition for turbulent transition is the existence of zero velocity gradient in the averaged flow profile. It is shown that turbulent transition can be effected via a singularity of the energy gradient function which may be associated with the chaotic attractor in dynamic system. The role of disturbance in the transition is also clarified in causing the energy gradient function to approach the singularity. Finally, it is interesting that turbulence can be controlled by modulating the distribution of the energy gradient function. △ Less

Submitted 2 June, 2009; originally announced June 2009.

Comments: The Third International Symposium on Physics of Fluids (ISPF3) 15-18 June, 2009, Jiuzhaigou, China. 6 pages; 1 figure

Mechanism of Tubular Pinch Effect of Dilute Suspension in a Pipe Flow

Authors: Hua-Shu Dou, Boo Cheong Khoo

Abstract: Experiments have shown that in dilute suspension flow at laminar state through a circular tube particles migrate towards a concentric annular region with a mean radius of about 0.6 of the tube radius. This phenomenon is well-known as the tubular pinch effect, which is still not fully understood. In this study, the energy gradient method is used to study this phenomenon. It is found that at low R… ▽ More Experiments have shown that in dilute suspension flow at laminar state through a circular tube particles migrate towards a concentric annular region with a mean radius of about 0.6 of the tube radius. This phenomenon is well-known as the tubular pinch effect, which is still not fully understood. In this study, the energy gradient method is used to study this phenomenon. It is found that at low Reynolds number particles will move to the position of r/R=0.58. Based on the result, the mechanism of this phenomenon is well explained. △ Less

Submitted 11 November, 2008; originally announced November 2008.

Comments: 10 pages; 3 figures

Incipient Separation in Shock Wave Boundary Layer Interactions as Induced by Sharp Fin

Authors: Hua-Shu Dou, Boo Cheong Khoo, Khoon Seng Yeo

Abstract: The incipient separation induced by the shock wave turbulent boundary layer interaction at the sharp fin is the subject of present study. Existing theories for the prediction of incipient separation, such as those put forward by McCabe (1966) and Dou and Deng (1992), can have thus far only predicting the direction of surface streamline and tend to over-predict the incipient separation condition… ▽ More The incipient separation induced by the shock wave turbulent boundary layer interaction at the sharp fin is the subject of present study. Existing theories for the prediction of incipient separation, such as those put forward by McCabe (1966) and Dou and Deng (1992), can have thus far only predicting the direction of surface streamline and tend to over-predict the incipient separation condition based on the Stanbrook's criterion. In this paper, the incipient separation is firstly predicted with Dou and Deng (1992)'s theory and then compared with Lu and Settles (1990)' experimental data. The physical mechanism of the incipient separation as induced by the shock wave/turbulent boundary layer interactions at sharp fin is explained via the surface flow pattern analysis. Furthermore, the reason for the observed discrepancy between the predicted and experimental incipient separation conditions is clarified. It is found that when the wall limiting streamlines behind the shock wave becomes\ aligning with one ray from the virtual origin as the strength of shock wave increases, the incipient separation line is formed at which the wall limiting streamline becomes perpendicular to the local pressure gradient. The formation of this incipient separation line is the beginning of the separation process. The effects of Reynolds number and the Mach number on incipient separation are also discussed. Finally, a correlation for the correction of the incipient separation angle as predicted by the theory is also given. △ Less

Submitted 11 November, 2008; originally announced November 2008.

Comments: 34 pages; 9 figures

Journal ref: Shock Waves, Vol. 15, No.6, Oct., 2006, 425-436

Numerical Simulation of Fibre Suspension Flow through an Axisymmetric Contraction and Expansion Passages by Brownian Configuration Field Method

Authors: Zhumin Lu, Boo Cheong Khoo, Hua-Shu Dou, Nhan Phan-Thien, Khoon Seng Yeo

Abstract: In this paper, the finite element method is combined with the Brownian Configuration Field (BFC) method to simulate the fibre suspension flow in axisymmetric contraction and expansion passages. In order to solve for the high stress at high concentration, the Discrete Adaptive Viscoelastic Stress Splitting (DAVSS) method is employed. For the axisymmetric contraction and expansion passages with di… ▽ More In this paper, the finite element method is combined with the Brownian Configuration Field (BFC) method to simulate the fibre suspension flow in axisymmetric contraction and expansion passages. In order to solve for the high stress at high concentration, the Discrete Adaptive Viscoelastic Stress Splitting (DAVSS) method is employed. For the axisymmetric contraction and expansion passages with different geometry ratios, the results obtained are compared to available constitutive models and experiments. The predicted vortex length for dilute suspensions agrees well with experimental data in literature. Our numerical results show clearly the effect on vortex enhancement with increase of the volume fractions and the aspect ratios. Effect of aspect ratio of fibres on the vortex length is also studied. It is found that for the lower expansion ratio flows the vortex dimension in the corner region is fairly independent of fibre concentration and aspect ratio of fibres while the said vortex dimension increases with the increase of fibre concentration for contraction flows. The finding suggests that the aligned fibre approximation traditionally employed in previous work does not exactly describe the effect of fibre motion, and the present BFC method is deemed more suitable for the flow of dilute fibre suspensions. In terms of numerics, the employment of DAVSS enhances numerical stability in the presence of high concentration of fibre in the flow. △ Less

Submitted 10 November, 2008; originally announced November 2008.

Comments: 35 pages; 16 figures

Journal ref: Chemical engineering science, 2006, Vol.61, 2006, 4998-5009

Simulations of Fibre Orientation in Dilute Suspensions with Front Moving in the Filling Process of a Rectangular Channel Using Level Set Method

Authors: Hua-Shu Dou, Boo Cheong Khoo, Nhan Phan-Thien, Khoon Seng Yeo, Rong Zheng

Abstract: The simulation of fibre orientation in dilute suspension with a front moving is carried out using the projection and level set methods. The motion of fibres is described using the Jeffery equation and the contribution of fibres to the flow is accounted for by the configuration field method. The governing Navier-Stokes equation for the fluid flow is solved using the projection method with finite… ▽ More The simulation of fibre orientation in dilute suspension with a front moving is carried out using the projection and level set methods. The motion of fibres is described using the Jeffery equation and the contribution of fibres to the flow is accounted for by the configuration field method. The governing Navier-Stokes equation for the fluid flow is solved using the projection method with finite difference scheme, while the fibre-related equations are directly solved with the Runge-Kutta method. Our findings indicate that the fibre motion has strong influence on the distributions of the streamwise and transverse velocities in the fountain flow. Fibre motion produces strong normal stress near the wall which leads to the reduction of transversal velocity as compared to the Newtonian flow without fibres and in turn the streamwise velocity near the wall is increased. Thus, the fibre addition to the flow weakens the strength of the fountain flow. It is also found that the fibre orientation is not always along the direction of velocity vector in the process of mold filling. In the region of the fountain flow, the fibre near the centerline is more oriented cross the streamwise direction comparing to that in the region far behind the flow front. This leads to that the fibre near the centreline in the region of fountain flow is more extended along the transverse direction. Since fibre orientation in the suspension flow and the shape of the flow front have great bearing on the quality of the product made from injection molding, this study has much implications for engineering applications. These results can also be useful in other field dealing with fibre suspensions. △ Less

Submitted 10 November, 2008; originally announced November 2008.

Comments: 48 pages; 16 figures

Journal ref: Rheologica Acta, Vol.46, No.4, March 2007, 427-447

Mechanism of Wall Turbulence in Boundary Layer Flow

Authors: Hua-Shu Dou, Boo Cheong Khoo

Abstract: The energy gradient method is used to analyze the turbulent generation in the transition boundary layer flow. It is found that the maximum of the energy gradient function occurs at the wall for the Blasius boundary layer flow. At this location under a sufficiently high Reynolds number, even a low level of free-stream disturbance can cause the turbulent transition and sustain the flow to be in a… ▽ More The energy gradient method is used to analyze the turbulent generation in the transition boundary layer flow. It is found that the maximum of the energy gradient function occurs at the wall for the Blasius boundary layer flow. At this location under a sufficiently high Reynolds number, even a low level of free-stream disturbance can cause the turbulent transition and sustain the flow to be in a state of turbulence. This is an excellent explanation of the physics of self-sustenance of wall turbulence. The mechanism of receptivity for boundary layer flow can also be understood from the energy gradient criterion. That is, the free-stream disturbance can propagate towards the wall by the "energy gradient" process to cause turbulent transition, and the transition point in boundary layer can be moved forward towards the leading edge when the level of external disturbance increases. △ Less

Submitted 10 November, 2008; originally announced November 2008.

Comments: Four pages; Four figures. Presented at The Second International Symposium on Physics of Fluids (ISPF2), 9-12 June 2008, Nanjing, China

Shear Instabilities in Granular Flows down on Inclined Plane

Authors: Hua-Shu Dou, Boo Cheong Khoo, Nhan Phan-Thien

Abstract: Instabilities at interface of two stream granular flows have been reported in recent experiment [1] that breaking waves can form at the interface between two streams of identical grains flowing on an inclined plane downstream of a splitter plate. In this report, the theory of hydrodynamic instability is used to analyze the shear flow of granular materials. It is shown that the shear instability… ▽ More Instabilities at interface of two stream granular flows have been reported in recent experiment [1] that breaking waves can form at the interface between two streams of identical grains flowing on an inclined plane downstream of a splitter plate. In this report, the theory of hydrodynamic instability is used to analyze the shear flow of granular materials. It is shown that the shear instability in two-stream granular flows actually comes from the competition between the energy gradients in transverse and streamwise directions as well as the interaction of two streams. We argue that the flow energy loss in the streamwise direction has a stabilizing effect, while the transverse component of the friction force formed by grain surface friction acts as the source of instabilities. An equation has been derived to characterize the transition between steady and wavy flows. Good qualitative agreement with the experimental data is obtained. △ Less

Submitted 2 May, 2005; originally announced May 2005.

Comments: 13 pages, 3 figures

Determining the Critical Condition for Flow Transition in a Full-Developed Annulus Flow

Authors: Hua-Shu Dou, Boo Cheong Khoo, Her Mann Tsai

Abstract: Axial flow in an annulus between two concentric cylinders is commonly seen in various flow devices used in chemical processing industries and petroleum science and engineering. The flow state in the annulus strongly influences the performance of fluid transportation in the devices. Therefore, the determination of flow state which is laminar flow or turbulent flow is an important task to predict th… ▽ More Axial flow in an annulus between two concentric cylinders is commonly seen in various flow devices used in chemical processing industries and petroleum science and engineering. The flow state in the annulus strongly influences the performance of fluid transportation in the devices. Therefore, the determination of flow state which is laminar flow or turbulent flow is an important task to predict the performance of the flow devices. In previous works, we have proposed an energy gradient method for studying the flow instability and turbulent transition. In this method, it is shown that the flow instability and turbulent transition in wall bounded shear flows depend on the relative magnitude of the gradient of the total mechanical energy in transverse direction and the rate of loss of the total mechanical energy along the streamwise direction for a given imposed disturbance. For pipe and plane Poiseuille flows, it has been demonstrated that the transition to turbulence for these wall bounded parallel flows occurs at a consistent value of the energy gradient parameter (Kmax). In present study, the critical condition for turbulent transition in annulus flow is calculated with the energy gradient method for various radius ratios. The critical flow rate and critical Reynolds number are given for various radius ratios. Then, the analytical results are compared with the experiments in the literature. Finally, the implication of the result is discussed in terms of the drag reduction and mixing as well as heat transfer in practical industrial applications of various fluid delivery devices. △ Less

Submitted 7 June, 2010; v1 submitted 27 April, 2005; originally announced April 2005.

Comments: 20 gapes; 4 figures (2010, in press)

Journal ref: Journal of Petrolum Science and Engineering, Vol.71, 2010

Threshold amplitudes for transition to turbulence in a pipe

Authors: Hua-Shu Dou, Boo Cheong Khoo, Khoon Seng Yeo

Abstract: Threshold amplitude of disturbance for transition to turbulence in a pipe Poiseuille flow is investigated. Based on the energy gradient theory, we argued that the transition to turbulence depends on magnitudes of the energy gradient of mean flow and the disturbance energy. Furthermore, the threshold disturbance energy required for turbulence transition is expressed as a function of the complemen… ▽ More Threshold amplitude of disturbance for transition to turbulence in a pipe Poiseuille flow is investigated. Based on the energy gradient theory, we argued that the transition to turbulence depends on magnitudes of the energy gradient of mean flow and the disturbance energy. Furthermore, the threshold disturbance energy required for turbulence transition is expressed as a function of the complementary angle to the energy angle which also characterizes the behaviour of the energy gradient of mean flow. With some mathematical treatments, the variation of the threshold energy of disturbance versus the Reynolds number is obtained. Then, it is found for a fixed disturbance frequency that the normalized amplitude of disturbance is scaled by Re with an exponent of -3/2 for the transition occurrence in a pipe. This value of exponent agrees well with several recent results reported in the literature. Finally, the mechanism of transition to turbulence is suggested for different disturbance levels. △ Less

Submitted 25 June, 2005; v1 submitted 18 February, 2005; originally announced February 2005.

Comments: 19 pages; 6 figures. This paper has been presented at the 7th China National Conference on Turbulence & Flow Instability, Aug.10-13, 2004, Beijing, China

Instability of Taylor-Couette Flow between Concentric Rotating Cylinders

Authors: Hua-Shu Dou, Boo Cheong Khoo, Koon Seng Yeo

Abstract: The energy gradient theory is used to study the instability of Taylor-Couette flow between concentric rotating cylinders. This theory has been proposed in our previous works. In our previous studies, the energy gradient theory was demonstrated to be applicable for wall-bounded parallel flows. It was found that the critical value of the energy gradient parameter Kmax at turbulent transition is abou… ▽ More The energy gradient theory is used to study the instability of Taylor-Couette flow between concentric rotating cylinders. This theory has been proposed in our previous works. In our previous studies, the energy gradient theory was demonstrated to be applicable for wall-bounded parallel flows. It was found that the critical value of the energy gradient parameter Kmax at turbulent transition is about 370-389 for wall-bounded parallel flows (which include plane Poiseuille flow, pipe Poiseuille flow and plane Couette flow) below which no turbulence occurs. In this paper, the detailed derivation for the calculation of the energy gradient parameter in the flow between concentric rotating cylinders is provided. The calculated results for the critical condition of primary instability (with semi-empirical treatment) are found to be in very good agreement with the experiments in the literature. A possible mechanism of spiral turbulence generation observed for counter-rotation of two cylinders can also be explained using the energy gradient theory. The energy gradient theory can serve to relate the condition of transition in Taylor-Couette flow to that in plane Couette flow. The latter reasonably becomes the limiting case of the former when the radii of cylinders tend to infinity. It is our contention that the energy gradient theory is possibly fairly universal for analysis of flow instability and turbulent transition, and is found valid for both pressure and shear driven flows in parallel and rotating flow configurations. △ Less

Submitted 8 June, 2010; v1 submitted 14 February, 2005; originally announced February 2005.

Comments: 38 pages; 12 figures

Journal ref: Int.J.Thermal Sci.47:1422-1435,2008

Energy Loss Distribution in the Plane Couette Flow and the Taylor-Couette Flow between Concentric Rotating Cylinders

Authors: Hua-Shu Dou, Boo Cheong Khoo, Khoon Seng Yeo

Abstract: The distribution of energy loss due to viscosity friction in plane Couette flow and Taylor-Couette Flow between concentric rotating cylinders are studied in detail for various flow conditions. The energy loss is related to the industrial processes in some fluid delivery devices and has significant influence on the flow efficiency, flow stability, turbulent transition, mixing, and heat transfer beh… ▽ More The distribution of energy loss due to viscosity friction in plane Couette flow and Taylor-Couette Flow between concentric rotating cylinders are studied in detail for various flow conditions. The energy loss is related to the industrial processes in some fluid delivery devices and has significant influence on the flow efficiency, flow stability, turbulent transition, mixing, and heat transfer behaviours, etc. Therefore, it is important to know about the energy loss distribution in the flow domain and to know its influence on the flow for better understanding of the flow physics. The calculation or methodology of calculating the energy loss distribution in the Taylor-Couette flow between concentric rotating cylinders is not readily found in the open literature. In this paper, the principle and the calculation are given for single cylinder rotation of either the inner or outer cylinder, and counter and same direction rotation of two cylinders. For comparison, the distribution of energy loss in a plane Couette flow is also derived for various flow conditions. Discussions of the effect of energy loss on the flow behaviour are carried out from which some findings are suggested. △ Less

Submitted 15 June, 2010; v1 submitted 28 January, 2005; originally announced January 2005.

Comments: 35 pages; 16 figures. (a few words have been corrected in this version)

Journal ref: Int.J.Thermal Sci.46:262-275,2007

Investigation of Turbulent transition in plane Couette flows Using Energy Gradient Method

Authors: Hua-Shu Dou, Boo Cheong Khoo

Abstract: The energy gradient method has been proposed with the aim of better understanding the mechanism of flow transition from laminar flow to turbulent flow. In this method, it is demonstrated that the transition to turbulence depends on the relative magnitudes of the transverse gradient of the total mechanical energy which amplifies the disturbance and the energy loss from viscous friction which damps… ▽ More The energy gradient method has been proposed with the aim of better understanding the mechanism of flow transition from laminar flow to turbulent flow. In this method, it is demonstrated that the transition to turbulence depends on the relative magnitudes of the transverse gradient of the total mechanical energy which amplifies the disturbance and the energy loss from viscous friction which damps the disturbance, for given imposed disturbance. For a given flow geometry and fluid properties, when the maximum of the function K (a function standing for the ratio of the gradient of total mechanical energy in the transverse direction to the rate of energy loss due to viscous friction in the streamwise direction) in the flow field is larger than a certain critical value, it is expected that instability would occur for some initial disturbances. In this paper, using the energy gradient analysis, the equation for calculating the energy gradient function K for plane Couette flow is derived. The result indicates that K reaches the maximum at the moving walls. Thus, the fluid layer near the moving wall is the most dangerous position to generate initial oscillation at sufficient high Re for given same level of normalized perturbation in the domain. The critical value of K at turbulent transition, which is observed from experiments, is about 370 for plane Couette flow when two walls move in opposite directions (anti-symmetry). This value is about the same as that for plane Poiseuille flow and pipe Poiseuille flow (385-389). Therefore, it is concluded that the critical value of K at turbulent transition is about 370-389 for wall-bounded parallel shear flows which include both pressure (symmetrical case) and shear driven flows (anti-symmetrical case). △ Less

Submitted 7 June, 2010; v1 submitted 27 January, 2005; originally announced January 2005.

Comments: 18 pages; 7 figures. (2010, in press)

Journal ref: Advances in Applied Mathematics and Mechanics, Vol.2, 2010