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Boo Cheong Khoo

Publications and source records attributed to Boo Cheong Khoo.

At least 19 recordsLinked to original sources

Using Large Language Models for Parametric Shape Optimization

Recent advanced large language models (LLMs) have showcased their emergent capability of in-context learning, facilitating intelligent decision-making through natural language prompts without retraining. This new machine learning paradigm has shown promise in various fields, including general control and optimization problems. Inspired by these advancements, we explore the potential of LLMs for a specific and essential engineering task: parametric shape optimization (PSO). We develop an optimization framework, LLM-PSO, that leverages an LLM to determine the optimal shape of parameterized engineering designs in the spirit of evolutionary strategies. Utilizing the ``Claude 3.5 Sonnet'' LLM, we evaluate LLM-PSO on two benchmark flow optimization problems, specifically aiming to identify drag-minimizing profiles for 1) a two-dimensional airfoil in laminar flow, and 2) a three-dimensional axisymmetric body in Stokes flow. In both cases, LLM-PSO successfully identifies optimal shapes in agreement with benchmark solutions. Besides, it generally converges faster than other classical optimization algorithms. Our preliminary exploration may inspire further investigations into harnessing LLMs for shape optimization and engineering design more broadly.

cs.CE

Application of a variational hybrid quantum-classical algorithm to heat conduction equation

The prosperous development of both hardware and algorithms for quantum computing (QC) potentially prompts a paradigm shift in scientific computing in various fields. As an increasingly active topic in QC, the variational quantum algorithm (VQA) leads a promising direction for solving partial differential equations on Noisy Intermediate Scale Quantum (NISQ) devices. Although a clear perspective on the advantages of QC over classical computing techniques for specific mathematical and physical problems exists, applications of QC in computational fluid dynamics to solve practical flow problems, though promising, are still in an early stage of development. To explore QC in practical simulation of flow problems, this work applies a variational hybrid quantum-classical algorithm, namely the variational quantum linear solver (VQLS), to resolve the heat conduction equation through finite difference discretization of the Laplacian operator. Details of VQLS implementation are discussed by various test instances of linear systems. Finally, the successful statevector simulations of the heat conduction equation in one and two dimensions demonstrate the validity of the present algorithm by proof-of-concept results. In addition, the heuristic scaling for the heat conduction problem indicates that the time complexity of the present approach is logarithmically dependent on the precision ε and linearly dependent on the number of qubits n.

quant-ph

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

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.

physics.flu-dyn

Flow-excited membrane instability at moderate Reynolds numbers

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.

physics.flu-dyn

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

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.

physics.flu-dyn

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

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.

physics.flu-dyn

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

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.

cs.CE

Numerical study on wide gap Taylor Couette flow with flow transition

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.

physics.flu-dyn

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

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.

physics.flu-dyn

Stability of boundary layer flow based on energy gradient theory

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.

physics.flu-dyn

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

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.

physics.flu-dyn

Instability of Taylor-Couette Flow between Concentric Rotating Cylinders

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.

physics.flu-dyn

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

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.

physics.flu-dyn

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

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).

nlin.CD

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

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.

physics.flu-dyn

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

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.

physics.flu-dyn

Criteria Of Turbulent Transition In Parallel Flows

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.

physics.flu-dyn

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

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.

physics.flu-dyn