ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Thu, 05 Aug 2021 06:08:44 GMT2021-08-05T06:08:44Z501821- Pressure/flow behaviour in collapsible tube subjected to forced downstream pressure fluctuationshttps://scholarbank.nus.edu.sg/handle/10635/58637Title: Pressure/flow behaviour in collapsible tube subjected to forced downstream pressure fluctuations
Authors: Low, T.H.; Chew, Y.T.; Winoto, S.H.; Chin, R.
Abstract: An experimental investigation, has been made into the pressure/flow behaviour of a collapsible tube subjected to downstream pressure fluctuations. These downstream pressure waves are observed to be transmitted upstream beyond the point of collapse. The mean flow rate is not significantly affected by the amplitude or frequency of pressure fluctuations. However, the oscillatory flow amplitude is reduced at the higher frequency. The mean flow rate also remains independent of the mean driving pressure.
Sat, 01 Jul 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/586371995-07-01T00:00:00Z
- Lattice Boltzmann interface capturing method for incompressible flowshttps://scholarbank.nus.edu.sg/handle/10635/60637Title: Lattice Boltzmann interface capturing method for incompressible flows
Authors: Zheng, H.W.; Shu, C.; Chew, Y.T.
Abstract: A lattice Boltzmann interface capturing method for incompressible flows is proposed in this paper. The interface is naturally captured by minimizing the free energy functional. It is easily implemented and does not require interface reconstruction as required by most of the traditional interface tracking methods such as the volume of fluid method. Moreover, the method does not require the isotropic property of the fourth order lattice tensor as do other lattice Boltzmann methods. Thus the D2Q5 (D2 means two dimensional, Q5 means five velocity model) discrete velocity model is applied in the method. The interface profile along the flat interface and coexistence curve can be given analytically. The proposed method is validated for some test cases, and compared to the volume of fluid and level set methods. Numerical results show that the present method performs very well and can generate very sharp interfaces. © 2005 The American Physical Society.
Tue, 01 Nov 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/606372005-11-01T00:00:00Z
- Taylor-series expansion and least-squares-based lattice Boltzmann method: Two-dimensional formulation and its applicationshttps://scholarbank.nus.edu.sg/handle/10635/61463Title: Taylor-series expansion and least-squares-based lattice Boltzmann method: Two-dimensional formulation and its applications
Authors: Shu, C.; Niu, X.D.; Chew, Y.T.
Abstract: An explicit lattice Boltzmann method (LBM) is developed in this paper to simulate flows in an arbitrary geometry. The method is based on the standard LBM, Taylor-series expansion, and the least-squares approach. The final formulation is an algebraic form and essentially has no limitation on the mesh structure and lattice model. Theoretical analysis for the one-dimensional (ID) case showed that the version of the LBM could recover the Navier-Stokes equations with second order accuracy. A generalized hydrodynamic analysis is conducted to study the wave-number dependence of shear viscosity for the method. Numerical simulations of the 2D lid-driven flow in a square cavity and a polar cavity flow as well as the "no flow" simulation in a square cavity have been carried out. Favorable results were obtained and compared well with available data in the literature, indicating that the present method has good prospects in practical applications. ©2002 The American Physical Society.
Fri, 01 Mar 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/614632002-03-01T00:00:00Z
- Three-dimensional lattice Boltzmann BGK model and its application to flows with heat transfer in a rectangular microchannelhttps://scholarbank.nus.edu.sg/handle/10635/61577Title: Three-dimensional lattice Boltzmann BGK model and its application to flows with heat transfer in a rectangular microchannel
Authors: Chew, Y.T.; Niu, X.D.; Shu, C.
Abstract: In this paper, we present a 3D lattice Boltzmann BGK model for simulation of micro flows with heat transfer. This model is an extension of the two-dimensional model that is based on the kinetic theory and the thermal lattice Boltzmann method. The kinetic relations of the relaxation times in this model were linked with the Knudsen number, and a diffuse scattering boundary condition for the velocity and thermal fields was presented for the 3D lattice Boltzmann method. The present 3D lattice Boltzmann model was successfully applied to simulate the flow and heat transfer in rectangular channels using the 3D TLLBM developed by the authors. Numerical results obtained by the present method show that the LBM can give a good prediction of the micro fluidic behaviours with thermal effects. Copyright © 2005 John Wiley & Sons, Ltd.
Thu, 20 Apr 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/615772006-04-20T00:00:00Z
- Simulation of shock-wave propagation with finite volume lattice boltzmann methodhttps://scholarbank.nus.edu.sg/handle/10635/86078Title: Simulation of shock-wave propagation with finite volume lattice boltzmann method
Authors: Qu, K.; Shu, C.; Chew, Y.T.
Abstract: A new approach was recently proposed to construct equilibrium distribution functions (fα eq) of the lattice Boltzmann method for simulation of compressible flows. In this approach, the Maxwellian function is replaced by a simple function which satisfies all needed relations to recover compressible Euler equations. With Lagrangian interpolation polynomials, the simple function is discretized onto a fixed velocity pattern to construct f α eq. In this paper, the finite volume method is combined with the new lattice Boltzmann models to simulate ID and 2D shock-wave propagation. The numerical results agree well with available data in the literatures. © World Scientific Publishing Company.
Sun, 01 Apr 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/860782007-04-01T00:00:00Z
- Heat transfer in turbulent channel flow over protrusionshttps://scholarbank.nus.edu.sg/handle/10635/73501Title: Heat transfer in turbulent channel flow over protrusions
Authors: Chen, Y.; Chew, Y.T.; Khoo, B.C.
Abstract: In this study, heat transfer and flow structures over protrusions in turbulent channel flow were systematically investigated by DES numerically. It is found that larger height ratio induces higher friction factor and heat transfer. However the performance factor, on the other hand, increases first then reaches its asymptotic limit and even decreases with an increase of height ratio. The highest local Nusselt number was found at the upstream portion of protrusion. Additionally, the local Nusselt number distribution is symmetric for low protrusion while asymmetric for high protrusion because of asymmetric flow structure. © 2012 American Institute of Physics.
Sun, 01 Jan 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/735012012-01-01T00:00:00Z
- Particle number per cell and scaling factor effect on accuracy of DSMC simulation of micro flowshttps://scholarbank.nus.edu.sg/handle/10635/61049Title: Particle number per cell and scaling factor effect on accuracy of DSMC simulation of micro flows
Authors: Shu, C.; Mao, X.H.; Chew, Y.T.
Abstract: Purpose - This paper aims to give some guidance on the selection of particle numbers per cell and the number of molecules per particle in the micro flow simulation by using DSMC method. Design/methodology/approach - The numerical investigation is performed to study the effects of particle number per cell and the scaling factor of real molecules to a simulated particle on accuracy of DSMC simulation of two-dimensional micro channel flows in the "slip flow" and "transition flow" regimes. Findings - Numerical results show that both the particle number per cell and the scaling factor have effect on the accuracy of the DSMC results from the statistical error and the physical aspects. In the "slip flow" regime, a larger value of scaling factor can be used to obtain accurate results as compared to the "transition flow" regime. However, in the "transition flow" regime, much less number of particles in each cell can be used to generate accurate DSMC results as compared to the "slip flow" regime. Research limitations/implications - The present work is limited to the two-dimensional case. Practical implications - The results of this paper are very useful for the two-dimensional micro flow simulation by DSMC. Originality/value - The work in this paper is original and provides guidance on micro flow simulation. © Emerald Group Publishing Limited.
Sat, 01 Jan 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/610492005-01-01T00:00:00Z
- Numerical simulation of flows past a rotational circular cylinder by taylor-series-expansion and least squares-based lattice Boltzmann methodhttps://scholarbank.nus.edu.sg/handle/10635/60947Title: Numerical simulation of flows past a rotational circular cylinder by taylor-series-expansion and least squares-based lattice Boltzmann method
Authors: Shu, C.; Qu, K.; Niu, X.D.; Chew, Y.T.
Abstract: An explicit Taylor series expansion and least square-based lattice Boltzmann method (TLLBM) is used to simulate the two-dimensional unsteady viscous incompressible flows. TLLBM is based on the well-known Taylor series expansion and the least square optimization. It has no limitation on mesh structure and lattice model. Its marching in time is accurate. Therefore, it is very suitable for simulation of time dependent problems. Numerical experiments are performed for simulation of flows past a rotational circular cylinder. Good agreement is achieved between the present results and available data in the literature. © World Scientific Publishing Company.
Tue, 01 Nov 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609472005-11-01T00:00:00Z
- A novel immersed boundary velocity correction-lattice Boltzmann method and its application to simulate flow past a circular cylinderhttps://scholarbank.nus.edu.sg/handle/10635/54615Title: A novel immersed boundary velocity correction-lattice Boltzmann method and its application to simulate flow past a circular cylinder
Authors: Shu, C.; Liu, N.; Chew, Y.T.
Abstract: A novel immersed boundary velocity correction-lattice Boltzmann method is presented and validated in this work by its application to simulate the two-dimensional flow over a circular cylinder. The present approach is inspired from the conventional immersed boundary method (IBM). In the conventional IBM, the effect of rigid body on the surrounding flow is modeled through a forcing term, which is in turn used to correct the surrounding velocity field. It was found that this process is actually an iterative procedure, trying to satisfy the non-slip boundary condition at the solid wall. In this work, a new concept of immersed boundary velocity correction approach is proposed, which directly corrects the velocity to enforce the physical boundary condition. The main advantage of the new method is that it is simple in concept and easy for implementation, and the convergence of numerical computation is faster and more stable than the conventional IBM. One challenging issue of conventional IBM is that some streamlines may pass through the solid body since there is no mechanism to enforce the non-slip condition at the boundary. As shown in the present numerical results, this unphysical phenomenon is avoided in our new method since the non-slip condition is enforced. The present results for the steady and unsteady flows compare very well with available data in the literature. © 2007 Elsevier Inc. All rights reserved.
Mon, 01 Oct 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/546152007-10-01T00:00:00Z
- A momentum exchange-based immersed boundary-lattice Boltzmann method for simulating incompressible viscous flowshttps://scholarbank.nus.edu.sg/handle/10635/54439Title: A momentum exchange-based immersed boundary-lattice Boltzmann method for simulating incompressible viscous flows
Authors: Niu, X.D.; Shu, C.; Chew, Y.T.; Peng, Y.
Abstract: A momentum exchange-based immersed boundary-lattice Boltzmann method is presented in this Letter for simulating incompressible viscous flows. This method combines the good features of the lattice Boltzmann method (LBM) and the immersed boundary method (IBM) by using two unrelated computational meshes, an Eulerian mesh for the flow domain and a Lagrangian mesh for the solid boundaries in the flow. In this method, the non-slip boundary condition is enforced by introducing a forcing term into the lattice Boltzmann equation (LBE). Unlike the conventional IBM using the penalty method with a user-defined parameter or the direct forcing scheme based on the Navier-Stokes (NS) equations, the forcing term is simply calculated by the momentum exchange of the boundary particle density distribution functions, which are interpolated by the Lagrangian polynomials from the underlying Eulerian mesh. Numerical examples show that the present method can provide very accurate numerical results. © 2006 Elsevier B.V. All rights reserved.
Mon, 29 May 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/544392006-05-29T00:00:00Z
- A lattice Boltzmann BGK model for simulation of micro flowshttps://scholarbank.nus.edu.sg/handle/10635/54295Title: A lattice Boltzmann BGK model for simulation of micro flows
Authors: Niu, X.D.; Shu, C.; Chew, Y.T.
Abstract: We propose a lattice Boltzmann BGK model for simulation of micro flows. This model is based on the kinetic theory and the entropic lattice Boltzmann method (S. Ansumali and I. V. Karlin, J. Stat. Phys., 107 (2002) 291) but the relaxation time is re-defined in terms of the Knudsen number, and a diffuse-scattering boundary condition (DSBC) is adopted to consider the velocity slip at the wall. Simple theoretical analysis and numerical validation show that the proposed model gives good predictions of the micro fluidic behaviors.
Sun, 01 Aug 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/542952004-08-01T00:00:00Z
- A new differential lattice Boltzmann equation and its application to simulate incompressible flows on non-uniform gridshttps://scholarbank.nus.edu.sg/handle/10635/73057Title: A new differential lattice Boltzmann equation and its application to simulate incompressible flows on non-uniform grids
Authors: Chew, Y.T.; Shu, C.; Niu, X.D.
Abstract: A new differential lattice Boltzmann equation (LBE) is presented in this work, which is derived from the standard LBE by using Taylor series expansion only in spatial direction with truncation to the second-order derivatives. The obtained differential equation is not a wave-like equation. When a uniform grid is used, the new differential LBE can be exactly reduced to the standard LBE. The new differential LBE can be applied to solve irregular problems with the help of coordinate transformation. The present scheme inherits the merits of the standard LBE. The 2-D driven cavity flow is chosen as a test case to validate the present method. Favorable results are obtained and indicate that the present scheme has good prospects in practical applications.
Tue, 01 Jan 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/730572002-01-01T00:00:00Z
- Numerical study of 2D multiphase flows over grooved surface by lattice Boltzmann methodhttps://scholarbank.nus.edu.sg/handle/10635/51631Title: Numerical study of 2D multiphase flows over grooved surface by lattice Boltzmann method
Authors: Huang, J.J.; Shu, C.; Chew, Y.T.; Zheng, H.W.
Abstract: In this paper, the effects of surface wettability and topography on a droplet, which is driven by a body force to pass through grooved walls, are studied by using the multiphase lattice Boltzmann model. At small scale, the shape and velocity of the droplet were found to be strongly affected by the wettability and configuration of the wall. The drag on the droplet moving over grooved surfaces was found to decrease as the wall hydrophobicity increases. It was also found that the wettability decides whether the droplet fills or does not fill the whole grooves. © World Scientific Publishing Company.
Sun, 01 Apr 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/516312007-04-01T00:00:00Z
- A lattice Boltzmann study on the large deformation of red blood cells in shear flowhttps://scholarbank.nus.edu.sg/handle/10635/54297Title: A lattice Boltzmann study on the large deformation of red blood cells in shear flow
Authors: Sui, Y.; Chew, Y.T.; Low, H.T.
Abstract: The transient deformation of a liquid-filled elastic capsule, simulating a red blood cell, was studied in simple shear flow. The simulation is based on a hybrid method which introduces the immersed boundary concept in the framework of the multi-block lattice Boltzmann model. The method was validated by the study on deformation of an initially circular capsule with Hooke's membrane. Also studied were capsules with Skalak membrane of initially elliptical and biconcave shapes, which are more representative of a red blood cell. Membrane tank treading motion is observed. As the ratio between membrane dilation modulus and shear modulus increases, the capsule shows asymptotic behavior. For an initially elliptical capsule, it is found that the steady shape is independent of initial inclination angle. For an initially biconcave capsule, the tank treading frequency from two-dimensional modeling is comparable to that of real cells. Another interesting finding is that the tank treading velocity has not attained steady state when the capsule shape becomes steady; and at this state there is the internal vortex pair. The treading velocity continues to decrease and reaches a steady value when the internal vortex pair has developed into a single vortex. © World Scientific Publishing Company.
Fri, 01 Jun 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/542972007-06-01T00:00:00Z
- Characteristics of square cylinder wake transition flowshttps://scholarbank.nus.edu.sg/handle/10635/84902Title: Characteristics of square cylinder wake transition flows
Authors: Luo, S.C.; Chew, Y.T.; Ng, Y.T.
Abstract: The wake transition regime in square cylinder flow is investigated experimentally. Mode A and B transitions similar to those captured in circular cylinder flows are found to exist in this regime and by using dye- and laser-induced fluorescence visualization, the spanwise and streamwise vortex structures are captured. Upon comparison with their corresponding modes in circular cylinder flows, some differences are noted. Most notably, the critical Reynolds numbers at which Mode A and B occur for square cylinder flows were estimated to be approximately 160 and 200, respectively, and are lower than those found in circular cylinder flows, which are generally agreed to be approximately 188-190 and 230-260, respectively. Also, the spanwise wavelengths for the two modes in square cylinder flows (5.2 Ds and 1.2 Ds for Modes A and B, respectively) are longer than their counterparts in circular cylinder flows (3-4D0 and 0.8-1 D0, respectively). Furthermore, by using a hot wire to measure the shedding frequency, it was found that the Mode A transition here does not exhibit a hysteresis phenomenon, which is unlike the case in circular cylinder flows. The Mode A to B transition here is also not associated with a sudden increase in Strouhal number, which is again unlike the corresponding situation in circular cylinder flows. However, the vortical structures are noted to be similar between the corresponding modes for the two bluff body shapes, and hence the formation mechanism can be deemed to be similar. © 2003 American Institute of Physics.
Mon, 01 Sep 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/849022003-09-01T00:00:00Z
- Simulation of unsteady incompressible flows by using Taylor series expansion- and least square-based lattice Boltzmann methodhttps://scholarbank.nus.edu.sg/handle/10635/61311Title: Simulation of unsteady incompressible flows by using Taylor series expansion- and least square-based lattice Boltzmann method
Authors: Chew, Y.T.; Shu, C.; Niu, X.D.
Abstract: In this work, an explicit Taylor series expansion- and least square-based lattice Boltzmann method (LBM) is used to simulate two-dimensional unsteady incompressible viscous flows. The new method is based on the standard LBM with introduction of the Taylor series expansion and the least squares approach. The final equation is an explicit form and essentially has no limitation on mesh structure and lattice model. Since the Taylor series expansion is only applied in the spatial direction, the time accuracy of the new method is kept the same as the standard LBM, which seems to benefit for unsteady flow simulation. To validate the new method, two test problems, that is, the vortex shedding behind a circular cylinder at low Reynolds numbers and the oscillating flow in a lid driven cavity, were considered in this work. Numerical results obtained by the new method agree very well with available data in the literature.
Mon, 01 Jul 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/613112002-07-01T00:00:00Z
- Enhancement of heat transfer in turbulent channel flow over dimpled surfacehttps://scholarbank.nus.edu.sg/handle/10635/85149Title: Enhancement of heat transfer in turbulent channel flow over dimpled surface
Authors: Chen, Y.; Chew, Y.T.; Khoo, B.C.
Abstract: A systematic numerical investigation of heat transfer in turbulent channel flow over dimpled surface is conducted. Both symmetric (or spherical) and asymmetric dimple with different depth ratios (h/D) and skewness (Dx and Dz) are considered for a series of Reynolds numbers Re 2H (based on bulk velocity and full channel height) between 4000 and 6000 while Prandtl number Pr is fixed at 0.7. It is found that the optimum dimple configuration for enhancing heat transfer measured in terms of the volume goodness factor is obtained for the case of asymmetric dimple with a depth ratio of h/D = 15% and stream-wise skewness of Dx = 15%. The heat transfer capacity in terms of Nusselt number is significantly increased, while the associated pressure loss is kept almost to the same level as the symmetric dimple with the same depth ratio. The present study also suggests that the heat transfer enhancement is closely related to ejection with counter-rotating flow, intensified secondary flow and vortex structures at the downstream rim of asymmetric dimple. All these findings suggest that a carefully designed asymmetric dimpled surface presents a viable means of enhancing heat transfer compared to the symmetric dimple. © 2012 Elsevier Ltd. All rights reserved.
Sat, 01 Dec 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/851492012-12-01T00:00:00Z
- Numerical and theoretical study of gaseous flows in a micro tubehttps://scholarbank.nus.edu.sg/handle/10635/73694Title: Numerical and theoretical study of gaseous flows in a micro tube
Authors: Shu, C.; Mao, X.H.; Chew, Y.T.
Abstract: The 3-D axisymmetric, steady flow in a micro-tube with Kn number in the 'slip flow' regime is investigated by analytical method and DSMC (direct simulation Monte Carlo) approach. The analytical formulations are derived by dimensional analysis. Under different pressure ratios of the inlet to outlet, the streamwise velocity, nonlinear pressure distributions and volumetric flow rates along the streamwise direction are obtained analytically, which are compared with those given from DSMC computation. Good agreement was achieved. The causes of deviation in pressure distributions between analytical and DSMC results are discussed in terms of compressibility and rarefaction effects.
Mon, 01 Jan 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/736942001-01-01T00:00:00Z
- An isoparametric transformation-based interpolation-supplemented lattice boltzmann method and its applicationhttps://scholarbank.nus.edu.sg/handle/10635/73182Title: An isoparametric transformation-based interpolation-supplemented lattice boltzmann method and its application
Authors: Qu, K.; Shu, C.; Chew, Y.T.
Abstract: Isoparametric transformation was applied to the interpolation-supplemented lattice Boltzmann method (ISLBM). As a result, it can be applied to arbitrarily structural grids and reserve the simple procedure of ISLBM. The simulation of the lid-driven cavity flow and the flow around a cylinder were performed to validate the proposed method. The numerical results agree excellently with the data in the literature. © 2010 World Scientific Publishing Company.
Sun, 30 May 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/731822010-05-30T00:00:00Z
- An object-oriented and quadrilateral-mesh based solution adaptive algorithm for compressible multi-fluid flowshttps://scholarbank.nus.edu.sg/handle/10635/59501Title: An object-oriented and quadrilateral-mesh based solution adaptive algorithm for compressible multi-fluid flows
Authors: Zheng, H.W.; Shu, C.; Chew, Y.T.
Abstract: In this paper, an object-oriented and quadrilateral-mesh based solution adaptive algorithm for the simulation of compressible multi-fluid flows is presented. The HLLC scheme (Harten, Lax and van Leer approximate Riemann solver with the Contact wave restored) is extended to adaptively solve the compressible multi-fluid flows under complex geometry on unstructured mesh. It is also extended to the second-order of accuracy by using MUSCL extrapolation. The node, edge and cell are arranged in such an object-oriented manner that each of them inherits from a basic object. A home-made double link list is designed to manage these objects so that the inserting of new objects and removing of the existing objects (nodes, edges and cells) are independent of the number of objects and only of the complexity of O(1). In addition, the cells with different levels are further stored in different lists. This avoids the recursive calculation of solution of mother (non-leaf) cells. Thus, high efficiency is obtained due to these features. Besides, as compared to other cell-edge adaptive methods, the separation of nodes would reduce the memory requirement of redundant nodes, especially in the cases where the level number is large or the space dimension is three. Five two-dimensional examples are used to examine its performance. These examples include vortex evolution problem, interface only problem under structured mesh and unstructured mesh, bubble explosion under the water, bubble-shock interaction, and shock-interface interaction inside the cylindrical vessel. Numerical results indicate that there is no oscillation of pressure and velocity across the interface and it is feasible to apply it to solve compressible multi-fluid flows with large density ratio (1000) and strong shock wave (the pressure ratio is 10,000) interaction with the interface. © 2008 Elsevier Inc. All rights reserved.
Tue, 01 Jul 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/595012008-07-01T00:00:00Z
- Simulation of natural convection in a square cavity by Taylor series expansion- and least squares-based lattice Boltzmann methodhttps://scholarbank.nus.edu.sg/handle/10635/61305Title: Simulation of natural convection in a square cavity by Taylor series expansion- and least squares-based lattice Boltzmann method
Authors: Shu, C.; Peng, Y.; Chew, Y.T.
Abstract: The Taylor series expansion- and least squares-based lattice Boltzmann method (TLLBM) was used in this paper to extend the current thermal model to an arbitrary geometry so that it can be used to solve practical thermo-hydrodynamics in the incompressible limit. The new explicit method is based on the standard lattice Boltzmann method (LBM), Taylor series expansion and the least squares approach. The final formulation is an algebraic form and essentially has no limitation on the mesh structure and lattice model. Numerical simulations of natural convection in a square cavity on both uniform and nonuniform grids have been carried out. Favorable results were obtained and compared well with the benchmark data. It was found that, to get the same order of accuracy, the number of mesh points used on the nonuniform grid is much less than that used on the uniform grid.
Sun, 01 Dec 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/613052002-12-01T00:00:00Z
- On using high-order polynomial curve fits in the quasi-steady theory for square-cylinder gallopinghttps://scholarbank.nus.edu.sg/handle/10635/86131Title: On using high-order polynomial curve fits in the quasi-steady theory for square-cylinder galloping
Authors: Ng, Y.T.; Luo, S.C.; Chew, Y.T.
Abstract: Quasi-steady theory shows that the galloping response of a square cylinder exhibits a hysteresis phenomenon. The equation of motion, which was derived based on a seventh-order polynomial curve fit on the side force (Cy) versus angle of attack (α) curve, shows that the number of positive real roots corresponds to the number of stationary oscillation amplitudes. In this investigation, we use polynomials of even higher order (ninth and eleventh) to curve fit the Cy versus α curve, in an attempt to see if additional positive real roots occur, which may reveal even more flow physics. The results show that only extra negative real roots and/or complex roots are obtained when higher than seventh-order polynomial curve fits are used. Hence, the use of a seventh-order polynomial curve fit in the quasi-steady theory is shown to be sufficient in describing the flow physics which includes the prediction of the hysteresis phenomenon. © 2004 Elsevier Ltd. All rights reserved.
Sat, 01 Jan 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/861312005-01-01T00:00:00Z
- Critical evaluation of the explicit data analysis algorithm for a crossed wire anemometer in highly turbulent isotropic flowhttps://scholarbank.nus.edu.sg/handle/10635/58050Title: Critical evaluation of the explicit data analysis algorithm for a crossed wire anemometer in highly turbulent isotropic flow
Authors: Chew, Y.T.; Ha, S.M.
Abstract: An experiment was conducted in a grid generated, nominally homogeneous, isotropic turbulent flow, in a wind tunnel of 0.33 m × 0.33 m cross-sectional area, in order to evaluate the performance of an explicit non-real time data analysis algorithm for a crossed wire anemometer in highly turbulent flow. The 7000 data values acquired at various downstream locations with different turbulence intensities were analysed by the explicit non-real time method. It was found that the simple explicit non-real time method is adequate in computing the Reynolds stresses up to a turbulence intensity of 40% when compared with the tedious lookup table method. The discrepancies are less than ±6% for Reynolds normal stresses and less than ±15% for Reynolds shear stress. A theoretical model based on the Gaussian probability density function was also developed to predict the probability distribution of the yaw angle of flow at various turbulence intensities in order to refine the explicit non-real time and look-up table methods of data analysis.
Wed, 01 Aug 1990 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/580501990-08-01T00:00:00Z
- Generalized transport vortex methodhttps://scholarbank.nus.edu.sg/handle/10635/60401Title: Generalized transport vortex method
Authors: Zhao, J.; Chew, Y.T.; Luo, S.C.; Pan, L.S.; Wu, J.K.
Abstract: In this paper, a novel vortex method - generalized transport vortex method is introduced. Being a Lagrangian-Eulerian Approach, this method determines the vorticity field through studying the vortex/circulation's generalized transport in an artificial velocity (generalized velocity) field of Lagrangian frame. The velocity field is then determined through the use of Poisson's equation in Eulerian frame. The "generalized transport process" refers the movement and area variation of vortex/circulation, which takes consideration of both diffusion and convection processes. Comparing with traditional vortex-in-cell methods and hybrid vortex methods, it does not use splitting algorithm in math, instead, handle diffusion as a part of the convection process. There is no region decomposition issue in the computation, and its expression is rather simple and easy to realize numerically. Being a numerical application, the present method is used to compute flow past one impulsively started circular cylinder. It is capable of calculating the evolution of the fine structure of the flow field with time precisely. © 2006 Elsevier Ltd. All rights reserved.
Sun, 01 Jul 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/604012007-07-01T00:00:00Z
- A 3D incompressible thermal lattice Boltzmann model and its application to simulate natural convection in a cubic cavityhttps://scholarbank.nus.edu.sg/handle/10635/53887Title: A 3D incompressible thermal lattice Boltzmann model and its application to simulate natural convection in a cubic cavity
Authors: Peng, Y.; Shu, C.; Chew, Y.T.
Abstract: A 3D incompressible thermal lattice Boltzmann model is proposed in this paper to solve 3D incompressible thermal flow problems. Two different particle velocity models of D3Q15 and D3Q19 are incorporated in our thermal model. It is indicated that the present thermal model is simple and easy for implementation. It is validated by its application to simulate the 3D natural convection of air in a cubical enclosure, which is heated differentially at two vertical side walls. Good agreement was obtained between the present results and those from a Navier-Stokes solver. © 2003 Elsevier B.V. All rights reserved.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/538872004-01-01T00:00:00Z
- Taylor series expansion and least squares-based lattice Boltzmann method: Three-dimensional formulation and its applicationshttps://scholarbank.nus.edu.sg/handle/10635/85736Title: Taylor series expansion and least squares-based lattice Boltzmann method: Three-dimensional formulation and its applications
Authors: Shu, C.; Niu, X.D.; Chew, Y.T.
Abstract: The two-dimensional form of the Taylor series expansion- and least square-based lattice Boltzmann method (TLLBM) was recently presented by Shu et al. 8 TLLBM is based on the standard lattice Boltzmann method (LBM), Taylor series expansion and the least square optimization. The final formulation is an algebraic form and essentially has no limitation on the mesh structure and lattice model. In this paper, TLLBM is extended to the three-dimensional case. The resultant form keeps the same features as the two-dimensional one. The present form is validated by its application to simulate the three-dimensional lid-driven cavity flow at Re = 100, 400 and 1000. Very good agreement was achieved between the present results and those of Navier-Stokes solvers.
Mon, 01 Sep 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/857362003-09-01T00:00:00Z
- Amino acid modified xanthone derivatives: Novel, highly promising membrane-active antimicrobials for multidrug-resistant gram-positive bacterial infectionshttps://scholarbank.nus.edu.sg/handle/10635/127508Title: Amino acid modified xanthone derivatives: Novel, highly promising membrane-active antimicrobials for multidrug-resistant gram-positive bacterial infections
Authors: Koh Jun Jie; Lin Shuimu; Aung Thet Tun; Lim Fanghui; Zou Hanxun; Bai Yang; Li Jianguo; Lin Huifen; Pang Li Mei; Koh Wee Luan; Shuhaida Mohammed Salleh; Lakshminarayanan Rajamani; Zhou Lei; Qiu Shengxiang; Pervushin Konstantin; Chandra Verma; Tan Tiang Hwee; Cao Derong; Liu Shouping; Beuerman Roger W.
Thu, 01 Jan 2015 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1275082015-01-01T00:00:00Z
- Numerical simulation of capsule deformation in simple shear flowhttps://scholarbank.nus.edu.sg/handle/10635/60940Title: Numerical simulation of capsule deformation in simple shear flow
Authors: Sui, Y.; Chen, X.B.; Chew, Y.T.; Roy, P.; Low, H.T.
Abstract: The transient deformation of two-dimensional non-circular and three-dimensional non-spherical capsules in simple shear flow is studied numerically, using the hybrid immersed boundary and multi-block lattice Boltzmann method recently proposed by the present authors. The capsules are modeled as Newtonian liquid drops enclosed by elastic membranes; the liquids inside and outside the capsule have the same physical properties. The present results show important different behaviors between two-dimensional and three-dimensional capsules in shear flow. For two-dimensional non-circular capsules without considering the membrane bending rigidity, or considering the bending with the minimum bending-energy configurations (shapes at which the bending-energy has a global minimum) being uniform-curvature shapes, the capsules will always achieve the steady tank treading motion (a capsule deforms to a steady shape with a steady inclination and the membrane rotates around the liquid inside). However, for three-dimensional non-spherical capsules without membrane bending rigidity, such a steady mode does not exist; with the shear rate decreasing, the three-dimensional capsules' motion changes from swinging mode (a capsule undergoes periodic shape deformation and inclination oscillation while its membrane is rotating around the liquid inside) to flipping mode. The deformation of two-dimensional capsules, with their initial non-circular shapes taken as the minimum bending-energy configurations, is also considered. It is quite interesting to find such two-dimensional capsules behave qualitatively similar to three-dimensional capsules: there is a swinging-to-flipping transition induced by lowering the shear rate. © 2009 Elsevier Ltd. All rights reserved.
Mon, 01 Feb 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609402010-02-01T00:00:00Z
- Numerical investigation of transporting droplets by spatiotemporally controlling substrate wettabilityhttps://scholarbank.nus.edu.sg/handle/10635/51484Title: Numerical investigation of transporting droplets by spatiotemporally controlling substrate wettability
Authors: Huang, J.J.; Shu, C.; Chew, Y.T.
Abstract: Droplet behaviors on substrates with wettability controlled both in space and in time are numerically investigated by using the lattice Boltzmann method. Several typical droplet responses are found under different designs of substrate wettability control. Special attention is drawn to the conditions under which rapid transport of droplets can be achieved. It is found that on alternating non-wetting-wetting units with proper non-wetting confining stripes, this objective can be realized when the frequency of wettability switch approximately matches that of the droplet to move across one unit. The variation of the "optimal" frequency with the size of the confining stripe is sought within certain ranges. The various types of droplet movement are analyzed by looking at the three-phase lines on the substrate, as well as the droplet shapes under different conditions. The results may provide useful implications for droplet manipulation in microfluidic devices. © 2008 Elsevier Inc. All rights reserved.
Mon, 01 Dec 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/514842008-12-01T00:00:00Z
- An axisymmetric lattice Boltzmann model for simulation of Taylor-Couette flows between two concentric cylindershttps://scholarbank.nus.edu.sg/handle/10635/59429Title: An axisymmetric lattice Boltzmann model for simulation of Taylor-Couette flows between two concentric cylinders
Authors: Niu, X.D.; Shu, C.; Chew, Y.T.
Abstract: Recently, Halliday et al. presented an idea by inserting the "source" terms into the two-dimensional (2D) lattice Boltzmann equation (LBE) so that the emergent dynamics of the lattice fluid can be transformed into the cylindrical polar system. This paper further extends the idea of Halliday et al. to include the effect of azimuthal rotation. The terms related to the azimuthal effect are considered as inertia forces. By using our recently developed Taylor-series-expansion and least-square-based lattice Boltzmann method (TLLBM) for the transformed LBE and a second order explicit finite difference method for the azimuthal moment equation, Taylor-Couette flows between two concentric cylinders with the inner cylinder rotating were simulated. To show the performance of the proposed model, the same problem was also simulated by the three-dimensional (3D) LBM. Numerical results showed that the present axisymmetric model is much more efficient than the 3D model for an axisymmetric flow problem.
Tue, 01 Jul 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/594292003-07-01T00:00:00Z
- Dynamic response of a hot-wire anemometer. Part II: A flush-mounted hot-wire and hot-film probes for wall shear stress measurementshttps://scholarbank.nus.edu.sg/handle/10635/58130Title: Dynamic response of a hot-wire anemometer. Part II: A flush-mounted hot-wire and hot-film probes for wall shear stress measurements
Authors: Chew, Y.T.; Khoo, B.C.; Lim, C.P.; Teo, C.J.
Abstract: The present study continues the work described in Part I of this paper in utilizing a specifically designed apparatus for generating a known near-wall fluctuating flow field for the purpose of quantifying the dynamic response of a flush-mounted hot-element wall shear stress gauge. Two different types of wall shear stress gauges were tested: a flush-mounted hot wire in contact with the wall substrate and commercially available quartz substrate hot-film gauges with different thicknesses of quartz coating. It is found that the flush-mounted hot-wire gauge has a much higher dynamic response than the quartz substrate gauges, although it is still lower than the marginally elevated hot-wire configuration. This may suggest the possible use of a marginally elevated hot wire as a wall shear shress gauge to ensure sufficient responsiveness for time-resolved wall shear stress measurements.
Fri, 01 May 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/581301998-05-01T00:00:00Z
- Determination of hot-wire position from a solid wall in an opaque channelhttps://scholarbank.nus.edu.sg/handle/10635/59885Title: Determination of hot-wire position from a solid wall in an opaque channel
Authors: Tay, C.M.J.; Khoo, B.C.; Chew, Y.T.
Abstract: Hot-wire experiments sometimes require the position of the hot-wire probe with respect to a solid wall to be known, as is often the case with boundary layer flow measurements. However, optical access to determine the position of the wall visually may not always be available. The current paper describes a procedure to allow a hot wire to determine accurately the location of the solid wall with respect to any arbitrary reference position using the response of the hot wire when it approaches a solid wall under no flow conditions. An accurate probe-positioning system can then be used to position the probe with respect to this wall. The method is applied to both an aluminum wall and a Perspex wall, and both give consistent measurements with an accuracy of better than ±0.005mm. © 2012 IOP Publishing Ltd.
Wed, 01 Aug 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/598852012-08-01T00:00:00Z
- A solution adaptive simulation of compressible multi-fluid flows with general equation of statehttps://scholarbank.nus.edu.sg/handle/10635/59269Title: A solution adaptive simulation of compressible multi-fluid flows with general equation of state
Authors: Zheng, H.W.; Shu, C.; Chew, Y.T.; Qin, N.
Abstract: The unstructured quadrilateral mesh-based solution adaptive method is proposed in this article for simulation of compressible multi-fluid flows with a general form of equation of state (EOS). The five equation model (J. Comput. Phys. 2002; 118:577-616) is employed to describe the compressible multi-fluid flows. To preserve the oscillation-free property of velocity and pressure across the interface, the non-conservative transport equation is discretized in a compatible way of the HLLC scheme for the conservative Euler equations on the unstructured quadrilateral cell-based adaptive mesh. Five numerical examples, including an interface translation problem, a shock tube problem with two fluids, a solid impact problem, a two-dimensional Riemann problem and a bubble explosion under free surface, are used to examine its performance in solving the various compressible multi-fluid flow problems with either the same types of EOS or different types of EOS. The results are compared with those calculated by the following methods: the method with ROE scheme (J. Comput. Phys. 2002; 118:577-616), the seven equation model (J. Comput. Phys. 1999; 150:425-467), Shyue's fluid-mixture model (J. Comput. Phys. 2001; 171:678-707) or the method in Liu et al. (Comp. Fluids 2001; 30:315-337). The comparisons for the test problems show that the proposed method seems to be more accurate than the method in Allaire et al. (J. Comput. Phys. 2002; 118:577-616) or the seven-equation model (J. Comput. Phys. 1999; 150:425-467). They also show that it can adaptively and accurately solve these compressible multi-fluid problems and preserve the oscillation-free property of pressure and velocity across the material interface. © 2010 John Wiley & Sons, Ltd.
Thu, 20 Oct 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/592692011-10-20T00:00:00Z
- Application of lattice Boltzmann method to simulate microchannel flowshttps://scholarbank.nus.edu.sg/handle/10635/59557Title: Application of lattice Boltzmann method to simulate microchannel flows
Authors: Lim, C.Y.; Shu, C.; Niu, X.D.; Chew, Y.T.
Abstract: Microflow has become a popular field of interest due to the advent of microelectromechanical systems. In this work, the lattice Boltzmann method, a particle-based approach, is applied to simulate the two-dimensional isothermal pressure driven microchannel flow. Two boundary treatment schemes are incorporated to investigate their impacts to the entire flow field. We pay particular attention to the pressure and the slip velocity distributions along the channel in our simulation. We also look at the mass flow rate which is constant throughout the channel and the overall average velocity for the pressure-driven flow. In addition, we include a simulation of shear-driven flow in our results for verification. Our numerical results compare well with those obtained analytically and experimentally. From this study, we may conclude that the lattice Boltzmann method is an efficient approach for simulation of microflows. © 2002 American Institute of Physics.
Mon, 01 Jul 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/595572002-07-01T00:00:00Z
- Time-resolved near-wall hot-wire measurements: Use of laminar flow wall correction curve and near-wall calibration techniquehttps://scholarbank.nus.edu.sg/handle/10635/58838Title: Time-resolved near-wall hot-wire measurements: Use of laminar flow wall correction curve and near-wall calibration technique
Authors: Khoo, B.C.; Chew, Y.T.; Li, G.L.
Abstract: This work describes the use of the laminar flow wall correction method and near-wall calibration technique to account for the influence of wall effects on near-wall hot-wire measurements thereby enabling the realization of a true instantaneous velocity field. Most previous works for near-wall hot-wire correction have only concentrated on obtaining the true mean velocity. Experiments were carried out with different wall substrates in both turbulent channel and boundary layer flows. The corrected instantaneous velocity is evaluated in terms of the higher order moments of fluctuation and their respective distributions for comparison to published works in the literature. There is good agreement between results which supports the use of the said methods on near-wall hot-wire measurements to obtain a true instantaneous velocity field.
Mon, 01 Apr 1996 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/588381996-04-01T00:00:00Z
- A hybrid method to study flow-induced deformation of three-dimensional capsuleshttps://scholarbank.nus.edu.sg/handle/10635/54274Title: A hybrid method to study flow-induced deformation of three-dimensional capsules
Authors: Sui, Y.; Chew, Y.T.; Roy, P.; Low, H.T.
Abstract: A hybrid method is proposed to study the transient deformation of liquid filled capsules with elastic membranes under flow. In this method, the immersed boundary concept is introduced into the framework of lattice Boltzmann method, and the multi-block strategy is employed to refine the mesh near the capsule to increase the accuracy and efficiency of computation. A finite element model is incorporated to obtain the forces acting on the membrane nodes of the three-dimensional capsule which is discretized into flat triangular elements. The present method was validated by studying the transient deformation of initially spherical and oblate-spheroidal capsules with various membrane constitutive laws under shear flow; and there were good agreements with previous theory or numerical results. The versatility of the present method was demonstrated by studying the effects of inertia on the deformation of capsules in shear flow; and the inertia effects were found to be significant. The transient deformation of capsules with initially biconcave discoid shape in shear flow was also studied. The unsteady tank-treading motion was observed, in which the capsule undergoes periodic shape deformation and inclination oscillation while its membrane is rotating around the liquid inside. To our knowledge, this motion of three-dimensional biconcave discoid capsules has not been fully recovered by numerical simulation so far. © 2008 Elsevier Inc. All rights reserved.
Sun, 01 Jun 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/542742008-06-01T00:00:00Z
- Development of an immersed boundary-phase field-lattice boltzmann method for neumann boundary condition to study contact line dynamicshttps://scholarbank.nus.edu.sg/handle/10635/59921Title: Development of an immersed boundary-phase field-lattice boltzmann method for neumann boundary condition to study contact line dynamics
Authors: Shao, J.Y.; Shu, C.; Chew, Y.T.
Abstract: The implementation of Neumann boundary condition in the framework of immersed boundary method (IBM) is presented in this paper to simulate contact line dynamics using a phase field-lattice Boltzmann method. Immersed boundary method [10] is known as an efficient algorithm for modelling fluid-solid interaction. Abundance of prominent works have been devoted to refine IBM [1,11,12]. However, they are mainly restricted to problems with Dirichlet boundary condition. Research that implements the Neumann boundary condition inIBM is very limited to the best of our knowledge. This deficiency significantly limits the application of IBM in computational fluiddynamics (CFD) since physical phenomena associated with Neumann boundary conditions are extremely diverse. The difficulty is attributed to the fact that implementation of Neumann boundary condition is much more complex than that of Dirichlet boundary condition. Inthe present work, we initiate the first endeavour to implement Neumann boundary condition in IBM with assistance of its physical interpretation rather than simple mathematical manipulation. Concretely speaking, rooted from physical conservation law, the Neumann boundary condition is considered as contribution of flux from the boundary to its relevant physical parameter in a control volume. Moreover, the link between the flux and its corresponding flow field variable is directly manipulated through the immersed boundary concept. In this way, the Neumann boundary conditions can be implemented in IBM. The developed method is applied together with phase field-lattice Boltzmann method to study contact line dynamics. The phase field method [27,39], which becomes increasingly popular inmultiphase flow simulation, can efficiently capture complex interface topology and naturally resolve the contact line singularity. Meanwhile, the lattice Boltzmann method is known as an alternative to model fluid dynamics and holds good prospect to simulate multiphase flows with complex geometry [38]. In this context, the developed immersed boundary-phase field-LBM is verified in detail for bothsteady and unsteady contact line problems. Tests show that the proposed method can correctly reproduce both equilibrium results and dynamic process.©2012 Elsevier Inc.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/599212013-01-01T00:00:00Z
- Uniform flow past one (or two in tandem) finite length circular cylinder(s)https://scholarbank.nus.edu.sg/handle/10635/58881Title: Uniform flow past one (or two in tandem) finite length circular cylinder(s)
Authors: Luo, S.C.; Gan, T.L.; Chew, Y.T.
Abstract: The problem of uniform flow past one (or two in tandem formation) finite length circular cylinder(s) is investigated experimentally in the present paper. For the case of flow past a single finite length cylinder, it is found that the flow that separates from the free end interacts strongly with those that separate from the sides and results in a three-dimensional flow. This separated flow from the free end delays the interaction between the separated flows from the sides, resulting in a less negative wake pressure and a lower drag when compared with flow past an infinitely long cylinder. Spanwise effects were found to be stronger over the longest (h/d = 8) cylinder investigated, with the spanwise drag coefficient increasing towards the free end. Vortex shedding with some regularity can be detected only at y/h = 0.95 and y/h ≤ 0.5 (with weak spectral peak) of the h/d = 8 cylinder, with corresponding Strouhal numbers equal to 0.08 and 0.191, respectively. For flow past two tandem finite length circular cylinders, although the transition from reattached flow to co-shedding flow is still observed, due to the highly three-dimensional nature of the flow, the change no longer takes place over the entire span simultaneously. When the dimensionless spacing between the cylinders increases from one (cylinders touching), the co-shedding flow structure was first detected near the free end and its extent later expands towards the mounted end with further increase in cylinders spacing, at the expense of the reattached flow region. At very small spacing between the cylinders, a third flow structure which is characterised by having lower surface pressure over the windward part of the downstream cylinder was also observed. Spanwise variation of the drag that acts on both cylinders was observed. The magnitude of the drag coefficient was found to be strongly dependent on the flow structure involved and normally varies as a direct consequence of the variation in the wake pressure. The overall trend is that of higher local drag towards the free end.
Mon, 01 Jan 1996 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/588811996-01-01T00:00:00Z
- Near-wall hot-wire measurements. Part II: Turbulence time scale, convective velocity and spectra in the viscous sublayerhttps://scholarbank.nus.edu.sg/handle/10635/85477Title: Near-wall hot-wire measurements. Part II: Turbulence time scale, convective velocity and spectra in the viscous sublayer
Authors: Khoo, B.C.; Chew, Y.T.; Teo, C.J.
Abstract: This work continues the studies of Khoo et al. (Exp. Fluids 29: 448-460, 2001), where experiments were performed in turbulent-channel and flat-plate boundary-layer flows using near-wall hot-wire probes. The probability density function (pdf) of the wall-shear stress and streamwise velocity fluctuations in the viscous sublayer, buffer region and beyond were compared and analyzed. The convective velocity Uc of the streamwise velocity fluctuations in the very near-wall region was obtained using a two-point correlation technique. It was found that in the viscous sublayer, Uc is approximately constant at 13uτ and 15uτ, respectively, for the channel and boundary-layer flows. Spectra data for the viscous sublayer are presented for the first time, and the normalized spectral plots for different flow conditions collapse at high frequencies or wavenumbers, thus indicating the possible presence of small-scale universality at different Reynolds numbers. The integral time scale corresponding to the streamwise velocity fluctuations in the viscous sublayer is also presented.
Thu, 01 Nov 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/854772001-11-01T00:00:00Z
- Use of DES in mildly separated internal flow: dimples in a turbulent channelhttps://scholarbank.nus.edu.sg/handle/10635/141054Title: Use of DES in mildly separated internal flow: dimples in a turbulent channel
Authors: Chien Ming Tay; Boo Cheong Khoo; Yong Tian Chew
Mon, 23 Oct 2017 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1410542017-10-23T00:00:00Z
- Influence of the Reynolds number on chaotic mixing in a spatially periodic micromixer and its characterization using dynamical system techniqueshttps://scholarbank.nus.edu.sg/handle/10635/60539Title: Influence of the Reynolds number on chaotic mixing in a spatially periodic micromixer and its characterization using dynamical system techniques
Authors: Xia, H.M.; Shu, C.; Wan, S.Y.M.; Chew, Y.T.
Abstract: A micromixer is a key component of various microfluidic systems, such as microreactors and μ-total analysis systems. One important strategy for passive mixer design is to generate chaotic advection using channel geometry, which usually has spatially periodic structures. In this paper, the influence of the Reynolds number on chaotic mixing in such mixers is studied with three mixer models. Characterization of the mixer with dynamical system techniques is also studied. The influence of fluid inertial effects on the occurrence of chaotic advection is first discussed. It is found that at low Re(Re < 1), the flow could become reversible in the mixer, which raises the difficulty to generate chaotic advection. In this case, specific fluid manipulations, such as stretching and folding processes, are necessary. This study also proposes a characterization method using Lyapunov exponent (λ) and Poincaré mapping information, which allows us to analyze the mixing performance of the mixer with one single mixer unit. Results show that it objectively reflects the dynamical properties of the mixers, such as being globally chaotic, partially chaotic or stable. So it can be used as an analytical tool to differentiate, evaluate and optimize various chaotic micromixers.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/605392006-01-01T00:00:00Z
- A front-tracking lattice Boltzmann method to study flow-induced deformation of three-dimensional capsuleshttps://scholarbank.nus.edu.sg/handle/10635/54176Title: A front-tracking lattice Boltzmann method to study flow-induced deformation of three-dimensional capsules
Authors: Sui, Y.; Low, H.T.; Chew, Y.T.; Roy, P.
Abstract: In this paper, a hybrid method is proposed to study the flow-induced deformation of three-dimensional capsules. The capsules consist of Newtonian liquid drops enclosed by thin elastic membranes. In the proposed approach, the front-tracking method is coupled with the lattice Boltzmann method. The fluids inside and outside the capsule is treated as one fluid with varying physical properties, and is modeled by the lattice Boltzmann equation. The capsule membrane is explicitly tracked by the membrane nodes that are advected by the flow. The multi-block strategy of the lattice Boltzmann method is employed to refine the mesh near the capsule, which greatly increase the accuracy and efficiency of the three-dimensional computation. The capsule membrane is discretized into unstructured flat triangular elements, and a finite element model is incorporated to account for the membrane mechanics. With the present method, the transient deformation of initially spherical capsules with membrane following Neo-Hookean constitutive laws is simulated in shear flow, under various dimensionless shear rates and ratios of internal to surrounding liquid viscosities. The present results, including the Taylor shape parameter, the capsule inclination angle and the tank-treading frequency, agree well with previously published numerical results. © 2009 Elsevier Ltd. All rights reserved.
Mon, 01 Mar 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/541762010-03-01T00:00:00Z
- Hysteresis phenomenon in the galloping oscillation of a square cylinderhttps://scholarbank.nus.edu.sg/handle/10635/85287Title: Hysteresis phenomenon in the galloping oscillation of a square cylinder
Authors: Luo, S.C.; Chew, Y.T.; Ng, Y.T.
Abstract: It is well known that a square cylinder with one side normal to a uniform stream will gallop when a critical flow velocity is exceeded. It is also quite well known that there is a hysteresis phenomenon in the variation of the cylinder's galloping amplitude with the flow velocity. However, little is known about the cause of this hysteresis phenomenon, and the objective of this paper is to study it more closely. In the present study, flow over a stationary square cylinder at different angle of attack (α) and at Reynolds number (Re) of 250 and 1000 was investigated numerically by using a 2-D hybrid vortex computation scheme. The study reveals that the well known point of inflection which exists in the side force (Cy) versus α plots at high Reynolds number only occurs at Re = 1000, α = 4° in the present numerical simulation. Nonlinear analysis further reveals that this point of inflection is the cause of the hysteresis phenomenon. By further analysing the computed flow field, it is noted that at Re = 1000, α = 4°, intermittent flow reattachment takes place at alternate vortex shedding cycle on one side of the cylinder. This results in larger side force fluctuation, and it is conjectured that such large side force fluctuation affects the increasing trend of the side force with angle of attack, resulting in the point of inflection reported earlier. The above-mentioned alternate cycle flow reattachment was much less prominent at α = 2° and 6° (Re = 1000), and was not observed at Re = 250. Finally, dye flow visualization was carried out in a recirculating water tunnel and the results at Re = 1000 confirms the existence of the intermittent flow reattachment. However, in the experiment, flow reattachment does not take place in a very regular alternate cycle manner as in the computation. Instead, it occurs intermittently, possibly due to three-dimensional effects in real flow. © 2003 Elsevier Ltd. All rights reserved.
Fri, 01 Aug 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/852872003-08-01T00:00:00Z
- Application of Taylor series expansion and Least-squares-based lattice Boltzmann method to simulate turbulent flowshttps://scholarbank.nus.edu.sg/handle/10635/59571Title: Application of Taylor series expansion and Least-squares-based lattice Boltzmann method to simulate turbulent flows
Authors: Shu, C.; Peng, Y.; Zhou, C.F.; Chew, Y.T.
Abstract: Lattice Boltzmann method (LBM) has become an alternative method of computing a variety of fluid flows, ranging from low Reynolds number laminar flows to highly turbulent flows. For turbulent flows, non-uniform grids are preferred. Taylor series expansion- and least-squares-based LBM (TLLBM) is an effective and convenient way to extend standard LBM to be used on arbitrary meshes. In order to show its ability to solve turbulent flows, we combine it with k-ω and S-A turbulence models. To validate these combinations, the benchmark problems of the turbulent channel flow and the turbulent flow over a backward facing step at Re = 44 000 are simulated. Our results compare well with the analytical solution and the experimental results of Kim et al. [22]. This shows that the combination of TLLBM with turbulence model can solve turbulent flows effectively. © 2006 Taylor & Francis.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/595712006-01-01T00:00:00Z
- Tank-treading, swinging, and tumbling of liquid-filled elastic capsules in shear flowhttps://scholarbank.nus.edu.sg/handle/10635/61461Title: Tank-treading, swinging, and tumbling of liquid-filled elastic capsules in shear flow
Authors: Sui, Y.; Low, H.T.; Chew, Y.T.; Roy, P.
Abstract: The dynamic motion of three-dimensional (3D) capsules in a shear flow is investigated by direct numerical simulation. The capsules are modeled as Newtonian liquid droplets enclosed by elastic membranes, with or without considering the membrane-area incompressibility. The internal liquid of the capsules is the same as that outside. The dynamic motion of capsules with initially spherical and oblate spheroidal unstressed shapes is studied under various shear rates. The results show that spherical capsules deform to stationary configurations and then the membranes rotate around the liquid inside (steady tank-treading motion). Such a steady mode is not observed for oblate spheroidal capsules. It is shown that with the shear rate decreasing, the motion of oblate spheroidal capsules changes from the swinging mode (a capsule undergoes periodic shape deformation and inclination oscillation while its membrane is rotating around the liquid inside) to tumbling mode. © 2008 The American Physical Society.
Thu, 31 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/614612008-01-31T00:00:00Z
- Effect of membrane bending stiffness on the deformation of elastic capsules in extensional flow: A lattice Boltzmann studyhttps://scholarbank.nus.edu.sg/handle/10635/60054Title: Effect of membrane bending stiffness on the deformation of elastic capsules in extensional flow: A lattice Boltzmann study
Authors: Sui, Y.; Chew, Y.T.; Roy, P.; Low, H.T.
Abstract: The transient deformation of liquid capsules enclosed by elastic membranes in twodimensional extensional flow is studied numerically, using an improved immersed boundary-lattice Boltzmann method. The purpose of the present study is to investigate the effect of interfacial bending stiffness on the deformation of such capsules, under the subcritical elasticity capillary number conditions. The present model can simulate flow-induced deformation of capsules with arbitrary resting shapes (concerning the inplane tension) and bending-free configurations. The deformation of capsules with initially circular, elliptical, and biconcave resting shapes was investigated in the present study; the capsules' bending-free configurations were considered as either circular shapes or their initially resting shapes. The results show that for capsules with bending-free configuration as circles, membrane bending rigidity has significant rounding effect on the steady deformed profiles. For elliptical and biconcave capsules with resting shapes as the bending-free configurations, it is found that with the bending stiffness increasing, the capsules' steady shapes are more akin to their initial shapes. © World Scientific Publishing Company.
Wed, 01 Aug 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/600542007-08-01T00:00:00Z
- Numerical study of eccentric Couette-Taylor flows and effect of eccentricity on flow patternshttps://scholarbank.nus.edu.sg/handle/10635/60959Title: Numerical study of eccentric Couette-Taylor flows and effect of eccentricity on flow patterns
Authors: Shu, C.; Wang, L.; Chew, Y.T.; Zhao, N.
Abstract: In this study, the differential quadrature (DQ) method was used to simulate the eccentric Couette-Taylor vortex flow in an annulus between two eccentric cylinders with rotating inner cylinder and stationary outer cylinder. An approach combining the SIMPLE (semiimplicit method for pressure-linked equations) and DQ discretization on a non-staggered mesh was proposed to solve the time-dependent, three-dimensional incompressible Navier-Stokes equations in the primitive variable form. The eccentric steady Couette-Taylor flow patterns were obtained from the solution of three-dimensional Navier-Stokes equations. The reported numerical results for steady Couette flow were compared with those from Chou and San and Szeri. Very good agreement was achieved. For steady eccentric Taylor vortex flow, detailed flow patterns were obtained and analyzed. The effect of eccentricity on the eccentric Taylor vortex flow pattern was also studied.
Fri, 01 Oct 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609592004-10-01T00:00:00Z
- Three-dimensional lattice Boltzmann interface capturing method for incompressible flowshttps://scholarbank.nus.edu.sg/handle/10635/61578Title: Three-dimensional lattice Boltzmann interface capturing method for incompressible flows
Authors: Zheng, H.W.; Shu, C.; Chew, Y.T.; Sun, J.H.
Abstract: This paper presents a new three-dimensional lattice Boltzmann interface capturing method for incompressible flows following the work of Zheng et al. (Phys. Rev. E 2005; 72:056705). As shown in the paper, the fourth rank isotropic property of the lattice tensor is not needed for interface capturing. As a result, a new D3Q7 (D3 means three dimensional, Q7 means seven velocity bits) lattice velocity model and its associated equilibrium distribution functions are proposed in the paper. The proposed model is validated by comparing its numerical results with those of an existing lattice Boltzmann interface capturing model (J. Comput. Phys. 2004; 198:628-644) and three-dimensional direction split flux-corrected transport method (Int. J. Numer. Meth. Fluids 1997; 24:671-691). Numerical results showed that the present model performs better than the existing methods in capturing the interface. It greatly improves the computational efficiency and saves at least half of the memory as compared to other lattice Boltzmann interface capturing models. Copyright © 2007 John Wiley & Sons, Ltd.
Sun, 30 Mar 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/615782008-03-30T00:00:00Z
- Particle image velocimetry in the investigation of flow past artificial heart valveshttps://scholarbank.nus.edu.sg/handle/10635/58608Title: Particle image velocimetry in the investigation of flow past artificial heart valves
Authors: Lim, W.L.; Chew, Y.T.; Chew, T.C.; Low, H.T.
Abstract: Full-field measurement of instantaneous velocities in the flow field of artificial heart valves is vital as the flow is unsteady and turbulent. Particle image velocimetry (PIV) provides us the ability to do this as compared to other point measurement devices where the velocity is measured at a single point in space over time. In the development of a PIV system to investigate the flow field of artificial heart valves, many problems associated with the project arose and were subsequently resolved. Experience gained in the setting up of an environment conducive for PIV studies of artificial heart valves; from seed particle selection to refractive index matching, and the evolution of computer algorithms to satisfy the varied flow conditions in artificial heart valves are presented here. Velocity profiles and distributions are computed and drawn for a porcine tissue heart valve based on measurements with the PIV system developed.
Sat, 01 Jan 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/586081994-01-01T00:00:00Z
- Transient deformation of elastic capsules in shear flow: Effect of membrane bending stiffnesshttps://scholarbank.nus.edu.sg/handle/10635/61608Title: Transient deformation of elastic capsules in shear flow: Effect of membrane bending stiffness
Authors: Sui, Y.; Chew, Y.T.; Roy, P.; Chen, X.B.; Low, H.T.
Abstract: The transient deformation of liquid capsules enclosed by elastic membranes with bending rigidity in shear flow has been studied numerically, using an improved immersed boundary-lattice Boltzmann method. The purpose of the present study is to investigate the effect of interfacial bending stiffness on the deformation of such capsules. Bending moments, accompanied by transverse shear tensions, usually develop due to a preferred membrane configuration or its nonzero thickness. The present model can simulate flow induced deformation of capsules with arbitrary resting shapes (concerning the in-plane tension) and arbitrary configurations at which the bending energy has a global minimum (minimum bending-energy configurations). The deformation of capsules with initially circular, elliptical, and biconcave resting shapes was studied; the capsules' minimum bending-energy configurations were considered as either uniform-curvature shapes (like circle or flat plate) or their initially resting shapes. The results show that for capsules with minimum bending-energy configurations having uniform curvature (circle or flat plate), the membrane carries out tank-treading motion, and the steady deformed shapes become more rounded if the bending stiffness is increased. For elliptical and biconcave capsules with resting shapes as minimum bending-energy configurations, it is quite interesting to find that with the bending stiffness increasing, the capsules' motion changes from tank-treading mode to flipping mode, and resembles Jeffery's flipping mode at large bending stiffness. © 2007 The American Physical Society.
Tue, 05 Jun 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/616082007-06-05T00:00:00Z