ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sat, 08 Aug 2020 21:12:44 GMT2020-08-08T21:12:44Z50511- Parallelized FVM algorithm for three-dimensional viscoelastic flowshttps://scholarbank.nus.edu.sg/handle/10635/85534Title: Parallelized FVM algorithm for three-dimensional viscoelastic flows
Authors: Dou, H.-S.; Phan-Thien, N.
Abstract: A parallel implementation for the finite volume method (FVM) for three-dimensional (3D) viscoelastic flows is developed on a distributed computing environment through Parallel Virtual Machine (PVM). The numerical procedure is based on the SIMPLEST algorithm using a staggered FVM discretization in Cartesian coordinates. The final discretized algebraic equations are solved with the TDMA method. The parallelisation of the program is implemented by a domain decomposition strategy, with a master/slave style programming paradigm, and a message passing through PVM. A load balancing strategy is proposed to reduce the communications between processors. The three-dimensional viscoelastic flow in a rectangular duct is computed with this program. The modified Phan-Thien-Tanner (MPTT) constitutive model is employed for the equation system closure. Computing results are validated on the secondary flow problem due to non-zero second normal stress difference N2. Three sets of meshes are used, and the effect of domain decomposition strategies on the performance is discussed. It is found that parallel efficiency is strongly dependent on the grid size and the number of processors for a given block number. The convergence rate as well as the total efficiency of domain decomposition depends upon the flow problem and the boundary conditions. The parallel efficiency increases with increasing problem size for given block number. Comparing to two-dimensional flow problems, 3D parallelized algorithm has a lower efficiency owing to largely overlapped block interfaces, but the parallel algorithm is indeed a powerful means for large scale flow simulations.
Sat, 01 Mar 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/855342003-03-01T00:00:00Z
- Squeezing flow of a viscoelastic solidhttps://scholarbank.nus.edu.sg/handle/10635/58718Title: Squeezing flow of a viscoelastic solid
Authors: Phan-Thien, N.
Abstract: We report an exact solution structure to the plane and axi-symmetric squeezing flows of a viscoelastic solid-like material modelled by a three-dimensional analogue of the Kelvin-Meyer-Voigt equation, consisting of the neo-Hookean rubber-like finite deformation and the Upper Convected Maxwell models. Although the solution is valid for any prescribed time function for the plate velocity, we choose to focus on the oscillatory squeezing flow, where the top plate is displaced sinusoidally with an arbitrary amplitude. It is found that the load can exhibit a significant degree of asymmetry. This is largely due to the rubber-like elasticity in the response, resulting in a larger force in the downward phase of the displacement. This, however, can be reversed at a higher Reynolds number, where material inertia dominates. This is the first time a non-trivial solution for this class of material is reported.
Mon, 25 Dec 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/587182000-12-25T00:00:00Z
- Numerical simulation of fibre suspension flow through an axisymmetric contraction and expansion passages by Brownian configuration field methodhttps://scholarbank.nus.edu.sg/handle/10635/60943Title: Numerical simulation of fibre suspension flow through an axisymmetric contraction and expansion passages by Brownian configuration field method
Authors: Lu, Z.; Khoo, B.C.; Dou, H.-S.; Phan-Thien, N.; Seng Yeo, K.
Abstract: In this paper, the finite element method is combined with the Brownian configuration field (BFC) method to simulate the fibre suspension flow in axisymmetric contraction and expansion passages. In order to solve for the high stress at high concentration, the discrete adaptive viscoelastic stress splitting (DAVSS) method is employed. For the axisymmetric contraction and expansion passages with different geometry ratios, the results obtained are compared to available constitutive models and experiments. The predicted vortex length for dilute suspensions agrees well with experimental data in literature. Our numerical results show clearly the effect on vortex enhancement with increase of the volume fractions and the aspect ratios. Effect of aspect ratio of fibres on the vortex length is also studied. It is found that for the lower expansion ratio flows the vortex dimension in the corner region is fairly independent of fibre concentration and aspect ratio of fibres while the said vortex dimension increases with the increase of fibre concentration for contraction flows. The finding suggests that the aligned fibre approximation traditionally employed in previous work does not exactly describe the effect of fibre motion, and the present BFC method is deemed more suitable for the flow of dilute fibre suspensions. In terms of numerics, the employment of DAVSS enhances numerical stability in the presence of high concentration of fibre in the flow. © 2006 Elsevier Ltd. All rights reserved.
Tue, 01 Aug 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609432006-08-01T00:00:00Z
- Simulations of fibre orientation in dilute suspensions with front moving in the filling process of a rectangular channel using level-set methodhttps://scholarbank.nus.edu.sg/handle/10635/61314Title: Simulations of fibre orientation in dilute suspensions with front moving in the filling process of a rectangular channel using level-set method
Authors: Dou, H.-S.; Khoo, B.C.; Phan-Thien, N.; Yeo, K.S.; Zheng, R.
Abstract: The simulation of fibre orientation in dilute suspension with front moving is carried out using the projection and level-set methods. The motion of fibres is described using the Jeffery equation, and the contribution of fibres to the flow is accounted for by the configuration-field method. The dilute suspension of short fibres in Newtonian fluids is considered. The governing Navier-Stokes equation for the fluid flow is solved using the projection method with finite difference scheme, while the fibre-related equations are directly solved with the Runge-Kutta method. In the present study for fibres in dilute suspension flow for injection molding, the effects of various flow and material parameters on the fibre orientation, the velocity distributions and the shapes of the leading flow front are found and discussed. Our findings indicate that the presence of fibre motion has little influence on the front shape in the ranges of fibre parameters studied at the fixed Reynolds number. Influence of changing fibre parameters only causes variation of front shape in the region near the wall, and the front shape in the central core area does not vary much with the fibre parameters. On the other hand, the fibre motion has strong influence on the distributions of the streamwise and transverse velocities in the fountain flow. Fibre motion produces strong normal stress near the wall which leads to the reduction of transversal velocity as compared to the Newtonian flow without fibres, which in turn, leads to the increased streamwise velocity near the wall. Thus, the fibre addition to the flow weakens the strength of the fountain flow. The Reynolds number has also displayed significant influence on the distribution of the streamwise velocity behind the flow front for a given fibre concentration. It is also found that the fibre orientation is not always along the direction of the velocity vector in the process of mold filling. In the region of the fountain flow, the fibre near the centreline is more oriented across the streamwise direction compared to that in the region far behind the flow front. This leads to the fact that the fibre near the centreline in the region of fountain flow is more extended along the transverse direction. As the fibre orientation in the suspension flow and the shape of the flow front have great bearing on the quality of the product made from injection molding, this study has much implications for engineering applications. These results can also be useful in other fields dealing with fibre suspensions. © Springer-Verlag 2006.
Thu, 01 Mar 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/613142007-03-01T00:00:00Z
- Dynamic simulation of sphere motion in a vertical tubehttps://scholarbank.nus.edu.sg/handle/10635/60014Title: Dynamic simulation of sphere motion in a vertical tube
Authors: Yu, Z.; Phan-Thien, N.; Tanner, R.I.
Abstract: In this paper, the sedimentation of a sphere and its radial migration in a Poiseuille flow in a vertical tube filled with a Newtonian fluid are simulated with a finite-difference-based distributed Lagrange multiplier (DLM) method. The flow features, the settling velocities, the trajectories and the angular velocities of the spheres sedimenting in a tube at different Reynolds numbers are presented. The results show that at relatively low Reynolds numbers, the sphere approaches the tube axis monotonically, whereas in a high-Reynolds-number regime where shedding of vortices takes place, the sphere takes up a spiral trajectory that is closer to the tube wall than the tube axis. The rotation motion and the lateral motion of the sphere are highly correlated through the Magnus effect, which is verified to be an important (but not the only) driving force for the lateral migration of the sphere at relatively high Reynolds numbers. The standard vortex structures in the wake of a sphere, for Reynolds number higher than 400, are composed of a loop mainly located in a plane perpendicular to the streamwise direction and two streamwise vortex pairs. When moving downstream, the legs of the hairpin vortex retract and at the same time a streamwise vortex pair with rotation opposite to that of the legs forms between the loops. For Reynolds number around 400, the wake structures shed during the impact of the sphere on the wall typically form into streamwise vortex structures or else into hairpin vortices when the sphere spirals down. The radial, angular and axial velocities of both neutrally buoyant and non-neutrally buoyant spheres in a circular Poiseuille flow are reported. The results are in remarkably good agreement with the available experimental data. It is shown that suppresion of the sphere rotation produces significant large additional lift forces pointing towards the tube axis on the spheres in the neutrally buoyant and more-dense-downflow cases, whereas it has a negligible effect on the migration of the more dense sphere in upflow. © 2004 Cambridge University Press.
Wed, 10 Nov 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/600142004-11-10T00:00:00Z
- The influence of inlet velocity profile on three-dimensional three-generation bifurcating flowshttps://scholarbank.nus.edu.sg/handle/10635/61500Title: The influence of inlet velocity profile on three-dimensional three-generation bifurcating flows
Authors: Zhang, C.H.; Liu, Y.; So, R.M.C.; Phan-Thien, N.
Abstract: The influence of inlet velocity profile on the three-dimensional three-generation bifurcating flow has been numerically studied using a CFD code based on finite volume method. The bifurcating airways simulated the branches of human lung. The axial length and cross-sectional diameter of the three-generation airway are taken from the anatomic data of the 5th-7th generation airway of an averaged height man. The curvature and bifurcating angle of each junction are taken as 2.265 diameter of the next generation and 70°, respectively, from physiological consideration. Computations are carried out for eight Reynolds numbers ranging from 200 to 1600, each under uniform and parabolic inlet velocity profiles, to consider the relations between the entry flow patterns and the overall flow characteristics including mainstream flow pattern, secondary flow vortices, asymmetrical flow partition and pressure drop. The mass flow ratio between the medial and lateral branch, and the total pressure drop are closely related to the entry flow patterns for larger Reynolds numbers.
Tue, 01 Oct 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/615002002-10-01T00:00:00Z
- Criteria of negative wake generation behind a cylinderhttps://scholarbank.nus.edu.sg/handle/10635/59817Title: Criteria of negative wake generation behind a cylinder
Authors: Dou, H.-S.; Phan-Thien, N.
Abstract: It was demonstrated by simulation in our previous study that both the normal stress and its gradient are responsible for the negative wake generation (overshoot in the axial velocity) and streamline shifting. Extensional properties of the fluids dominate the generation of the negative wake, while other factors strengthen or weaken the formation of velocity overshoot. In this study, the criteria for the negative wake generation are discussed in detail for various fluid models, including the PTT, the FENE-CR, the FENE-P, and the Giesekus models. With the FENE-CR fluid, it is easier to generate negative wake than with the FENE-P fluid. This confirms that the constant shear viscosity FENE-CR fluid enhances the velocity overshoot, and that the shear-thinning viscosity FENE-P fluid delays the negative wake generation. The Giesekus fluid has a similar behaviour to the PTT fluid with regarding to the critical conditions of negative wake generation when appropriate fluid parameters are selected. The mechanism of wall proximity in enhancing the negative wake generation is also demonstrated with the analysis for the first time. © Springer-Verlag 2004.
Sat, 01 May 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/598172004-05-01T00:00:00Z
- Simulating flow of DNA suspension using dissipative particle dynamicshttps://scholarbank.nus.edu.sg/handle/10635/61294Title: Simulating flow of DNA suspension using dissipative particle dynamics
Authors: Fan, X.; Phan-Thien, N.; Chen, S.; Wu, X.; Ng, T.Y.
Abstract: We simulate DNA suspension microchannel flows using the dissipative particle dynamics (DPD) method. Two developments make this simulation more realistic. One is to improve the dynamic characteristics of a DPD system by modifying the weighting function of the dissipative force and increasing its cutoff radius, so that the Schmidt number can be increased to a practical level. Another is to set up a wormlike chain model in the DPD framework, according to the measured extension properties of a DNA molecule in uniform flows. This chain model is then used to study flows of a DNA suspension through microchannels. Interesting results on the conformation evolution of DNA molecules passing through the microchannels, including periodic contraction-diffusion microchannels, are reported. © 2006 American Institute of Physics.
Thu, 01 Jun 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612942006-06-01T00:00:00Z
- Rotation of a spheroid in a Couette flow at moderate Reynolds numbershttps://scholarbank.nus.edu.sg/handle/10635/61255Title: Rotation of a spheroid in a Couette flow at moderate Reynolds numbers
Authors: Yu, Z.; Phan-Thien, N.; Tanner, R.I.
Abstract: The rotation of a single spheroid in a planar Couette flow as a model for simple shear flow is numerically simulated with the distributed Lagrangian multiplier based fictitious domain method. The study is focused on the effects of inertia on the orbital behavior of prolate and oblate spheroids. The numerical orbits are found to be well described by a simple empirical model, which states that the rate of the spheroid rotation about the vorticity axis is a sinusoidal function of the corresponding projection angle in the flow-gradient plane, and that the exponential growth rate of the orbit function is a constant. The following transitions in the steady state with increasing Reynolds number are identified: Jeffery orbit, tumbling, quasi-Jeffery orbit, log rolling, and inclined rolling for a prolate spheroid; and Jeffery orbit, log rolling, inclined rolling, and motionless state for an oblate spheroid. In addition, it is shown that the orbit behavior is sensitive to the initial orientation in the case of strong inertia and there exist different steady states for certain shear Reynolds number regimes. © 2007 The American Physical Society.
Tue, 21 Aug 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612552007-08-21T00:00:00Z
- Rheology of polymers in many-body dissipative particle dynamics simulations: Schmidt number effecthttps://scholarbank.nus.edu.sg/handle/10635/143068Title: Rheology of polymers in many-body dissipative particle dynamics simulations: Schmidt number effect
Authors: Jiayi Zhao; Shuo Chen; Nhan Phan-Thien
Fri, 13 Apr 2018 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1430682018-04-13T00:00:00Z
- Normal stress differences behavior of polymeric particle suspension in shear flowhttps://scholarbank.nus.edu.sg/handle/10635/85488Title: Normal stress differences behavior of polymeric particle suspension in shear flow
Authors: Lin, Y.; Phan-Thien, N.; Cheong Khoo, B.
Abstract: Normal stress differences behavior of polymeric particle suspension in shear flow is investigated. The first normal stress difference (N1) in shear flow is found to mainly arise from the polymer matrix with a relaxation behavior altered by the presence of particles, which has its origin from the polymer chains trapped in the near-contact asymmetric regions of closely packed particles, while the second normal stress difference (N2) is dominated by the microstructure of particles at high volume fractions. It is proposed that the stress tensor in a polymeric suspension is the sum of a particle-contributed stress arising from the microstructure interaction and a polymeric matrix contributed stress with a relaxation spectrum altered by the particle microstructure. A simple model is proposed to describe how the relaxation behavior of a polymer matrix can be changed by the confinements of particles in suspension. © 2014 The Society of Rheology.
Wed, 01 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/854882014-01-01T00:00:00Z
- Microchannel flow of a macromolecular suspensionhttps://scholarbank.nus.edu.sg/handle/10635/60737Title: Microchannel flow of a macromolecular suspension
Authors: Fan, X.; Phan-Thien, N. Nhan; Yong, N.T.; Wu, X.; Xu, D.
Abstract: In the delivery of DNA molecules by microfluidic devices, the channel width is very often in the same order as the size of the DNA molecules and the applicability of continuum mechanics at this level may be questioned. In this paper we use finitely extendable nonlinear elastic (FENE) chains to model the DNA molecules and employ the dissipative particle dynamics (DPD) method to simulate their behavior in the flow. Simple DPD fluids are found to behave just like a Newtonian fluid in Poiseuille flow. However, the velocity profiles of FENE chain suspensions can be fitted with power-law curves, especially for dilute suspensions. Some results on the conformation and migration of FENE chains are also reported. © 2003 American Institute of Physics.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/607372003-01-01T00:00:00Z
- Distributed Lagrange multiplier/fictitious domain method in the framework of lattice Boltzmann method for fluid-structure interactionshttps://scholarbank.nus.edu.sg/handle/10635/59975Title: Distributed Lagrange multiplier/fictitious domain method in the framework of lattice Boltzmann method for fluid-structure interactions
Authors: Shi, X.; Phan-Thien, N.
Abstract: A new implementation of the lattice Boltzmann method (LBM) for fluid-structure interactions is presented. The idea of the distributed-Lagrange-multiplier/fictitious-domain method (DLM/FD) is introduced in the framework of the lattice Boltzmann algorithm. This implementation employs a fixed mesh for the solution of the fluid problem and a Lagrangian formulation for the solid problem. The main advantage of the method is that the re-meshing procedure normally required in the ALE method is circumvented. Numerical examples are provided to verify the algorithm and illustrate the capacity of the method to deal with the fluid/elastic-solid interactions. © 2005 Elsevier Inc. All rights reserved.
Fri, 10 Jun 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/599752005-06-10T00:00:00Z
- Exponential-time differencing schemes for low-mass DPD systemshttps://scholarbank.nus.edu.sg/handle/10635/60265Title: Exponential-time differencing schemes for low-mass DPD systems
Authors: Phan-Thien, N.; Mai-Duy, N.; Pan, D.; Khoo, B.C.
Abstract: Several exponential-time differencing (ETD) schemes are introduced into the method of dissipative particle dynamics (DPD) to solve the resulting stiff stochastic differential equations in the limit of small mass, where emphasis is placed on the handling of the fluctuating terms (i.e., those involving random forces). Their performances are investigated numerically in some test viscometric flows. Results obtained show that the present schemes outperform the velocity-Verlet algorithm regarding both the satisfaction of equipartition and the maximum allowable time step. © 2013 Elsevier B.V. All rights reserved.
Wed, 01 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/602652014-01-01T00:00:00Z
- Negative wake in the uniform flow past a cylinderhttps://scholarbank.nus.edu.sg/handle/10635/60876Title: Negative wake in the uniform flow past a cylinder
Authors: Dou, H.-S.; Phan-Thien, N.
Abstract: The upstream/downstream streamline shift and the associated negative wake generation (streamwise velocity overshoot in the wake) in a viscoelastic flow past a cylinder are studied in this paper, for the Oldroyd-B, UCM, PTT, and FENECR fluids, using the Discrete Elastic Viscous Split Stress Vorticity (DEVSS-ω) scheme (Dou HS, Phan-Thien N (1999). The flow of an Oldroyd-B fluid past a cylinder in a channel: adaptive viscosity vorticity (DAVSS-ω) formulation. J Non-Newtonian Fluid Mech 87:47-73). The numerical algorithm is a parallelized unstructured Finite Volume Method (FVM), running under a distributed computing environment through the Parallel Virtual Machine (PVM) library. It is demonstrated that both the normal stress and its gradient are responsible for the negative wake generation and streamline shifting. Fluid extensional rheology plays an important role in the generation of the negative wake. The negative wake can occur in flows where the fluid extensional viscosity does not increase rapidly with strain rate. The formation of the negative wake does not depend on whether the streamlines undergo an upstream or a downstream shift. Shear-thinning viscosity weakens the velocity overshoot and while shear-thinning first normal stress coefficient enhances the velocity overshoot. Wall proximity is not necessary for the velocity overshoot; however, it enhances the strength of the negative wake. For the Oldroyd-B fluid, the ratio of the solvent viscosity to the zero-shear viscosity plays an important role in the streamline shift. In addition, mesh dependent behaviour of normal stresses along the centreline at high De in most cylinder/sphere simulations is due to the convection of normal stress from the cylinder to the wake, which results in the maximum of the normal stress being located off the centreline by a short distance at high De.
Mon, 01 Sep 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/608762003-09-01T00:00:00Z
- Oscillatory torsional flow of a viscoelastic solidhttps://scholarbank.nus.edu.sg/handle/10635/61039Title: Oscillatory torsional flow of a viscoelastic solid
Authors: Phan-Thien, N.
Abstract: We show that a voscoelastic solid, modelled by a three-dimensional analogue of the Kelvin-Meyer-Voigt equation (the neo-Hookean rubber-like body and the Oldroyd-B element in parallel), has an exact similarity solution in the torsional flow geometry, including inertia. Numerical results obtained by a finite difference method for the oscillatory torsional flow indicate that the flow loses its stability at high amplitudes of oscillations. This is partly explained by a linear stability analysis of a simpler flow generated by a constant angular displacement.
Thu, 01 Aug 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/610392002-08-01T00:00:00Z
- Simulation of front evolving liquid film flowing down an inclined plate using level set methodhttps://scholarbank.nus.edu.sg/handle/10635/61300Title: Simulation of front evolving liquid film flowing down an inclined plate using level set method
Authors: Dou, H.-S.; Phan-Thien, N.; Khoo, B.C.; Yeo, K.S.; Zheng, R.
Abstract: In this paper, the projection/level set method is used to simulate the liquid film flow down on an inclined plate for both Newtonian and non-Newtonian fluids. Special numerical treatments are designed to cope with the viscous terms. The simulation is carried out for different inclined plate angles ranging from 20° to 90°. The effects of the inclined angle, surface tension, and shear thinning on the flow are discussed. It is found that surface tension reduces the contact angle, increases the film thickness, and promotes a fuller flow front. The effect of shear thinning is to increase the velocity near the wall, make the contact angle larger, and reduce the film thickness. It is also found that the variation of rheological properties and surface tension may produce a complex flow behaviour due to the swollen flow front. © Springer-Verlag 2004.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/613002004-01-01T00:00:00Z
- Molecular dynamics simulation of a liquid in a complex nano channel flowhttps://scholarbank.nus.edu.sg/handle/10635/60824Title: Molecular dynamics simulation of a liquid in a complex nano channel flow
Authors: Fan, X.-J.; Phan-Thien, N.; Yong, N.T.; Diao, X.
Abstract: We report some molecular dynamics simulation results for a complex nano channel flow. In certain flow geometry, some of the flow features cannot be predicted by the Navier-Stokes equations with no-slip boundary conditions. The results show a loss of dynamic similarity for flows with similar geometry and global dimensionless flow parameters. Nano-sized vortex flow can be developed at low Reynolds numbers due to near-wall molecules having large enough momenta, resulting in qualitatively different flow field from that predicted by the Navier-Stokes equations. © 2002 American Institute of Physics.
Fri, 01 Mar 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/608242002-03-01T00:00:00Z
- Viscoelastic mobility problem using a boundary element methodhttps://scholarbank.nus.edu.sg/handle/10635/61679Title: Viscoelastic mobility problem using a boundary element method
Authors: Phan-Thien, N.; Fan, X.-J.
Abstract: In this paper, the complete double layer boundary integral equation formulation for Stokes flows is extended to viscoelastic fluids to solve the mobility problem for a particle in an unbounded body of fluid, where the non-linearity is handled by using particular solutions of the Stokes inhomogeneous equation. Some meshless techniques are employed and a point-wise solver is used to solve the viscoelastic constitutive equation, avoiding volume meshing. The method is tested against the numerical solution for a sphere settling in an unbounded Oldroyd-B fluid. Some results on a prolate motion in shear flow for the Oldroyd-B fluid are reported and compared qualitatively with some theoretical and experimental results. © 2002 Elsevier Science B.V. All rights reserved.
Thu, 15 Aug 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/616792002-08-15T00:00:00Z
- Numerical investigations on the compressibility of a DPD fluidhttps://scholarbank.nus.edu.sg/handle/10635/60932Title: Numerical investigations on the compressibility of a DPD fluid
Authors: Pan, D.; Phan-Thien, N.; Mai-Duy, N.; Khoo, B.C.
Abstract: The compressibility of a dissipative particle dynamics (DPD) fluid is studied numerically through several newly developed test models, where both the density and the divergence of the velocity field are considered. In the case of zero conservative force, the DPD fluid turns out to be compressible. Effects of the compressibility are observed to be reduced as the particle mass is chosen to be smaller and the system temperature to be higher. In the case of non-zero conservative force, the condition of constant density and divergence-free of velocity can be approximately achieved at large values of the repulsion parameter (i.e., weakly compressible flow). Furthermore, the speed of sound and local Mach number are computed and found to be in good agreement with the theoretical estimation. © 2013 Elsevier Inc.
Sat, 01 Jun 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/609322013-06-01T00:00:00Z
- Short-term and long-term irreversibility in particle suspensions undergoing small and large amplitude oscillatory stresshttps://scholarbank.nus.edu.sg/handle/10635/61284Title: Short-term and long-term irreversibility in particle suspensions undergoing small and large amplitude oscillatory stress
Authors: Lin, Y.; Phan-Thien, N.; Khoo, B.C.
Abstract: The short-term and long-term irreversible behaviors of suspensions of rigid particles in oscillatory shear flow are studied by measuring the evolution of complex viscosity in time and applying of nonlinear analysis of the responded strain signal under the controlled-stress mode, and complemented by optical measurements on the particle motion. The short-term transition time for the system to reach a quasisteady state is an approximately bell-shaped function of the amplitude of the strain response, thus showing a critical strain amplitude accounting for the peak transition time. The short-term behavior is caused by the particle self-organization due to collisions between particles. At longer time scales, the complex viscosity of the suspension increases when probed by forces that elicit small strain amplitudes and decreases when stresses that result in large strain amplitudes are applied. It is proposed that the long-term behavior for stresses eliciting small strain amplitude is induced by the shear-induced diffusion of particles which self-organize into a crystal-like microstructure that can be easily annulled in oscillatory flow with large strain amplitude, while for stresses causing large strain amplitude the dominant microstructure is formed immediately via the oscillation. © 2013 The Society of Rheology.
Sun, 01 Sep 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612842013-09-01T00:00:00Z
- Using oscillatory squeezing flow to measure the viscoelastic properties of dental composite resin cements during curinghttps://scholarbank.nus.edu.sg/handle/10635/85824Title: Using oscillatory squeezing flow to measure the viscoelastic properties of dental composite resin cements during curing
Authors: Jiang, P.; See, H.; Swain, M.V.; Phan-Thien, N.
Abstract: We have investigated the rheological changes in two particulate-filled dental composite resin cements during the curing process using a Micro-Fourier Rheometer (MFR). In the MFR, the sample was sandwiched between two parallel plates, and pseudorandom small amplitude squeezing was applied by oscillating the upper plate over a range of frequencies. Fourier transforms of the displacement signal and the resulting time dependent force signal enabled the rapid determination of the dynamic properties G′ and G″ over the frequency range 2π-200π rad/s. This technique permitted us to follow changes in the rheological properties of the resin cements through the setting period. A typical result was that G′ increased from 2×103 Pa to 2×105 Pa after about 120 s, and that G″ changed from 4×103 Pa to 4×104 Pa over the same period at frequency 40π rad/s. We also found that the dental composite resin cements show linear viscoelastic behaviour over a range of strain amplitudes before curing, but the response becomes distinctly non-linear at the later stages of curing for strain amplitudes γ > 0.067%.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/858242003-01-01T00:00:00Z
- Concentration dependence of yield stress and dynamic moduli of kaolinite suspensionshttps://scholarbank.nus.edu.sg/handle/10635/123522Title: Concentration dependence of yield stress and dynamic moduli of kaolinite suspensions
Authors: Lin, Yuan; PHAN-THIEN,NHAN; Lee, Jason Boon Ping; Khoo, Boo Cheong
Thu, 01 Jan 2015 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1235222015-01-01T00:00:00Z
- Dissipative particle dynamics simulation of polymer drops in a periodic shear flowhttps://scholarbank.nus.edu.sg/handle/10635/85016Title: Dissipative particle dynamics simulation of polymer drops in a periodic shear flow
Authors: Chen, S.; Phan-Thien, N.; Fan, X.-J.; Khoo, B.C.
Abstract: The steady-state and transient shear flow dynamics of polymer drops in a microchannel are investigated using the dissipative particle dynamics (DPD) method. The polymer drop is made up of 10% DPD solvent particles and 90% finite extensible non-linear elastic (FENE) bead spring chains, with each chain consisting of 16 beads. The channel's upper and lower walls are made up of three layers of DPD particles, respectively, perpendicular to Z -axis, and moving in opposite directions to generate the shear flow field. Periodic boundary conditions are implemented in the X and Y directions. With FENE chains, shear thinning and normal stress difference effects are observed. The "colour" method is employed to model immiscible fluids according to Rothman-Keller method; the χ -parameters in Flory-Huggins-type models are also analysed accordingly. The interfacial tension is computed using the Irving-Kirkwood equation. For polymer drops in a steady-state shear field, the relationship between the deformation parameter ( Ddef) and the capillary number ( Ca ) can be delineated into a linear and nonlinear regime, in qualitative agreement with experimental results of Guido et al. [J. Rheol. 42 (2) (1998) 395]. In the present study, Ca < 0.22, in the linear regime. As the shear rate increases further, the drop elongates; a sufficiently deformed drop will break up; and a possible coalescence may occur for two neighbouring drops. Dynamical equilibrium between break-up and coalescence results in a steady-state average droplet-size distribution. In a shear reversal flow, an elongated and oriented polymer drop retracts towards a roughly spherical shape, with a decrease in the first normal stress difference. The polymer drop is found to undergo a tumbling mode at high Schmidt numbers. A stress analysis shows that the stress response is different from that of a suspension of solid spheres. An overshoot in the strain is observed for the polymer drop under extension due to the memory of the FENE chains. © 2004 Elsevier B.V. All rights reserved.
Sat, 20 Mar 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/850162004-03-20T00:00:00Z
- Three-dimensional roll-up of a viscoelastic mixing layerhttps://scholarbank.nus.edu.sg/handle/10635/61583Title: Three-dimensional roll-up of a viscoelastic mixing layer
Authors: Yu, Z.; Phan-Thien, N.
Abstract: In this paper, the three-dimensional roll-up of a viscoelastic mixing layer is numerically simulated with the pseudospectral method using the FENE-P model. An artificial diffusion is self-adaptively introduced into the constitutive equation to stabilize the time integration. In the three-dimensional mixing layer and within the parameter ranges studied, the effect of the polymer additives on the formation of the coherent structures such as the ribs and the cups is found to be negligible. The polymer normal stresses develop wherever there exist extensional strains that are produced by the coherent structures and they then hinder the development of these structures. Stretching by the quadrupoles and the ribs together gives rise to an enormous enhancement of the polymer normal stress differences in the symmetrical plane between the quadrupole pair. These normal stress differences directly or indirectly weaken all large-scale structures occurring in the flow including the quadrupoles, the cups, the ribs, the spanwise vortices which rotate in the opposite direction to that of the cups, and the thin spanwise vortical sheets. Attenuation of these large-scale structures leads to a diminution of small-scale structures after their breakdown in the secondary roll-up process of the thin sheets. There is a tendency for the small-scale structures in the core region to merge into a large-scale one in the viscoelastic case. As a result, a flat and inclined vortex forms at the end of the simulation which resembles the type of structure observed in an experimental mixing layer with surfactants injected. In addition, the results show that the extensional viscosity is an important quantity to determine the extent to which the coherent structures in a mixing layer are modified by polymer additives. © 2004 Cambridge University Press.
Tue, 10 Feb 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/615832004-02-10T00:00:00Z
- Folgar-Tucker constant for a fibre suspension in a Newtonian fluidhttps://scholarbank.nus.edu.sg/handle/10635/60354Title: Folgar-Tucker constant for a fibre suspension in a Newtonian fluid
Authors: Phan-Thien, N.; Fan, X.-J.; Tanner, R.I.; Zheng, R.
Mon, 25 Mar 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/603542002-03-25T00:00:00Z
- Viscometric flow for a many-body dissipative particle dynamics (MDPD) fluid with Lees?Edwards boundary conditionhttps://scholarbank.nus.edu.sg/handle/10635/141540Title: Viscometric flow for a many-body dissipative particle dynamics (MDPD) fluid with Lees?Edwards boundary condition
Authors: Jiayi Zhao; Shuo Chen; PHAN-THIEN,NHAN
Thu, 03 Aug 2017 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1415402017-08-03T00:00:00Z
- Tangential flow and advective mixing of viscoplastic fluids between eccentric cylindershttps://scholarbank.nus.edu.sg/handle/10635/61460Title: Tangential flow and advective mixing of viscoplastic fluids between eccentric cylinders
Authors: Fan, Y.; Phan-Thien, N.; Tanner, R.I.
Abstract: This is a study on the tangential flow and advective mixing of viscoplastic fluids (Bingham plastics) between two eccentric, alternately rotating cylinders. Two geometrical configurations and various rotation modes are considered for a relatively large range of the yield stress. The hp-type finite element method with the mixed formulation is used to solve for the steady velocity and pressure fields. The bi-viscosity and the Papanastasiou models agree quantitatively with each other in predicting the velocity fields and the practically unyielded zones. However, the Papanastasiou model is more robust and economic than the bi-viscosity model in the computation using Newton iteration. In the steady flows, in addition to the motionless zones, we have discovered some plugs with rigid rotation, including rotating plugs stuck onto the outer cylinder and rotating, even counter-rotating, plugs disconnected from both cylinders. The unsteady, periodic flow is composed of a sequence of the steady flows, which is valid in the creeping flow regime. The characteristics of advective mixing in these flows have been studied by analysing the asymptotic coverages of a passive tracer, the distributions of the lineal stretching in the flow and the variations of the mean stretching of the flow with time. The tracer coverage is intuitive but qualitative and, occasionally, it depends on the initial location of the tracer. On the other hand, the distribution of stretching is quantitative and more reliable in reflecting the mixing characteristics. Interestingly, the zones of the lowest stretching in the distribution graphs are remarkably well matched with the regular zones in the tracer-coverage graphs. Furthermore, the mixing efficiency proposed by Ottino (1989) is used to characterize the advective mixing in the two geometrical configurations with various rotation modes. It is important to realize that, for plastic fluids, a major barrier to effective mixing is the unyielded fluid plugs which are controlled by the yield stress and geometrical configurations. Therefore, when designing an eccentric helical annular mixer it is important to pay attention first to the geometric issues then to the operating issues.
Sun, 25 Mar 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/614602001-03-25T00:00:00Z
- Stretching and relaxation of malaria-infected red blood cellshttps://scholarbank.nus.edu.sg/handle/10635/61385Title: Stretching and relaxation of malaria-infected red blood cells
Authors: Ye, T.; Phan-Thien, N.; Khoo, B.C.; Lim, C.T.
Abstract: The invasion of red blood cells (RBCs) by malaria parasites is a complex dynamic process, in which the infected RBCs gradually lose their deformability and their ability to recover their original shape is greatly reduced with the maturation of the parasites. In this work, we developed two types of cell model, one with an included parasite, and the other without an included parasite. The former is a representation of real malaria-infected RBCs, in which the parasite is treated as a rigid body. In the latter, where the parasite is absent, the membrane modulus and viscosity are elevated so as to produce the same features present in the parasite model. In both cases, the cell membrane is modeled as a viscoelastic triangular network connected by wormlike chains. We studied the transient behaviors of stretching deformation and shape relaxation of malaria-infected RBCs based on these two models and found that both models can generate results in agreement with those of previously published studies. With the parasite maturation, the shape deformation becomes smaller and smaller due to increasing cell rigidity, whereas the shape relaxation time becomes longer and longer due to the cell's reduced ability to recover its original shape. © 2013 Biophysical Society.
Tue, 03 Sep 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/613852013-09-03T00:00:00Z
- Dissipative particle dynamics modeling of low Reynolds number incompressible flowshttps://scholarbank.nus.edu.sg/handle/10635/85014Title: Dissipative particle dynamics modeling of low Reynolds number incompressible flows
Authors: Mai-Duy, N.; Pan, D.; Phan-Thien, N.; Khoo, B.C.
Abstract: This paper is concerned with the numerical modeling of a slow (creeping) flow using a particle-based simulation technique, known as dissipative particle dynamics (DPD), in which the particles' mass is allowed to approach zero to simultaneously achieve a high sonic speed, a low Reynolds number, and a high Schmidt number. This leads to a system of stiff stochastic differential equations, which are solved efficiently by an exponential time differencing (ETD) scheme. The ETD-DPD method is first tested in viscometric flows, where the particle mass is reduced down to 0.001. The method is then applied for the modeling of rigid spheres in a Newtonian fluid by means of two species of DPD particles, one representing the solvent particles and the other, the suspended particle. Calculations are carried out at particle mass of 0.01, with corresponding Mach number of 0.08, Reynolds number of 0.05, and Schmidt number of 6.0 × 103. Stokes results are used to determine the DPD parameters for the solvent-sphere interaction forces. The method obeys equipartition and yields smooth flows around the sphere with quite uniform far-field velocities. © 2013 The Society of Rheology.
Fri, 01 Mar 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/850142013-03-01T00:00:00Z
- Completed double layer boundary element method for periodic fibre suspension in viscoelastic fluidhttps://scholarbank.nus.edu.sg/handle/10635/59746Title: Completed double layer boundary element method for periodic fibre suspension in viscoelastic fluid
Authors: Nguyen-Hoang, H.; Phan-Thien, N.; Khoo, B.C.; Fan, X.-J.; Dou, H.-S.
Abstract: A numerical method to simulate the periodic fibre suspension in viscoelastic fluid is developed with the completed double layer boundary element method (CDLBEM). The periodic summations that arise in the formulation were well handled by Ewald summation technique to speed up the convergence rate in the computation. The formulation for velocity field in periodic fibre suspension in viscoelastic fluid is derived and is used to simulate multiple fibres suspended in a viscoelastic shear flow. Simulations are carried out for various fibre aspect ratios and volume fractions ranging from dilute to concentrated regimes. Numerical results of macroscopic rheological properties of the system are compared to available experiments on viscoelastic fibre suspensions, and are found to agree reasonably well with the experimental data. © 2008 Elsevier Ltd. All rights reserved.
Fri, 01 Aug 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/597462008-08-01T00:00:00Z
- Flow around spheres by dissipative particle dynamicshttps://scholarbank.nus.edu.sg/handle/10635/60341Title: Flow around spheres by dissipative particle dynamics
Authors: Chen, S.; Phan-Thien, N.; Khoo, B.C.; Fan, X.J.
Abstract: The dissipative particle dynamics (DPD) method is used to study the flow behavior past a sphere. The sphere is represented by frozen DPD particles while the surrounding fluids are modeled by simple DPD particles (representing a Newtonian fluid). For the surface of the sphere, the conventional model without special treatment and the model with specular reflection boundary condition proposed by Revenga [Comput. Phys. Commun. 121-122, 309 (1999)] are compared. Various computational domains, in which the sphere is held stationary at the center, are investigated to gage the effects of periodic conditions and walls for Reynolds number (Re)=0.5 and 50. Two types of flow conditions, uniform flow and shear flow are considered, respectively, to study the drag force and torque acting on the stationary sphere. It is found that the calculated drag force imposed on the sphere based on the model with specular reflection is slightly lower than the conventional model without special treatment. With the conventional model the drag force acting on the sphere is in better agreement with experimental correlation obtained by Brown and Lawler [J. Environ. Eng. 129, 222 (2003)] for the case of larger radius up to Re of about 5. The computed torque also approaches the analytical Stokes value when Re<1. For a force-free and torque-free sphere, its motion in the flow is captured by solving the translational and rotational equations of motion. The effects of different DPD parameters (a, γ, and σ) on the drag force and torque are studied. It shows that the dissipative coefficient (γ) mainly affects the drag force and torque, while random and conservative coefficient have little influence on them. Furthermore the settling of a single sphere in square tube is investigated, in which the wall effect is considered. Good agreement is found with the experiments of Miyamura [Int. J. Multiphase Flow 7, 31 (1981)] and lattice-Boltzmann simulation results of Aidun [J. Fluid Mech. 373, 287 (1998)]. © 2006 American Institute of Physics.
Sun, 01 Oct 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/603412006-10-01T00:00:00Z
- An implementation of no-slip boundary conditions in DPDhttps://scholarbank.nus.edu.sg/handle/10635/59467Title: An implementation of no-slip boundary conditions in DPD
Authors: Duong-Hong, D.; Phan-Thien, N.; Fan, X.
Abstract: We report an implementation of the no-slip boundary condition in the modeling of solid boundaries by dissipative particle dynamics (DPD) method. Stimulated by a model for several types of solid boundaries, we develop an implementation that satisfies no-slip boundary condition with practically no-density distortion near the boundaries. The model is implemented to simulate the planar Poiseuille and Couette flows, as well as the flow through a contraction and diffusion channel. Results compare excellently with the previous methods.
Wed, 01 Dec 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/594672004-12-01T00:00:00Z
- A numerical study of strongly overdamped Dissipative Particle Dynamics (DPD) systemshttps://scholarbank.nus.edu.sg/handle/10635/54676Title: A numerical study of strongly overdamped Dissipative Particle Dynamics (DPD) systems
Authors: Mai-Duy, N.; Phan-Thien, N.; Khoo, B.C.
Abstract: In this paper, we investigate the behaviour of a Dissipative Particle Dynamics (DPD) system in the overdamped limit where the particles approach zero mass limit. In this limit, the DPD system becomes singular. We propose two numerical schemes to deal with this system - one results in a fully-populated but well-conditioned matrix system, while the other employs a deflation technique to handle the system in an iterative manner, where the eigenvalue of -1 corresponding to a rigid-body motion is mapped to zero. The latter iterative scheme is to be preferred, with the possibility of parallel implementations. Some numerical results are presented to verify the two proposed schemes. © 2013 Elsevier Inc..
Mon, 15 Jul 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/546762013-07-15T00:00:00Z
- Non-linear oscillatory flow of a soft solid-like viscoelastic materialhttps://scholarbank.nus.edu.sg/handle/10635/58532Title: Non-linear oscillatory flow of a soft solid-like viscoelastic material
Authors: Phan-Thien, N.; Newberry, M.; Tanner, R.I.
Abstract: Large-amplitude oscillatory shearing flow data are reported for a wheat-flour dough, which is highly shear-thinning in its dynamic data. The large-amplitude data are analysed using a recently proposed constitutive equation for bread dough, in which dough is considered as a solid-like material. It is found that the model predictions agree well with experimental data, giving a certain degree of confidence in the constitutive equation. Furthermore, the markedly non-linear response of the material, even at as low an amplitude as 0.05, is mainly due to the strain softening behaviour of the material; and this non-linearity cannot be predicted by a model with shear rate dependent parameters alone. (C) 2000 Elsevier Science B.V. All rights reserved.
Tue, 01 Aug 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/585322000-08-01T00:00:00Z
- Rheology of bubble suspensions using dissipative particle dynamics. Part I: A hard-core DPD particle model for gas bubbleshttps://scholarbank.nus.edu.sg/handle/10635/61238Title: Rheology of bubble suspensions using dissipative particle dynamics. Part I: A hard-core DPD particle model for gas bubbles
Authors: Tran-Duc, T.; Phan-Thien, N.; Cheong Khoo, B.
Abstract: In this paper, the rheology of dilute bubble suspensions is studied using dissipative particle dynamics (DPD). Each gas bubble is modeled by a hard-core DPD particle. The approach addresses the issue of zero-viscosity arising from modeling of a gas bubble by a set of DPD particles. Moreover, it helps to reduce significantly the computational demand due to a much less number of DPD particles required in the simulation. A dissipative layer is created outside the effective region of the hard-core DPD particle to manage the hydrodynamic force acting on it, and different phases can be defined accordingly. The model is further examined in the simulation of dilute bubble suspensions, and measurements on the Newtonian viscosity for the volume fraction less than 15% are consistent with experimental results and results from theoretical models in continuum mechanics at low Ca limit. © 2013 The Society of Rheology.
Fri, 01 Nov 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612382013-11-01T00:00:00Z
- Fully developed viscous and viscoelastic flows in curved pipeshttps://scholarbank.nus.edu.sg/handle/10635/60383Title: Fully developed viscous and viscoelastic flows in curved pipes
Authors: FAN, Y.; Tanner, R.I.; Phan-Thien, N.
Abstract: Some h-p finite element computations have been carried out to obtain solutions for fully developed laminar flows in curved pipes with curvature ratios from 0.001 to 0.5. An Oldroyd-3-constant model is used to represent the viscoelastic fluid, which includes the upper-convected Maxwell (UCM) model and the Oldroyd-B model as special cases. With this model we can examine separately the effects of the fluid inertia, and the first and second normal-stress differences. From analysis of the global torque and force balances, three criteria are proposed for this problem to estimate the errors in the computations. Moreover, the finite element solutions are accurately confirmed by the perturbation solutions of Robertson & Muller (1996) in the cases of small Reynolds/Deborah numbers. Our numerical solutions and an order-of-magnitude analysis of the governing equations elucidate the mechanism of the secondary flow in the absence of second normal-stress difference. For Newtonian flow, the pressure gradient near the wall region is the driving force for the secondary flow; for creeping viscoelastic flow, the combination of large axial normal stress with streamline curvature, the so-called hoop stress near the wall, promotes a secondary flow in the same direction as the inertial secondary flow, despite the adverse pressure gradient there; in the case of inertial viscoelastic flow, both the larger axial normal stress and the smaller inertia near the wall promote the secondary flow. For both Newtonian and viscoelastic fluids the secondary volumetric fluxes per unit of work consumption and per unit of axial volumetric flux first increase then decrease as the Reynolds/Deborah number increases; this behaviour should be of interest in engineering applications. Typical negative values of second normal-stress difference can drastically suppress the secondary flow and in the case of small curvature ratios, make the flow approximate the corresponding Poiseuille flow in a straight pipe. The viscoelasticity of Oldroyd-B fluid causes drag enhancement compared to Newtonian flow. Adding a typical negative second normal-stress difference produces large drag reductions for a small curvature ratio δ = 0.01; however, for a large curvature ratio δ = 0.2, although the secondary flows are also drastically attenuated by the second normal-stress difference, the flow resistance remains considerably higher than in Newtonian flow. It was observed that for the UCM and Oldroyd-B models, the limiting Deborah numbers met in our steady solution calculations obey the same scaling criterion as proposed by McKinley et al. (1996) for elastic instabilities; we present an intriguing problem on the relation between the Newton iteration for steady solutions and the linear stability analyses.
Fri, 10 Aug 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/603832001-08-10T00:00:00Z
- Viscoelastic flow past a confined cylinder: Instability and velocity inflectionhttps://scholarbank.nus.edu.sg/handle/10635/61677Title: Viscoelastic flow past a confined cylinder: Instability and velocity inflection
Authors: Dou, H.-S.; Phan-Thien, N.
Abstract: The flow of the Oldroyd-B fluid past a circular cylinder in a channel is simulated by a parallelized finite volume method (FVM) using fine unstructured meshes. Convergent solutions are obtained up to a high Deborah number of De = 1.6 with a high aspect ratio mesh, while it can only be obtained for De = 0.6 with a more or less equi-lateral mesh. The governing equations are re-written using the discrete elastic viscous split stress formulation together with an independent interpolation of the vorticity (DEVSS-ω). The numerical procedure is based on the semi-implicit method for pressure-linked equations revised (SIMPLER) algorithm, using a collocated FVM discretization. To enhance the numerical accuracy, mixed meshes (structured and unstructured) are used and adapted to the geometry and the flow physics. The results show that an instability featuring stress oscillations on the top of the cylinder is produced at a critical De number (De ≈ 0.6). This critical value is in agreement with the experiments in the literature. The oscillation generated in the shear layer on the cylinder surface at high De number is convected downstream into the wake behind the cylinder. This feature of flow oscillation agrees well with experiments reported in the literature. The mechanism of these phenomena can be explained reasonably by a boundary layer analysis. The key finding is that the instability is due to an inflectional velocity profile, near the cylinder, generated by normal stress on the cylinder surface at high De number, which can only be captured with fine meshes. According to a newly proposed theory (so called energy gradient theory), inflectional velocity profile leads to flow instability and consequently allows the convection of the oscillatory flow within the shear layer downstream of the wake and results in the flow pulsation in the spanwise direction. Therefore, the origin of the instability for the flow around a cylinder is in the shear layer on the cylinder and not in the wake itself. In addition, it seems that the increasing rate of numerical perturbation is related to the mesh aspect ratio. Computing using thin-long meshes could get convergence even if the flow is oscillatory, while the computing is prohibited by the inflectional instability when equi-lateral triangular meshes are used. © 2007 Elsevier Ltd. All rights reserved.
Wed, 01 Aug 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/616772007-08-01T00:00:00Z
- A numerical study of viscoelastic effects in chaotic mixing between eccentric cylindershttps://scholarbank.nus.edu.sg/handle/10635/54681Title: A numerical study of viscoelastic effects in chaotic mixing between eccentric cylinders
Authors: Fan, Y.; Tanner, R.I.; Phan-Thien, N.
Abstract: In this paper, we are concerned with the effect of fluid elasticity and shear-thinning viscosity on the chaotic mixing of the flow between two eccentric, alternately rotating cylinders. We employ the well-developed h-p finite element method to achieve a high accuracy and efficiency in calculating steady solutions, and a full unsteady algorithm for creeping viscoelastic flows to study the transient process in this periodic viscoelastic flow. Since the distribution of periodic points of the viscoelastic flow is not symmetric, we have developed a domain-search algorithm based on Newton iteration for locating the periodic points. With the piecewise-steady approximation, our computation for the upper-convected Maxwell fluid predicts no noticeable changes of the advected coverage of a passive tracer from Newtonian flow, with elasticity levels up to a Deborah number of 1.0. The stretching of the fluid elements, quantified by the geometrical mean of the spatial distribution, remains exponential up to a Deborah number of 6.0, with only slight changes from Newtonian flow. On the other hand, the shear-thinning viscosity, modelled by the Carreau equation, has a large impact on both the advection of a passive tracer and the mean stretching of the fluid elements. The creeping, unsteady computations show that the transient period of the velocity is much shorter than the transient period of the stress, and from a pragmatic point of view, this transient process caused by stress relaxation due to sudden switches of the cylinder rotation can be neglected for predicting the advective mixing in this time-periodic flow. The periodic points found up to second order and their eigenvalues are indeed very informative in understanding the chaotic mixing patterns and the qualitative changes of the mean stretching of the fluid elements. The comparison between our computations and those of Niederkorn and Ottino (1993) reveals the importance of reducing the discretization error in the computation of chaotic mixing. The causes of the discrepancy between our prediction of the tracer advection and Niederkorn and Ottino's (1993) experiment are discussed, in which the influence of the shear-thinning first normal stress difference is carefully examined. The discussion leads to questions on whether small elasticity of the fluid has a large effect on the chaotic mixing in this periodic flow.
Sat, 10 Jun 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/546812000-06-10T00:00:00Z
- A numerical simulation of suspension flow using a constitutive model based on anisotropic interparticle interactionshttps://scholarbank.nus.edu.sg/handle/10635/54668Title: A numerical simulation of suspension flow using a constitutive model based on anisotropic interparticle interactions
Authors: Phan-Thien, N.; Fan, X.-J.; Zheng, R.
Abstract: We report a Brownian configuration field implementation of a recent constitutive equation for suspensions, reported by Phan-Thien et al. 1999. The numerical method is a hybrid technique, which combines a modification of the Brownian configuration field method described by Hulsen et al. 1997 and the adaptive viscosity split stress formulation proposed by Sun et al. 1996. The implementation is used to examine the flow past a sphere in a tube. The relative viscosity derived from the drag force/sedimentation velocity agrees well with a well-known empiricism. In addition, the ratio of the pressure force to the drag on the sphere seems to be weakly dependent on the volume fraction, and is somewhat higher than Brenner's results of 1962, which were derived for Newtonian fluids.
Sat, 01 Jan 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/546682000-01-01T00:00:00Z
- A Numerical technique based on integrated RBFs for the system evolution in molecular dynamicshttps://scholarbank.nus.edu.sg/handle/10635/54684Title: A Numerical technique based on integrated RBFs for the system evolution in molecular dynamics
Authors: Mai-Duy, N.; Tran-Cong, T.; Phan-Thien, N.
Abstract: This paper presents a new numerical technique for solving the evolution equations in molecular dynamics (MD). The variation of the MD system is represented by radial-basis-function (RBF) equations which are constructed using integrated multiquadric basis functions and point collocation. The proposed technique requires the evaluation of forces once per time step. Several examples are given to demonstrate the attractiveness of the present implementation. Copyright © 2011 Tech Science Press.
Sat, 01 Jan 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/546842011-01-01T00:00:00Z
- Simulation of a polymer drop in a shear field with dissipative particle dynamics methodhttps://scholarbank.nus.edu.sg/handle/10635/73849Title: Simulation of a polymer drop in a shear field with dissipative particle dynamics method
Authors: Chen, S.; Phan-Thien, N.; Fan, X.J.; Khoo, B.C.
Abstract: The dynamics of polymer drops in micro channel in steady-state and transient shear flow conditions are investigated using the Dissipative Particle Dynamics (DPD) method. The polymer drop is made up of 10% DPD solvent particles and 90% finite extensible non-linear elastic (FENE) bead spring chains. With FENE chains, shear thinning and normal stress difference effects are observed. For a polymer drop in steady-state shear field, the relationship between the deformation rate (D) and the Capillary number ( Ca ) could be divided into linear and nonlinear regimes. As the shear rate increases further, the drop begins to elongate; a sufficiently deformed drop will break up, and in the mean time, possible coalescence may occur for two neighboring drops. In a shear reversal, an elongated and oriented polymer drop retracts towards a roughly spherical shape. Due to the soft interaction of DPD particles, the tumbling phenomena of the polymer drop is not apparent even for large Ca.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/738492003-01-01T00:00:00Z
- Oscillatory squeezing flow of a biological materialhttps://scholarbank.nus.edu.sg/handle/10635/133193Title: Oscillatory squeezing flow of a biological material
Authors: Phan-Thien; Nasseri, S.; Bilston, L.E.
Abstract: Large-amplitude oscillatory squeezing flow data are reported for a complex biological material, which is highly shear-thinning in oscillatory shear flow. This soft tissue has a linear viscoelastic limit at a strain of approximately 0.2%. The oscillatory squeezing flow data at large strain are analyzed using two constitutive models: a bi-viscosity Newtonian model, and a non-linear Maxwell model. It is found that although both models may have the same response in shape, the later matches with our non-linear experimental data better. It is also concluded that the non-linear response of the material in large amplitude oscillatory flow is mainly due to the shear thinning of the material.
Sat, 01 Jan 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1331932000-01-01T00:00:00Z
- Viscoelastic mobility problem of a system of particleshttps://scholarbank.nus.edu.sg/handle/10635/61678Title: Viscoelastic mobility problem of a system of particles
Authors: Yu, Z.; Phan-Thien, N.; Fan, Y.; Tanner, R.I.
Abstract: In this paper we present a new implementation of the distributed Lagrange multiplier/fictitious domain (DLM) method by making some modifications over the original algorithm for the Newtonian case developed by Glowinski et al. [Int. J. Multiphase Flow 25 (1999) 755], and its extended version for the viscoelastic case by Singh et al. [J. Non-Newtonian Fluid Mech. 91 (2000) 165]. The key modification is to replace a finite-element triangulation for the velocity and a "staggered" (twice coarser) triangulation for the pressure with a rectangular discretization for the velocity and the pressure. The sedimentation of a single circular particle in a Newtonian fluid at different Reynolds numbers, sedimentation of particles in the Oldroyd-B fluid, and lateral migration of a single particle in a Poiseuille flow of a Newtonian fluid are numerically simulated with our code. The results show that the new implementation can give a more accurate prediction of the motion of particles compared to the previous DLM codes and even the boundary-fitted methods in some cases. The centering of a particle and the well-organized Karman vortex street are observed at high Reynolds numbers in our simulation of a particle sedimenting in a Newtonian fluid. Both results obtained using the DLM method and the spectral element method reveal that the direct contribution of the viscoelastic normal stress to the force on a particle in the Oldroyd-B fluid is very important. © 2002 Elsevier Science B.V. All rights reserved.
Sat, 01 Jun 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/616782002-06-01T00:00:00Z
- Modelling of solid bodies in dissipative particle dynamicshttps://scholarbank.nus.edu.sg/handle/10635/73636Title: Modelling of solid bodies in dissipative particle dynamics
Authors: Mai-Duy, N.; Phan-Thien, N.; Khoo, B.C.; Tran-Cong, T.
Abstract: This paper is concerned with the use of oscillating particles instead of the usual frozen particles to model a suspended particle (solid body) in a Dissipative Particle Dynamics (DPD) particle-based simulation method. A suspended particle is represented by a set of basic DPD particles connected to reference sites by linear springs. The reference sites are moved as a whole with the imposed displacement that is calculated using data from the previous time step, while the velocities of their associated DPD particles are found by solving the DPD equations at the current time step. In this way, a specified Boltzmann temperature can also be maintained in the region occupied by the suspended particles and this parameter can be utilised to control the size of suspended particles. Several numerical results in two dimensions are presented to demonstrate attractiveness of the proposed model.
Tue, 01 Jan 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/736362013-01-01T00:00:00Z
- Stagnation and rotating-disk flows over a compliant surfacehttps://scholarbank.nus.edu.sg/handle/10635/61364Title: Stagnation and rotating-disk flows over a compliant surface
Authors: Phan-Thien, N.; Yeo, K.S.
Abstract: In this paper, exact solutions to the stagnation flow over a compliant surface and the flow about a rotating disk coated by a compliant layer are discussed. The compliant layer is modelled by the Mooney rubberlike material. In the stagnation flow, the deformation of the rubber layer depends on two dimensionless groups: one is the ratio of pressure to the shear modulus of the material, and the other is the ratio of the boundary layer thickness to the thickness of the layer. It is shown that this deformation has a limit point at a critical value of the pressure force, beyond which the solution does not exist. In the rotating-disk flow, the deformation of the rubber layer depends on two dimensionless groups, one is a material parameter, and the other is a flow loading parameter, the ratio of the flow-induced viscous stress to the shear modulus multiplied by the ratio of the thickness of the layer to the boundary layer thickness. At large value of a material parameter, it is found that this deformation has a limit point at a critical value of the flow loading parameter.
Sat, 01 Sep 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/613642001-09-01T00:00:00Z
- Simulating the continuous flow separation element for dna extractionhttps://scholarbank.nus.edu.sg/handle/10635/73847Title: Simulating the continuous flow separation element for dna extraction
Authors: Chen, S.; Phan-Thien, N.; Fan, X.J.; Khoo, B.C.
Abstract: The particle suspension flow under external magnetic force in bifurcating micro-channel has been numerically studied using mesoscopic technique, dissipative particle dynamics. The bifurcating micro-channel simulates the continuous flow separation element (CFSE) which is used in the isolation of DNA from whole blood for Polymerase Chain Reaction (PCR). The rotational motion of the spheres is studied in the present study by using the Denavit-Hartenberg representation of linkages in robot arm kinematics. The surrounding fluids are simulated by simple DPD particles. The computational technique is tested by simulating the settling of one sphere in a channel and the rotation of one sphere in shear flow. Migration of the sphere subjecting to the magnetic force is investigated in detail. The conformation evolution of DNA molecule passing through the periodic contraction and diffusion slit is also investigated.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/738472003-01-01T00:00:00Z
- On the Rayleigh problem for a viscoelastic fluidhttps://scholarbank.nus.edu.sg/handle/10635/58571Title: On the Rayleigh problem for a viscoelastic fluid
Authors: Phan-Thien, N.; Chew, Y.T.
Fri, 01 Jan 1988 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/585711988-01-01T00:00:00Z
- A spring model for suspended particles in dissipative particle dynamicshttps://scholarbank.nus.edu.sg/handle/10635/84818Title: A spring model for suspended particles in dissipative particle dynamics
Authors: Phan-Thien, N.; Mai-Duy, N.; Khoo, B.C.
Abstract: This paper is concerned with the use of oscillating particles instead of the usual frozen particles to model a suspended particle in the dissipative particle dynamics (DPD) method. A suspended particle is represented by a set of basic DPD particles connected to reference sites by linear springs of very large stiffness. The reference sites, collectively modeling a rigid body, move as a rigid body motion calculated through their Newton-Euler equations, using data from the previous time step, while the velocities of their associated DPD particles are found by solving the DPD equations at the current time step. In this way, a specified Boltzmann temperature (specific kinetic energy of the particles) can be maintained throughout the computational domain, including the region occupied by the suspended particles. This parameter can also be used to adjust the size of the suspended and solvent particles, which in turn affect the strength of the shear-thinning behavior and the effective maximal packing fraction. Furthermore, the suspension, comprised of suspended particles in a set of solvent particles all interacting under a quadratic soft repulsive potential, can be simulated using a relatively large time step. Several numerical examples are presented to demonstrate attractiveness of the proposed model. © 2014 The Society of Rheology.
Wed, 01 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/848182014-01-01T00:00:00Z
- Advanced Recycled Polyethylene Terephthalate Aerogels from Plastic Waste for Acoustic and Thermal Insulation Applications.https://scholarbank.nus.edu.sg/handle/10635/155199Title: Advanced Recycled Polyethylene Terephthalate Aerogels from Plastic Waste for Acoustic and Thermal Insulation Applications.
Authors: Koh, Hong Wei; Le, Duyen K; Ng, Gek Nian; Zhang, Xiwen; Phan-Thien, Nhan; Kureemun, Umeyr; Duong, Hai M
Abstract: This work presents for the first time, a simple, practical and scalable approach to fabricating recycled polyethylene terephthalate (rPET) aerogels for thermal and acoustic insulation applications. The rPET aerogels were successfully developed from recycled PET fibers and polyvinyl alcohol (PVA) and glutaraldehyde (GA) cross-linkers using a freeze-drying process. The effects of various PET fiber concentrations (0.5, 1.0 and 2.0 by wt.%), fiber deniers (3D, 7D and 15D) and fiber lengths (32 mm and 64 mm) on the rPET aerogel structures and multi-properties were comprehensively investigated. The developed rPET aerogels showed a highly porous network structure (98.3⁻99.5%), ultra-low densities (0.007⁻0.026 g/cm³), hydrophobicity with water contact angles of 120.7⁻149.8°, and high elasticity with low compressive Young's modulus (1.16⁻2.87 kPa). They exhibited superior thermal insulation capability with low thermal conductivities of 0.035⁻0.038 W/m.K, which are highly competitive with recycled cellulose and silica-cellulose aerogels and better than mineral wool and polystyrene. The acoustic absorption results were also found to outperform a commercial acoustic foam absorber across a range of frequencies.
Thu, 17 May 2018 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1551992018-05-17T00:00:00Z