ORCID Profile
0000-0002-2481-0994
Current Organisation
National University of Singapore
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Publisher: AIP Publishing
Date: 05-2022
DOI: 10.1063/5.0090898
Abstract: An immersed boundary-lattice Boltzmann method is employed to simulate a squirmer (a classical self-propelled model) array swimming in a Newtonian fluid. The swimming Reynolds number Res is set in the range 0.05 ≤ Res ≤ 5 to study three typical arrays (i.e., the two-squirmer, triangular-squirmer, and quadrilateral-squirmer arrays) in their swimming speed, their power expenditure (P), and their hydrodynamic efficiency (η). Our results show that the two-pusher array with a smaller ds (the distance between the squirmers) yields a slower speed in contrast to the two-puller array, where a smaller ds yields a faster speed at Res ≥ 1 (“pusher” is propelled from the rear and “puller” from the front). The regular triangular-pusher (triangular-puller) array with θ = −60° (the included angle between the squirmers) swims faster (slower) than that with θ = 60° the quadrilateral-pusher (quadrilateral-puller) array with model 2 swims faster (slower) than model 1 (the models are to be defined later). It is also found that a two-puller array with a larger ds is more likely to become unstable than that with a smaller ds. The triangular-puller array with θ = 60° is more likely to become unstable than that with θ = 60° the quadrilateral-puller array with model 1 becomes unstable easier than that with model 2. In addition, a larger ds generally results in a less energy expenditure. A faster squirmer array yields a higher η, except for two extraordinarily puller arrays. A quantitative relation for η with ReU & 1 is obtained approximately, in that the increasing ratio of η is proportional to an exponent of the motion Reynolds number ReU.
Publisher: AIP Publishing
Date: 12-2019
DOI: 10.1063/1.5130711
Abstract: The mechanical and thermal behavior of nonisothermal fiber-filled composites in a three-dimensional printing process is studied numerically with a smoothed particle hydrodynamics method. A classical microstructure-based fiber suspension model with a temperature-dependent power-law viscosity model and a microstructure constitutive model is implemented to model a fiber-filled system. The fiber microstructure is described by a second-order tensor A2 which describes the spatially averaged orientation of the fibers. Two benchmark cases are presented to validate the reliability of the present implementation. Three typical printing modes are tested to assess the characteristics of printed layers. The results show that the printed layer becomes thicker, and the fiber alignment in the printing direction is enhanced in the bottom half of the layer and reduced in the top half due to the existence of nonisothermal effects in the process. The variation in fiber orientation becomes larger with increasing fiber concentration. By increasing the Peclet number, the deposited layer thickness reduces and the fiber alignment in the printing direction is enhanced in the top half and reduced in the bottom half. The evolution of the orientation and the velocity gradient tensors projected along several streamlines are discussed to illustrate the effects of the temperature and different printing modes on the deposited layer.
Publisher: AIP Publishing
Date: 12-2020
DOI: 10.1063/5.0031352
Abstract: The deformation and breakup of viscoelastic drops in simple shear flows of Newtonian liquids are studied numerically. Our three-dimensional numerical scheme, extended from our previous two-dimensional algorithm, employs a diffusive-interface lattice Boltzmann method together with a lattice advection–diffusion scheme, the former to model the macroscopic hydrodynamic equations for multiphase fluids and the latter to describe the polymer dynamics modeled by the Oldroyd-B constitutive model. A block-structured adaptive mesh refinement technique is implemented to reduce the computational cost. The multiphase model is validated by a simulation of Newtonian drop deformation and breakup under an unconfined steady shear, while the coupled algorithm is validated by simulating viscoelastic drop deformation in the shear flow of a Newtonian matrix. The results agree with the available numerical and experimental results from the literature. We quantify the drop response by changing the polymer relaxation time λ and the concentration of the polymer c. The viscoelasticity in the drop phase suppresses the drop deformation, and the steady-state drop deformation parameter D exhibits a non-monotonic behavior with the increase in Deborah number De (increase in λ) at a fixed capillary number Ca. This is explained by the two distribution modes of the polymeric elastic stresses that depend on the polymer relaxation time. As the concentration of the polymer c increases, the degree of suppression of deformation becomes stronger and the transient result of D displays an overshoot. The critical capillary number for unconfined drop breakup increases due to the inhibitive effects of viscoelasticity. Different distribution modes of elastic stresses are reported for different De.
Publisher: AIP Publishing
Date: 03-2021
DOI: 10.1063/5.0042693
Abstract: The bubble velocity discontinuity (BVD), when single bubble rising in shear-thinning viscoelastic fluids, is studied numerically. Our three-dimensional numerical scheme employs a phase-field lattice Boltzmann method together with a lattice Boltzmann advection-diffusion scheme, the former to model the macroscopic hydrodynamic equations for multiphase fluids, and the latter to describe the polymer dynamics modeled by the exponential Phan–Thien–Tanner (ePTT) constitutive model. An adaptive mesh refinement technique is implemented to reduce computational cost. The multiphase solver is validated by simulating the buoyant rise of single bubble in a Newtonian fluid. The critical bubble size for the existence of the BVD and the velocity-increasing factor of the BVD are accurately predicted, and the results are consistent with the available experiments. Bubbles of different sizes are characterized as subcritical (smaller than critical size) and supercritical (larger than critical size) according to their transient rising velocity behaviors, and the polymeric stress evolution affecting the local flow pattern and bubble deformation is discussed. Pseudo-supercritical bubbles are observed with transition behaviors in bubble velocity, and their sizes are smaller than the critical value. The formation of bubble cusp and the existence of negative wake are observed for both the pseudo-supercritical and the supercritical bubbles. For the supercritical bubble, the trailing edge cusp and the negative wake arise much earlier. The link between the BVD, the bubble cusp, and the negative wake is discussed, and the mechanism of the BVD is explained.
Publisher: AIP Publishing
Date: 05-2020
DOI: 10.1063/5.0004527
Abstract: A smoothed particle hydrodynamics method is employed to study the mechanical and thermal behaviors of a fiber-filled composite with an anisotropic thermal conductivity (which is coupled to the orientation of the fibers) in a three-dimensional printing process for one- and two-layer deposition. Using a microstructure-based fiber suspension model with a fiber orientation-dependent thermal conductivity model, a temperature-shear-thinning viscosity model, and a microstructure constitutive model, the effect of the nozzle temperature on the fiber alignment when printing one layer and the mechanical and thermal interactions between two printed layers are investigated. It is found that the anisotropic thermal conductivity (fiber-orientation-dependent) enhances the fiber alignment in the printing direction in the upper half layer and reduces it in the lower half at a relatively high fiber concentration (Φ = 0.2). For the one-layer deposition, the fiber alignment in the printing direction is enhanced in the lower half of the layer with an increase in the nozzle temperature. This tendency is more pronounced with the increase in both the fiber concentration and the aspect ratio. On the two-layer deposition, the fiber alignment of the first layer experiences a “reciprocating” evolution due to the squeezing from the second layer, thus creating an enhancement in the upper half and a reduction in the lower half in the fiber alignment in the first layer (with respect to the printing direction). Increasing the fiber concentration or the aspect ratio lifies this variation for the first layer. Increasing the substrate velocity also leads to some variations in the fiber alignment.
Publisher: AIP Publishing
Date: 11-2021
DOI: 10.1063/5.0071693
Abstract: We employ an immersed boundary-lattice Boltzmann (IB-LB) scheme to simulate a cylindrical (a classical self-propelled model) and a rod-shaped squirmer swimming in a channel filled with power-law fluids. The power-law index n, the channel blocking ratio κ (squirmer diameter/channel width), and the swimming Reynolds number Re are, respectively, set at 0.8 ≤ n ≤ 1.2, 0.2 ≤ κ ≤ 0.5 and 0.05 ≤ Re ≤ 5 to investigate the microswimmer' swimming speed, its power expenditure (P), and its hydrodynamic efficiency (η). The results show that increasing n yields a faster squirmer at a low Re (Re ≤ 0.5). On further increasing Re (Re ≥ 1), a larger n results in a slower pusher (a squirmer propelled from the rear), or a faster puller (a squirmer propelled from the front). Increasing the channel's width (decreasing κ) can lead to a slower puller or a puller rod squirmer. A definition of puller usher will be provided later. It is also found that, with shear-thinning, it is easier to unstabilize a puller than with shear-thickening, when increasing Re. Swimming in a shear-thinning fluid expends more power P than in a shear-thickening fluid, and P is scaled with Re according to P ∼ Ren-1 (0.05 ≤ Re ≤ 1). In addition, a stronger channel constraint (κ = 0.5) yields a higher η for the puller and the weak inertial pusher, whereas a weaker channel constraint (κ = 0.2) results in a higher η for the pusher with the increased fluid inertia.
No related grants have been discovered for Zhenyu Ouyang.