Search

The number of topological defects in the shear flow of active nematic fluids is numerically investigated in this study. The evolution of the flow state of extensile active nematic fluids is explored by increasing the activity of active nematic fluids. Evidently, medium-activity active nematic fluids exhibit a highly ordered vortex lattice fluid state. However, high-activity active nematic fluids exhibit a meso-scale turbulent flow accompanied by topological defects. The number of topological defects (Ndef) increases with increasing shear Reynolds number (Res). Fluid viscosity strongly influences Ndef, while the influence of fluid density is relatively weak. Ndef decreases with increasing activity length scale (lζ) value. A small Res value strongly influences Ndef, whereas a large lζ value only weakly influences Ndef. As the activity increases, Ndef in contractile active nematic fluids becomes larger than that of extensile active nematic fluids. [ABSTRACT FROM AUTHOR]

In order to study the dynamic characteristics of micro-organisms or engineered swimmers, the simulations of the motion and interaction of a pair of squirmers in a shear flow are conducted using the lattice Boltzmann method (LBM) in the range of squirmer-type factor −5 ≤ β ≤ 5, self-propulsion strength 0.1 ≤ α ≤ 0.8, initial spacing between two squirmers 0.5d ≤ d′ ≤ 3d (d is the diameter of the squirmers). The interactions and motion mechanisms of squirmers in puller–puller, pusher–pusher, puller–pusher, and pusher–puller configurations are analyzed. The results show that there are three typical motion patterns in the puller–puller configuration, i.e., steady tilting motion (STM), large-amplitude oscillation motion (LAO), and small-amplitude oscillation motion (SAO). The motion of pullers has a large vertical range, and the swimming angle continues to increase in the LAO, while the pullers oscillate near the plate and have a small range of changes in swimming angles in the SAO. The situation is more complex in the pusher–pusher configuration, and there are five motion patterns, i.e., STM, LAO, SAO, limit cycle motion (LCM), and open limit cycle motion (OLCM). Three different STMs are found according to the pressure distribution around the pushers. d′ is an important parameter affecting the interaction of squirmers. In the puller–puller configuration, the backflow area on the right side of puller 1 is completely suppressed when d′ = 0.5d, but this suppression gradually weakens as d′ increases. Changes in d′ result in the differences in the final motion patterns of pullers on the upper and low plates. In the pusher–pusher configuration, changes in d′ result in transitions between different motion patterns. There exist a critical swimming angle θc when d′ changes, and the pattern changes from the STM to the LCM when θ > θc. At d′ = 0.5d, in the puller–pusher configuration, there exists a stable structure formed by the mutual repulsion caused by the high pressure area between the puller and pusher. In the pusher–puller configuration, there is a low-pressure area on one side of the pusher, which attracts the low-pressure area on the head of the puller and affects their subsequent motion patterns. [ABSTRACT FROM AUTHOR]

Migration of a particle in a confined shear flow of Giesekus fluids is investigated numerically with the method of direct forcing/fictitious domain. We focus on the migration direction for the particle with initial lateral position y0 and determination of critical point yc of a particle moving towards the center line or wall. The effect of viscosity ratio μr, shear-thinning parameter α, Weissenberg number Wi, and blocking rate β on the value yc is analyzed. The results showed that when μr ≤ 0.5, the particle will migrate towards the wall regardless of the value of y0. When μr > 0.5, yc increases with increasing μr, and some particles will migrate towards the center line with the increase in μr. The particle is more likely to migrate towards the center line at small values of Wi and α but at large values of μr. The impact of Wi and β on the particle migration direction is more obvious. The particle will migrate towards the wall for β = 0.3 and is more likely to migrate towards the wall with increasing β. α and Wi have little influence on the pressure distribution in the case of the same β and μr. The particle near the wall will migrate faster because large positive pressure and negative pressure appear around the particle. [ABSTRACT FROM AUTHOR]

The hydrodynamic properties of a squirmer type of self-propelled particle in a simple shear flow are investigated using the immersed boundary-lattice Boltzmann method in the range of swimming Reynolds number 0.05 ≤ Res ≤ 2.0, flow Reynolds number 40 ≤ Rep ≤ 160, blocking rate 0.2 ≤ κ ≤ 0.5. Some results are validated by comparing with available other results. The effects of Res, Rep and κ on the hydrodynamic properties of squirmer are discussed. The results show that there exist four distinct motion modes for the squirmer, i.e., horizontal mode, attractive oscillation mode, oscillation mode, and chaotic mode. Increasing Res causes the motion mode of the squirmer to change from a constant tumbling near the centerline to a stable horizontal mode, even an oscillatory or appealing oscillatory mode near the wall. Increasing the swimming intensity of squirmer under the definite Res will induce the squirmer to make periodic and stable motion at a specific distance from the wall. Increasing Rep will cause the squirmer to change from a stable swimming state to a spiral motion or continuous rotation. Increasing κ will strengthen the wall's attraction to the squirmer. Increasing swimming intensity of squirmer will modify the strength and direction of the wall's attraction to the squirmer if κ remains constant. [ABSTRACT FROM AUTHOR]

The lateral migration of a spherical particle in a sheared Giesekus fluid is numerically studied by the direct-forcing fictitious domain method. The model is first validated by comparing the simulation results with the available data in the literature. Effects of the viscosity ratio, shear thinning, Weissenberg number, and wall confinement on the particle migration are examined. Results show that the particle migrates toward the wall, irrespective of whether the fluid is shear thinning. The wall confinement, shear thinning, and high Weissenberg number could respectively facilitate the particle lateral migration. While the effect of viscosity ratio on the particle migration is not monotonic, a separatrix value is found which divides the viscosity ratio into two ranges. Moreover, effects of rheological properties on the particle angular velocity and its variation throughout the migration pathway are also explored, and the position where the angular velocity starts to decrease is affected by fluid rheological properties. [ABSTRACT FROM AUTHOR]

The immersed boundary-lattice Boltzmann method is used to study the inertial migration of particles in Poiseuille flow of a power-law fluid. The effects of Reynolds number, power-law index, and blockage ratio on the formation of particle trains are explored. The results show that single particle with different initial positions reach the same equilibrium position for the same power-law index. The stable equilibrium position moves closer to the centerline under the higher power-law index and blockage ratio. One-line of eight particles distributed initially at a vertical position will migrate laterally to the vicinity of the wall and form single-line particle trains. The particle spacing is unstable and increases when particles migrate to the equilibrium position. The inertial focusing length is an important factor for analyzing the formation of particle trains, which will be longer with increasing the power-law index. The mean particle spacing will be reduced with increasing Re and blockage ratio. Two-lines of 12 particles distributed initially and abreast along both sides of the centerline will migrate to the vicinity of the wall and form staggered particle trains. Due to the multiparticles interaction, the final particle equilibrium position will deviate from the single particle equilibrium position. The axial spacing between two neighboring particles is stable or fluctuates within a certain range. The particle spacing decreases with increasing the power-law index and blockage ratio, and with decreasing Re. The shear-thinning fluid is beneficial to the formation of single-line particle trains and staggered particle trains. [ABSTRACT FROM AUTHOR]

The finite volume method is used to numerically simulate the dynamics of cylindrical particles in the contraction flow of a second-order fluid. The effects of Stokes number, Deborah number, particle aspect ratio and contraction ratio on the spatial and orientation distributions of cylindrical particles are analyzed. Some numerical results are compared with the available experimental results in the contraction flow of Newtonian fluids. The results show that the spatial and orientation distributions of the particles are dependent on the competition among the inertia, viscoelasticity and effect of confined wall. The result of competition is responsible for the force and torque inducing a cross-streamline migration and rotation of particles. Along the flow direction, the particle spatial distribution in the cross section changes non-monotonically. The particle orientation distribution changes monotonically and particles gradually align themselves with the flow direction, and this phenomenon is more obvious than the case of the Newtonian fluid under the same conditions. High shear rate makes the particles align with their major axis near to the flow direction. With the increase of the Stokes number, the particle spatial distribution becomes more uniform, and orientation distribution function of particles becomes flatter and wider. With the increase of Deborah number and the contraction ratio, the particle spatial distribution becomes more non-uniform, in the meantime, more particles tend to align themselves with the flow direction. With the increases of the particle aspect ratio, the particle spatial distribution in the cross section becomes more uniform, and more particles align with their major axis near to the flow direction. In the parameter region studied here, the particle aspect ratio has a weaker effect on the spatial and orientation distributions of particles than Stokes number, Deborah number and contraction ratio. [ABSTRACT FROM AUTHOR]

Flow past a center-pinned freely rotatable cylinder asymmetrically confined in a two-dimensional channel is simulated with the lattice Boltzmann method for a range of Reynolds number 0.1 ≤ Re ≤ 200, eccentricity ratio 0/8 ≤ ε ≤ 7/8, and blockage ratio 0.1 ≤ β ≤ 0.5. It is found that the inertia tends to facilitate the anomalous clockwise rotation of the cylinder. As the eccentricity ratio increases, the cylinder rotates faster in the counterclockwise direction and then slows down at a range of Re < 10. At a range of Re > 40, there exists an anomalous clockwise rotation for the cylinder at a low eccentricity ratio and the domain where the cylinder rotates anomalously becomes larger with the increase in the Reynolds number. In a channel with a higher blockage ratio, the rotation of the cylinder is more sensitive to the change of cylinder lateral position, and the separatrix at which the cylinder remains a state of rest moves upward generally. The cylinder is more likely to rotate counterclockwise and the rotating velocity is larger. At a lower blockage ratio, the anomalous clockwise rotation is more likely to occur, and the largest rotating velocity occurs when the blockage ratio is equal to 0.3. The mechanism of distinct rotational behavior of the cylinder is attributed to the transformation of distribution of shear stress which is resulted from the variation of pressure drop, the shift of maximum or minimum pressure zones along the upper and lower semi-cylinder surface, and the movement of stagnant point and separate point. Finally, the effects of the cylinder rotation on the flow structure and hydrodynamic force exerted on the cylinder surface are analyzed as well. [ABSTRACT FROM AUTHOR]

The viscosity properties of the semi-concentrated suspensions of fibers with small aspect ratio were investigated in transient shear flow using a concentric cylinder geometry. Experimental results were compared with predictions by utilizing the constructive model developed by Batchelor in conjunction with the hybrid closure proposed by Advani and Tucker. To rectify the discrepancy between the theoretical prediction available at the present time and the measured data, a new expression for the specific viscosity in the semi-concentrated shear flows of fiber suspension is proposed allowing for the effects of the fiber–fiber interactions and the finite-aspect-ratio fiber. The results show that the model predictions for the fiber suspensions of different aspect ratio and concentration are fairly good. [ABSTRACT FROM AUTHOR]

The equation for the orientation probability function of slender cylindrical particles suspended in planar turbulent flows was investigated. After ensemble averaging, the equations for the mean and fluctuating probability function were derived. The equation for the fluctuating probability function appearing in the equation of mean probability function was solved by using the method of characteristics analysis. The orientational dispersion terms due to the random motion of cylindrical particles in the equation of mean probability function are related to the mean probability function and the Lagrangian fluid velocity correlations. The evolution of the mean probability function was described by a modified orientation-space-convection equation, where the dispersion terms account for the randomizing effect of the turbulence. [ABSTRACT FROM AUTHOR]

The available expressions for the effective viscosity can not provide good predictions compared with the experiment data measured in the semi-dilute shear flows of fiber suspension with small aspect ratio. The departure of the theoretical prediction from the measured data increases with the decrease of the fiber aspect ratio. Therefore, by experiment for the fiber with 20 μm diameter, a new expression for the effective viscosity in the semi-dilute shear flows of fiber suspension with small aspect ratio is proposed, the relationship between the shear viscosity of fiber suspensions and the fiber concentration is given. The results show that the effective viscosity is not a linear function of the fiber concentration. [ABSTRACT FROM AUTHOR]

• The pattern of particle migrations can be roughly classified into "returning" and "passing". • The effects of some factors on the particle migrations are explored. • The separatrix between "returning" and "passing" pattern is dependent on some factors. In this paper, the migrations of two interacting particles in a three-dimensional bounded shear flow of Giesekus fluids are numerically investigated using the direct forcing/fictitious domain method for the Weissenberg number ranging from 0.1 to 1.0, the mobility parameter α which quantifies the shear-thinning effect ranging from 0.01 to 0.7, and the ratio of the solvent viscosity to the total viscosity being 0.1. The model is first validated by comparing the numerical results with the available data in the literature. The effects of the Weissenberg number, the shear-thinning effect, and the initial vertical distance between two particles on the particle migrations are explored. Some of the results are in agreement with the experimental ones. The results show that the pattern of particle migrations can be roughly classified into "returning" and "passing". The variations of particle velocity and pressure field in the "returning" pattern are totally different from those in the "passing" pattern. The separatrix between "returning" and "passing" pattern is dependent on the initial vertical distance between two particles, the Weissenberg number and the shear-thinning effect. With other parameters fixed, the trajectories of particle change from the "returning" pattern to the "passing" pattern as the initial vertical distance between two particles and the Weissenberg number increase, but as the shear-thinning effect decreases. [ABSTRACT FROM AUTHOR]