Lin Jianzhong

Numerical research on the orientation distribution of fibers immersed in laminar and turbulent pipe flows

The spatial and orientational distributions of fibers in laminar and turbulent pipe flows are simulated numerically. The simulated results are consistent with the experimental data available in the literature. In the laminar flow regime, more fibers are aligned with the flow direction with increasing Reynolds number. The shear rate of fluid around a fiber is the key factor for determining the orientation distribution of fibers. The fibers aspect ratio and fiber density have insignificant influence, while the fiber velocity and the fiber Stokes number have marginal influence, on the orientation distribution of fibers. In the turbulent regime, the spatial and orientational distributions become more homogeneous with increasing Reynolds number, and the fluctuation intensity of fiber velocity in the streamwise direction is larger than that in the other two directions, in contrast to the fluctuation intensity of the fiber angular velocity, which is weaker in the streamwise direction.

Effects of the aspect ratio on the sedimentation of a fiber in Newtonian fluids

Lattice Boltzmann method is used to investigate sedimentation of a single fiber in a Newtonian fluid. The effects of the aspect ratio of a fiber are carefully examined. The computational results show that the stable orientation of a single fiber is the horizontal direction. The terminal Reynolds number increases as the aspect ratio increases, and basically remains invariable when the aspect ratio is high enough. The lateral drift of the fiber is more apparent at higher aspect ratios and the orientation of the fiber at which the lateral drifting velocity reaches the maximal is not sensitive to the value of the aspect ratio. When the aspect ratio is around 2.8, the fiber rotates fastest from the vertical location to the horizontal. Some of the results are compared with the experimental data and good agreements are found.

Numerical research on the coherent structure in the viscoelastic second-order mixing layers

Numerical simulations have been performed in time-developing plane mixing layers of the viscoelastic second-order fluids with pseudo-spectral method. Roll-up, pairing and merging of large eldies were examined at high Reynolds numbers and low Deborah numbers. The effect of viscoelastics on the evolution of the large coherent structure was shown by making a comparison between the second-order and Newtonian fluids at the same Reynolds numbers.

Nanoparticle coagulation in a planar jet via moment method

Large eddy simulations of nanoparticle coagulation in an incompressible planar jet were performed. The particle is described using a moment method to approximate the particle general dynamics equations. The time-averaged results based on 3000 time steps for every case were obtained to explore the influence of the Schmidt number and the Damkohler number on the nanoparticle dynamics. The results show that the changes of Schmidt number have the influence on the number concentration of nanoparticles only when the particle diameter is less than 1 nm for the fixed gas parameters. The number concentration of particles for small particles decreases more rapidly along the flow direction, and the nanoparticles with larger Schmidt number have a narrower distribution along the transverse direction. The smaller nanoparticles coagulate and disperse easily, grow rapidly hence show a stronger polydispersity. The smaller coagulation time scale can enhance the particle collision and coagulation. Frequented collision and coagulation bring a great increase in particle size. The large the Damkohler number is, the higher the particle polydispersity is.

Numerical Research on the Fiber Suspensions in a Turbulent T-shaped Branching Channel Flow*

Abstract The concentration and orientation of fiber in a turbulent T-shaped branching channel flow are investigated numerically. The Reynolds averaged Navier-Stokes equations together with the Reynolds stress turbulent model are solved for the mean flow field and the turbulent kinetic energy. The fluctuating velocities of the fluid are assumed as a random variable with Gaussian distribution whose variance is related to the turbulent kinetic energy. The slender-body theory is used to simulate the fiber motion based on the known mean and fluctuating velocities of the fluid. The results show that at low Reynolds number, fiber concentration is high in the flow separation regions, and fiber orientation throughout the channel is widely distributed with a slight preference of aligning along the horizontal axis. With increasing of Re, the high concentration region disappears, and fiber orientation becomes homogeneous without any preferred direction. At high Reynolds number, fiber concentration increases gradually along the flow direction. The differences in the distribution of concentration and orientation between different fiber aspect ratio are evident only at low Re. Both Re and fiber aspect ratio have small effect on the variance of orientation angle.

New approach to minimize dispersion induced by turn in capillary electrophoresis channel flows

The mechanism of dispersion induced by turn in the capillary electrophoresis channel flows was analyzed firstly. Then the mathematical model of electroosmotic flow is built, and the dispersion of the flow, with different distribution of charge at inner and outer wall in the turns, was simulated numerically using the finite differential method. A new approach of altering the distribution of charge at inner and outer wall in the turns was presented, based on the computational results, to minimize the dispersion induced by turn. Meanwhile, an optimization algorithm to analyze the numerical results and determine the optimal distribution of charge in the turns was also developed. It is found that the dispersion induced by turn in the capillary electrophoresis channel flows could be significantly suppressed by this approach.

The collision efficiency of spherical dioctyl phthalate aerosol particles in the Brownian coagulation

The collision efficiency in the Brownian coagulation is investigated. A new mechanical model of collision between two identical spherical particles is proposed, and a set of corresponding collision equations is established. The equations are solved numerically, thereby obtaining the collision efficiency for the monodisperse dioctyl phthalate spherical aerosols with diameters ranging from 100 to 760 nm in the presence of van der Waals force and the elastic deformation force. The calculated collision efficiency, in agreement with the experimental data qualitatively, decreases with the increase of particle diameter except a small peak appearing in the particles with a diameter of 510 nm. The results show that the interparticle elastic deformation force cannot be neglected in the computation of particle Brownian coagulation. Finally, a set of new expressions relating collision efficiency to particle diameter is established.

Sedimentation of Rigid Cylindrical Particles with Mechanical Contacts

A collision model of two cylindrical particles is put forward. Based on the model the sedimentation of rigid cylindrical particles with mechanical contacts is simulated numerically by using the lattice Boltzmann method. Some numerical results are compared with the experimental ones given by us and others, and good agreements are found. In the sedimentation process, particles will rotate and drift at any initial orientation and terminal Reynolds number. The orientation, lateral position, drifty and sedimenting velocity of particles change periodically at small terminal Reynolds number. With the increasing terminal Reynolds number, the periodicity disappears, and an inverted T-structure forms. This structure appears more quickly and lasts for a longer time at larger terminal Reynolds number.

Effects of tensor closure models and 3-D orientation on the stability of fiber suspensions in a channel flow

Three different kinds of closure model of fiber orientation tensors were applied to sumulate numerically the hydrodynamic stability of fiber suspensions in a channel flow.The effects of closure roodels and three-dimensional(3-D)orientation distribution of fibers on the results of stability analysis were examined.It is found that the relationship of the behaviro in hydrodynamic stability and the parameter of the fiber given by all the three models are the same.However,the attenuation of flow instability is most distinct using 3-D hybrid model because the orientation of the fiber departures from the flow direction,and least apparent using its 2-D counterpart for that the fibers show a tendency towards alignment with the flow direction in this case.

Stability Analysis in Spatial Mode for Channel Flow of Fiber Suspensions

Different from previous temporal evolution assumption, the spatially growing mode was employed to analyze the linear stability for the channel flow of fiber suspensions. The stability equation applicable to fiber suspensions was established and solutions for a wide range of Reynolds number and angular frequency were given numerically. The results show that, the flow instability is governed by a parameter H which represents a ratio between the axial stretching resistance of fiber and the inertial force of the fluid. An increase of H leads to a raise of the critical Reynolds number, a decrease of corresponding wave number, a slowdown of the decreasing of phase velocity, a growth of the spatial attenuation rate and a diminishment of the peak value of disturbance velocity. Although the unstable region is reduced on the whole, long wave disturbances are susceptible to fibers.