Takahiro Yasuda

Effect of collision and velocity model of lattice Boltzmann model on three-dimensional turbulent flow simulation

ABSTRACT Various collision and velocity models of the lattice Boltzmann model (LBM) were compared to determine their effects on the efficiency of a three-dimensional homogeneous isotropic decaying turbulent flow simulation. We determined that a decrease in the number of velocities, in particular, 13-velocities, which can be used in the quasi-equilibrium lattice Boltzmann and in the multiple-relaxation time models (MRT), could considerably decrease the computational effort. However, decreasing the number of velocities deteriorates the stability and the accuracy of the results. By comparing the collision models, we also determined that the stability of the entropic lattice Boltzmann model (ELBM), and 19- and 27- velocity MRT is much higher than in other models. However, the numerical viscosity introduced by the ELBM underestimates the enstrophy, and the computational effort increases because of the calculation overhead required to solve the additional equations if special care is not given to the calculation.

Comparison of Local Computational Approaches for Unsteady Viscous Incompressible Flows

Numerical simulations of 2D decaying homogeneous isotropic turbulence are presented using three local computational approaches, ACM, KRLNS equations and LBM, in order to investigate accuracy, efficiency and the capability to capture the correct transient behavior of the incompressible flows and the results are compared with the solutions obtained by the Pseudo-Spectral Method (PSM), which is the standard method for this problem. Parallel computations are carried out on a GPU by using the CUDA library provided by the NVIDIA and the speedup is investigated. It is found that all three methods can capture the transient flow fields and LBM is the most accurate and efficient approach for this kind of unsteady incompressible flow.

Entropic Lattice Boltzmann Simulation for Unsteady Flow Around two Square Cylinders Arranged Side by Side in a Channel

Entropic lattice Bolzmann model (ELBM) was developed in recent years. The method enhances stability by satisfying the second principle of the thermodynamics by imposing the monotonicity and the minimality of the H-function. But the qualitative and quantitative validities of ELBM results, especially for unsteady flow, were not confirmed sufficiently. In our previous study, we applied ELBM to a 2-dimensional channel flow past single square cylinder, and found that the validity of ELBM is guaranteed under the condition of rms value of viscosity difference ratio \(\varDelta_{\mathrm{0rms}},\ \varDelta_{\mathrm{0rms}}\leq 0.24\). But the generality of the condition has not been confirmed. In this study, we applied ELBM to a 2-dimensional channel flow past two square cylinders arranged side by side in Reynolds number region \(\mathit{Re}=100\) and 1000 and investigated the effect of number of cylinder. As a results, it was found that as well as the in the case of single square cylinder, the condition \(\varDelta_{\mathrm{0rms}}\leq 0.24\) is also almost reasonable in the case of two square cylinder.

The Effect of Fine Particles Added Into Water on the Motion of Bubbles Rising Helically

LDV, PIV and some methods using ultrasonic sound have been often employed to measure multiphase flows. Fine particles are usually added into flows as tracer or scattering particles. The effect of particles added in on the flow characteristics is, however, not examined in detail. Because multiphase flows, especially gas-liquid systems, have gas-liquid interfaces, where impurities are known to aggregate, fine particles may aggregate in gas-liquid interfaces, and may affect the flow situation. Therefore, we measured the movement of helically rising bubbles to investigate the effect of fine particles mixed into the liquid phase. Polyethylene particles of 160μm and 10.6μm medium diameters are used. The reductions of helical sizes and rising and moving velocities are recognized by adding particles. The effect of particle size is also discussed.Copyright

Three-Dimensional Numerical Study on Flow Around a Square Cylinder on Oscillatory Flow

Flow around a square cylinder in oscillatory flow was studied by a numerical simulation using a finite-volume method. The computations were carried out by assuming 3-dimensional (3-d), unsteady, incompressible and viscous flow through Keulegan-Carpenter number (KC) in the range of 2.5 ? KC ? 25 at Stokes number (β) = 95. The region in KC ? 4, “longitudinal vortex pairs” periodically arranged in span-direction were found out as well as in the case of circular cylinder. On the other hand, KC ? 5, the flow patterns called “transverse-vortex street”, “double-pair” which appeared in the case of the circuler cylinder at KC =12 and KC =16, respectively, were not formed. The drag and inertia coefficients of the Morison equation CD and CM, were estimated and it was found that decreasing of CD as KC increases in the region 2.5 ? KC ? 12 is caused by the change in the position of rolling up of the flow in the wake. By calculating spanwise correlation, the effect of 3-D flow structures on the transvers force were investigated. The effect remarkably appears in the range of 2.5 ? KC ? 8 in which the “longitudinal vortex pairs” are formed.

Fluid-Dynamic Force Characteristics of an Auto Rotating Flat Plate

When a thin flat pate is released in still air, the plate automatically sets into a rotational motion. This phenomenon is called autorotation. The autorotating flat plate is loaded by unsteady fluid-dynamic forces. It is guessed that the forces are contributed by one due to vortex shedding from the edges of the plate and one due to the plate rotation, but the detail force characteristics have not been clear yet. In this study, we calculated the two types of flow by the discrete vortex method, one is flow around a rotating flat plate hinged about its center at constant rotating frequency in the uniform flow and the other is flow around a freely falling flat plate. The computed result in both types of flow agree well with the experiment. In the case of freely falling plate, autorotation phenomena could be predicted. By computed fluid dynamic forces, the contributions of vortex shedding from the plate to fluid dynamic forces and feedback effect of the fluid dynamic forces were found.© 2007 ASME

3-D Flow Structures Around and Fluid-Dynamic Forces Acting on a Rectangular Cylinder in Oscillatory Flow

Three-dimensional flow structures around and fluiddynamic forces acting on a rectangular cylinder in oscillatory flow were investigated by numerical simulation using finite volume method. The computations were carried out for three kinds of cross-sections with width/height ratio (d/H) d/H = 0.6, 1.0 and 2.0 and for the amplitude of oscillating flow in the range of 2.5 ≤ the Keulegan-Carpenter number (KC) ≤ 25, the Stokes number (β) = 95. The calculated flow patterns and the drag and inertia force coefficients of Morison equation acting on the cylinder were compared with the experimental ones using a U-tube water tank. In this paper, we note how the KC number and the width/height ratio of the cylinders affect the unsteady and three-dimensional flow structures such as the “longitudinal vortices” and “transverse street” which formed in the case of a circular cylinder fixed in oscillatory flow, and how the CD and the CM values of Morison coefficients change corresponding to the change of the behavior of the flow patterns. Furthermore the relationship between spanwise correlation coefficient of the transverse force R(x3 ), where x3 is the spanwise position from the bottom of the cylinder, and three-dimensional vortex structures were investigated.Copyright