Purpose -This study is concerned with the direct numerical simulation (DNS) of a turbulent channel flow by an improved vortex in cell (VIC) method. The paper aims to discuss these issues. Design/methodology/approach -First, two improvements for VIC method are proposed to heighten the numerical accuracy and efficiency. A discretization method employing a staggered grid is presented to ensure the consistency among the discretized equations as well as to prevent the numerical oscillation of the solution. A correction method for vorticity is also proposed to compute the vorticity field satisfying the solenoidal condition. Second, the DNS for a turbulent channel flow is conducted by the improved VIC method. The Reynolds number based on the friction velocity and the channel half width is 180. Findings -It is highlighted that the simulated turbulence statistics, such as the mean velocity, the Reynolds shear stress and the budget of the mean enstrophy, agree well with the existing DNS results. It is also shown that the organized flow structures in the near-wall region, such as the streaks and the streamwise vortices, are favourably captured. These demonstrate the high applicability of the improved VIC method to the DNS for wall turbulent flows. Originality/value -This study enables the VIC method to perform the DNS for wall turbulent flows.
The control of the motion of small gas bubbles by a vortex ring is explored through numerical simulation. Hydrogen bubbles with a diameter of 0.2 mm are arranged in quiescent water, forming a bubble cluster. A vortex ring is launched vertically, passing through the bubble cluster. The behaviors of the vortex ring and bubble motion are analyzed. The diameter of the vortex ring at launch is 42.5 mm, and the bubble volume fraction is less than 0.04. As the vortex ring convects through the bubble cluster, it displaces and entrains the bubbles. After the vortex ring passes through the cluster, the bubbles are involved in the vortex ring. The research also clarifies the evolution of the diameter and circulation of the vortex ring.
The possibility to control the motion of small gas bubbles by a vortex ring is explored through a numerical simulation. Hydrogen bubbles with diameter of 0.2 mm are arranged in quiescent water, and a vortex ring is launched toward the bubbles. The behavior of the vortex ring and the bubble motion are analyzed. The diameter of the vortex ring at the launch is 42.5 mm, the Reynolds number is 500, and the bubble volume fraction at the launch is less than 0.04. The simulation highlights that the vortex ring convects in the bubble cluster with shoving the bubbles and that the bubbles are entrained and involved by the vortex ring. It also clarifies the changes of the diameter and circulation of the vortex ring.
SummaryThe flow in a two-dimensional cavity with a free surface and a bottom opening on a channel stream is numerically calculated. The calculated results are compared with experimental data measured in the vertical openings of ship models. The calculations are performed using GSMAC-FEM. The arbitrary Lagrangian-Eulerian formulation is applied to deal with the free surface motion. To test the algorithm used in the present calculation, three cases of the fluid problem are examined: normal cavity flow, free oscillation of water in a tank, and the run up height of a solitary wave on a tank wall. In the three cases, the calculation results show good agreements with previously published data. Using the present calculation algorithm, the flow in a cavity with a free surface and a bottom opening to a channel flow is calculated and the following conclusions are obtained: (1) Vertical oscillations of the free surface in the cavities are observed in the numerical calculation when there are two vertical cavities with the free surface on the upper wall of a channel flow. (2) The stream velocities of the free surface motion in the numerical calculation are the same as those of the experimental results measured in the vertical opening of ship models at the dimensionless speed, 2U/(Lω 0 ) or 1/(πSt).
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