Direct Numerical Simulation Of Particulate Flows With The Parallel Fictitious Domain Method

Particulate flows commonly exist in nature and industry. A comprehensive understanding of the mechanism of particulate flows is helpful to know the nature for people and to provide related theoretical guidance for the industries. Particulate flow is one of the hotspots in fluid mechanics research. However, it is still poor understanding of the interaction between particles and turbulence because of the complex of particulate flow. In recent years, the direct numerical simulation (DNS), which can calculate hydrodynamic force on the particles theoretically, becomes one powerful tool to probe the mechanism of the interaction between particles and turbulence. The DNS attracts the attention of more and more researchers. However, the computional cost of the DNS is huge and traditional serial computation hardly meets its requirement. In the present thesis, a new parallel fictitious domain method (FD) is proposed, and then applied to study turbulent channel flow and turbulent duct flow.The new method, which is based on the uniform grid, takes the domain decomposition as the parallel strategy. A fractional-step time scheme is used to decouple the combined system into two sub-problems:fluid sub-problem and particle sub-problem. The fluid sub-problem is solved with multi-grid iterative method. The particle sub-problem is handled by two particle lists for each sub-domain. Testing cases show that the parallel efficiency of new method is excellent.For the particle-laden channel flow, firstly the effects of the particles inertia (density) on the turbulence modulation are investigated at the particle-fluid density ratio ranging from 1 to 1042. The results show that the addition of the particles first increases and then reduces the flow resistance, with the increasing of particle density. The significant drag-reduction by the particles is observed for large particle density ratios at the transient stage, but not at the statistically steady state. The particles with large inertia significantly suppress the large-scale hairpin vortices and consequently attenuate the turbulence intensity at the bulk region. Secondly, the effects of the ellipsoidal particles on the turbulence modulation in the channel flow are investigated. The results show that the effects of ellipsoidal particles are similar with sphere particles in the same volume. Although there is no drag-reduction effect, the flow resistance reduces with the increasing of aspect ratio of ellipsoidal particles.Regarding the particle-laden turbulent flows in a square duct, the effects of the finite-size particles on the mean and root-mean-square (RMS) velocities are investigated firstly. Our results show that the mean secondary flow is enhanced and its circulation center shifts closer to the center of the duct cross-section when the particles are added. The reason is analyzed by examining the terms in the mean streamwise vorticity equation. It is observed that the particles enhance the wall-tangential component of the RMS velocity (i.e. Reynolds normal stress) more than its wall-normal component in the near-wall region near the corners, resulting in the enhancement in the gradients of the normal stress difference, which we think is mainly responsible for the enhancement in the mean secondary flow. In addition, the particles accumulate preferentially in the near-corner region, which may be caused by the turbophoresis effect and the mean secondary flow effect together. Secondly, the depositon of finite-size heavy particles in turbulent duct flow are investigated. Our results show that the particle sedimentation breaks the up-down symmetry of the mean secondary vortices. The suppression of the first vortex in vicinity of the bottom wall is expected to be primarily caused by the hindering effect of the sediments. And the second and forth vortex appear to merge in a stronger circulation which transports the fluids downward in the bulk center region and upward along the side walls. This circulation has a significant impact on the distribution of the mean streamwise velocity, whose maximum value occurs in the lower half duct. The particles accumulate preferentially at the face center of the bottom wall and the corner region.