Research on Local Control of Generation of Quasi-streamwise Vortex in Near Wall Turbulence

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To obtain an algorithm for a local control of organized structure in the near wall region, we conducted direct numerical simulations for turbulent flows in a plane channel. By adding some modifications, we observed the modulation of the vortex structures in the near wall region.By the spanwise oscillation of the wall, the transform of vorticity from spanwise to streamwise components was prevented. It resulted in the significant decrease in the generation of streamwise vortices.For the wall roughness, we considered sand-like roughness and ribs. The streamwise vortices in the near wall region was replaced by the typical structure depend on the roughness features. In the outer region, however, the global scale of flow passage characterized the turbulence structure.By increasing the viscous resistance especially in the spanwise direction the significant decrease in hydrodynamic drag was attained. The mass flow rate was increased by the addition of such an anisotropic viscosity only in the vicinity of the wall such as within the buffer layer, for both high and low Reynolds numbers.To mimic the drag reduction by adding a small amount of polymers, a direct numerical simulation was carried out, in couple with discrete element models. The polymer in the turbulent flow is represented by beads-spring-dashpot element. The viscosity represented by a dashpot is found to be the primary factor for the drag reduction. Experimental findings were reproduced consistently in our simulation.On the basis of the above-mentioned numerical experiments, we confirmed that the control of quasi-streamwise vortices was effective for the modification of wall bounded turbulence.