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

为了得到局部控制近壁区组织结构的算法，我们对平面通道内的湍流流动进行了直接的数值模拟。通过增加一些修正，我们观察到了近壁区涡旋结构的调制，通过壁面的展向振荡，阻止了涡度从展向分量向流向分量的转变。对于壁面粗糙度，我们考虑了沙质粗糙度和肋状粗糙度。根据粗糙度特性，用典型结构代替了近壁区的流向涡。然而，在外部区域，流道的整体尺度是湍流结构的特征，通过增加粘性阻力，特别是在展向方向的粘性阻力，流体动力阻力显著减小。在高雷诺数和低雷诺数情况下，只有在壁面附近(如缓冲层内)加入这种各向异性粘性，才能提高质量流量。为了模拟加入少量聚合物的减阻效果，结合离散元模型进行了直接数值模拟。湍流中的聚合物用珠子-弹簧-阻尼器单元来表示。研究发现，阻尼器所代表的粘度是减阻的主要因素。在上述数值实验的基础上，我们证实了准流向涡的控制对壁面边界湍流的修正是有效的。

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.