Wall Vibration Control Based On The Spanwise Turbulent Channel Flow Structure

A turbulent channel flow structure and drag reduction mechanism induced by spanwise wall oscillation are investigated using direct numerical simulation (DNS) method. The database of turbulent channel flow is produced by DNS. The basic turbulence statistics and the near-wall turbulence structure are analyzed, and the main results are as follows:(1)The computational results are firstly validated by experimental data, and the good agreements are obtained and imply the reliability of the algorithm. Then, the analysis of basic turbulence statistics indicates that the near-wall average flow velocity gradient is decreased, the buffer layer is incrassated and the log layer is upward shifted when the periodic motion of the wall is presented in the spanwise direction. Correspondingly, the root-mean-square velocity fluctuations and Reynolds shear stress are decreased in the whole region and the peak is outward shifted.(2)Through the analysis of the transportation of Reynolds stresses, it is found that the attenuation of turbulence intensities and skin friction at the statistically stationary stage is mainly due to the sustained reduction of the redistribution terms. It indicates that the inter-component transfer of turbulent kinetic energy is hampered by the periodic motion of the wall.(3) An investigation of typical coherent structures, such as streamwise vortices and low-speed streaks, is carried out. It is found that the streaks and the vortex structures of the near-wall region almost disappear. The steaks get wider and the space between steaks increases in spanwise direction as far away from the wall under the influence of the oscillating wall. These results imply that the wall motion break the self-sustaining mechanism of wall turbulence.(4)The detection of the turbulence burst events with VITA (variable interval time average) method is performed and indicates that the turbulence burst events are suppressed under the influence of the oscillating wall.(5)Through quadrant analysis, it is founded that the ejection and the sweep are significantly decreased and contribute less to Reynolds stress which means that main production mechanism of turbulence is suppressed.(6)The analysis of turbulence fluctuations spectrum indicates spanwise wall oscillation causes energy in less wave number region centralizing obviously to a specific direction, the surface of two-dimension energy spectrum getting a little more smooth, the kinetic energy fluctuations decreasing in the whole region, which indicates that turbulence fluctuations is suppressed by oscillating wall.The above compuatations and corresponding analysis reveal that the wall motion in spanwise direction can suppress the streamwise vortex structure in the near-wall region, due to the viscosity effect, and therefore suppress the ejection and sweep motions of fluid which is involved in streamwise vortices. As a result, turbulence events and skin friction are significantly reduced.