Detection of near-wall streamwise vortices by measurable information at wall

Abstract

The relationship between near-wall streamwise vortices and flow quantities that can be measured at wall, i.e., streamwise and spanwise wall shear stress τwx, τwz and fluctuating wall pressure p′w, is studied using the DNS databases of fully developed turbulent channel flow. It is found that all the three quantities τwx, τwz and p′w are closely related with near-wall streamwise vortices. τwx corresponds to the sweep and ejection motion induced by the downstream streamwise vortex. τwz is the spanwise footprint of the overhead vortical structure. Compared with the wall shear stresses, the relationship between p′w and the near-wall streamwise vortex is more complex. In the upstream of the detecting point, the high pressure region (p′w > 0) corresponds to the sweep motion on the down-wash side of the streamwise vortex, while the low pressure region (p′w < 0) corresponds to the ejection on the up-wash side of the structure; at the detecting point, a low pressure region is formed just underneath the streamwise vortex; in the downstream, the correspondence between p′w and the sweep/ejection motion is in opposite to that in the upstream. Considering the practical requirement of turbulence control, a random blowing/suction at the wall is introduced to examine the robustness of the relationship. The results show that τwx is greatly contaminated, while p′w and τwz still exhibit an excellent correspondence with near-wall streamwise vortices.