Experimental study on drag reduction control of porous media wall turbulence

Abstract

In order to study the control law and effect of the related mechanism of porous media on the turbulent drag reduction of a plate wall, experimental research of a plate laid with porous media was carried out in a low-turbulence, re-flow wind tunnel. First, the hot wire was used to acquire the time-averaged statistics and fluctuation data inside the boundary layer on the plate surface by single point measurement. Then, the fluorescenct oil flow was applied to obtain the global skin friction distribution downstream of the porous media. Finally, through Time-resolved Particle Image Velocimetry technique, the normal velocity flow field on the surface and the structural information of the near-wall strip were captured in quantitative terms, and the Dynamic Mode Decomposition (DMD) was adopted to analyze the strip’s structural features with the mode reduction. The results showed that the drag reduction effect of porous media had a trend of first increasing and then decreasing in the flow direction, with the maximum drag reduction rate on the surface of porous media. Meanwhile, the porous media changed the internal structure of the turbulent boundary layer, reduced the velocity gradient of the viscous bottom layer, and decreased the momentum exchange between the inner and outer layers, which suppressed the development of large-scale strip structure and promoted the lift of the spanwise vortices. The results of DMD analysis further explained that the porous media could effectively weaken the strip energy fluctuation and accelerate its spatio–temporal evolution process. Moreover, the second and higher order modes could reach a stable state in shorter time.