Effect of surface roughness element on near wall turbulence with zero-pressure gradient

Abstract

In the present paper, we present an investigation on the effect of roughness elements onto near-wall kinematics of a zeropressure- gradient turbulent boundary layer. An array of spanwisely-aligned cylindrical roughness elements was attached to the wall surface to regulate the near-wall low-speed streaky structures. With both qualitative visualization and quantitative measurement, we found that the regularization only occurs in the region below the height of the roughness elements. Statistical analysis on the probability distribution of the streak spanwise spacing showed that the mean spanwise streak spacing is dominated by the roughness elements; however, the latter’s effect is in competition with the intrinsic streak generation mechanisms of smooth wall turbulence. Below the top of the roughness elements, local streamwise turbulent fluctuation intensity can be reduced by about 10%. We used POD analysis to depict such regularization effect in terms of near-wall structure modulation. We further found that if the spanwise spacing of roughness elements increased to be larger than the mean streak spacing in the smooth wall turbulence, there is no streak-regularization effect in the buffer region, so that the near-wall streamwise turbulent fluctuation intensity doesn’t reduce.