Modulation of Near-Wall Turbulence by Textured Surfaces for Drag Control

Surface texturing is a promising way to passively control near-wall flow features, to achieve desirable drag characteristics. In this work, we explore the effects of staggered three-dimensional cubical elements placed in a turbulent boundary layer (TBL) through direct numerical simulations (DNS). A finite length of textured patch is utilized which is periodic in the spanwise direction, and which has a staggered arrangement of roughness elements in successive rows, in the streamwise direction. Parametric sensitivity is addressed by varying the element height and by placing the element strip at two different streamwise locations, altogether varying the roughness Reynolds number from k+ ≈ 19 to k+ ≈ 47. The chosen k+ values fall within the transitionally rough regime, with an “intermediate-type” aspect ratio, creating a unique combination that is previously unexplored. Compared with the baseline smooth-wall case, the roughness elements alter the near-wall velocity distribution, and the deviation broadens with roughness height. In the near wake zone, incorporation of surface texturing alters the baseline velocity profile with a prominent effect in the log region. Consistent with previous literature, the induction of higher hairpin vortices induces a low-speed region close to the wall, and these differences decay at downstream locations. A detailed analysis is carried out to quantify the separation and recirculation regions behind the roughness elements, which are significant in dictating the stability characteristics. Further, the changes in the TBL characteristics are quantified through a Reynolds stress analysis, which identifies a second peak close to the boundary layer edge associated with hairpin vortices induced by the roughness element. Also, in the surface texture wake, the quadrant decomposition of the Reynolds shear stress term identifies intensified sweep and ejection motions due to modulation in near-wall streak generation mechanism. Finally, the impact of surface texture on the turbulent viscous force is estimated by comparing the streamwise variation of skin friction drag coefficient, which shows a reduction of almost 11% in the highest k+. In comparison, a reduction of around 4% is observed for lower k+ cases.