Skin-friction drag reduction by local porous uniform blowing in spatially developing compressible turbulent boundary layers

This paper investigates the effects of local porous uniform blowing on the skin-friction drag reduction in the spatially developing compressible turbulent boundary layers through direct numerical simulations. Under the influence of uniform blowing, the skin-friction drag decreases drastically within the controlled regions, and the effect extends downstream. The drag reduction rate exceeds 10% for the uniform blowing velocities considered herein. The intrinsic mechanism of decreased skin-friction drag is further explored, in terms of the various contributions to this quantity, the Reynolds shear stress budget, and the multi-scale distributions and transfer of enhanced turbulent motions. The numerical results for the identity of Renard and Deck ["A theoretical decomposition of mean skin friction generation into physical phenomena across the boundary layer,"J. Fluid Mech. 790, 339-367 (2016)] indicate that the decreased skin-friction drag is reflected in a negative contribution in the streamwise developing direction. The enhanced turbulent motion is investigated through the Reynolds shear stress budget, and uniform blowing is found to strengthen the magnitudes of all budget terms. We conclude that uniform blowing promotes the energy cascade process in the near-wall region, promoting the appearance of smaller turbulent structures and the formation of large outer scales. Similar skin-friction drag reduction in the subsonic case is also observed in a supersonic case. The upstream transition process is delayed, which leads to a relatively larger skin-friction drag reduction rate in a supersonic case. Finally, we investigate the porosity effects on the skin-friction drag and conclude that the blowing flux is a decisive factor.