Assessment and parameterization of upstream shielding effect in quasi-roughness induced transition with direct numerical simulations

Boundary layer transition control is an essential technology for developing natural laminar flow airfoils, and one of the effective strategies is shielding effect that transition induced by large roughness can be delayed by low amplitude roughness around it. In this paper, we study the shielding effect brought by a two-dimensional strip placed upstream of a discrete quasi-roughness in transonic Blasius boundary layer flow with direct numerical simulations (DNS). With a comparison between cases with or without the shielding strip, we find that the strip can decrease the roughness Reynolds number Rekby lifting up the boundary layer, and reduce the stronger left vortex in wake structure which sheds from the top of the discrete quasi-roughness. Meanwhile, the strip enhances the weaker right vortex trailing from the horseshoe vortex. We also conduct a parametric study on strip height k(up), and results show a trend that transition is gradually delayed in the left vortex, and meanwhile gets earlier in the right one with a k(up) increment in specific range. When k(up) exceeds a critical value, a reversal phenomenon is observed that transition happens much earlier, because the strip provides an excessively large disturbance which strengthens weak vortices that should have been dissipated rapidly. This work improves the understanding of shielding effect in transonic regime, and figures out the influence of shielding strip height to offer a useful reference for future studies and applications. (C) 2020 Elsevier Masson SAS. All rights reserved.