Impacts of rough surface location and skewness on laminar-turbulent transition with pressure gradient
Phys. Rev. Fluids 10, 073903 – Published 23 July, 2025
DOI: https://doi.org/10.1103/qkw2-vwct
Abstract
Here we employ direct numerical simulation to examine the transitional flow characteristics influenced by a three-dimensional rough surface and pressure gradient. Three types of skewness are designed for the rough surface, strategically positioned upstream of the laminar separation bubble (LSB), adjacent to the separation point, and within the bubble itself, aiming to elucidate the disparities in the transition mechanisms beneath varying skewness and surface locations, as well as the underlying reasons. The findings indicate that the frictional resistance associated with positive skewness exceeds that of negative skewness, and the surface force is more sensitive to negative skewness than to positive skewness. These are independent of the rough surface's location relative to the LSB. Negative skewness not only enhances the continuity of the transverse secondary flow, but also amplifies the spatial heterogeneity in streamwise and wall-normal directions when the rough surface is positioned in proximity to the separation point. The underlying mechanism is an increase in the velocity gradient within the free shear layer (FSL), which provokes spanwise rollers at the LSB's trailing edge and fosters the growth of inviscid instability. Conversely, positive skewness has a contrasting impact. Provided the overall height of the rough surface does not transcend the FSL, positioning the rough surface within the LSB markedly extends the LSB and postpones the downstream onset of transition, a phenomenon unaffected by skewness or the flow characteristics over the rough surface. Within the context of linear dynamic approximation, rough surfaces located near the separation point may augment the intensity of disturbances, yet the degree of quasiorthogonality in the modes is inferior to that observed with rough surfaces situated ahead of the separation point. This aligns with the resonant disturbance induced by positive skewness, essentially indicating that convective instability is mitigated by backflow.
