Characterizing Surface Roughness Effects on Supersonic Turbulent Boundary Layers

Flight vehicles traveling at supersonic or hypersonic speeds are vulnerable to the onset of surface roughness, which can result in changes in the state of the boundary layer, ultimately affecting the performance of the vehicle. While the majority of the wetted surface area of a vehicle is relatively smooth, every vehicle will contain roughness on some level. The concept of similarity between smooth- and rough-wall flows is of great practical importance as most computational and analytical modeling tools rely on it either explicitly or implicitly in predicting flows over rough walls. While a number of important questions have yet to be answered, significant progress has been made in the understanding of flows over rough surfaces in recent years. This paper will be conducting numerical research in rough-wall-bounded turbulent flows in supersonic regimes. Wall-modeled Large Eddy Simulation (WMLES) on a flat plate with various roughness ratios will be conducted at M∞ = 2 to evaluate the boundary layer responses. These responses will be characterized in ensemble averaged mean velocity characteristics as well as turbulent intensity responses through the Reynolds Stresses. The second goal is to characterize the streamwise development of mechanical distortions in the domain. In addition, the near-wall coherent structures will be analyzed to determine the impact of roughness effects. The mean and turbulent statistics scaled by the roughness friction velocity will be compared to other results.

The Effects of Upstream Wall Roughness On the Spatio-Temporal Characteristics of Flow Separations Induced by a Forward-Facing Step

Separating and reattaching turbulent flows induced by a forward-facing step submerged in thick oncoming turbulent boundary layers (TBL) developed over smooth and rough upstream walls were investigated using time-resolved particle image velocimetry. The examined upstream walls resulted in smooth, transitionally rough and fully rough wall conditions. The upstream boundary layer thicknesses were 4.3 and 6.7 times the step height in the smooth and rough wall cases, respectively. The Reynolds number based on the step height and free-stream velocity was 7800. The effects of upstream wall roughness on the mean flow characteristics, Reynolds stresses defined in both Cartesian and curvilinear coordinate systems as well as the unsteadiness of the turbulent separation bubbles were critically examined. The results show that upstream wall roughness increases the boundary layer thickness and turbulence intensity and consequently, promotes early mean flow reattachment over the step. Distinct regions of significantly elevated vertical Reynolds normal stress and Reynolds shear stress were observed upstream of the step in the fully rough wall case compared to the smooth wall case. Proper orthogonal decomposition (POD) and the reverse flow area over the step were employed to investigate the unsteadiness of the separation bubbles. The first POD mode coefficient and the reverse flow area over the step were well correlated and exhibited the same dominant frequency.