Interaction of Tollmien-Schlichting waves with Forward-Facing Steps

Abstract

Aerodynamic surfaces are subject to several spanwise-invariant irregularities which can promote early transition and a consequent increase in skin friction. A common surface irregularity is panel joints, which are usually preferred in a Forward-Facing configuration after several studies proved that Backward-Facing steps have more detrimental effects on transition. Nevertheless, how a Forward-Facing Step (FFS) locally interacts with incoming Tollmien-Schlichting (TS) waves remains partially unknown. Prior investigations focused on establishing semi-empirical laws to describe the onset of transition based on several step parameters. However, recent numerical results signal important flow features at the step location, which might play a role in the subsequent growth of TS waves. The present study concerns the close-examination of TS waves interaction with an FFS by means of experiments performed at TU Delft and Direct Numerical Simulations (DNS) conducted by DLR. Experimentally, the localized effect of the step on the incoming instabilities is determined using phase-locked Planar Particle Image Velocimetry (2C-2DPIV) and Hot-Wire Anemometry (HWA) measurements. In the workshop, the results for subcritical, critical and supercritical steps will be presented to show how the topological differences at the step are related to the explosive growth of TS waves downstream of the FFS. Additionally, Power Spectral Density results from HWA measurements will be included to track the changes in the disturbance energy distribution along the frequency domain as the TS waves encounter the step.