Computational Investigation of Step Excrescence Sensitivity in a Swept-Wing Boundary Layer

Abstract

The interaction between an existing stationary crossflow instability and a two-dimensional step excrescence placed into a swept boundary layer on a lifting surface has been investigated computationally. Backward-facing steps were found to not amplify the stationary modes. The existence of an additional local traveling instability was found in the recirculation region following the step. Further time-resolved calculations were recommended to confirm this mode as the cause of breakdown. Forward-facing steps were found to affect the transition location only once a critical step height was exceeded. This height was associated with sudden amplification of stationary crossflow modes and transition moving forward to the step. A physics-based correlation based on linear stability theory and associated with the height of the core of the stationary crossflow vortex was proposed for that critical height, which agreed well with results for the geometry and Reynolds numbers tested experimentally. Critical forward-facing step heights were in the majority of cases higher than those associated with backward-facing steps for the same conditions. Moreover, the physics associated with two-dimensional excrescences in swept-wing flow was fundamentally different from that associated with other types of roughness, as well as all roughnesses in two-dimensional flowfields. Upon further validation, the proposed correlation for forward-facing steps would provide a reasonable manufacturing criteria available to airframe designers that would prove less restrictive and more accurate than existing empirical criteria.