Computational Investigation of Step Excrescence Sensitivity in a Swept-Wing Boundary Layer
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2015, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. The interaction between an existing stationary crossflow instability and a two-dimensional (2-D) step excrescence placed into a swept boundary layer on a lifting surface has been investigated computationally. By calculating the flowfield surrounding a step excrescence using boundary conditions taken from spatially larger simulations and highly resolved grids, the development of stationary disturbances can be tracked. 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. The observed experimental and computational results are consistent with breakdown caused by nonlinear interactions between stationary crossflow modes and this additional traveling mode. Further time-resolved calculations are necessary 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 was proposed for that critical height which agrees very well with the geometry and Reynolds numbers tested experimentally. Critical forward-facing step heights are in the majority of cases higher than those associated with backward-facing steps for the same conditions. Moreover, the physics associated with 2-D excrescences in swept-wing flow is fundamentally different from that associated with other types of roughness as well as all roughnesses in 2-D flowfields. Upon further validation, the proposed correlation for forward-facing steps will provide a reasonable manufacturing criteria available to airframe designers that will prove less restrictive and more accurate than existing criterion.
