Tufts, Matthew W (2015-09). Computational Investigation of Sensitivity of the Crossflow Instability to Two-Dimensional Surface Imperfections. Doctoral Dissertation.
Thesis
The interaction between two-dimensional steps and gaps with stationary crossflow vortices on a swept laminar wing has been investigated using computational methods. Using flow conditions as experienced in complementary experiments using the SWIFTER and SWIFTEST airfoils as found in flight and as installed in the KSWT facility, the growth of stationary crossflow disturbances were calculated using direct simulation. Forward-facing steps were found to amplify stationary crossflow waves significantly once a threshold step height had been exceeded. This value was found to correlate well with experimentally observed movement of the transition front forward. A correlation based on a physical explanation of the mechanisms involved was proposed, and also was found to correlate well with the SWIFTER and SWIFTEST experimental observations. Backward-facing steps were found not to amplify stationary waves for step heights tested in the computational regime. Local stability analyses of the flowfield surrounding the backward-facing steps reveal the existence of a traveling mode similar to traveling crossflow vortices. A mechanism whereby this leads to transition to turbulence was hypothesized.
