103rd NIA CFD Seminar: Topic: Study of High-Speed Transition due to Roughness Elements

Speaker: Prakash Shrestha Speaker Bio: Prakash Shrestha is a doctoral candidate in the Department of Aerospace Engineering and Mechanics at University of Minnesota-Twin Cities. Currently, he is working at National Institute of Aerospace (NIA) as a summer visiting student with Scott Berry, NASA Langley Research Center (LaRC), in high-speed transition due to wall-injectors. His research interests include boundary-layer stability, transition to turbulence, modal analysis, complex grid-generation, supersonics, and hypersonics. Abstract: Transitional hypersonic boundary layers due to diamond-shaped and cylindrical roughness elements (passive tripping) are studied using direct numerical simulations (DNS). A low Reynolds number experiment, consisting of an array of diamond-shaped roughness elements (Semper & Bowersox 2017), and a high Reynolds number experiment, consisting of an array of cylindrical roughness elements (Williams et al. 2018), are used to validate our simulations. Three dynamically prominent flow structures are consistently observed in both arrays as well as in their respective isolated roughness configurations. These flow structures are the upstream vortex system, the shock system, and the shear layers and the counter-rotating streamwise vortices from the wake of the roughness elements. Analysis of the power spectral density (PSD) reveals the dominant source of instability due to the diamond-shaped roughness elements as a coupled system of the shear layers and the counter-rotating streamwise vortices irrespective of spanwise roughness-spacing (isolated roughness and roughness-array). However, the dominant source of instability due to the cylindrical roughness elements is observed to be the upstream vortex system irrespective of spanwise roughness-spacing. Therefore, the shape of a roughness element plays an important role in the instability mechanism. Furthermore, dynamic mode decomposition (DMD) of three-dimensional snapshots of pressure fluctuations unveil globally dominant modes consistent with the PSD analysis in all the roughness configurations.​ Additional information, including the webcast link, can be found at the NIA CFD Seminar website: http://www.hiroakinishikawa.com/niacfds/index.html