Center for Turbulence Research Proceedings of the Summer Program 2010 1 Hypersonic flows with discrete oblique surface roughness and their stability properties By G. Groskopf † , M. J. Kloker † , K. A. Stephani ‡ , O. Marxen AND G. Iaccarino Laminar hypersonic boundary-layer flows deformed by an asymmetrically placed dis- crete surface roughness about half the boundary-layer thickness high are investigated applying the compressible bi-global linear stability theory (B-LST) in flow crossplanes. Non-perfect gas properties such as variable chemical composition, heat capacity, ther- mal conductivity, or thermal energy relaxation are included. The steady base flows are extracted from spatial direct numerical simulations (DNS) for reacting and non-reacting air, accounting for chemical as well as thermal nonequilibrium. Rarefaction effects are considered based on direct simulation Monte Carlo (DSMC). Roughness setup and flow configuration follow the STS-119 flight experiment with an obliquely placed fence-type trip element. A cold-flow non-reacting gas case and a hot-flow reacting gas case are con- sidered. Local and integral growth of instabilities in the wake of the roughness element is compared for cold and hot hypersonic flow. 1. Introduction For atmospheric (re-)entry as well as sustained hypersonic flight, drag and heat load of the vehicle significantly depend on the state of the boundary layer. Therefore, it is essen- tial to understand the causes for the transition of the boundary layer from laminar flow to turbulence. Nowadays emphasis is put on the influence of discrete three-dimensional roughness elements on transition. In addition to the experimental approach, see, e.g., the STS-119 flight experiment or the work of Casper et al. (2008), there is the possibility of direct numerical simulation, see, e.g., Marxen & Iaccarino (2008), Birrer et al. (2008), and Stephani et al. (2010), or a theoretical analysis of the flow, see, e.g., Choudhari et al. (2008). The latter method aims at investigating the disturbance growth, whereas the former often delivers only steady base flows without simulation of instabilities because this is more demanding. Groskopf et al. (2008) investigated the influence of discrete three-dimensional rough- ness in a symmetric setup at wind tunnel conditions by applying their B-LST code BIGSTAB for two-dimensional eigenfunctions in crossplanes of the flow. The wall-normal and spanwise shear layers induce enhanced growth of first-mode boundary-layer insta- bilities, which results in earlier transition of the flow. One of the identified unstable eigenmodes was convincingly confirmed by an unsteady DNS, see Groskopf et al. (2010). The horseshoe vortices play only a minor role for the investigated cases with a roughness height of half the undisturbed boundary-layer thickness, in accordance with the results of Bartkowicz et al. (2010). Horseshoe vortices cause bypass transition for element heights in the order of or greater than the undisturbed boundary-layer thickness, whereas the flow is subject to enhanced instability for lower heights. † Institut f¨ ur Aerodynamik und Gasdynamik, Universit¨at Stuttgart, Stuttgart, Germany ‡ Department of Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, USA