Numerical computations of hypersonic boundary layer roughness induced transition on a flat plate Gennaro Serino * , Fabio Pinna † and Patrick Rambaud ‡ von Karman Institute for Fluid Dynamics, Chausse de Waterloo 72, B-1640 Rhode-St-Genese, Belgium The work is focused on numerical simulations of roughness induced transition for hypersonic flow on a flat plate wall mounted roughness element. Numerical simulations are compared to experimental results in order to resemble the physics highlighted in the tests. In particular, stress has been placed on the detection of the vortices in the wake behind the roughness element and on the onset of transition. I. Introduction Since the experiment of Reynolds in 1883, the scientific community has demonstrated a great interest in transition to turbulence due to its influence on crucial fluid dynamic quantities such as drag and heat trans- fer. In high subsonic conditions, keeping the flow laminar on a commercial aircraft wing means reduction of drag and fuel consumption with substantial cost savings. In addition, higher heat flux in case of turbulent flow makes transition prediction crucial for the survival of the vehicle and of the crew during a re-entry at hypersonic speed. The mechanism of transition is influenced by several parameters which can be related both to free stream conditions, as the noise or turbulence level, and to the body itself, as the presence of surface irregularities or vibrations. Linked to the originating cause of disturbances, there are several path which actually leads to transition (Reshotko 1 ). For small enough disturbances we usually observe natural transition through differ- ent well defined stages (White 2 ). When initial disturbances entrained in the boundary layer are sufficiently strong, all these stages could be by-passed leading to a much quicker transition to turbulence. The latter case is defined by Schlichting 3 as bypass transition as opposed to the mechanism of linear amplification of unstable waves (Morkovin 4 ) which characterizes natural transition. When transition is caused by surface irregularities, it is defined as Roughness Induced Transition (hereafter RIT) and it plays a crucial role in space applications as, during re-entry, misaligned tiles or undesired gaps may promote turbulent flows with a following increase in heat flux and an abnormal wear of the heat shield. The understanding of the physics of RIT has sensibly improved in these years thanks to numerous exper- imental investigations as those reviewed by Ergin and White. 5 In many of the cases observed by them, around an isolated three dimensional roughness element, the flow showed similar and repetitive structures. Upstream of the obstacle (a cylinder or a diamond roughness element), a steady horseshoe vortex was ob- served going around the element and wrapping it while, on the lateral parts, two steady counter-rotating legs were observed. The steady vortices (first generation ) rapidly evolved downstream into streaks of low or high intensity whose footprints can be obtained with oil-sublimation or infrared thermography visualization techniques. The transition location is generally recognized in experiments where the turbulent wake starts to widen with an half angle of 10 ◦ respect to the flow direction as it can be seen in Fig. 1. Finally, when the Reynolds number is sufficiently high, as in hypersonic conditions, unsteady vortices (secondary generation ) originate from the separation region in the back of the roughness element accelerating the transition mech- anism. In parallel with experiments, numerous efforts have been put in Computational Fluid Dynamics (hereafter CFD) in order to develop a valid transitional model capable of reproducing the physics of RIT. The impor- tance of this numerical tool relies in the possibility to predict transition and its effect, as the increase in the * PhD Candidate, Aerospace Department, gennaro.serino@vki.ac.be, AIAA Student Membership † PhD Candidate, Aerospace Department, fabio.pinna@vki.ac.be ‡ Assistant Professor, Aerospace Department, patrick.rambaud@vki.ac.be 1 of 16 American Institute of Aeronautics and Astronautics