Hypersonic Transition in High Enthalpy Facilities

The present manuscript supports a lecture given in the scope of the VKI Lecture Series ’Multiphysics phenomena analysis on boundary layer stability in hypersonic regime’. It is limited to hypersonic transition phenomena in hypersonic shock tunnels at low enthalpy conditions. The manuscript provides a brief overview over existing high enthalpy shock tunnels and discusses the role of such facilities with respect to hypersonic boundary layer transition research. The operating principle of a typical shock tunnel is explained based on the High Enthalpy Shock Tunnel G¨ottingen (HEG). Furthermore, techniques to assess the free-stream disturbances in the challenging test environment of a shock tunnels are discussed and results obtained by means of a wedge shaped probe a presented. Moreover, transition studies on the second mode instability in HEG are introduced. The presence of the second mode and its dominance in the transition process on a cone allows to study transition control strategies targeting the suppression of the second mode. A passive control strategy applying ultrasonically absorptive carbon matrix composites led to promising results and encourages the development of the next generation of, high temperature resistant, porous ceramics.