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Previous investigations in the High Enthalpy Shock Tunnel Göttingen (HEG) of the German Aerospace Center (DLR) show that carbon fiber reinforced carbon ceramic (C/C) surfaces can be utilized to damp hypersonic boundary layer instabilities resulting in a delay of boundary layer transition onset. Numerical stability analyses confirmed these experimental results. However, C/C has some disadvantages, especially the limited oxidation resistance and its low mechanical strength, which could be critical during hypersonic flights. Thus, an ultrasonically absorptive fiber reinforced ceramic material based on a silicon carbide (C/C-SiC) was developed in the past years to fulfill this need. The present paper addresses the numerical rebuilding of the C/C-SiC absorber properties using impedance boundary conditions together with linear stability analysis. The focus of this paper is on the numerical comparison of the original C/C material and the improved C/C-SiC material, referred to as OCTRA in the literature. The influence on the second modes and the transition itself is investigated. The numerical results are compared with HEG wind tunnel tests. The wind tunnel model tested in HEG is a $$7^\circ$$ 7 ∘ half-angle blunted cone with an overall model length of about $$1.1 \,\textrm{m}$$ 1.1 m and a nose tip radius of 2.5 mm. These experiments were performed at Mach 7.5 and at different freestream unit Reynolds numbers.

Three-dimensional shock wave reflection comprises flow physics that is significantly different from the well-documented two-dimensional cases in a number of aspects. The most important differentiating factor is the sweep of the shock system. In particular, this work examines the nature of flow fields in which there is a transition of shock reflection configuration in three-dimensional space. The flow fields investigated have been made to exist in the absence of edge effects influencing the shock interaction and transition, as found previously to exist in conventional double-wedge studies. In general, the shock configurations are those with central regular and peripheral Mach reflection portions. It is shown that the sweep angle of the portions on either side of the transition point is subject to a cusp, as per an analytical model that is developed. This is confirmed with the use of numerical models with additional evidence provided by experimental results using oblique shadow photography. Further application of the principles of three-dimensional shock analysis and those pertaining to the sweep cusp model yield important insights regarding the overall shock geometry and that at transition.

Given the challenges of obtaining a natural turbulent boundary layer on common test model geometries in a shock tunnel, this work aims to investigate the influence of roughness elements on the boundary layer with respect to transition to turbulence. The experiments were conducted in the High Enthalpy Shock Tunnel Göttingen at the German Aerospace Center on a 1100-mm-long, 7°-half-angle cone and a 602-mm-long flat plate at Mach 7.4. Roughness elements were applied on the nosetip of the cone and near the leading edge of the flat plate. The roughness elements were scanned with a laser profilometer, allowing their specification in terms of a roughness Reynolds number based on the 70th-percentile element height and an exceedance probability distribution. Transition was examined for the cone geometry using streamwise-aligned coaxial thermocouples on the 0° meridian. This assisted sizing the roughness elements required for transition to occur as far upstream as detectable. Breakdown of roughness-induced vortical structures generated by the roughness elements with a similar roughness Reynolds number was then examined using the flat plate geometry with temperature-sensitive paint applied downstream of the roughness elements. It was found that roughness-induced vortices required a finite distance (persistence length) to break down into turbulent structures. The persistence length was successfully reduced by interspersing roughness elements with smaller ones.

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