OCTRA as ultrasonically absorptive thermal protection material for hypersonic transition suppression

Abstract

Previous investigations in the High Enthalpy Shock Tunnel Göttingen (HEG) of the German Aerospace Center (DLR) show that carbon fber 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 confrmed 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 fights. Thus, an ultrasonically absorptive fber reinforced ceramic material based on a silicon carbide (C/C-SiC) was developed in the past years to fulfll 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 infuence 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° half-angle blunted cone with an overall model length of about 1.1 m and a nose tip radius of 2.5 mm. These experiments were performed at Mach 7.5 and at diferent freestream unit Reynolds numbers.