OCTRA as Ultrasonically Absorptive Thermal Protection Material for Hypersonic Transition Suppression analyzed on a 7° Cone

The increase of the laminar portion of a boundary layer is of critical importance to the design and optimization of future hypersonic transport vehicles. This motivates the development of concepts to control hypersonic boundary layer transition. In the present paper an ultrasonically absorptive porous coating with random microstructure is used to passively control boundary layer transition. The second mode instability, commonly referred to as Mack mode [1], is the dominant mode for essentially 2D boundary layers at high local Mach number (Me > 4) and/or cold walls. A strong stabilization effect of the second mode instability above porous surface models with regular, cylindrical pores was shown theoretically and experimentally by Fedorov et al. [2] and Rasheed et al. [3]. Analogous results were presented by e.g. Lukashevich et al. [4], who investigated randomly structured felt metal. First studies with randomly structured carbon-carbon ceramic (C/C) were conducted by Wagner et al. [5] in the HEG and were compared with numerical linear stability theory, short LST, predictions by Wartemann et al. [6]. In all cases a stabilization effect on the second mode instability was observed, resulting in a significant delay of transition onset. The starting material C/C (Wagner et al. [5]) has two main disadvantages: its limited oxidation resistance and the low mechanical strength, which could be critical for real hypersonic flight applications. Carbon fiber reinforced silicon carbide (C/C-SiC) is highly suitable as thermal protection material (TPS) and was successfully tested as TPS during multiple flight test programs (see e.g. Weihs et al. [7]). This dense C/C-SiC TPS material was not developed for the application as an acoustic absorber. To close the gap between the porous C/C and the dense C/C-SiC, a new material based on C/C-SiC was developed [8]. Contrary to the dense C/C-SiC materials, the new C/C-SiC material, also known as OCTRA (Optimized Ceramic for Hypersonic Applications), has a porosity and permeability like C/C. The present paper addresses the numerical rebuilding of the OCTRA absorber behavior using an impedance boundary conditions together with linear stability analysis. The numerical results are compared with wind tunnel tests, which were performed in the HEG at Mach 7.5 and different unit Reynolds numbers. A 7° half-angle cone model with a nose radius of 2.5 mm and a total length of about 1.1 m was used. The measurements are compared with the numerical calculations of the original C/C material and the improved OCTRA material. The influence on the second modes and the transition itself are investigated