Experiments on passive hypersonic boundary layer control using ultrasonically absorptive carbon–carbon material with random microstructure

For the first time, the influence of ultrasonically absorptive carbon–carbon material on hypersonic laminar to turbulent boundary layer transition was investigated experimentally. A 7° half-angle blunted cone with a nose radius of 2.5 mm and a total length of 1,077 mm was tested at zero angle of attack in the High Enthalpy Shock Tunnel Göttingen of the German Aerospace Center (DLR) at Mach 7.5. One-third of the metallic model surface in circumferential direction was replaced by DLR in-house manufactured ultrasonically absorptive carbon–carbon material with random microstructure for passive transition control. The remaining model surface consisted of polished steel and served as reference surface. The model was equipped with coaxial thermocouples to determine the transition location by means of surface heat flux distribution. Flush-mounted piezoelectric fast-response pressure transducers were used to measure the pressure fluctuations in the boundary layer associated with second-mode instabilities. The free-stream unit Reynolds number was varied over a range of Re _m = 1.5 × 10⁶ m⁻¹ to Re _m = 6.4 × 10⁶ m⁻¹ at a stagnation enthalpy of h ₀ ≈ 3.2 MJ/kg and a wall temperature ratio of T _w/T ₀ ≈ 0.1. The present study revealed a clear damping of the second-mode instabilities and a delay of boundary layer transition along the ultrasonically absorptive carbon–carbon insert.