Skin Friction Sensor Development, Validation, and Application for High-Speed, High-Enthalpy Flow Conditions

This investigation concerns design and testing of skin friction sensors for high-speed, high-enthalpy conditions, such as re-entry vehicles, scramjets, jet engines, material testing, etc. Understanding these flows requires numerical and analytical modeling and reliable instrumentation. The sensor design is a direct measuring, non-nulling, cantilever beam arrangement. A multistep program tested the sensor through various facilities, culminating in simulated scramjet flight conditions. The first phase concerned calibrations for static, dynamic, pressure, thermal, and vibratory responses and electromagnetic interference. This phase concluded under Mach 3 conditions: stagnation temperatures and pressures of 300-655K and 750-1000 kPa. Wall shear was measured as 132-248 Pa for skin friction coefficients 0.0010-0.0014. The measurements demonstrated agreement with flat-plate analytical estimations, numerical predictions, and historical test results. Total uncertainty was +/- 8.7% at 95% confidence. The second phase involved testing at Air Force scramjet facilities. Mach 2.0-2.2 flows were studied at stagnation pressures and temperatures (172-995 kPa and 294-1000 K). The sensors determined wall shear measurements (94-600 Pa) for skin friction coefficients (0.0016-0.0028). The sensors clearly indicated transient flow of a scramjet operability cycle with shock train movement and separation zones. The sensor demonstrated full functionality under sustained high-enthalpy flow conditions upward of 1053 kJ/kg.