A Shear-Stress Sensor for Hypersonic Flow Measurement

A shear-stress or skin-friction sensor for hypersonic flow measurements is presented. The sensor measures the displacement of a spring-mounted rectangular plate using a differential capacitance-based detection scheme. The 1.8-mm × 1.8-mm sensor is fabricated using a silicon-on-insulator wafer with a 20-μm-thick epitaxial layer and packaged into a cylindrical form-factor with a 1/4-in diameter. Wall-shear-stress measurements are performed at a Mach-2.3 supersonic wind-tunnel facility at The University of Arizona. The sensors observe several features anticipated from hypersonic/supersonic flow physics, including the passing of normal shocks and the reversal of measured shear polarity when flow separation is deliberately induced via the use of a compression ramp. Furthermore, the measured value for dc wall-shear stress agrees with the model predictions for fully developed turbulent boundary layers.