A Floating Microsensor With Thermally Stable Grating Films for High-Temperature Wall Shear Stress Measurement

Fluid wall shear stress is a direct assessment of friction drag in turbulent boundary layers, which plays a crucial role in investigating aerodynamic optimization and drag reduction. Although optical sensors feature high sensitivities and strong immunity to electromagnetic interference, the conventional grating films undergo significant degradation at high temperatures, leading to deterioration of the sensing performance. To address the issue, this work proposes an optical wall shear stress sensor with a thermally stable high-reflectivity thin film structure, which adopts an aluminum oxide-gold-aluminum oxide (Al2O3-Au-Al2O3 ) three-layer structure. The grating film both alleviates stress concentration at the Si-Au interface and inhibits gold film agglomeration at high temperatures. A multiobjective genetic algorithm (MOGA) is employed to optimize the folded beam structure of the sensor, aimed at enhancing overall performance by balancing the sensitivity and response frequency. Based on microelectromechanical systems (MEMS) technology, sensor prototypes are fabricated for experimental verification. Under 1-kHz excitation, the sensor exhibits a dynamic wall shear stress sensitivity of 30.16 mV/Pa, and its coherence coefficient with the reference sensor reaches 0.997. Static calibration shows a sensitivity of 23.65 mV/Pa with a nonlinear error of 1.67% over a 100-Pa range. Furthermore, tests in an arc-heated wind tunnel confirm the sensor's thermal stability and reliable dynamic response at 400~° C, validating its applicability in high-enthalpy environments. This article provides a promising approach for measuring wall shear stress in high-temperature aerodynamic environments, such as aircraft engine combustion chambers and rocket engine nozzles.