Fiber-Optic Vector Skin Friction Systems for Cryogenic Shear Stress Measurements

The Interdisciplinary Consulting Corporation (IC2) proposes to develop dual-axis shear stress sensors that are applicable in a variety of environmental conditions such as those encountered in high-Reynolds number and high-speed ground-test facilities in response to NASA SBIR 2020 Phase I solicitation subtopic A1.08: Aeronautics Ground Test and Measurements Technologies.nbsp;nbsp;The proposed sensing system addresses a critically unmet measurement need in NASArsquo;s technology portfolio, specifically the ability to make time-resolved, continuous, direct, vector measurements of mean and fluctuating wall shear stress in high-Reynolds number, cryogenic transonic facilities as well as high-temperature supersonic and hypersonic wind tunnels.nbsp;nbsp;The proposed innovation is a dual-axis, instrumentation-grade, robust, high-bandwidth, high-resolution, micromachined optical shear stress sensor with a remote photodiode/fiber-optic array readout capable of operation in both low- and high-temperature environments.nbsp; The sensor system will enable localized vector measurement of the wall shear stress for characterization of complex boundary layer flows in ground-test facilities with temperatures ranging from 144-1215deg;R (80-675K).nbsp; The proposed dual-axis shear stress sensor consists of a floating element with optical gratings on the backside and on the top surface of a support substrate to permit backside optical transduction. This design represents a robust, flush-mounted, miniature, direct wall shear stress sensing system that possesses immunity from electromagnetic interference (EMI) and minimal sensitivity to normal pressure fluctuations and/or vibrations.nbsp;nbsp;Optical transduction of the floating element motion in two orthogonal directions is achieved by imaging the patterned optical gratings via a custom optical fiber array.nbsp; This fiber array is attached to a photodiode array on the distal end, allowing the electronics to be located away from the extreme temperatures at the measurement surface.