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. The proposed sensing system addresses a critically unmet measurement need in NASA’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. 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. 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 144-1215°R (80-675K). 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. 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. 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.

The proposed instrumentation technology has the potential to be transportable across multiple NASA facility classes. The target application is the NASA Aeronautics Test Program to be used as instrumentation for turbulent vector skin friction measurements within test facilities ranging from subsonic to hypersonic wind tunnels such as those at NASA Langley, Glenn, and Ames Research Centers. Potential facilities include the 0.3-M TCT, NTF, and 20-Inch Mach 6 Air Tunnel located at NASA Langley.

External customers for cryogenic/high-temperature, dual-axis shear stress measurement systems include universities, government agencies such as the Army and Air Force, and industry aircraft manufacturers such as Boeing. Customers seeking or currently designing next-generation civilian or defense aircraft have an identical unmet measurement need as NASA’s aeronautical ground-test facilities.

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