Capacitive Vector Skin Friction Measurement Systems for Complex Flow Fields

The Interdisciplinary Consulting Corporation (IC2) proposes to continue development of dual-axis shear stress sensors that are applicable in ground test facilities covering a large range of flow speeds in response to NASA SBIR 2018 Phase I solicitation subtopic A1.08. The proposed sensing system addresses a critically unmet measurement need in NASArsquo;s technology portfolio, specifically the ability to make time-resolved, continuous, direct, two-dimensional measurements of mean and fluctuating wall shear stress in wall-bounded turbulent and transitional flows in subsonic and transonic facilities. The realization of this capability not only benefits advanced air vehicle development but also impacts fundamental compressible boundary layer physics research areas such as transition to turbulence in three-dimensional flows, extending the current capabilities of NASArsquo;s ground test facilities.The proposed innovation is a dual-axis, instrumentation-grade, robust, high-bandwidth, high-resolution, silicon micromachined differential capacitive shear stress sensor for subsonic and transonic applications. The sensor system will enable localized, non-intrusive, vector measurement of mean and fluctuating wall shear stress for characterization of complex boundary-layer flows in ground-test facilities. The differential capacitive measurement scheme offers high sensitivity to in-plane shear stress as well as common-mode rejection of pressure fluctuations. Two sets of differential capacitors provide shear stress measurement capability in two orthogonal directions to provide the wall shear stress vector. Backside electrical contacts using IC2rsquo;s patent-pending fabrication and packaging process enable the sensor to remain flush with the test article surface while significantly reducing fabrication complexity and cost. The modeling aspects of the proposed design approach facilitate design optimization for various applications and flow conditions.