Real-Time Measurement of Skin Friction Using Calibrated Multi-Element Hotfilm Arrays
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This paper describes a general procedure for real-time calibrated measurement of skin friction using hotfilm sensors controlled by a constant voltage anemometer (CVA). Hot-films are sensitive to air and substrate temperature and compensating for temperature dependence requires in situ estimates of heated and unheated hotfilm resistance. This can be done by rapidly alternating between overheat ratios. To verify the usefulness of this approach, a CVA was constructed to make in situ estimates of conductive dissipation into the substrate. Following previous work, the analysis approach assumes the air and substrate are in thermal equilibrium. Wind tunnel tests were conducted within a 34 foot wind tunnel at Texas A&M University. Hotfilms were installed along the chord of a NACA 0018 airfoil and calibrated to measure shear stress. The validity of the calibration approach using multiple overheats was tested for changing air and substrate temperature over 12C ranges. The calibration remains valid over temperature changes of this magnitude when the air and model remain in thermal equilibrium. However, when the air and substrate were not in thermal equilibrium over this temperature range the calibration failed. Future work must consider a more general formulation that accommodates unequal air and substrate temperatures and avoids electronic noise. 2012 by Robert R. Long and Edward B. White. Published by the American Institute of Aeronautics and Astronautics, Inc.
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28th Aerodynamic Measurement Technology, Ground Testing, and Flight Testing Conference
