Development of a direct-force-reading, thin-film shear stress gage

Abstract

A thin-film gage was designed, constructed, and tested for the measurement of skin friction in steady and non-steady flows. This direct-force-reading gage is designed to have a high frequency response (> 1 kHz), high temperature capability (800 K), and full directional sensitivity in both subsonic and supersonic flows. The thin-film gage consists of a floating element attached to the wall surface by four tabs. The floating element consists of a thin metal film mounted flush to the wall surface. Shear forces acting on the thin-film surface cause strain concentrations to develop in the attachment tabs where strain sensors are located. The shear forces are related to the differential output of the strain sensors located on each of the orthogonal axes of the thin-film gage. A large-scale (10X) prototype was constructed and statically calibrated for actual shear stress measurements. Subsonic, steady flow tests demonstrated the directional sensitivity capability and gave reasonable shear measurements compared to Preston tube results. Subsonic, non-steady flow tests demonstrated the high frequency response of the thin-film gage. Supersonic flow testing was performed which identified design flaws in the original design which were corrected in the development of an actual-size (1X) prototype. Survival of the thin-film gage in a supersonic environment was demonstrated with the 1X prototype. Finally, the 1X prototype was statically calibrated for measurements of shear stress in a supersonic environment.