Experimental Study of Error Sources in Skin-Friction Balance Measurements

Abstract

An experimental study has been performed to determine potential error sources in skin-friction balance measurements. A floating-element balance, large enough to contain the instrumentation needed to systematically investigate these error sources has been constructed and tested in the thick turbulent boundary layer on the sidewall of a large supersonic wind tunnel. Test variables include element-to-case misalignment, gap size, and Reynolds number. The effects of these variables on the friction, lip, and normal forces have been analyzed. Results indicate that some of the intuitive assumptions generally held by designers and users of skin-friction balances may not be valid. For example, it was found that larger gap sizes were preferable to smaller ones; that small element recession below the surrounding test surface produced errors comparable to the same amount of protrusion above the test surface; and that normal forces on the element were, in some cases, large compared to the friction force. The principle contributions of this paper are (1) that users of skin-friction balances can use the data contained in this paper to estimate the errors involved in their measurements, and to make corrections if necessary, and (2) that the results of this study may lead to the development of new and improved devices for direct skin-friction measurement.