Development of a floating element wall shear transducer

A highly linear passive floating element wall shear transducer has been developed which uses a sensitive frequency modulation technique to measure a capacitance change proportional to the displacement of the floating element for small deflections. The floating element is 4mm square surrounded by a gap nominally 0.18mm and protrudes into the boundary layer 9 micrometer. Scaled on inner parameters, each side of the sensing head measures about 290, the mean gap width is 13, and the element protrusion is 0.7. The transducer was tested in a turbulent boundary layer on a flat plate at momentum thickness Reynolds numbers of 3140, 3500, and 3760 and the results compared to wall shear stress estimates obtained from the law of the wall. The transducer was found to predict the steady mean wall shear stress within the limits of experimental uncertainty in all cases; transducer measurements of mean wall shear stress for momentum thickness Reynolds numbers of 3140 and 3500 were within 2 percent of the actual value. Transducer measurement of the unsteady mean wall shear stress was severely limited by a mechanical resonance in the transducer at 63 Hz. The operating principles and general characteristics of the transducer are discussed, its performance is evaluated, and recommendations are made concerning future work.