Surface shear stress measurement system for boundary layer flow over a salt playa
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abstract
A novel method for measuring shear stress via a floating surface element beneath a turbulent boundary layer has been tested and implemented. The final design is used for measuring the surface shear stress in the boundary layer that forms over the salt playa of western Utah. Wind tunnel model studies were conducted to establish the characteristics of two competing sub-designs, and to determine their sensitivities and potential error sources. The wind tunnel based experiments included independent comparisons with the results of wall velocity gradient based measurements using hot-wire data. Overall, the results show that a floating element on a pool of liquid, here water, provides a more accurately measured surface shear stress than that determined utilizing an air bearing. In addition, the water pool technique is simpler to implement and produces a higher degree of repeatability. The wind tunnel results provided the design criteria needed to build a larger-scale device used to measure the surface shear stresses over the Great Salt Lake Desert in western Utah. This final design has been implemented and field results are presented and compared with sonic anemometer based estimates of the wall shear stress.
