Development of a direct measurement device for the local wall shear stress in boundary layer flows

Abstract

A new device for direct measurement of the local wall shear stress in boundary layers has been developed with a square friction surface and higher accuracy. The uncertainties were estimated from experiments in boundary layers at moderate Reynolds numbers. The analysis states that the main sources of uncertainty are temperature variation, gap size, and misalignment. The expected errors or uncertainties are represented as a function of gap size and misalignment. The use of the material Super Invar in the force sensor considerably reduces the uncertainty due to temperature variation by a factor of 6. Regarding the direct measurement of the local shear stress in a flat-plate turbulent boundary layer, there are no differences in the skin-friction coefficient measured by circular and square shaped floating elements. In this experiment, the maximum relative standard uncertainty of the developed direct measurement device is 0.65% for the local skin friction coefficient.