Comparison of four methods to measure wall shear stress in a turbulent boundary layer with separation

Three indirect methods of measuring skin friction (Preston tube, sublayer fence, and wall pulsed wire) were compared with a direct method (floating-element balance) in a boundary layer with a strong adverse pressure gradient. In one case skin-friction measurements were performed in a boundary layer with separation and reattachment. Agreement between the three methods and the balance was satisfactory, with differences between the indirect methods and the balance reading high in regions with a strong decrease of τ w and low where τ w increased in streamwise direction. There existed a tendency of the differences to increase with increasing h+, but no general correlation with h+ could be found.