Measurement of wall shear stress in turbulent boundary layers subject to strong pressure gradients

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Abstract
Measurements of the wall shear stress with a floating element and with Preston tubes were conducted in turbulent boundary layers. Sudden application and removal of adverse pressure gradients resulted in boundary layers far from equilibrium. Positive and negative errors of the Preston-tube results were observed for adverse pressure gradients. The negative errors occurred mainly in regions with dτw/dx > 0. The relation between the error, the pressure gradient and the tube size (1.1) suggested by Frei & Thomann (1980) predicts only positive errors for dp/dx > 0. Therefore, it cannot be used for the present pressure distributions and is not as general as was expected. The present results show that indirect methods to determine the wall shear stress should not extend beyond y+ - 3 if accuracies of ± 1% are required for pressure distributions similar to the ones used in the present investigation. Predictions from Ludwieg & Tillmann's relation (4.21) agree to within ± 10% with the present measurements. The Preston-tube readings indicate velocities below the law of the wall in regions with a decreasing adverse pressure gradient. No local parameters could be found that correlated the errors of the Preston-tube results for the large pressure gradients used in the present investigation.