Surface shear stress fluctuations in turbulent boundary layers
Abstract
Evaluation of the timedependent surface shear stress in turbulent boundary layers indicates the fluctuations are equal to or greater than the mean shear. Measurements of the probability distribution of the surface shear fluctuations show highly skewed fluctuations even for the zeropressuregradient boundary layer. For both zero and adverse pressure gradients large positive surface shear pulses are observed. The rootmeansquare of the shear fluctuations were found to be 0.3 times the mean value for the zeropressuregradient flow. For the adverse pressure gradient flows the positive pulses of surface shear may be as great as four times the mean value. The most probable value of shear was found to be less than the mean value of the surface shear. Measurements of the fluctuating surface shear stress in the separation region are still lacking due to instrumentation difficulties. However, examination of limited data on velocity fluctuations near the surface is used to infer that the surface shear stress fluctuations will be similar to those observed in the nonseparating regions. It is proposed that the region identified as intermittent (transitory) separation or detachment may well be the location where the most probable value of the surface shear is zero.
 Publication:

2nd Symposium on Turbulent Shear Flows
 Pub Date:
 1979
 Bibcode:
 1979stsf.procQ...4S
 Keywords:

 Open Channel Flow;
 Shear Stress;
 Surface Properties;
 Turbulent Boundary Layer;
 HotWire Anemometers;
 Pressure Gradients;
 Separated Flow;
 Fluid Mechanics and Heat Transfer