Abstract
A turbulent boundary layer undergoing smooth-wall separation via an adverse pressure gradient was investigated using a novel combination of laser-Doppler anemometry and pulsed-wire anemometry to simultaneously measure instantaneous wall shear stress along with the velocity fluctuations immediately above. The aim of this experiment was to obtain the degree of correlation of instantaneous wall shear stress with fluctuations of velocity at various distances from the surface as well as from the time-averaged location of flow separation. It was found that there is no correlation of the wall shear stress with wall-normal velocity fluctuations while there is a relatively strong correlation with the tangential velocity. This trend held both upstream of separation and into the recirculation region. Time-lagged correlations of wall shear stress and tangential velocity revealed that the velocity fluctuations in the outer region always lead the wall shear stress, lending support to the concept that the near wall dynamics are primarily driven by large scale outer motions, independent of the mean profiles. A simple model involving an oscillating external pressure gradient forcing of an unsteady viscous sublayer was developed to elucidate basic physics underlying the observed correlation between wall shear stress and fluctuating velocity. This simple model was able to qualitatively capture the near wall behaviour and it demonstrated how finite spatial scale of the forcing was essential to realistically model the behaviour of the correlation coefficient at further distance from the wall.