Characteristics of turbulent boundary layer large scale motions using direct fluctuating wall shear stress measurements – Physical Review Fluids Nov 2018

This experimental work studies the impact large scale motions in a zero pressure gradient turbulent boundary layer have on the fluctuating streamwise wall shear stress component using a recently developed 1×1 mm^2 floating element differential capacitive shear stress sensor. The sensing system allows for a flat band response with a bandwidth up to 1.8 kHz (based on a ±3 dB limit). The streamwise velocity is measured using single component hot-wire anemometry. The experimental setup is first verified to have a canonical zero pressure gradient turbulent boundary layer using the mean and fluctuating velocity profiles as well as fits for the mean wall shear stress with respect to the operating Reynolds number. Characteristics of the large scale structure are examined spatially using Taylor’s frozen field hypothesis and the lag time of peak levels of correlation between the shear stress and velocity signals. The large scale motion inclination angle is determined to be 16deg. The coherence between the signals demonstrate that low frequency motions dominate most of the boundary layer except nearest the wall. In addition, conditional sampling of velocity on shear stress provides conditional velocity statistics profiles which reveal information on the entire boundary layer during shear stress events, representing the qualitative features of the bursting-sweeping process.

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