On the skewness of streamwise velocity fluctuations in the turbulent boundary layer

The skewness of streamwise velocity fluctuations is a conventional turbulence statistic which contains valuable information about turbulent boundary layer dynamics. For instance, skewness represents an average measure of the phase difference between triadically coupled scales of motion, making it useful for quantifying near-wall non-linear interscale interactions, including classical amplitude modulation phenomena. However, by analysing a range of non-canonical turbulent boundary layers, we find that inferences regarding amplitude modulation effects require careful interpretation and a broader range of interscale interactions should be considered when modelling non-canonical boundary layer dynamics. In the outer region, a characteristic skewness profile arises due to characteristic velocity fluctuations associated with the turbulent non-turbulent interface. Specifically, the wall-normal location where the skewness changes sign from negative to positive is found to nominally coincide with the uppermost extent of this interface, on average. As such, we propose that this skewness sign change can be leveraged to define an appropriate turbulent boundary layer thickness which was observed to have a consistent relationship with previous definitions, such as the classical 99% thickness. Critically, this new definition can be applied to conventional single point streamwise velocity measurements, does not rely on thresholds, and is relatable to key turbulent boundary layer dynamics.