Pressure gradient effects on the distribution of velocity fluctuations in turbulent boundary layers*

This research investigates the wall-normal distribution of the streamwise and wall-normal velocity fluctuations in turbulent boundary layers with zero pressure gradient (ZPG) and a strong adverse pressure gradient (APG). High-spatial-resolution (HSR) two-component – two- dimensional (2C-2D) particle image velocity (PIV) measurements of the strong APG-TBL at Re_δ2 = 9, 650 and β = 30, are compared with the 2C-2D PIV measurements of the ZPG-TBL at Re_δ2 = 7,750 (where Re_δ2 is the momentum-thickness based Reynolds number and β is the Clauser’s pressure gradient parameter). The streamwise velocity fluctuations are skewed towards the larger values and the wall-normal velocity fluctuations towards the smaller values whereas this trend is reversed in the wake region. The extent of asymmetry in the near-wall and wake regions increases with an increase in the imposed APG. The locations of the zero skewness and the minimum flatness of the streamwise velocity fluctuations collapse with the location of the maximum Reynolds streamwise stress in both boundary layers, irrespective of the imposed pressure gradient. The same is observed for the wall-normal velocity fluctuations too.