Characterisation of uniform momentum zones in adverse pressure gradient turbulent boundary layers

Abstract

Uniform momentum zones (UMZs) for an adverse pressure gradient turbulent boundary layer (APG-TBL) are characterised in this paper. The database described by Cuvier et al. [2017], which was obtained using 2-component 2-dimensional (2C-2D) particle image velocimetry (PIV) at two free-stream velocities of 5 m/s and 9 m/s, is used for this study. The methodology described by Adrian et al. [2000] is used to identify and hence, statistically characterise the UMZs and UMZ interfaces in the APG-TBL. This study shows that the number of detected UMZs increases in the streamwise direction for both free-stream velocities. This increase is primarily thought to be due to the adverse pressure gradient (APG) since a comparison of the results between the two free-stream velocity APG-TBLs did not show any statistically significant difference with Re_\theta for the range of Reynolds number examined in this study. The thickness of the UMZs, \delta w and the jump in streamwise velocity across the UMZ interface were found to increase in the wall-normal direction, whereas the width of the UMZ interface, \delta u, was found to decrease. The number of UMZs, N_umz, identified appeared to influence the modal velocities/uniform velocities, geometrical properties of the UMZs and statistics of the flow. With increasing N_umz, the modal velocities of the UMZs moved to higher values to accommodate new UMZs that developed closer to the wall and the thickness of each UMZ and \delta u across the UMZ interface decreased, whereas \delta w increased. A slight difference in the magnitude of streamwise Reynolds stress and Reynolds shear stress was observed in the inner and outer regions, and a slight difference in the magnitude of turbulence production was also observed in the inner region between the velocity fields having a low or high number of UMZs indicating that the flow statistics are influenced by the number of UMZs present in the APG-TBL.