Structural differences between small and large momentum-defect turbulent boundary layers

We analyse how coherent structures in turbulent boundary layers (TBL) change in response to a strong adverse pressure gradient by using two direct numerical simulation databases. One zone of a zero-pressure-gradient TBL and three zones of a strongly decelerated TBL, corresponding to three ranges of mean velocity defect, with the shape factor varying from 1.54 to 3.75, are considered. We investigate the properties of three-dimensional sweeps, ejections and streamwise-velocity structures. The identified sweeps and ejections contribute everywhere more than 30% to the Reynolds shear stress in both flows. The effect of increasing mean velocity defect in the adverse-pressure-gradient TBL is significant. Streamwise-velocity structures and all wall-attached sweeps and ejections, that is near-wall ones and taller ones that reach the wall region (important in the logarithmic layer of the zero-pressure-gradient TBL), lose their streamwise elongation and become less organised. Wall-attached sweeps and ejections become progressively less numerous and spanwise elongated structures become more frequent. Their strength diminishes in comparison to wall-detached ones and they lose their role as the main contributors to the Reynolds shear stress. In terms of spatial organisation, a pair of side-by-side sweep and ejection is still the dominant configuration, but the probability of such an event has decreased. This fact, together with other results of spatial organisation of structures, does not point toward the presence of a Kelvin-Helmholtz-type instability or a varicose instability of low-speed structures in the outer region, two instabilities that have been suggested in the literature to explain the turbulence regeneration process in large velocity defect turbulent boundary layers.