Structure of the turbulent boundary layer over a grooved bump

Coherent structures in a turbulent boundary layer over a smooth (SW) transverse bump and also with fine-scale longitudinal square grooves (GW) are studied using DNS. The spanwise coherence of the rollers in the shear layer above the separation bubble downstream of the bump peak was explored via a phase-aligned ensemble average of 250 realizations for both SW and GW. In GW, counter-rotating streamwise swirling jets emanating at the same x downstream of the bump peak impinge on the shear layer and increase the spanwise coherence of the downstream shear layer rollup – also decreasing turbulence intensity, Reynolds shear stress and production with respect to SW. Interestingly, wall-attached minibubbles occur within the separation bubble with opposite circulation, adding further to the form-drag of the bump – by 20% over SW. In contrast, in SW, the minibubbles are intermittent, smaller, and occur less frequently, presumably contributing less to the SW form-drag. The minibubbles caused by the separation of the upstream wall flow move downstream due to induction by the image vortices. Zones of time-averaged negative production are explained in terms of the local dynamics. Negative production within the grooves is due to counter-gradient (positive) Reynolds shear stress, while outside the grooves, it is due to Reynolds normal stress and flow acceleration. Our study should explain various flows over rough bumps in nature and technology, including flows over mountains with trees, sand dunes, and subaqueous dunes.