Transformation of a separated turbulent flow in a conical dimple at the wall of a narrow channel and reduction in hydraulic losses as conicity increases

A separated flow in a narrow channel with a conical dimple at the wall with an incline angle of $\theta$ is calculated based on the solution to the unsteady Reynolds-averaged Navier–Stokes equations (URANS) closed using the modified shear stress transport (SST) model. It is noted that a sharp transition from the periodic mode with two alternately changing vortices in dimples close to cylindrical shape ($\theta$ = 10$^\circ$ – 22.5$^\circ$) to modes with periodic low-frequency vibrations of inclined single-vortex structures at moderate angles $\theta$ (30$^\circ$ – 37.5$^\circ$) and to modes with steady vortex structures at angles $\theta$ larger than 45$^\circ$ is observed. The relative hydraulic losses at the channel segment with the dimple remain at a level of 1.23 – 1.24 as $\theta$ increases to $\theta$ = 45$^\circ$ and, then, rapidly decrease by almost 1.5 times to 1.16 at $\theta$ = 60$^\circ$.