Experimental analysis of Reynolds effect on flow detachment and sudden flow release on a wall-mounted hump

Abstract

The boundary layer detachment limits compact power plants' operation at moderate Reynolds number in adverse pressure conditions. Flow detachment is promoted by the lack of momentum in the near-wall region when exposed to adverse pressure gradients. Transient flows or periodic flow perturbations may delay or prevent the flow detachment. The present investigation experimentally analyzes the behavior of separated flows based on an ad-hoc wall-mounted hump. The near wall flow region detachment and recirculated flow evolution under sudden flow release were experimentally characterized. The extension of the separated region and its dynamic development were monitored through surface pressure, temperature measurements, and hot-wire traverses. Comparing the 3D hump performance during steady state experiments against sudden flow acceleration runs, we report for the first time the temporal response of a diffusive passage exposed to a sudden flow acceleration and the impact of the acceleration on the boundary layer detachment. Due to the sudden flow release, the near-wall region can overcome the adverse pressure gradient. However, as the flow acceleration dilutes, the boundary layer detaches, and the recirculated flow region develops. The comparison of experimental results against 3D transient Computational Fluid Dynamics simulations, using the standard gas turbine industry approach, demonstrates the ability of Unsteady Reynolds Averaged Navier-Stokes models to predict the dynamic performance of this phenomenon.