Numerical investigation of highly unsteady accelerated/decelerated flows for blunt bodies experiencing impulsive motion

Abstract

Scale-resolving simulations (SRSs) such as large-eddy simulation (LES) and hybrid LES/Reynolds-averaged Navier-Stokes are applied to analyze the unsteady characteristics of the drag and flow fields for blunt bodies subject to impulsive motion. These simulations reveal highly transient behaviors of the primary and secondary vortices triggered by inertial forces during impulsive motion. A distinctive characteristic of the transient drag, namely, the existence of a plateau region, induced by a circular cylinder during impulsive acceleration is also revealed. It is found that among the SRS methods, the LES one-equation eddy method is able to more precisely capture the intrinsic local pressure gradient arising from unsteadiness of the vortices together with the instantaneous vorticity magnitude. The secondary vortices created by inertial forces and shear stresses due to the impulsive motion and the interactions of these vortices with the primary vortices turn out to play a key role in the transient behaviors of the flow field and the drag. The mechanism causing the highly unsteady flow field and its relationship to the bluntness of the shape configuration are also explored using the LES one-equation eddy method. It is found from the range of retained local Cp values and the transient vorticity distribution along the cylinder surface that the mobility of the unsteady flow separation point depends on the bluntness of the configuration.