Influence of the approach boundary layer on the flow over an axisymmetric hill at a moderate Reynolds number

TL;DR

This study uses Large Eddy Simulations to examine how different inflow boundary layer conditions affect the flow patterns and vortical structures around an axisymmetric hill at a moderate Reynolds number, revealing consistent flow features and some dependence on inflow conditions.

Contribution

It provides new insights into the influence of boundary layer thickness and laminar versus turbulent inflow conditions on flow topology and vortical structures over a three-dimensional hill.

Findings

Flow features are similar across different inflow conditions.

Vortical structures include horseshoe and hairpin vortices.

Downstream flow becomes independent of inflow conditions.

Abstract

Large Eddy Simulations of a flow at a moderate Reynolds number over and around a three-dimensional hill have been performed. The main aim of the simulations was to study the effects of various inflow conditions (boundary layer thickness and laminar versus turbulent boundary layers) on the flow behind the hill. The main features of the flow behind the hill are similar in all simulations, however various differences are observed. The topology of the streamlines (friction lines) on the surface adjacent to the lower wall was found to be independent of the inflow conditions prescribed and comprised four saddle points and four nodes (of which two are foci). In all simulations a variety of vortical structures could be observed, ranging from a horseshoe vortex - that was formed at the foot of the hill - to a train of large hairpin vortices in the wake of the hill. In the simulation with a thick incoming laminar boundary layer also secondary vortical structures (i.e. hairpin vortices) were observed to be formed at either side of the hill, superposed on the legs of the horseshoe vortex. Sufficiently far downstream of the hill, at the symmetry plane the mean velocity and the rms of the velocity fluctuations were found to become quasi-independent of the inflow conditions, while towards the sides the influence of the hill decreases and the velocity profiles recover the values prevailing at the inflow.