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

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.

Keywords:

• large eddy simulation
• flow separation
• boundary layer
• wake
• turbulence

Acknowledgements

The authors are grateful to the steering committee of the supercomputing facilities in Stuttgart for granting computing time on the NEC SX-8. MGV acknowledges the financial support of the German Research Foundation (DFG).