Large-eddy simulation of impinging jets on smooth and rough surfaces

ABSTRACT

We performed large-eddy simulations (LES) of forced impinging jets over smooth and rough surfaces, containing large-scale, azimuthal vortices generated by the enhanced primary instability in the jet shear layer. The interaction between these vortices and the turbulence in the wall jet that is formed downstream of the impingement region determines their rate of decay. To explore the surface-roughness effects on the evolution of the vortices, sand-grain-like surfaces are generated using uniformly distributed but randomly oriented ellipsoids. The flow is compared to our previous LES of jets impinging on a smooth surface. In spite of the severe modification caused by the roughness on the near-wall flow, the vortex development is not significantly altered. Slightly faster decay of the primary vortices is observed in the rough-wall case compared to the smooth-wall one; the secondary vortex that detaches from the wall and is lifted up has larger vorticity. The highly disturbed near-wall flow is advected outward and affects the evolution of the primary vortex for a longer period during the vortex interaction. The robust turbulent generation mechanism in the outer shear layer, however, mitigates the changes in vortex behaviour. The momentum deficit and the enhancement of turbulence due to the surface roughness play a key role during this process.

KEYWORDS:

• Large-eddy simulation
• impinging jet
• wall jet
• roughness
• immersed-boundary method
• brownout

Acknowledgments

The authors thank the High Performance Computing Virtual Laboratory (HPCVL), Queen’s University site, for the computational support. The US Government is authorised to reproduce and distribute reprints for Governmental purposes, notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Air Force Office of Scientific Research or the US Government.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work has been supported by Air Force Office of Scientific Research [grant number FA9550-10-1-0502], monitored by Dr. Rengasamy Ponnappan.