On LES Grid Criteria for Spray Induced Turbulence

Using non-viscosity dynamic structure Large Eddy Simulations (LES) turbulence model, spray=induced turbulence is investigated on a number of different Computational Fluid Dynamics (CFD) grids of varying mesh sizes (from 0.5 to 2 mm mesh). Turbulent flow is induced inside a quiescent chamber by liquid fuel spray and then left to decay after end of injection by virtue of its molecular viscosity and turbulent dissipation. Coherent structures (CS) of this turbulent flow are constructed and visualized using λ2 definition. Using CS, analysis is performed on the turbulent flow around the liquid spray jet. These CS from LES are then compared against the results from RANS calculations as well. The visualization of CS helps to explain the mechanism of fuel-air mixing obtained from LES results and its difference with RANS calculations. It is found that LES model from finer CFD mesh predicts fuel-air mixing by virtue of breaking down of large eddies to number of smaller eddies while RANS model predicts diffusion of fuel-rich pockets resulting in fuel-air mixing predictions.

It is observed that with finer CFD grids, further details of the turbulent flow structures induced by spray jet are obtained. Additionally RANS model is unable to reproduce the CS that is obtained from fine mesh LES results. Based on these observations, a set of criteria on CFD grid is proposed for LES-based models for spray-induced turbulence study.