On LES Grid Criteria for Spray Induced Turbulence 2012-01-0141
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.