A LES Simulation of Atomisation

Large Eddy Simulation has been used with a lot of success for single phase flows. Its extension to multiphase flows is underway. As far as liquid-gas flows are concerned, two limit cases have been addressed: In one hand, if the liquid phase corresponds to a set of droplets with diameters smaller than the LES filter size, a subgrid spray is described. In the other hand, if the characteristic sizes of the surface wrinkles are greater than the LES filters size, the surface is resolved and LES models concern the velocity field. An example of the first approach is a dilute spray and an example of the second approach is waves at ocean surface. The issue with LES simulation of atomization is that a surface resolved LES is expected close to the injector together with a subgrid sprays LES far from the injector when the spray is finally formed. If only a resolved LES is used, the drop diameter cannot be smaller than the LES filter size. It follows that smaller diameters cannot be described and the breakup process is blocked numerically at a size related to the filter size. At the contrary if a subgrid spray LES is used a model is necessarily used that is accurate only for given type of injector. The present work addresses the problem induced by this transition between resolved and unresolved spray. A first approach is proposed that is able to reach both limits. The transition is performed using a filtered surface density equation to avoid the assumption that ligaments, sheets and other surface topologies becomes spherical droplet abruptly at the subgrid level. Results will be shown to demonstrate the ability of the model to recover the essential characteristics of a spray in a Diesel like application.