(129i) An a Priori Analysis of Liquid-Gas Flows for the Large-Eddy Simulation of Turbulent Sprays

We are interested in the modeling and simulation of turbulent, reacting, multi-phase (liquid-vapor), multi-scale flows. These flows are of great research interest because of their presence in a variety of engineering problems, including drug synthesis, combustion dynamics, and agricultural spraying. The difficulties in solving these types of problems lie in the complexity of the highly non-linear governing equations — the large dimensionality vis a vis a large number of chemical species — and the difficulty involved in resolving the highly dynamic, small length-scale phenomena associated with surface and interface dynamics. In this work we adopt a coupled level-set and volume-of-fluid (CLSVOF) approach to capture/represent the liquid-gas dynamics in an Eulerian manner. The VOF approach will be used to distinguish the different fluid phases — liquid and gas — in  a computationally-affordable manner while the LS method is quite accurate in the tracking of interfaces as they evolve in space and time. Direct numerical simulation (DNS) is a promising tool for obtaining information in the dense zone of the spray, where little physical/experimental data are available. However, DNS is too compute-intensive to be used as a research and design tool. 

The more practical approach is to perform large eddy simulation (LES), instead of DNS. LES is advantageous in that it facilitates the simulation of turbulent flows in a temporally and spatially accurate manner. This is accomplished via the use of subgrid-scale (SGS) models to capture the small-scale dynamics, while the large-scale dynamics are resolved explicitly. Besides the traditional and familiar SGS terms arising from the single-phase turbulent flow problem, there are terms that comes from the multiphase and interface effects. Modeling of these terms is our ultimate goal.

In this work, we are going to perform a preliminary study on the SGS terms, by first performing a decomposition of the flow quantities into resolved and unresolved, or SGS, components. This allows us to identify the contribution of terms which need to be modeled.  Past effort to do this has focused on liquid-liquid systems. The liquid-gas system will physically exhibit a different break-up process than liquid-liquid system which might affect the numerical modeling, and thus worth performing a separate study. The flows are three-dimensional circular and elliptical water jets issuing into air. Our aim is modeling and simulating the primary break-up of liquid jets and the many topological changes that occur, including interface pinching, droplet coalescence or secondary break-up. In addition to evaluating the large/resolved-scale and the SGS components, we will assess the performance of the simulations via comparison to physical data from experiments performed at Dow Chemical.