Supergrid thermochemical closure of large-eddy simulation of turbulent combustion

To illustrate turbulent combustion modeling challenges that influence the formulation of a large-eddy-simulation (LES) closure, a strategy for thermochemical closure of LES of turbulent combustion that is currently under development is outlined, with emphasis on requirements for premixed combustion closure. The approach is intermediate between conventional closures, in which each LES cell contains an instantiation of the closure model, and the ‘representative interactive’ closure approach. The latter involves modeled time advancement of a single instantiation of a reference thermochemical field that is interrogated by each LES cell to obtain a cell-specific closure that takes into account limited state information residing at the LES level. This LES-level information is generated by time advancing modeled transport equations for low moments of a progress variable (e.g., normalized local temperature) and, for cases that are not uniformly premixed, the mixture fraction (related to the local fuel mass fraction). In the intermediate approach, LES cells are grouped into clusters, e.g. using standard domain decomposition that is commonly used for parallelization. This forms a supergrid of LES cell clusters, where each cluster is associated with one instantiation of the time-evolving reference field. In order for each reference-field instance to correctly represent conditions in its associated cluster, the reference-field instances flux fluid to each other as specified by LES property fluxes through supergrid faces, including inlet, outlet, and boundary treatments. This fluxing is akin to fluxing in typical linear-eddy-model (LEM) subgrid combustion closure of LES, but the representative interactive thermochemical closure method differs from the current use of LEM information for closure. The new approach will improve fidelity while reducing cost. Cost will be further reduced by using hierarchical parcel swapping (HiPS) instead of LEM to generate reference fields.