Progress in limiting the impact of numerical error on reacting les simulations

A method for assessing the impact of numerical error on LES solutions by enabling numerical convergence is demonstrated in a non-reacting bluff body flow. This is envisioned as the first step to the extension of this approach to reacting flows which is to follow. In this Explicit Filter method the filter size which appears in the subgrid modeling terms in the LES equations is explicitly specified and it is not assumed to be equal to the grid size. It is distinct from Explicit Filtering methods introduced in the literature which use complicated filtering operations of the non-linear terms in the equations to enable converged solutions. As a result, it is readily implementable in complex geometries and in irregular grids. The method is demonstrated in two different computational codes, namely University research code LESLIE3D and the commercially available Fluent, both employing uniform grids of various sizes. Results indicate that convergence is achieved in both codes as the grid is refined. This is contrasted to traditional Implicitly Filtered LES simulations where the grid size is used as the filter, which yielded effectively different solutions for each grid size over the same grid size range. The convergence studies using the Explicit Filter approach indicate that, for the cases studied, the numerical error did not change the qualitative nature of the solution but it had significant quantitative impacts with local errors exceeding 100% in some instances. The computational cost of the Explicit Filter approach is high as compared to the Implicit Filter approach for the same physical resolution. It is anticipated that the approach is going to be primarily be used for error assessment and enhanced grid utilization in engineering application of LES rather than for routine execution.