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Investigation of the Grid spacing calculation effects on the performance of Detached Eddy Simulation

Since its introduction in 1997, the use of Detached Eddy Simulation (DES) and similar hybrid turbulence techniques have become increasingly popular in the field of CFD. However, with increased use some of the limitations of the DES model have become apparent. One of these is the dependence of DES on grid construction, particularly regarding the point of transition between the Reynolds-Averaged Navier-Stokes and Large Eddy Simulation models. An additional issue that arises with unstructured grids is the definition of the grid spacing in the implementation of a DES length scale. For a hexahedral grid the conventional definition of Δ can be applied in a fairly direct fashion, but more complicated cell volumes (such as tetrahedral or combined hexahedral-tetrahedral) do not have that luxury. If other means of defining the grid spacing are used, the region of RANS-LES transition can shift significantly. This has implications for both the location of the grey area and the magnitude of the dissipation term in the LES region.

To investigate the grey area effects and also the effects of changing the methods of defining the grid spacing, the authors have implemented the one-equation model of Spalart-Allmaras (SA), SA based DES and the Menter’s Shear Stress Transport (SST) model based DES turbulence models in the in-house CFD code UNCLE, it is a two/three-dimensional, finite-volume, unstructured incompressible Navier-Stokes solver for steady and unsteady flow fields. Even though the ultimate goal is to achieve an insight on the grey area effects, but this paper will be confined to study the effects of the calculation of the grid spacing particularly in unstructured grids.