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[FAQ]

Both length scales are useful for a LES calculation with the standard Smagorinsky model: 1. The SGS mixing length is the length scale noted L_s that is directly used in the definition of the eddy viscosity as described in Eq. 4-278 (see section 4.15.2.1. Smagorinsky-Lilly Model in Fluent Theory guide). 2. The “filter width” is the quantity delta in Eq. 4-278 and corresponds to a local sizing of the cell, that is the cube root of cell volume for the LES standard Smagorinsky model. The SGS mixing length relies on delta but also introduces a dependency on near-wall eddy sizing to take into account the decrease of eddy sizing near the wall. Both quantities can be accessed through: – 1. SGS length scale : use the UDF macro: Compute_L_sgs(cell_t c, Thread *t, C_VOLUME(c,t), C_WALL_DIST(c,t), C_SMAGO) where C_SMAGO is the Smagorinsky constant (0.1 by default), C_VOLUME(c,t) is the cell volume and C_WALL_DIST(c,t) the nearest wall distance to the cell center. – 2. “Filter width” : select “subgrid filter length” in the GUI or use the UDF macro: Compute_Filter_L_sgs(cell_t c,Thread *t,C_VOLUME(c,t)) Keywords: SRS, LES, SGS, Smagorinsky, filter width, UDF