What is the difference between the SGS mixing length and the filter width for the LES standard Smagorinsky model, and how can I access them via UDF?
January 25, 2023 at 7:16 amFAQParticipant
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 126.96.36.199. 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
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