We have an exciting announcement about badges coming in May 2025. Until then, we will temporarily stop issuing new badges for course completions and certifications. However, all completions will be recorded and fulfilled after May 2025.
General

General

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?

    • FAQFAQ
      Participant

      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