### 12.6.2. Appendix 1: Summary of Numerics Settings with ANSYS Fluent

| Unsteady Simulation | Comment
--- | --- | ---
Convection Terms | CD / BCD | CD on simple geometries (also inside LES regions). In case of wiggles in solution use BCD (most industrial cases)
Pressure Discretization | Any except PRESTO | Use PRESTO only if required for other reasons. Note that the initial formation of turbulence structures can be delayed (inhibited) with PRESTO.
Velocity Gradients | Least Squares Cell Based | No significant impact on SRS, typically Least Square Cell Based. For the SAS model one should use the ‘Least Square Cell Based, or the Green-Gauss Node Based (GGNB). The latter allows a slightly higher sensitivity to initial instabilities.

Iterative Method

SIMPLEC

NITA/Fractional step only for simple flows

Monitor convergence: at least 1 order in mass conservation. SIMPLEC with 2-5 inner loops.

For cases that are difficult to converge try the coupled solver. More expensive, but potentially lower inner iterations required.

Increase Under-Relaxation Factors to values ~1

Under-relaxation | URF˜1 | Start with all URF˜1 (typically 0.8–0.95). Reduce in case of convergence problems. Lower values for additional physics (combustion, multi-phase, and so on).
Time Discretization | Second order backward Euler | Use CFL<1 in LES zones if possible. This condition can also be relaxed depending on the flow and CFL~5 was used for the T-junction test case successfully. Bounded for second order turbulence quantities (, , ) and other positive quantities (volume fraction, and so on).

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