### 12.6.5. Appendix 4: Generic Flow Types and Modeling

Table 12.7: Globally Unstable Flows

Examples

- Flows past bluff bodies

  - Flow past buildings

  - Landing gears of airplanes

  - Baffles in mixers

  - Side mirrors of cars

  - Stalled wings/sails

  - Trains/trucks/cars in crossflow

  - Tip gap of turbomachinery blades

  - Flows past orifices, sharp nozzles, and so on.

- Flows with strong swirl instabilities

  - Flow in combustion chambers of gas turbines

  - Some tip vortex flows in adverse pressure gradients

  - Flows past vortex generators

- Flows with strong flow interaction

  - Impinging/colliding jets

Modeling

- SAS model is safest option because it has RANS fall-back position.

- DDES/SDES/SBES in case SAS does not show sufficient content of resolved turbulence. Provide suitable LES grid in LES region.

- Often SAS and DDES give very similar solutions.

- Optimal model SBES.

- ELES typically not required.

- The DDES model has been superseded by the SBES model family. The SBES modeling approach is recommended.

Critical

- Visually check turbulent structures

- Run flow until statistically converged

Table 12.8: Locally Unstable Flows

Examples

- Flows with large separation zones (< boundary layer thickness)

  - Backward-facing step type flows

  - Bump flows with large separation

  - Cavity flows

  - Mixing layer leaving plate/trailing edge

- Flows with weak swirl instabilities

  - Flames with low or zero swirl

- Flows with weak flow interaction

  - Jet in crossflow with low momentum ratio

Modeling

- Use ELES where geometry permits.

- DDES/SDES/SBES on high quality grids and low dissipation numerics (CD/BCD)

- The DDES model has been superseded by the SBES model family. The SBES modeling approach is recommended.

Critical

- Instability of Separating Shear Layer (SSL) must be resolved with DDES/SDES/SBES quickly. ELES is safer as it provides unsteady inlet to separation zone but generally much more expensive

- Visually check turbulent structures in SSL.

Table 12.9: Stable Flows

Examples

- Attached and mildly separated wall bounded flows

  - Boundary layers

  - Channel/pipe flows

Modeling

- LES in separate domain if possible

  - WMLES for higher Re numbers

  - Maybe interpolate larger domain RANS solution onto LES zone boundaries

  - Use unsteady (synthetic) turbulence at inlet, preferred Vortex Method

- ELES in combined RANS-LES simulation

  - Define LES zone. Extend LES zone to leave space around critical area.

  - Place RANS-LES interface into region of uncritical flows (such as equilibrium boundary layers)

- Global model

  - Use SBES model and place synthetic turbulence generator to force the model into WMLES mode

Critical

- Visually check turbulent structures

- Provide sufficient grid resolution in (WM)LES zones especially for wall-bounded flows (Section 12.2.3.3).

- CFL number<1.

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