Incompressible External Aerodynamics: Steady State RANS Approach
The following sections describe guidelines for estimation of external aerodynamic/hydrodynamic loads for incompressible flow using a steady-state RANS approach.
This approach is valid when the maximum Mach number in the domain is less than 0.2–0.3, and when unsteady wake effects are not important.
| Note | If unsteady wake effects are important, use a DES, LES, or URANS approach. |
This page contains the following sections:
Recommended Domain Settings
- Use a hexahedral or "bullet-shaped" far-field boundary, with domain extents around 8–10 body lengths or wing spans, whichever is larger, from the body. The mesh size on this boundary can be large: typically of the order of the body length.
- Use a velocity inlet boundary condition type for the upstream and lateral boundaries
- Use a pressure outlet boundary condition for the downstream boundary.
- Using a half-body with a symmetry plane is appropriate for cases with no sideslip. Take care to match the symmetry plane mesh growth with the mesh growth of the volume mesh.
- Optional: Angles of attack/sideslip are typically defined by changing the far-field boundary condition flow direction, not the orientation of the body.
Recommended Physics Setup
To set up an incompressible external aerodynamics model using the steady state RANS approach, choose the following physics models in order:
| Group Box | Model |
|---|---|
| Space | Three Dimensional |
| Time | Steady |
| Material | Select a material model that is appropriate for your simulation. |
| Flow | Segregated Flow |
| Equation of State | Constant Density |
| Viscous Regime | Turbulent |
| Turbulence | Reynolds-Averaged Navier-Stokes (Selected automatically) |
| Realizable K-Epsilon or Spalart-Allmaras |
To select a specific turbulence model, choose from the following:
- The
Realizable K-Epsilon model is preferred, especially if there is a blunt boundary at the end. Either the
low y+ or
all y+ boundary treatments can be used with this model, depending on the mesh.
This model works best when complex geometric protrusions exist, bluff body separation occurs, or both.
- The
Spalart-Allmaras turbulence model can be used in cases of streamlined geometries without large base separation regions.
This model works best for attached boundary layers or mildly separated flows (that is, flow past a wing at or below stall).
Recommended Mesh Settings
Both polyhedral and trimmed meshes have proven successful on this class of problem.
In either case, typical rules for good meshing practices apply and are important. The mesh should give proper resolution of geometry and flow field features, along with good cell quality.
For instance, refine the mesh:
- Around leading and trailing edges of lifting surfaces, control surfaces, and other protrusions.
- At tips of lifting surfaces and control surfaces in order to capture vortices that are shed in these locations.
- At body/lifting-surface junctions.
- To resolve any bluff-body wakes.
- On airfoil leading edges. Use aligned surface meshes in these regions when possible.
- If feasible, use at least two cells to resolve thin blunt leading edges, or other thin step-like protrusions.
| Note | External aerodynamics simulations can be sensitive to the quality of the surface geometry representation. Care must be taken when using the surface wrapper such that junctions/intersections result in smooth, well-defined edges. Whenever possible, the preferred approach is to start with clean CAD geometry and bypass surface wrapping. |
Polyhedral Meshes
- Polyhedra have the advantage of smooth growth away from the body.
- Due to the smooth growth behavior, polyhedral are typically preferred when aerodynamic coefficients are of key importance.
- Due to the pseudo-random orientation of the faces, polyhedra also generally give better results than trimmed meshes when large sweeps in flow direction (such as angle of attack or sideslip) are used.
- Typical polyhedral meshes contain 2–20 million cells, depending on the complexity of the geometry and flow field, and the near-wall treatment.
- Use Volume Growth Rate, with the expansion rate set between 1.05 and 1.15.
| Note | Be careful when using volumetric controls with polyhedral meshes: abrupt changes in cell size can occur if there is a significant difference in the sizes that are specified on the surface and in the volumetric control. |
Trimmed Meshes
- If you are modeling a single flow direction, or only small variation in flow, a trimmed mesh with the grid lines aligned with the flow often provides the most efficient approach to getting good answers.
- Typical trimmed meshes contain 4–40 million cells, depending on the complexity of the geometry and flow field, and the near-wall treatment.
- In order to capture the near field properly, set the volume growth rate to Slow or Very Slow.
Prism Layers
- Proper resolution of the boundary layer is critical. Choose the thickness of the prism layer such that the entire boundary layer is contained within it.
- The use of wall functions rather than integrating to the wall can be appropriate depending on several factors, including:
- Desired accuracy
- Relative importance of skin friction drag
- Importance of transition
- Existence/importance of separation/reattachment on smooth boundaries
- For integrating to the wall, 20–30 prism layers are typically used, with the near-wall y+ being of the order 1.
- For wall functions, 5–8 prism layers are typically used, with the near-wall y+ being of the order 50–150.
- When using the Spalart-Allmaras Turbulence model, integrate to the wall (do not use wall functions).