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bigfooted Aug 29, 2022
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36 changes: 36 additions & 0 deletions _docs_v7/Markers-and-BC.md
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Expand Up @@ -13,6 +13,8 @@ The term *Marker* refers to a named entity in your mesh file. Boundary condition
- [Heat Transfer or Convection (no-slip) Wall](#heat-transfer-or-convection-no-slip-wall)
- [Isothermal (no-slip) Wall](#isothermal-no-slip-wall)
- [Farfield Boundary Condition](#farfield-boundary-condition)
- [Turbulence Boundary Condition](#turbulence-boundary-condition)
- [Wall functions](#wall-functions)
- [Inlet Boundary Condition](#inlet-boundary-condition)
- [Total Conditions](#total-conditions)
- [Mass Flow Inlet](#mass-flow-inlet)
Expand Down Expand Up @@ -121,6 +123,40 @@ A marker can be defined as a Farfield boundary by addings its name to the `MARKE
MARKER_FAR= (farfield)
```

## Turbulence Boundary Condition ##

| Solver | Version |
| --- | --- |
| `RANS`, `INC_RANS`, | 7.3.0 |

The turbulence boundary conditions do not have a `MARKER_` keyword but can instead be set for inlet and freestream boundaries using the keywords:

For the SA turbulence model:
```
FREESTREAM_NU_FACTOR= 3
```

For the SST turbulence model:
```
FREESTREAM_TURBULENCEINTENSITY= 0.05
FREESTREAM_TURB2LAMVISCRATIO= 10
```

### Wall functions ###
Accurately resolving the turbulence close to walls requires very fine meshes and can be quite expensive. When the vertices of the first cell neighboring the wall have on average a normalized distance $$y^+ >1$$, wall functions can be used. For example to activate wall functions on the markers `wall1` and `wall2`, we write:
```
MARKER_WALL_FUNCTIONS=(wall1,STANDARD_WALL_FUNCTION,wall2,STANDARD_WALL_FUNCTION)
```
The wall functions will now be used automatically. all functions have 5 additional expert parameters:
```
WALLMODEL_KAPPA= 0.41
WALLMODEL_B= 5.5
WALLMODEL_MINYPLUS= 5.0
WALLMODEL_MAXITER= 200
WALLMODEL_RELFAC= 0.5
```
The constant `WALLMODEL_KAPPA` is the von Karman constant, and `WALLMODEL_B` is an additional constant describing the universal 'law of the wall'. The constants are supposed to be universal, and do not change. The setting `WALLMODEL_MINYPLUS= 5` will activate the wall model only when the local value of $$y^+$$ is higher than the value given (default: 5). Note that in principle, this implementation is valid for any $$y^+ < 100-500$$ and will also work correctly for very small values of $$y^+$$. the upper limit that can be used depends on (and increases with) the Reynolds number. The universal law of the wall is an implicit function and a Newton iterator is used to determine $$u^+(y^+)$$. The maximum number of iterations can be set by `WALLMODEL_MAXITER` and the relaxation factor can be set with `WALLMODEL_RELFAC`. When the Newton solver does not converge within the maximum number of iterations given, a warning message will appear during the computation. When these warning messages do not disappear, you might consider increasing `WALLMODEL_MAXITER` or decreasing `WALLMODEL_RELFAC`.

## Inlet Boundary Condition ##
Inlet boundary conditions are set using the option `MARKER_INLET`.

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