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12.20.2 $k$- $\epsilon$ Models and $k$- $\omega$ Models

When you are modeling turbulent flows in FLUENT using one of the $k$- $\epsilon$ models or one of the $k$- $\omega$ models, you must provide the boundary conditions for $k$ and $\epsilon$ (or $k$ and $\omega$) in addition to other mean solution variables. The boundary conditions for $k$ and $\epsilon$ (or $k$ and $\omega$) at the walls are internally taken care of by FLUENT, which obviates the need for your inputs. The boundary condition inputs for $k$ and $\epsilon$ (or $k$ and $\omega$) you must supply to FLUENT are the ones at inlet boundaries (velocity inlet, pressure inlet, etc.). In many situations, it is important to specify correct or realistic boundary conditions at the inlets, because the inlet turbulence can significantly affect the downstream flow.

See Section  7.2.2 for details about specifying the boundary conditions for $k$ and $\epsilon$ (or $k$ and $\omega$) at the inlets.

You may want to include the effects of the wall roughness on selected wall boundaries. In such cases, you can specify the roughness parameters (roughness height and roughness constant) in the panels for the corresponding wall boundaries (see Section  7.13.1).

Additionally, you can control whether or not to set the turbulent viscosity to zero within a laminar zone. If the fluid zone in question is laminar, the text command define/ boundary-conditions/fluid will contain an option called Set Turbulent Viscosity to zero within laminar zone?. By setting this option to yes, FLUENT will set both the production term in the turbulence transport equation and $\mu_t$ to zero. In contrast, when the Laminar Zone option is turned on in a Fluid boundary condition panel, only the production term is set to zero. See Section  7.17.1 for details about laminar zones.

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Note that the laminar zone feature is also available for the Spalart-Allmaras and RSM models.


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