Page 108 - ansys

6.5.1 k-ɛ Models and k-ω Models

When you are modeling turbulent flows in ANSYS FLUENT using one of the k- models or one of the k-ω
models, you must provide the boundary conditions for k and (or k and ω) in addition to other mean
solution variables. The boundary conditions for k and (or k and ω) at the walls are internally taken
care of by ANSYS FLUENT, which obviates the need for your inputs. The boundary condition inputs for
k and (or k and
ω), which you must enter in ANSYS 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.

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, ANSYS FLUENT will set both the production term in the turbulence transport equation and µt to
zero. In contrast, when the Laminar Zone option is enabled in a Fluid cell zone condition dialog box,
only the production term is set to zero.
You can set the Reynolds stresses by using constant values, profile functions of coordinates

(a) Large Eddy Simulation Model
It is possible to specify the magnitude of random fluctuations of the velocity components at an inlet
only if the velocity inlet boundary condition is selected. In this case, you must specify a Turbulence
Intensity that determines the magnitude of the random perturbations on individual mean velocity
components as describe in Inlet Boundary Conditions for the LES Model. For all boundary types other
than velocity inlets, the boundary conditions for LES remain the same as for laminar flows.

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