Boundary Conditions
The boundary condition parameters for the Turbulence module apply to Boundaries in the Flow Analysis Tree. The options also apply to interfaces for which the Flow module is blanked on one side of the interface, creating a Boundary.
The boundary conditions appear in the Properties panel when you select a boundary in the Flow Analysis Tree under General Boundaries.
Wall
The Wall boundary condition for Turbulence corresponds to a solid boundary. You can select the Wall boundary condition for Turbulence for a selected boundary or interface under the Turbulence module in the Properties panel. The Wall boundary condition is the default condition for the Turbulence module when you select a wall for the Flow module. The effect of wall roughness is modeled using the Wall Roughness Model. The Output available with the Wall boundary condition include Area, Normal, and Y Plus.
• Wall Function
You can select the various Wall Function for turbulence near-wall treatment for a selected Wall under the Turbulence module in the Flow Analysis Tree and implement in the Properties panel. The available wall functions are listed below:
◦ Standard
◦ Nonequilibrium(Kim)
◦ Unified(Shih)
The first Interior Node is placed at a distance from the wall which corresponds to whether a low Reynolds number approach or a high Reynolds number approach is used. Using the low Reynolds number approach (LRN) you place the first Interior node within the viscous sublayer and the outward cell face within the buffer layer. This approach requires a very fine mesh near the wall. Using the high reynolds number approach (HRN) you place the first interior node within the inertial sublayer. This approach requires a coarser mesh near the wall.
• Wall Roughness Model
You can use the Wall Roughness Model option to make the wall shear a function of wall roughness. You can model the wall finish for the Turbulence module as smooth or rough:
◦ Smooth
◦ Rough—Defines the desired roughness height.
Specified Pressure Outlet
Use the Specified Pressure Outlet boundary condition to set the turbulent kinetic energy and the turbulent energy dissipation rate at an opening where the flow can exit the domain. You can specify the Outlet condition and associated parameters for a selected boundary under the Turbulence module in the Properties panel. The turbulent kinetic energy and the turbulent energy dissipation rate of the fluid exiting the Outlet are based on the internal (upwind) turbulent kinetic energy and the turbulent energy dissipation rate. The turbulent kinetic energy and the turbulent energy dissipation rate of any fluid reentering the domain are based on the turbulent kinetic energy and the turbulent energy dissipation rate that you specify for this boundary. The specified turbulent kinetic energy and the turbulent energy dissipation rate at the Outlet can influence the internal turbulent kinetic energy and the turbulent energy dissipation rate by diffusion. Typically this effect is insignificant unless the outflow is very small or the conductivity is very high (low Péclet number). The Output available with the Outlet boundary condition include Area and Normal.
 |
Outlet is the default boundary condition for the Turbulence module when you select Specified Pressure Outlet or Resistor Capacitor boundary condition for the Flow module.
|
Specified Values
You can use Specified Values as a Turbulence boundary condition when the turbulent kinetic energy and the turbulent energy dissipation rate at a boundary are known or the default value is acceptable. You can specify the Specified Values boundary option for a selected Boundary under the Turbulence module in the Properties panel.
Specified Values is the default boundary condition for the Turbulence module when you select Specified Velocity, Specified Volumetric Flux, Specified Total Pressure, or Specified Pressure Inlet for the Flow module. The turbulent kinetic energy and the turbulent energy dissipation rate of any fluid exiting the domain is based on the internal (upwind) turbulent kinetic energy and the turbulent energy dissipation rate. The turbulent kinetic energy and the turbulent energy dissipation rate of any fluid entering the domain are based on the turbulent kinetic energy and the turbulent energy dissipation rate that you specify for this boundary. The specified turbulent kinetic energy and the turbulent energy dissipation rate at an outlet can influence the internal turbulent kinetic energy and the turbulent energy dissipation rate by diffusion. Typically this effect is insignificant unless the outflow is very small or the conductivity is very high (low Péclet number).
|
|
• Turbulent Kinetic Energy—Default is 0.01 percent of the Kinetic Energy of the fluid entering the domain.
• Turbulent Energy Dissipation Rate—Default is 1 percent of the Kinetic Energy of the fluid entering the domain.
|
Symmetry
The Symmetry boundary condition for the Turbulence module is input under the Turbulence module in the Properties panel. Symmetry for the Turbulence module means that there is no flux or gradient of the turbulent kinetic energy at the boundary. Symmetry is the default boundary condition for the Turbulence module when you select a Symmetry boundary condition for the Flow module. A Symmetry boundary condition for the Turbulence module usually corresponds to a physical symmetry in the model, but is not necessary if the effects of this boundary condition are acceptable.
Output
The integrated quantities available as output from the Turbulence module for the boundaries are shown in output variables.
Interface Conditions
The Interface attributes options and Output for the Turbulence module are the same as the Boundary attributes, if the interface separates a solid and fluid. If the interface separates two solids or two fluid volumes, the Interface treatment defaults to the standard treatment for the interface between two internal cells.
The Interface attributes for the Turbulence module are the same as the attributes for the boundary condition if one side of the Interface is Blanked for flow. If the Flow module is Active on both sides of an interface, then the interface can only be assigned as a Default Interface.
The Porosity is set in the Common module.