When Outlets are present, use the Outlet Editor panel to define the
conditions of the gas at the outlet boundary. For automatic meshing cases, the Outlet icon bar
offers 4 icons for managing outlets: Rename 
, Copy 
, Paste 
, and Delete 
. You can create a new outlet by copying and pasting an existing one or by
using the New Outlet 
icon on the Boundary Condition icon bar. However, for body-fitted cases the
presence of the outlet(s) is determined by the mesh flags upon import of the mesh, such that you
cannot create new ones within the Simulation interface. For body-fitted mesh projects, then,
only the Rename option is available here.
Options on the Outlet Editor panel include:
Location: Select the location of the outlet in the location list.
Note: By default, surfaces included in the same open boundary condition are required to be contiguous, but including non-contiguous surfaces in the same open boundary is supported as long as these surfaces are connected to the same region. To disable the contiguity check, go to Edit Preferences > General Settings > Validation Settings and check the box Skip Open Surface Contiguity Check.
Outlet Options: Select from the list of outlet types: Static Pressure, Total Pressure, or Continuative Outlet. If you select Static Pressure or Total Pressure, the boundary type is regarded as Pressure Outlet. It is different from the Continuative Outlet in their treatment of pressure and flow velocity at the boundary.
At a Pressure Outlet, pressure is specified by the user, and flow velocity is calculated by the flow solver based on the pressure difference across the boundary. The user must enter a constant pressure value, or, if the pressure is Time Varying, a pressure profile that varies as a function of time (or crank angle) (see profile data in Entering Profile Data). The Time Varying options can include a time-frame offset (see Time Frames). For Pressure Outlet, an option is provided for Offset Distance to Apply Pressure. This is the offset distance between the boundary and the location of the specified fixed (ambient) pressure, which enables the boundary to absorb acoustic waves and reduce any tendency the waves may have to be reflected back in.
At a Continuative Outlet, flow velocity is assumed to be continuative with zero pressure gradient across the outlet boundary. No user input is required in this case.
Turbulence: This parameter set is necessary for the possibility of reverse flow at the outlet. To use it, select from the list a method for indicating outlet turbulence kinetic energy (TKE) and TKE dissipation rate; using the pull-down list, and then specify the corresponding parameter values and units where appropriate. Specification options include:
The above options allow you to specify absolute values for the TKE and TKE Dissipation rates or to use the relative values of Turbulent Intensity and Turbulent Length Scale. Turbulence Intensity is a dimensionless measure of the turbulent kinetic energy and is defined such that:
Turbulent Kinetic Energy: = ( UI ) 2 /2,
where U is the local mean velocity magnitude and I is the turbulence intensity fraction specified.
Turbulent Length Scale: Defines the boundary value of epsilon in the k-epsilon turbulence model, through the relationship:
Ε = C mu k 3/2 /L
Note: An Outlet boundary does not require specification of fluid composition or temperature. If reverse flow occurs, fluid composition of the reverse flow is assumed to be the same as the initial composition of the fluid region adjacent to the boundary. Fluid temperature is computed by an isentropic relation, with the reference state picked as the initial state of the same fluid region.
Note: Specifying an outlet boundary does not guarantee that the local flow is always out of the boundary. In particular, at a pressure outlet, flow direction is mainly affected by the local pressure difference across the boundary, which could change during the simulation. Reverse flow is possible and allowed. If reverse flow is common during the simulation and you would like to explicitly specify the fluid composition and temperature for it, it is better to set the boundary type as an inlet rather than an outlet.