### 2.6.1. Mass and Momentum

#### 2.6.1.1. Cartesian Velocity Components

The Cartesian components of the flow velocity at the opening are specified in the same way as for an inlet. For details, see Cartesian Velocity Components. The components can be specified such that both inflow and outflow occur over different regions of the boundary.

#### 2.6.1.2. Cylindrical Velocity Components

The components and axis are specified in the same way as for an inlet. For details, see Cylindrical Velocity Components. The components can be specified such that both inflow and outflow occur over different regions of the boundary.

#### 2.6.1.3. Opening Pressure and Direction

When the flow direction is into the domain, the pressure value is taken to be total pressure based on the normal component of velocity. When it is leaving the domain, it is taken to be relative static pressure.

This option also requires that the Flow Direction is specified, and is used if the flow is into the domain. For details, see Flow Direction.

This is the most robust and stable setting for a pressure specified opening because it puts a constraint on the momentum transported through the boundary condition.

This option is available for both single and multiphase simulations. When running multiphase simulations, the total pressure is applied as if all phases are incompressible. For details, see Multiphase Total Pressure in the CFX-Solver Theory Guide.

#### 2.6.1.4. Static Pressure and Direction

The pressure value is taken to be static pressure for both the inflow and outflow. This option also requires that the Flow Direction is specified, and is used if the flow is into the domain. For details, see Flow Direction.

If the flow is coming into the domain, this setting is the least stable because it does not constrain the momentum allowed to flow through the boundary. In this case, any momentum flow satisfies the static pressure condition, which can cause severe robustness problems. Always consider a pressure-specified opening before using this option.

#### 2.6.1.5. Entrainment

When `Entrainment` is selected, the Pressure Option check box becomes available. When the latter is selected, you can set the sub-option to `Static Pressure` or `Opening Pressure`.

- Entrainment with the `Static Pressure` option is similar to the `Static Pressure and Direction` option, except that the direction is obtained by enforcing the velocity gradient perpendicular to the boundary to be zero.

- Entrainment with the `Opening Pressure` option has a similar behavior to a pressure-specified opening and has similar stability and robustness but does not require that a flow direction is specified. Instead, the flow solver locally calculates the flow direction based on the direction of the velocity field. When the flow direction is into the domain, the pressure value is taken to be total pressure based on the normal component of velocity. When it is leaving the domain, it is taken to be relative static pressure.

When the Pressure Option check box is not selected, the `Static Pressure` sub-option takes effect by default.

The `Entrainment` option can be quite useful for situations in which the main flow tends to pull fluid through the boundary where the flow direction is unknown.

#### 2.6.1.6. Fluid Velocity

This option is only available for an inhomogeneous multiphase simulation. When this option is selected, the Mass and Momentum information is set on a per-fluid basis using `Normal Speed`, `Cartesian Velocity Components` or `Cylindrical Velocity Components`.

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