### 1.9.5. Opening

An opening boundary condition allows the fluid to cross the boundary surface in either direction. For example, all of the fluid might flow into the domain at the opening, or all of the fluid might flow out of the domain, or a mixture of the two might occur. An opening boundary condition might be used where it is known that the fluid flows in both directions across the boundary.

#### 1.9.5.1. Mass and Momentum

##### 1.9.5.1.1. Cartesian Velocity Components

The Cartesian components of the flow velocity are specified at the opening boundary:

| (1–242)

##### 1.9.5.1.2. Cylindrical Velocity Components

These are handled the same way as for an Inlet (Subsonic) boundary condition.

##### 1.9.5.1.3. Pressure and Direction

An opening boundary condition can also be specified with a Relative Pressure value:

| (1–243)

The value is interpreted as relative total pressure for inflow and relative static pressure for outflow.

The direction component, normal to boundary condition or direction components, must also be specified. The magnitude of the velocity at the opening boundary is then part of the solution.

###### 1.9.5.1.3.1. Loss Coefficient

You can optionally specify a loss coefficient, :

| (1–244)

with a velocity component specification at an opening boundary. The pressure drop is calculated using:

| (1–245)

where is the magnitude of the velocity component normal to the opening boundary. For inflows, the constraint on pressure and velocity becomes:

| (1–246)

and for outflows, the constraint becomes:

| (1–247)

The coefficient can be used to model the pressure drop across a screen or other planar resistance.

#### 1.9.5.2. Heat Transfer

##### 1.9.5.2.1. Static Temperature

Opening temperature or Static temperature can be specified at the opening boundary:

| (1–248)

#### 1.9.5.3. Additional Variables

For flow into the domain, the value of the Additional Variable at an opening boundary is that specified on the Additional Variable Details tab:

| (1–249)

For flow out of the domain the specified value is not used. The value for flow out of the domain is calculated by the CFX-Solver and is the Additional Variable value taken from the solution field.

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