# Natural Convection Inlet-Outlet

The Natural Convection Inlet-Outlet defines an open surface to an external environment considered infinite.

This boundary condition is meant to be used at clear patches that are clear to the outer atmosphere. For example, this boundary condition type can be used for open windows when modeling a room.

These surfaces will not be defined as pure inlets or outlets. They will behave (partially or totally) in one way or the other, depending on the local fluid conditions. As a result, recirculations and eddies are allowed at these patches, always complying with the continuity equation for the whole domain

More specifically, the Boundary Conditions for each fluid variable are:

• Velocity: It is a zero gradient boundary condition at all times, combined with a zero value for the tangential velocity. While it is the standard condition for outflows, the normal velocity for inflows is calculated depending on local conditions.
• Pressure: A value can be specified for this variable called Ambient Pressure. This value is defined as the total pressure at the surface when the local velocity vector is pointing inside the domain. In the same way, it is defined as the static pressure when the local velocity vector is pointing outside. By doing this, we can specify the far-field static pressure of the environment, since Bernoulli’s Principle is satisfied for subsonic and incompressible cases. For example, when defining a window, you can specify the outer atmospheric pressure here.
• Temperature: The temperature takes an inlet-outlet behavior. This means that it becomes zero gradient when the velocity vector aims outside the domain, and switches to fixed value when it aims inside the domain. In this second case, the Temperature fixed value is the one specified for the Ambient Temperature.
• Turbulent Variables: They have similar behavior as the Temperature (inlet-outlet). This means that it becomes zero gradient when the velocity vector aims outside the domain, and switches to fixed value when it aims inside the domain. In this second case, the fixed value is taken from the initial value at every cell.

Important Information

When using the Natural Convection Inlet-Outlet Boundary Condition in Convective Heat Transfer simulation, you will realize that the default value for the Ambient Pressure changes when you enable the Compressible switch. When it is off, the Boussinesq Approximation is used in the process, which computes gauge pressures. You must take this into account for your Ambient Pressure value.