Inlets and Outlets

Flow conditions are used to define where flow is entering or exiting the solution domain.

Determining the type of flow specifications depends on the available flow information. The table below helps you determine the type of flow specification to apply for flow entering the domain.


Upstream conditions (Flow In)	Flow specification	Additional information
A plenum or device delivering flow.	Pressure	Solves the velocity profile and mass flow. Gauge total pressure is the absolute total pressure minus the operating pressure.
A positive displacement device delivering a certain mass or volume flow rate.	Mass flow rate	Solves for pressure. Velocity is derived based on local density. The mass flow rate is capable of overcoming the back pressure of the system being modeled.
	Velocity	Solves for pressure. Mass flow is derived based on the local density. If the fluid flow into the domain has rotating or twisting movement as it enters a flow inlet, select Swirling flow and specify the speed of fluid rotation. You can reverse the direction of rotation by entering a negative rotational speed or double-clicking the arrow showing the rotation direction.

The table below helps you determine the type of flow specification to apply for flow exiting the domain.


Downstream conditions (Flow Out)	Flow specification	Additional information
Fluid is exiting the domain into a large volume where the static pressure is known.	Pressure	A constant static (gauge) pressure will be applied over the entire face. The velocity and mass flow are functions of the solution.
The average velocity is known at the outlet.	Velocity	The velocity is constrained over the face according to the value provided. Pressure is calculated at the outlet as a function of the upstream solution.
When the mass flow through the location is known or expected and at least one pressure condition has been specified.	Mass flow rate	The pressure and velocity are allowed to vary naturally, while maintaining a net mass flow.

Note: Your setup must make sense from a physics perspective. For example, for steady-state flow, you can’t specify 10kg of fluid flowing into your model and only 2 kg leaving. For this reason, it is highly recommended that one of your flow conditions is defined by pressure.

To include the distribution of temperature within the fluid, you will need to specify the in-flow Temperature and the backflow temperature where applicable. For inlets, you will specify the total temperature (also known as the stagnation temperature) rather than the static temperature for compressible flow. For incompressible flow, specify the static temperature. The total temperature represents the temperature of the fluid in a plenum at rest upstream. The local static temperature will be lower, depending on the velocity of the fluid.

This section contains the following topics: