Pressure inlet and outlet conditions are typically assigned in computational fluid dynamic analysis (CFD), at the opposite end of the model to a flow rate or a different pressure.

Here are a couple of typical examples of combinations that are used:

Flow Driven Analyses

In those cases where we know the flow rate conditions, the recommended boundary condition combinations for the inlet and outlet region are the following:

Pressure Driven Analyses

In those cases where we know the pressure conditions, the recommended boundary condition combinations for the inlet and outlet region are the following:

Pressure Inlet

The pressure inlet boundary condition defines an inflow condition based on the known pressure value (P) at the boundary. It is used when no flow rate is known, or if a flow rate (or velocity) is assigned at the outlet.

Important

For compressible analyses, temperature properties are also required at the inlet.

The turbulent flow quantities are matched with the values specified for the Initial Conditions and are thus not required as inputs.

Pressure Outlet

The pressure outlet boundary condition defines an outflow condition based on the flow pressure (P) at the outlet. This is usually used when there is a flow rate (or velocity), or a higher pressure assigned at the inlet.

All relevant unknown flow quantities e.g. temperature in case of compressible flow, and other quantities including turbulence, are calculated from the interior of the domain with gradients of the quantities fixed to zero value.

Pressure Inlet and Outlet Input Methods

The pressure values at the inlet and the outlet can be assigned using the following types:

Total pressure (inlet only)

Fixed (inlet and outlet)

Mean (outlet only)

These three types are described in detail below:

1) Total Pressure

The total pressure expression, defined by velocity vector U and density \(\rho\):

$$ p =p_0 – 0.5 \rho\ |\vec{U}|^2 $$

The user could specify a parameter \(\gamma\) which is used in transonic and supersonic cases.

The pressure \(p\) is calculated at the boundary based on the fixed total pressure \(p_0\) that must be specified. Since the velocity is calculated, as velocity changes, pressure p is adjusted accordingly until it reaches a converged value.

Additionally, the user can also input values in the form of a table by uploading a .csv file or entering the values manually. The following figure highlights the icon which serves this purpose.

Correct dependencies should be chosen. In this case, the uploaded file contains values that are dependent on time only.

Note

When using pressure inlet of type total pressure, it is generally recommended to use pressure outlet boundary condition type.

2) Fixed Value

This prescribes the pressure value on a certain boundary of the domain. This value could be constant or dependent on time and/or space coordinates. Either static or gauge pressure can be used depending on the analysis type.

Similar to the Total pressure input value, Fixed value also allows you to input a table. In this case, the uploaded file contains values that are dependent on time or space coordinates.

3) Mean Value

This is only used at an outlet where it is not clear that flow passes over the boundary uniformly. The average of the pressure over the boundary would equal this assigned value. Unlike the fixed value, not every region on the boundary needs to have the same value.

Last updated: September 11th, 2020

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