# Density

## 2D empty

In 2D cases, this boundary condition should be assigned to boundaries which are normal to the reduced coordinate. It applies an empty condition on such boundaries to indicate a two-dimensional problem.

## Cyclic

This boundary condition provides cyclic conditions between two boundaries. For example, it could be used in systems with transitional periodicity.

This boundary condition applies a prescribed gradient value at the boundary. Depending on the variable, it requires a scalar or vector value to be provided as input. The value at the boundary is then calculated using the fixed gradient and internal field values.

## Fixed value boundary condition type

The fixed value boundary condition type prescribes the value of a field on a certain boundary of the domain. This value could be constant or dependent on time and/or space coordinates. Typical use cases are:

• Inlets and walls (of pipes etc.)

The flow velocity is normally set to a known value. At walls, the velocity is usually set to 0. Also, turbulent quantities such as turbulent kinetic energy and dissipation rate can be set to fixed values which have to be estimated first.

• Outlets (of pipes etc.)

The pressure is often set to a known value on outlets. In incompressible simulations, it is common to use a value of 0 for the Gauge pressure.

• Thermal walls and insulation

Temperature or heat transfer rate are often set to known values.

## Fixed value boundary condition for OPENFOAM® An example of a fixed value boundary condition for OPENFOAM® simulations where the flow velocity is set to zero.

The depicted boundary condition is a typical example for a wall which the fluid adheres to due to viscous effects. When converted to an OPENFOAM® input file, the relevant snippet will look similar to the following:

wall
{
type            fixedValue;
value           uniform (0 0 0);
}


Mathematically, the boundary condition can be formulated as

${\stackrel{\to }{U}}_{\mathrm{\Gamma }}=0$

where Γ represents the boundary. Additionally, the fixed value boundary condition allows to define the known value in form of a function: The x value of the vector quantity is defined as a function of space coordinate X.

Alternatively, it is possible to set boundary values by uploading a CSV file. For this purpose, the user should choose File Upload as the Input Type. Correct dependencies should be chosen. In this case, the uploaded file contains values that are dependent on X, Y, and Z.  It is very important to choose the correct set of variables. In OPENFOAM® cases, three type of dependencies are included:

• time only,
• X, Y, and Z,
• all four (time, X, Y, and Z)

The Column index of the value identifies the column number of value in the CSV file. For vector quantities, it is assumed that this value points to the first column (the rest of the columns are placed exactly after this column).

## Inlet-outlet

Inlet-outlet is a generic outflow condition based on the flux: if flux points out of domain, Inlet-outlet applies a zero gradient condition at the boundary. Otherwise, i.e. flux into of domain, it applies the value prescribed as user input. Depending on the variable, this value could be scalar or vector.

## Outlet-inlet

Outlet-inlet is a generic inflow condition based on the flux: if flux points out of domain, it applies the value prescribed as user input at the boundary. Otherwise, i.e. flux into of domain, it applies a zero gradient condition. Depending on the variable, this value could be scalar or vector.

This boundary condition applies a zero gradient condition at the boundary using the internal cells. It is a special case of the fixed gradient boundary condition.

Typical use cases include, velocity outlets (and other transport quantities, such as turbulent kinetic energy and dissipation rate), walls (pressure gradient is often set to 0), and adiabatic walls (zero gradient for temperature).

## Symmetry

This boundary condition is used to specify domain symmetry. It is used to reduce computational effort by replacing symmetric redundancies in the interior.

## Symmetry plane

This boundary condition is a special case of Symmetry boundary condition in that it is could be assigned to planes only.

## Wedge

This boundary condition operates in the same way as the Cyclic boundary condition. The difference is that is is applied to two-dimensional geometries.

## Disclaimer

This offering is not approved or endorsed by OpenCFD Limited, producer and distributor of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks. OPENFOAM® is a registered trade mark of OpenCFD Limited, producer and distributor of the OpenFOAM software.