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:
The displacement is normally set to zero.
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.
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.
Temperature or heat transfer rate are often set to known values.
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
$${\overrightarrow{U}}_{\mathrm{\Gamma}}=0$$
where Γ represents the boundary. Additionally, the fixed value boundary condition allows to define the known value in form of a function:
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.
A secondary menu will appear to facilitate the uploading process:
It is very important to choose the correct set of variables. In OPENFOAM® cases, three type of dependencies are included:
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).
The depicted boundary condition is a typical example for a fixture where the selected part of the domain is not allowed to move in any direction. See also the detailed descriptions for displacement and temperature fixed value boundary conditons: