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:
Mechanical fixtures (screw holes, bearings etc.)
The displacement is normally set to zero.
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:
value uniform (0 0 0);
Mathematically, the boundary condition can be formulated as
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.
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:
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).
A typical example of a fixed value boundary condition. All three degrees of freedom are prescribed to a displacement of 0. This means that the boundary is not allowed to move in the x, y and z direction.
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:
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