Boundary conditions

Boundary conditions for each mesh boundary should normally be set in the Parameter File (.par), using the boundaryTypeMap parameter. This is used within the VELOCITY, TEMPERATURE or SCALARXX sections to set the boundary conditions of the respective components of the case.

Available Types

The potential values are summarised in the command line help function for nekRS, parHelp.txt and below.

---------------------------------------------------------------------------------------------------------------------------------------------------------------------
Key                         Value(s)                                   Description / Comment
---------------------------------------------------------------------------------------------------------------------------------------------------------------------

boundaryTypeMap             <bcType for ID 1>, <bcType for ID 1>, ...  boundary type for each boundary ID

                            none                                       no-op (internal or periodic) 
                            interpolation                              neknek boundary (int)
                                                                       assign int values in codedFixedValue

                            zeroValue                                  velocity only (w, wall)
                            codedFixedValue                            user specified Dirichlet (v, t, inlet)
                                                                       specify okl function codedFixedValueVelocity 
                                                                       or codedFixedValueScalar in udf

                            zeroGradient                               (I, o, O, insulated, outlet, outflow) 
                            codedFixedGradient                         user specified scalar Neumann (f, flux)
                                                                       specify okl function codedFixedGradientScalar in udf 

                            zeroXValue/zeroGradient                    symmetry x-normal plane (slipx, symx)
                            zeroYValue/zeroGradient                    symmetry y-normal plane (slipy, symy)
                            zeroZValue/zeroGradient                    symmetry z-normal plane (slipz, symz)
                            zeroNValue/zeroGradient                    unaligned symmetry (slip, sym)
                            zeroYZValue/zeroGradient                   (onx) 
                            zeroXZValue/zeroGradient                   (ony)
                            zeroXYValue/zeroGradient                   (onz)

                            zeroXValue/codedFixedGradient              traction x-normal plane (tractionx, shlx)
                                                                       specify okl function codedFixedGradientVelocity in udf 
                            zeroYValue/codedFixedGradient              traction y-normal plane (tractiony, shly)
                                                                       specify okl function codedFixedGradientVelocity in udf 
                            zeroZValue/codedFixedGradient              traction z-normal plane (tractionz, shlz)
                                                                       specify okl function codedFixedGradientVelocity in udf 
                            zeroNValue/codedFixedGradient              unaligned traction (traction, shl)

Tip

To setup some cases, you may need to use the boundaryIDMap parameter of the MESH section to apply the boundaryTypeMap options to the correct boundary IDs of the mesh (By default NekRS assumes that the boundaryIDs start at 1). Below is an example where the four boundary conditions are applied to the boundary IDs 389 (codeFixedValue), 231 (zeroGradient), 4 (zeroValue) and 23 (zeroValue).

[VELOCITY]
boundaryTypeMap = codedFixedValue, zeroGradient, zeroValue, zeroValue

[MESH]
boundaryIDMap = 389, 231, 4, 23

User Defined Value/Gradient

If a boundary condition requires a value setting rather than just a type, a suitable function will need to be provided within the .udf file. The name of the function used should be in the form of codedFixed + Value/Gradient + Velocity/Scalar/Mesh, E.G. codedFixedValueVelocity, codedFixedGradientScalar and codedFixedValueMesh.

All of these functions are passed the bcData struct which has the following parameters available:

Name	Type	Description
idM	int
fieldOffset	int
id	int
time	double
x, y, z	dfloat
nx, ny, nz	dfloat	normals
1x, t1y, t1z, t2x, t2y, t2z	dfloat	tangential directions
tr1, tr2	dfloat
u, v, w	dfloat	velocity
p	dfloat
uinterp, vinterp, winterp	dfloat	interpolated velocity values
scalarId	int
s	dfloat
flux	dfloat
sinterp	dfloat	interpolated scalar value
meshu, meshv, meshw	dfloat
trans, diff	dfloat	properties
usrwrk	@globalPtr const dfloat*

Internal / Periodic

None is used when a internal boundary condition is required or a periodic boundary condition has been set as part of the mesh and it does not need to be considered as part of the standard processing of boundary conditions.

Perodicity is linked to the mesh connectivity and is handled by the meshing tool.

Turbulent Inflow

Velocity Recycling