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5.2 Boundaries
In this section we discuss the way in which boundaries are treated in OpenFOAM. The subject of boundaries is quite complex because their role in modelling is not simply that of a geometric entity but an integral part of the solution and numerics through boundary conditions or inter-boundary ‘connections’. A discussion of boundaries sits uncomfortably between a discussion on meshes, fields, discretisation, computational processing etc.
We first need to consider that, for the purpose of applying boundary conditions, a boundary is generally broken up into a set of patches. One patch may include one or more enclosed areas of the boundary surface which do not necessarily need to be physically connected. A type is assigned to every patch as part of the mesh description, as part of the boundary file described in section 5.1.2. It describes the type of patch in terms of geometry or a data ‘communication link’. An example boundary file is shown below for a sonicFoam case. A type entry is clearly included for every patch (inlet, outlet, etc.), with types assigned that include patch, symmetryPlane and empty.
18 6
19 (
20 inlet
21 {
22 type patch;
23 nFaces 50;
24 startFace 10325;
25 }
26 outlet
27 {
28 type patch;
29 nFaces 40;
30 startFace 10375;
31 }
32 bottom
33 {
34 type symmetryPlane;
35 inGroups 1(symmetryPlane);
36 nFaces 25;
37 startFace 10415;
38 }
39 top
40 {
41 type symmetryPlane;
42 inGroups 1(symmetryPlane);
43 nFaces 125;
44 startFace 10440;
45 }
46 obstacle
47 {
48 type patch;
49 nFaces 110;
50 startFace 10565;
51 }
52 defaultFaces
53 {
54 type empty;
55 inGroups 1(empty);
56 nFaces 10500;
57 startFace 10675;
58 }
59 )
60
61 // ************************************************************************* //
The user can scan the tutorials for mesh generation configuration files, e.g. blockMeshDict for blockMesh (see section 5.3) and snappyHexMeshDict for snappyHexMesh (see section 5.4, for examples of different types being used. The following example provides documentation and lists cases that use the symmetryPlane condition.
foamInfo -a symmetryPlane
find $FOAM_TUTORIALS -name snappyHexMeshDict | \
xargs grep -El "type[\t ]*wall"
5.2.1 Geometric (constraint) patch types
The main geometric types available in OpenFOAM are summarised below. This is not a complete list; for all types see $FOAM_SRC/finiteVolume/fields/fvPatchFields/constraint.
- patch: generic type containing no geometric or topological information about the mesh, e.g. used for an inlet or an outlet.
- wall: for patch that coincides with a solid wall, required for some physical modelling, e.g. wall functions in turbulence modelling.
- symmetryPlane: for a planar patch which is a symmetry plane.
- symmetry: for any (non-planar) patch which uses the symmetry plane (slip) condition.
- empty: for solutions in in 2 (or 1) dimensions (2D/1D), the type used on each patch whose plane is normal to the 3rd (and 2nd) dimension for which no solution is required.
- wedge: for 2 dimensional axi-symmetric cases, e.g. a cylinder, the geometry is specified as a wedge of small angle (e.g. ) and 1 cell thick, running along the centre line, straddling one of the coordinate planes, as shown in Figure 5.3; the axi-symmetric wedge planes must be specified as separate patches of wedge type.
- cyclic: enables two patches to be treated as if they are physically connected; used for repeated geometries; one cyclic patch is linked to another through a neighbourPatch keyword in the boundary file; each pair of connecting faces must have similar area to within a tolerance given by the matchTolerance keyword in the boundary file.
- cyclicAMI: like cyclic, but for 2 patches whose faces are non matching; used for sliding interface in rotating geometry cases.
- processor: the type that describes inter-processor boundaries for meshes decomposed for parallel running.
5.2.2 Basic boundary conditions
Boundary conditions are specified in field files, e.g. p, U, in time directories as described in section 4.2.8. An example pressure field file, p, is shown below for the sonicFoam case corresponding to the boundary file presented in section 5.2.1.
18
19 internalField uniform 1;
20
21 boundaryField
22 {
23 inlet
24 {
25 type fixedValue;
26 value uniform 1;
27 }
28
29 outlet
30 {
31 type waveTransmissive;
32 field p;
33 psi thermo:psi;
34 gamma 1.4;
35 fieldInf 1;
36 lInf 3;
37 value uniform 1;
38 }
39
40 bottom
41 {
42 type symmetryPlane;
43 }
44
45 top
46 {
47 type symmetryPlane;
48 }
49
50 obstacle
51 {
52 type zeroGradient;
53 }
54
55 defaultFaces
56 {
57 type empty;
58 }
59 }
60
61 // ************************************************************************* //
Every patch includes a type entry that specifies the type of boundary condition. They range from a basic fixedValue condition applied to the inlet, to a complex waveTransmissive condition applied to the outlet. The patches with non-generic types, e.g. symmetryPlane, defined in boundary, use consistent boundary condition types in the p file.
The main basic boundary condition types available in OpenFOAM are summarised below using a patch field named . This is not a complete list; for all types see $FOAM_SRC/finiteVolume/fields/fvPatchFields/basic.
- fixedValue: value of is specified by value.
- fixedGradient: normal gradient of () is specified by gradient.
- zeroGradient: normal gradient of is zero.
- calculated: patch field calculated from other patch fields.
- mixed: mixed fixedValue/ fixedGradient condition depending on valueFraction
where
(5.1) - directionMixed: mixed condition with tensorial valueFraction, to allow different conditions in normal and tangential directions of a vector patch field, e.g. fixedValue in the tangential direction, zeroGradient in the normal direction.
5.2.3 Derived types
There are numerous more complex boundary conditions derived from the basic conditions. For example, many complex conditions are derived from fixedValue, where the value is calculated by a function of other patch fields, time, geometric information, etc. Some other conditions derived from mixed/directionMixed switch between fixedValue and fixedGradient (usually zeroGradient).
There are a number of ways the user can list the available boundary conditions in OpenFOAM, with the -listScalarBCs and -listVectorBCs utility being the quickest. The boundary conditions for scalar fields and vector fields, respectively, can be listed for a given solver, e.g. simpleFoam, as follows.
simpleFoam -listScalarBCs -listVectorBCs
foamInfo totalPressure
5.2.3.1 The inlet/outlet condition
The inletOutlet condition is one derived from mixed, which switches between zeroGradient when the fluid flows out of the domain at a patch face, and fixedValue, when the fluid is flowing into the domain. For inflow, the inlet value is specified by an inletValue entry. A good example of its use can be seen in the damBreak tutorial, where it is applied to the phase fraction on the upper atmosphere boundary. Where there is outflow, the condition is well posed, where there is inflow, the phase fraction is fixed with a value of 0, corresponding to 100% air.
18
19 internalField uniform 0;
20
21 boundaryField
22 {
23 leftWall
24 {
25 type zeroGradient;
26 }
27
28 rightWall
29 {
30 type zeroGradient;
31 }
32
33 lowerWall
34 {
35 type zeroGradient;
36 }
37
38 atmosphere
39 {
40 type inletOutlet;
41 inletValue uniform 0;
42 value uniform 0;
43 }
44
45 defaultFaces
46 {
47 type empty;
48 }
49 }
50
51 // ************************************************************************* //
5.2.3.2 Entrainment boundary conditions
The combination of the totalPressure condition on pressure and pressureInletOutletVelocity on velocity is extremely common for patches where some inflow occurs and the inlet flow velocity is not known. That includes the atmosphere boundary in the damBreak tutorial, inlet conditions where only pressure is known, outlets where flow reversal may occur, and where flow in entrained, e.g. on boundaries surrounding a jet through a nozzle.
The totalPressure condition specifies
| (5.2) |
The idea behind this combination is that the condition is a standard combination in the case of outflow, but for inflow the normal velocity is allowed to find its own value. Under these circumstances, a rapid rise in velocity presents a risk of instability, but the rise is moderated by the reduction of inlet pressure, and hence driving pressure gradient, as the inflow velocity increases.
The specification of these boundary conditions in the U and p_rgh files, in the damBreak case, are shown below.
18 dimensions [0 1 -1 0 0 0 0];
19
20 internalField uniform (0 0 0);
21
22 boundaryField
23 {
24 leftWall
25 {
26 type noSlip;
27 }
28 rightWall
29 {
30 type noSlip;
31 }
32 lowerWall
33 {
34 type noSlip;
35 }
36 atmosphere
37 {
38 type pressureInletOutletVelocity;
39 value uniform (0 0 0);
40 }
41 defaultFaces
42 {
43 type empty;
44 }
45 }
46
47
48 // ************************************************************************* //
18
19 internalField uniform 0;
20
21 boundaryField
22 {
23 leftWall
24 {
25 type fixedFluxPressure;
26 value uniform 0;
27 }
28
29 rightWall
30 {
31 type fixedFluxPressure;
32 value uniform 0;
33 }
34
35 lowerWall
36 {
37 type fixedFluxPressure;
38 value uniform 0;
39 }
40
41 atmosphere
42 {
43 type totalPressure;
44 p0 uniform 0;
45 }
46
47 defaultFaces
48 {
49 type empty;
50 }
51 }
52
53 // ************************************************************************* //
5.2.3.3 Fixed flux pressure
In the above example, it can be seen that all the wall boundaries use a boundary condition named fixedFluxPressure. This boundary condition is used for pressure in situations where zeroGradient is generally used, but where body forces such as gravity and surface tension are present in the solution equations. The condition adjusts the gradient accordingly.
5.2.3.4 Time-varying boundary conditions
There are several boundary conditions for which some input parameters are specified by a function of time (using Function1 functionality) class. They can be searched by the following command.
find $FOAM_SRC/finiteVolume/fields/fvPatchFields -type f | \
xargs grep -l Function1 | xargs dirname | sort -u
The Function1 is specified by a keyword following the uniformValue entry, followed by parameters that relate to the particular function. The Function1 options are list below.
- constant: constant value.
- table: inline list of (time value) pairs; interpolates values linearly between times.
- tableFile: as above, but with data supplied in a separate file.
- csvFile: time-value data supplied in a file in CSV format.
- square: square-wave function.
- sine: sine function.
- one and zero: constant one and zero values.
- polynomial: polynomial function using a list (coeff exponent) pairs.
- scale: scales a given value function by a scalar scale function; both entries can be themselves Function1; scale function is often a ramp function (below), with value controlling the ramp value.
- linearRamp, quadraticRamp, halfCosineRamp, quarterCosineRamp and quarterSineRamp: monotonic ramp functions which ramp from 0 to 1 over specified duration.
Examples or a time-varying inlet for a scalar are shown below.
{
type uniformFixedValue;
uniformValue constant 2;
}
inlet
{
type uniformFixedValue;
uniformValue table ((0 0) (10 2));
}
inlet
{
type uniformFixedValue;
uniformValue polynomial ((1 0) (2 2)); // = 1*t^0 + 2*t^2
}
inlet
{
type uniformFixedValue;
uniformValue
{
type tableFile;
file "dataTable.txt";
}
}
inlet
{
type uniformFixedValue;
uniformValue
{
type csvFile;
nHeaderLine 4; // number of header lines
refColumn 0; // time column index
componentColumns (1); // data column index
separator ","; // optional (defaults to ",")
mergeSeparators no; // merge multiple separators
file "dataTable.csv";
}
}
inlet
{
type uniformFixedValue;
uniformValue
{
type square;
frequency 10;
amplitude 1;
scale 2; // Scale factor for wave
level 1; // Offset
}
}
inlet
{
type uniformFixedValue;
uniformValue
{
type sine;
frequency 10;
amplitude 1;
scale 2; // Scale factor for wave
level 1; // Offset
}
}
input // ramp from 0 -> 2, from t = 0 -> 0.4
{
type uniformFixedValue;
uniformValue
{
type scale;
scale linearRamp;
start 0;
duration 0.4;
value 2;
}
}