(

- 12.1 Reference State Structure Definition:
`ReferenceState_t` - 12.2 Reference State Example
- 12.3 Convergence History Structure Definition:
`ConvergenceHistory_t` - 12.4 Discrete Data Structure Definition:
`DiscreteData_t` - 12.5 Integral Data Structure Definition:
`IntegralData_t` - 12.6 Family Data Structure Definition:
`Family_t` - 12.7 Geometry Reference Structure Definition:
`GeometryReference_t` - 12.8 Family Boundary Condition Structure Definition:
`FamilyBC_t` - 12.9 Family Boundary Condition Data Set Structure Definition:
`FamilyBCDataSet_t` - 12.10 User-Defined Data Structure Definition:
`UserDefinedData_t` - 12.11 Gravity Data Structure Definition:
`Gravity_t`

This section contains miscellaneous structure types for describing reference states, convergence history, discrete field data, integral or global data, families, and user-defined data.

`ReferenceState_t` describes a reference state, which is a list
of geometric or flow-state quantities defined at a common location or
condition.
Examples of typical reference states associated with CFD calculations
are freestream, plenum, stagnation, inlet and exit.
Note that providing a `ReferenceState` description is
particularly important if items elsewhere in the CGNS database are
`NormalizedByUnknownDimensional`.

ReferenceState_t := { Descriptor_t ReferenceStateDescription ; (o) List( Descriptor_t Descriptor1 ... DescriptorN ) ; (o) List( DataArray_t<DataType, 1, 1> DataArray1 ... DataArrayN ) ; (o) DataClass_t DataClass ; (o) DimensionalUnits_t DimensionalUnits ; (o) List( UserDefinedData_t UserDefinedData1 ... UserDefinedDataN ) ; (o) } ;

- Default names for the
`Descriptor_t`,`DataArray_t`, and`UserDefinedData_t`lists are as shown; users may choose other legitimate names. Legitimate names must be unique within a given instance of`ReferenceState_t`and shall not include the names`DataClass`,`DimensionalUnits`, or`ReferenceStateDescription`.

Data-name identifiers associated with `ReferenceState` are shown in
the table below.

Data-Name Identifier |
| Description |
| Units |
---|---|---|---|---|

Mach | Mach number, M = q/c | - | ||

Mach_Velocity | Velocity scale, q | L/T
| ||

Mach_VelocitySound | Speed of sound scale, c | L/T
| ||

Reynolds | Reynolds number, Re = VL / _{R}ν | - | ||

Reynolds_Velocity | Velocity scale, V | L/T
| ||

Reynolds_Length | Length scale, L_{R} | L
| ||

Reynolds_ViscosityKinematic | Kinematic viscosity scale, ν | L^{2}/T
| ||

LengthReference | Reference length, L | L
| ||

In addition, any flowfield quantities (such as `Density`,
`Pressure`, etc.) can be included in the `ReferenceState`.

The reference length *L* (`LengthReference`) may be necessary for
`NormalizedByUnknownDimensional`
databases, to define the length scale used for nondimensionalizations.
It may be the same or different from the `Reynolds_Length` used to
define the Reynolds number.

Because of different definitions for angle of attack and angle of yaw,
these quantities are not explicitly defined in the SIDS.
Instead, the user can unambigouosly denote the freestream velocity
vector direction by giving `VelocityX`, `VelocityY`, and
`VelocityZ` in `ReferenceState`, (with the reference state
denoting the freestream).

Care should be taken when defining the reference state quantities to ensure consistency. (See the discussion in the section on Nondimensional Data Normalized by Unknown Dimensional Quantities.) For example, if velocity, length, and time are all defined, then the velocity stored should be length/time. If consistency is not followed, different applications could interpret the resulting data in different ways.

`DataClass` defines the
default for the class of data contained in the reference state.
If any reference state quantities are dimensional,
`DimensionalUnits`
may be used to describe the system of dimensional units employed.
If present, these two entities take precedence over all corresponding
entities at higher levels of the hierarchy, following the standard
precedence rules.

The `UserDefinedData_t`
data structure allows arbitrary user-defined data to be stored in
`Descriptor_t` and `DataArray_t` children without the
restrictions or implicit meanings imposed on these node types at other
node locations.

We recommend using the `ReferenceStateDescription` entity to
document the flow conditions.
The format of the documentation is currently unregulated.

An example is presented in this section of a reference state entity that contains dimensional data. An additional example of a nondimensional reference state is provided in the Structured Two-Zone Flat Plate Example.

A freestream reference state where all data quantities are dimensional.
Standard atmospheric conditions at sea level are assumed for static
quantities, and all stagnation variables are obtained using the
isentropic relations.
The flow velocity is 200 m/s aligned with the *x*-axis.
Dimensional units of kilograms, meters, and seconds are used.
The data class and system of units are specified at the
`ReferenceState_t` level rather
than attaching this information directly to the
`DataArray_t` entities for
each reference quantity.
Data-name identifiers are provided in the section
Conventions for Data-Name Identifiers.

ReferenceState_t ReferenceState = {{ Descriptor_t ReferenceStateDescription = {{ Data(char, 1, 45) = "Freestream at standard atmospheric conditions" ; }} ; DataClass_t DataClass = Dimensional ; DimensionalUnits_t DimensionalUnits = {{ MassUnits = Kilogram ; LengthUnits = Meter ; TimeUnits = Second ; TemperatureUnits = Kelvin ; AngleUnits = Radian ; }} ; DataArray_t<real, 1, 1> VelocityX = {{ Data(real, 1, 1) = 200. ; }} ; DataArray_t<real, 1, 1> VelocityY = {{ 0. }} ; DataArray_t<real, 1, 1> VelocityZ = {{ 0. }} ; DataArray_t<real, 1, 1> Pressure = {{ 1.0132E+05 }} ; DataArray_t<real, 1, 1> Density = {{ 1.226 }} ; DataArray_t<real, 1, 1> Temperature = {{ 288.15 }} ; DataArray_t<real, 1, 1> VelocitySound = {{ 340. }} ; DataArray_t<real, 1, 1> ViscosityMolecular = {{ 1.780E-05 }} ; DataArray_t<real, 1, 1> PressureStagnation = {{ 1.2806E+05 }} ; DataArray_t<real, 1, 1> DensityStagnation = {{ 1.449 }} ; DataArray_t<real, 1, 1> TemperatureStagnation = {{ 308.09 }} ; DataArray_t<real, 1, 1> VelocitySoundStagnation = {{ 351.6 }} ; DataArray_t<real, 1, 1> PressureDynamic = {{ 0.2542E+05 }} ; }} ;Note that all

Flow solver convergence history information is described by the
`ConvergenceHistory_t` structure.
This structure contains the number of iterations and a list of data
arrays containing convergence information at each iteration.

ConvergenceHistory_t := { Descriptor_t NormDefinitions ; (o) List( Descriptor_t Descriptor1 ... DescriptorN ) ; (o) int Iterations ; (r) List( DataArray_t<DataType, 1, Iterations> DataArray1 ... DataArrayN ) ; (o) DataClass_t DataClass ; (o) DimensionalUnits_t DimensionalUnits ; (o) List( UserDefinedData_t UserDefinedData1 ... UserDefinedDataN ) ; (o) } ;

- Default names for the
`Descriptor_t`,`DataArray_t`, and`UserDefinedData_t`lists are as shown; users may choose other legitimate names. Legitimate names must be unique within a given instance of`ConvergenceHistory_t`and shall not include the names`DataClass`,`DimensionalUnits`, or`NormDefinitions`. -
`Iterations`is the only required field for`ConvergenceHistory_t`.

`Iterations` identifies the number of iterations for which
convergence information is recorded.
This value is also passed into each of the
`DataArray_t` entities,
defining the length of the data arrays.

`DataClass` defines the
default for the class of data contained in the convergence history.
If any convergence-history data is dimensional,
`DimensionalUnits`
may be used to describe the system of dimensional units employed.
If present, these two entities take precedence over all corresponding
entities at higher levels of the hierarchy, following the standard
precedence rules.

The `UserDefinedData_t`
data structure allows arbitrary user-defined data to be stored in
`Descriptor_t` and `DataArray_t` children without the
restrictions or implicit meanings imposed on these node types at other
node locations.

Measures used to record convergence vary greatly among current
flow-solver implementations.
Convergence information typically includes global forces, norms of
equation residuals, and norms of solution changes.
Attempts to systematically define a set of convergence measures within
the CGNS project have been futile.
For global parameters, such as forces and moments, a set of
standardized data-array identifiers is used.
For equations residuals and solution changes, no such standard list
exists.
It is suggested that data-array identifiers for norms of equations
residuals begin with `RSD`, and those for solution changes begin with
`CHG`.
For example, `RSDMassRMS` could be used for the
*L*_{2}-norm (RMS) of mass conservation residuals.
It is also strongly recommended that `NormDefinitions` be utilized to
describe the convergence information recorded in the data arrays.
The format used to describe the convergence norms in `NormDefinitions`
is currently unregulated.

`DiscreteData_t` provides a description of generic discrete data (i.e.,
data defined on a computational grid); it is identical to
`FlowSolution_t`
except for its type name.
This structure can be used to store field data, such as fluxes or
equation residuals, that is not typically considered part of the flow
solution.
`DiscreteData_t` contains a list for data arrays, identification of
grid location, and a mechanism for identifying rind-point data included
in the data arrays.
All data contained within this structure must be defined at the same
grid location and have the same amount of rind-point data.

DiscreteData_t< int CellDimension, int IndexDimension, int VertexSize[IndexDimension], int CellSize[IndexDimension] > := { List( Descriptor_t Descriptor1 ... DescriptorN ) ; (o) GridLocation_t GridLocation ; (o/d) IndexRange<IndexDimension> PointRange ; (o) IndexArray<IndexDimension, ListLength[], int> PointList ; (o) Rind_t<IndexDimension> Rind ; (o/d) List( DataArray_t<DataType, IndexDimension, DataSize[]> DataArray1 ... DataArrayN ) ; (o) DataClass_t DataClass ; (o) DimensionalUnits_t DimensionalUnits ; (o) List( UserDefinedData_t UserDefinedData1 ... UserDefinedDataN ) ; (o) } ;

- Default names for the
`Descriptor_t`,`DataArray_t`, and`UserDefinedData_t`lists are as shown; users may choose other legitimate names. Legitimate names must be unique within a given instance of`DiscreteData_t`and shall not include the names`DataClass`,`DimensionalUnits`,`GridLocation`,`PointList`,`PointRange`, or`Rind`. - There are no required fields for
`DiscreteData_t`.`GridLocation`has a default of`Vertex`if absent.`Rind`also has a default if absent; the default is equivalent to having an instance of`Rind`whose`RindPlanes`array contains all zeros. - Both of the fields
`PointRange`and`PointList`are optional. Only one of these two fields may be specified. - The structure parameter
`DataType`must be consistent with the data stored in the`DataArray_t`entities. - For unstructured zones
`GridLocation`options are limited to`Vertex`or`CellCenter`, unless one of`PointRange`or`PointList`is present. - Indexing of data within the
`DataArray_t`structures, must be consistent with the associated numbering of vertices or elements.

`DiscreteData_t` requires four structure parameters;
`CellDimension` identifies the dimensionality of cells or elements,
`IndexDimension` identifies the dimensionality of the grid size
arrays, and `VertexSize` and `CellSize` are the number of
core vertices and cells, respectively, in each index direction,
excluding rind points.
For structured zones, core vertices and cells begin at `[1,1,1]`
(in 3-D) and end at `VertexSize` and `CellSize`,
respectively.
For unstructured zones, `IndexDimension` is always 1.

The arrays of discrete data are stored in the list of
`DataArray_t` entities.
The field `GridLocation`
specifies the location of the data with respect to the grid; if
absent, the data is assumed to coincide with grid vertices (i.e.,
`GridLocation = Vertex`).
All data within a given instance of `DiscreteData_t` must reside at the
same grid location.

For structured grids, the value of `GridLocation` alone specifies the
location and indexing of the discrete. Vertices are explicity indexed.
Cell centers and face centers are indexed using the minimum of the connecting
vertex indices, as described in the section
Structured Grid Notation and Indexing Conventions.

For unstructured grids, the value of `GridLocation` alone specifies
location and indexing of discrete data only for vertex and cell-centered data.
The reason for this is that element-based grid connectivity provided in the
`Elements_t` data structures explicitly indexes only
vertices and cells. For data stored at alternate grid locations (e.g., edges),
additional connectivity information is needed. This is provided by the optional
fields `PointRange` and `PointList`; these refer to vertices,
edges, faces or cell centers, depending on the values of `CellDimension`
and `GridLocation`. The following table shows these relations.

CellDimension | GridLocation | |||
---|---|---|---|---|

Vertex | EdgeCenter | *FaceCenter | CellCenter | |

1 | vertices | - | - | cells (line elements) |

2 | vertices | edges | - | cells (area elements) |

3 | vertices | edges | faces | cells (volume elements) |

*Note*: In the table, *FaceCenter stands for the possible types:
`IFaceCenter`, `JFaceCenter`, `KFaceCenter`,
or `FaceCenter`.

Although intended for edge or face-based discrete data for unstructured grids,
the fields `PointRange/List` may also be used to (redundantly) index vertex
and cell-centered data. In all cases, indexing of flow solution data corresponds
to the element numbering as defined in the `Elements_t`
data structures.

`Rind` is an optional field
that indicates the number of rind planes (for
structured grids) or rind points or elements (for unstructured grids)
included in the data.
Its purpose and function are identical to those described for the
`GridCoordinates_t` structure.
Note, however, that the `Rind` in this structure is independent
of the `Rind` contained in `GridCoordinates_t`.
They are not required to contain the same number of rind planes or
elements.
Also, the location of any flow-solution rind points is assumed to be
consistent with the location of the core flow solution points (e.g.,
if `GridLocation = CellCenter`,
rind points are assumed to be located at fictitious cell centers).

`DataClass` defines the
default for the class of data contained in the
`DataArray_t` entities.
For dimensional data,
`DimensionalUnits`
may be used to describe the system of dimensional units employed.
If present, these two entities take precedence over all corresponding
entities at higher levels of the hierarchy, following the standard
precedence rules.

The `UserDefinedData_t`
data structure allows arbitrary user-defined data to be stored in
`Descriptor_t` and `DataArray_t` children without the
restrictions or implicit meanings imposed on these node types at other
node locations.

dependencies:

`DiscreteData_t` requires the structure function `ListLength`,
which is used to specify the number of entities (e.g. vertices) corresponding
to a given `PointRange` or `PointList`. If `PointRange` is
specified, then `ListLength` is obtained from the number of points
(inclusive) between the beginning and ending indices of `PointRange`.
If `PointList` is specified, then `ListLength` is the number of
indices in the list of points. In this situation, `ListLength` becomes
a user input along with the indices of the list `PointList`.
By *user* we mean the application code that is generating the CGNS
database.

dependencies:

The function `DataSize[]` is the size of discrete-data arrays.
It is identical to the function
`DataSize[]` defined for
`FlowSolution_t`.

`IntegralData_t` provides a description of generic global or
integral data that may be associated with a particular zone or an entire
database.
In contrast to `DiscreteData_t`,
integral data is not associated with any specific field location.

IntegralData_t := { List( Descriptor_t Descriptor1 ... DescriptorN ) ; (o) List( DataArray_t<DataType, 1, 1> DataArray1 ... DataArrayN ) ; (o) DataClass_t DataClass ; (o) DimensionalUnits_t DimensionalUnits ; (o) List( UserDefinedData_t UserDefinedData1 ... UserDefinedDataN ) ; (o) } ;

- Default names for the
`Descriptor_t`,`DataArray_t`, and`UserDefinedData_t`lists are as shown; users may choose other legitimate names. Legitimate names must be unique within a given instance of`DiscreteData_t`and shall not include the names`DataClass`or`DimensionalUnits`. - There are no required fields for
`IntegralData_t`. - The structure parameter
`DataType`must be consistent with the data stored in the`DataArray_t`entities.

`DataClass` defines the
default class for data contained in the
`DataArray_t` entities.
For dimensional data,
`DimensionalUnits`
may be used to describe the system of units employed.
If present, these two entities take precedence over all corresponding
entities at higher levels of the hierarchy, following the standard
precedence rules.

`UserDefinedData_t`
data structure allows arbitrary user-defined data to be stored in
`Descriptor_t` and `DataArray_t` children without the
restrictions or implicit meanings imposed on these node types at other
node locations.

Geometric associations need to be set through one layer of indirection.
That is, rather than setting the geometry data for each mesh entity
(nodes, edges, and faces), they are associated to intermediate objects.
The intermediate objects are in turn linked to nodal regions of the
computational mesh.
We define a CFD *family* as this intermediate object.
This layer of indirection is necessary since there is rarely a 1-to-1
connection between mesh regions and geometric entities.

The `Family_t` data structure holds the CFD family data.
Each mesh surface is linked to the geometric entities of CAD databases
by a name attribute.
This attribute corresponds to a family of CAD geometric entities on which
the mesh face is projected.
Each one of these geometric entities is described in a CAD file and is not
redefined within the CGNS file.
A `Family_t` data structure may be included in the
`CGNSBase_t` structure for
each CFD family of the model.

A hierarchy of `Family_t` nodes can be build by adding a
`Family_t` node as child of an existing `Family_t` node.
One can mimic an existing CAD hierarchy or add another hierarchy for
special application purpose.

In case of a tree of `Family_t`, the actual reference to a family
in the tree should follow the pattern as described in
`Base Level Families`.

The `Family_t` structure contains all information pertinent to a
CFD family.
This information includes the name attribute or family name, the
boundary conditions applicable to these mesh regions, and the referencing
to the CAD databases.

Family_t := { List( Descriptor_t Descriptor1 ... DescriptorN ) ; (o) FamilyBC_t FamilyBC ; (o) List( GeometryReference_t GeometryReference1 ... GeometryReferenceN ) ; (o) RotatingCoordinates_t RotatingCoordinates ; (o) List( Family_t Family1 ... FamilyN ) ; (o) List( FamilyName_t FamilyName1 ... FamilyNameN ) ; (o) List( UserDefinedData_t UserDefinedData1 ... UserDefinedDataN ) ; (o) int Ordinal ; (o) } ;

- All data structures contained in
`Family_t`are optional. - Default names for the
`Descriptor_t`,`GeometryReference1_t`, and`UserDefinedData_t`lists are as shown; users may choose other legitimate names. Legitimate names must be unique at this level and must not include the names`FamilyBC`,`Ordinal`, or`RotatingCoordinates`. - The CAD referencing data are written in the
`GeometryReference_t`data structures. They identify the CAD systems and databases where the geometric definition of the family is stored. - The boundary condition type pertaining to a family is contained in
the data structure
`FamilyBC_t`. If this boundary condition type is to be used, the`BCType`specified under`BC_t`must be`FamilySpecified`. - For the purpose of defining zone properties, families are extended
to a volume of cells.
In such case, the
`GeometryReference_t`structures are not used. - The mesh is linked to the family by attributing a family name to a
BC patch or a zone in the data structure
`BC_t`or`Zone_t`, respectively. - A hierarchy of families is possible through the list of
`FamilyName_t`nodes. These nodes contain both a user defined node name and a family name. The node name`FamilyParent`may be used to specify the family name for the parent of the current`Family_t`node. -
`Ordinal`is defined in the SIDS as a user-defined integer with no restrictions on the values that it can contain. It may be used here to attribute a number to the family. - A
`Family_t`tree structure can be specified using the list of`Family_t`hildren nodes. Into each of these children nodes the note #7 can be used to have a back tracking of the node parent.

Rotation of the CFD family may be defined using the
`RotatingCoordinates_t`
data structure.

`UserDefinedData_t`
data structure allows arbitrary user-defined data to be stored in
`Descriptor_t` and `DataArray_t` children without the
restrictions or implicit meanings imposed on these node types at other
node locations.

The standard interface data structure identifies the CAD systems used
to generate the geometry, the CAD files where the geometry is stored,
and the geometric entities corresponding to the family.
The `GeometryReference_t` structures contain all the information
necessary to associate a CFD family to the CAD databases.
For each `GeometryReference_t` structure, the CAD format
is recorded in `GeometryFormat`, and the CAD file in
`GeometryFile`.
The geometry entity or entities within this CAD file that correspond to
the family are recorded under the `GeometryEntity_t` nodes.

GeometryReference_t := { List( Descriptor_t Descriptor1 ... DescriptorN ) ; (o) GeometryFormat_t GeometryFormat ; (r) GeometryFile_t GeometryFile ; (r) List (GeometryEntity_t GeometryEntity1 ... GeometryEntityN) ; (o/d) List( UserDefinedData_t UserDefinedData1 ... UserDefinedDataN ) ; (o) } ;

The `GeometryFormat` is an enumeration type that identifies the CAD
system used to generate the geometry.

GeometryFormat_t := Enumeration( GeometryFormatNull, GeometryFormatUserDefined, NASA-IGES, SDRC, Unigraphics, ProEngineer, ICEM-CFD ) ;

- Default names for the
`Descriptor_t`,`GeometryEntity_t`, and`UserDefinedData_t`lists are as shown; users may choose other legitimate names. Legitimate names must be unique at this level and must not include the names`GeometryFile`or`GeometryFormat`. - By default, there is only one
`GeometryEntity`and its name is the family name. - There is no limit to the number of CAD files or CAD systems referenced in a CGNS file. Different parts of the same model may be described with different CAD files of different CAD systems.
- Other CAD geometry formats may be added to this list as needed.

One of the main advantages of the concept of a layer of indirection (called a family here) is that the mesh density and the geometric entities may be modified without altering the association between nodes and intermediate objects, or between intermediate objects and geometric entities. This is very beneficial when handling boundary conditions and properties. Instead of setting boundary conditions directly on mesh entities, or on CAD entities, they may be associated to the intermediate objects. Since these intermediate objects are stable in the sense that they are not subject to mesh or geometric variations, the boundary conditions do not need to be redefined each time the model is modified. Using the concept of indirection, the boundary conditions and property settings are made independent of operations such as geometric changes, modification of mesh topology (i.e., splitting into zones), mesh refinement and coarsening, etc.

The `FamilyBC_t` data structure contains the boundary condition
type.
It is envisioned that it will be extended to hold both material and
volume properties as well.

FamilyBC_t := { BCType_t BCType; (r) List( FamilyBCDataSet_t<ListLength> BCDataSet1 ... BCDataSetN ) ; (o) } ;

- Default names for the
`FamilyBCDataSet_t`list are as shown; users may choose other legitimate names. Legitimate names must be unique within a given instance of`FamilyBC_t`and shall not include the name`BCType`.

`BCType` specifies the
boundary-condition type, which gives general information on the
boundary-condition equations to be enforced.
Boundary conditions are to be applied at the locations specified by the
`BC_t` structure(s) associated with the
CFD family.

The `FamilyBC_t` structure provides for a list of
boundary-condition data sets.
In general, the proper `FamilyBCDataSet_t`
instance to impose on the CFD family is determined by the
`BCType` association table.
The mechanics of determining the proper data set to impose is described
in the section Matching Boundary
Condition Data Sets.

For a few boundary conditions, such as a symmetry plane or polar
singularity, the value of `BCType`
completely describes the equations to impose, and no instances of
`FamilyBCDataSet_t` are needed.
For "simple" boundary conditions, where a single set of Dirichlet and/or
Neumann data is applied, a single `FamilyBCDataSet_t` will likely appear
(although this is not a requirement).
For "compound" boundary conditions, where the equations to impose
are dependent on local flow conditions, several instances of
`FamilyBCDataSet_t` will likely appear; the procedure for choosing
the proper data set is more complex as described in the section
Matching Boundary Condition Data Sets.

`FamilyBCDataSet_t` contains Dirichlet and Neumann data for a single set
of boundary-condition equations. Its intended use is for simple boundary-condition types,
where the equations imposed do not depend on local flow conditions.

FamilyBCDataSet_t := { List( Descriptor_t Descriptor1 ... DescriptorN ) ; (o) BCTypeSimple_t BCTypeSimple ; (r) BCData_t<1> DirichletData ; (o) BCData_t<1> NeumannData ; (o) ReferenceState_t ReferenceState ; (o) DataClass_t DataClass ; (o) DimensionalUnits_t DimensionalUnits ; (o) List( UserDefinedData_t UserDefinedData1 ... UserDefinedDataN ) ; (o) } ;

- Default names for the
`Descriptor_t`and`UserDefinedData_t`lists are as shown; users may choose other legitimate names. Legitimate names must be unique within a given instance of`FamilyBCDataSet_t`and shall not include the names`BCTypeSimple`,`DataClass`,`DimensionalUnits`,`DirichletData`,`NeumannData`or`ReferenceState`. -
`BCTypeSimple`is the only required field. All other fields are optional.

`BCTypeSimple` specifies the boundary-condition type,
which gives general information on the boundary-condition equations to be enforced.
`BCTypeSimple` is also used for matching boundary condition data sets as discussed
in the section Matching Boundary Condition Data Sets.

Boundary-condition data is separated by equation type into Dirichlet and Neumann conditions. Dirichlet boundary conditions impose the value of the given variables, whereas Neumann boundary conditions impose the normal derivative of the given variables. The mechanics of specifying Dirichlet and Neumann data for boundary conditions is covered in section Boundary Condition Specification Data.

The substructures `DirichletData` and `NeumannData` contain boundary-condition data
defined as globally constant over the family.

Reference quantities applicable to the set of boundary-condition data are contained in the
`ReferenceState` structure.
`DataClass` defines the default for the class of
data contained in the boundary condition data. If the boundary conditions contain dimensional data,
`DimensionalUnits` may be
used to describe the system of dimensional units employed. If present, these three entities take
precedence over all corresponding entities at higher levels of the hierarchy,
following the standard precedence rules.

`UserDefinedData_t` data structure allows arbitrary
user-defined data to be stored in `Descriptor_t` and `DataArray_t`
children without the restrictions or implicit meanings imposed on these
node types at other node locations.

Note that `FamilyBCDataSet_t` is similar to the data structure
`BCDataSet_t`. The
primary difference is that `FamilyBCDataSet_t` only allows for globally
constant Dirichlet and Neumann data.

Since the needs of all CGNS users cannot be anticipated,
`UserDefinedData_t` provides a means of storing arbitrary
user-defined data in `Descriptor_t`
and `DataArray_t`
children without the restrictions or implicit meanings imposed on these
node types at other node locations.

UserDefinedData_t := { List( Descriptor_t Descriptor1 ... DescriptorN ) ; (o) GridLocation_t GridLocation ; (o/d) IndexRange_t<IndexDimension> PointRange ; (o) IndexArray_t<IndexDimension, ListLength, int> PointList ; (o) List( DataArray_t<> DataArray1 ... DataArrayN ) ; (o) DataClass_t DataClass ; (o) DimensionalUnits_t DimensionalUnits ; (o) FamilyName_t FamilyName ; (o) List( AdditionalFamilyName_t AddFamilyName1 ... AddFamilyNameN ) ; (o) List( UserDefinedData_t UserDefinedData1 ... UserDefinedDataN ) ; (o) int Ordinal ; (o) } ;

- Default names for the
`Descriptor_t`,`DataArray_t`, and`UserDefinedData_t`lists are as shown; users may choose other legitimate names. Legitimate names must be unique within a given instance of`UserDefinedData_t`and shall not include the names`DataClass`,`DimensionalUnits`,`FamilyName`,`GridLocation`,`Ordinal`,`PointList`, or`PointRange`. -
`GridLocation`may be set to`Vertex`,`IFaceCenter`,`JFaceCenter`,`KFaceCenter`,`FaceCenter`,`CellCenter`, or`EdgeCenter`. If`GridLocation`is absent, then its default value is`Vertex`. When`GridLocation`is set to`Vertex`, then`PointList`or`PointRange`refer to node indices, for both structured and unstructured grids. When`GridLocation`is set to`FaceCenter`, then`PointList`or`PointRange`refer to face elements. -
`GridLocation`,`PointRange`, and`PointList`may only be used when`UserDefinedData_t`is located below a`Zone_t`structure in the database hierarchy. - Only one of
`PointRange`and`PointList`may be specified. - Both
`FamilyName`and`AdditionalFamilyName`should refer to a`CGNSBase_t`level`Family_t`, in the parent base or in another sibling base (see Base Level Families).

The `Gravity_t` data structure may be used to define the
gravitational vector.

Gravity_t := { List( Descriptor_t Descriptor1 ... DescriptorN ) ; (o) DataArray_t<real, 1, PhysicalDimension> GravityVector ; (r) DataClass_t DataClass ; (o) DimensionalUnits_t DimensionalUnits ; (o) List( UserDefinedData_t UserDefinedData1 ... UserDefinedDataN ) ; (o) } ;

- Default names for the
`Descriptor_t`and`UserDefinedData_t`lists are as shown; users may choose other legitimate names. Legitimate names must be unique within a given instance of`Gravity_t`and shall not include the names`DataClass`,`DimensionalUnits`, or`GravityVector`.

The only required field under the `Gravity_t` data structure is
`GravityVector`, which contains the components of the gravity
vector in the coordinate system being used.

`DataClass` defines the default
class for data contained in the
`DataArray_t` entity.
For dimensional data,
`DimensionalUnits` may be
used to describe the system of units employed.
If present, these two entities take precedence over the corresponding
entities at higher levels of the CGNS hierarchy, following the
standard precedence rules.

`UserDefinedData_t`
data structure allows arbitrary user-defined data to be stored in
`Descriptor_t` and `DataArray_t` children without the
restrictions or implicit meanings imposed on these node types at other
node locations.