momentum

Description

The momentum function object computes linear/angular momentum, reporting integral values and optionally writing the fields.

Operands

Operand	Type	Location
input	-	-
output file	dat	$FOAM_CASE/postProcessing/<FO>/<time>/<file>
output field	-	-

Usage

Example of the momentum function object by using functions sub-dictionary in system/controlDict file:

momentum1
{
    // Mandatory entries (unmodifiable)
    type            momentum;
    libs            (fieldFunctionObjects);

    // Optional entries (runtime modifiable)
    regionType      all;
    writeMomentum   yes;
    writePosition   yes;
    writeVelocity   yes;
    p               p;
    U               U;
    rho             rho;
    rhoRef          1.0;

    cylindrical     true;
    origin          (0 0 0);
    e1              (1 0 0);
    e3              (0 0 1);

    // Optional (inherited) entries
    writePrecision  8;
    writeToFile     true;
    useUserTime     true;
    region          region0;
    enabled         true;
    log             true;
    timeStart       0;
    timeEnd         1000;
    executeControl  timeStep;
    executeInterval 1;
    writeControl    timeStep;
    writeInterval   1;
}

where the entries mean:

Property	Description	Type	Required	Default
type	Type name: momentum	word	yes	-
libs	Library name: fieldFunctionObjects	word	yes	-
regionType	Selection type: all/cellSet/cellZone	word	no	all
writeMomentum	Write (linear, angular) momentum fields	bool	no	no
writePosition	Write angular position component fields	bool	no	no
writeVelocity	Write angular velocity fields	bool	no	no
p	Pressure field name	word	no	p
U	Velocity field name	word	no	U
rho	Density field name	word	no	rho
rhoRef	Reference density (incompressible)	scalar	no	1.0
cylindrical	Use cylindrical coordinates	bool	no	no
origin	Origin for cylindrical coordinates	vector	conditional	-
name	Name of cellSet/cellZone if required	word	conditional	-

The inherited entries are elaborated in:

• functionObject
• writeFile

Usage by the postProcess utility is not available.

Notes on entries

• For incompressible cases, the value of rhoRef is used.
• When specifying the cylindrical coordinate system, the rotation can be specified directly with e1/e2/e3 axes, or via a rotation sub-dictionary.

For example:

origin      (0 0 0);
rotation
{
    type    cylindrical;
    axis    (0 0 1);
}

Stored properties

Reduced data

The following data are stored by the function object, and can be used by other function objects:

• momentum_x : x-component of momentum
• momentum_y : y-component of momentum
• momentum_z : z-component of momentum
• momentum_r : r-component of angular momentum
• momentum_theta : theta-component of angular momentum
• momentum_axis : axial-component of angular momentum

Further information

Tutorial:

• cavity

Source code:

• momentum

API:

• Foam::functionObjects::momentum

History: Introduced in version v1812