MOMENTUM_SOURCE

Specifies the momentum source.

Type

AcuSolve Command

Syntax

MOMENTUM_SOURCE (“name”) {parameters...}

Qualifier

User-given name.

Parameters

type (enumerated) [=constant]

Type of the momentum_source.

none: No momentum_source.
constant or const: Constant momentum_source. Requires momentum_source.
piecewise_linear or linear: Piecewise linear curve fit. Requires curve_fit_values and curve_fit_variable.
cubic_spline or spline: Cubic spline curve fit. Requires curve_fit_values and curve_fit_variable.
user_function or user: User-defined function. Requires user_function, user_values and user_strings.

momentum_source (array) [={0,0,0}]

The constant value of the momentum source. This array must have exactly three components in the global xyz coordinate system. Used with constant type.

Note: momentum_source and gravitational_acceleration under EQUATION are cumulative.

curve_fit_values or curve_values (array) [={0,0,0,0}]

A four-column array of independent-variable/momentum source data values. Used with piecewise_linear and cubic_spline types.

curve_fit_variable or curve_var (enumerated) [no default]

Independent variable of the curve fit. Used with piecewise_linear and cubic_spline types.

x_coordinate or xcrd: X-component of coordinates.
y_coordinate or ycrd: Y-component of coordinates.
z_coordinate or zcrd: Z-component of coordinates.
x_reference_coordinate or xrefcrd: X-component of reference coordinates.
y_reference_coordinate or yrefcrd: Y-component of reference coordinates.
z_reference_coordinate or zrefcrd: Z-component of reference coordinates.
x_velocity or xvel: X-component of velocity.
y_velocity or yvel: Y-component of velocity.
z_velocity or zvel: Z-component of velocity.
velocity_magnitude or vel_mag: Velocity magnitude.
pressure or pres: Pressure.
temperature or temp: Temperature.
eddy_viscosity or eddy: Turbulence eddy viscosity.
kinetic_energy or tke: Turbulence kinetic energy.
velocity_scale or tvel: Transition velocity scale.
dissipation_rate or teps: Turbulence dissipation rate.
eddy_frequency or tomega: Turbulence frequency.
eddy_time or ttau: Turbulence eddy time.
intermittency or tintc: Transition intermittency.
transition_re_theta or treth: Transition Re-Theta.
species_1 or spec1: Species 1.
species_2 or spec2: Species 2.
species_3 or spec3: Species 3.
species_4 or spec4: Species 4.
species_5 or spec5: Species 5.
species_6 or spec6: Species 6.
species_7 or spec7: Species 7.
species_8 or spec8: Species 8.
species_9 or spec9: Species 9.
mesh_x_displacement or mesh_xdisp: X-component of mesh displacement.
mesh_y_displacement or mesh_ydisp: Y-component of mesh displacement.
mesh_z_displacement or mesh_zdisp: Z-component of mesh displacement.
mesh_displacement_magnitude or mesh_disp_mag: Mesh displacement magnitude.
mesh_x_velocity or mesh_xvel: X-component of mesh velocity
mesh_y_velocity or mesh_yvel: Y-component of mesh velocity.
mesh_z_velocity or mesh_zvel: Z-component of mesh velocity.
mesh_velocity_magnitude or mesh_vel_mag: Mesh velocity magnitude.

user_function or user (string) [no default]

Name of the user-defined function. Used with user_function type.

user_values (array) [={}]

Array of values to be passed to the user-defined function. Used with user_function type.

user_strings (list) [={}]

Array of strings to be passed to the user-defined function. Used with user_function type.

multiplier_function (string) [=none]

User-given name of the multiplier function for scaling the momentum source values. If none, no scaling is performed.

Description

This command specifies the momentum source or like terms applied to the momentum equation:(1)

where

ρ

is the density, u is the velocity vector, p is the pressure,

is the viscous stress tensor and g is the momentum source vector defined here. MOMENTUM_SOURCE commands are referenced by BODY_FORCE commands, which in turn are referenced by ELEMENT_SET commands:

MOMENTUM_SOURCE( "my gravity" ) {	
type	= constant
momentum_source	= { 0, -9.81, 0 }
}	
BODY_FORCE( "my body force" ) {	
momentum_source	= "my gravity"
...	
}	
ELEMENT_SET( "fluid with gravity" ) {	
body_force	= "my body force"
...	
}

This example defines a constant momentum source (acceleration) in the y direction.

A constant momentum_source applies a spatially uniform gravity vector to an element set, as shown in the example above.

Gravity models of types piecewise_linear and cubic_spline may be used to define momentum source (acceleration) as a function of a single independent variable. For example,

GRAVITY( "curve fit momentum source" ) {	
type	= piecewise_linear
curve_fit_values	= { 0., 0., 0., 0. ;
10., 0., 0., -1. ;
20., 0., 0., 0. ; }
curve_fit_variable	= x_coordinate
}

defines a body force in the z direction as a function of the x coordinate. The curve_fit_values parameter is a four-column array corresponding to the independent variable and the x, y and z components of body force in the global xyz coordinate system. The independent variable values must be given in ascending order. The limit point values of the curve fit are used when curve_fit_variable falls outside of the curve fit limits.

The curve_fit_values data may be read from a file. For the above example, the curve fit values may be placed in a file, say momentum_source.fit:

0.	0.	0.	0.
  10.	0.	0.	-1.
  20.	0.	0.	0.

and read by:

MOMENTUM_SOURCE( "curve fit gravity" ) {	
type	= piecewise_linear
curve_fit_values	=Read( "momentum_source.fit" )
curve_fit_variable	= x_coordinate
}

A body force of type user_function may be used to model more complex behaviors; see the AcuSolve User-Defined Functions Manual for a detailed description of user-defined functions.

For example, consider the case where body force is a linear function of species 1 and species 2. Then the input command may be given by:

GRAVITY( "UDF body force" ) {	
type	= user_function
user_function	= "usrBodyForceExample"
user_values	= { 1., 1.5 } # proportionality constants
}

where the user-defined function "usrBodyForceExample" may be implemented as follows:

#include "acusim.h"
#include "udf.h"
UDF_PROTOTYPE( usrBodyForceExample ) ; /* function prototype */ 
Void usrBodyForceExample (	    		
UdfHd	udfHd,	/* Opaque handle for accessing data	*/	
Real*    	outVec,	/* Output vector	*/	
Integer  	nItems,	/* Number of elements	*/	
Integer	vecDim	/* = 3 (for three components)	*/	
) {			
Integer 	elem ;	/* an element counter	*/	
Real 	coef1 ;	/* scaling factor 1	*/	
Real 	coef2 ;	/* scaling factor 2	*/	
Real* 	spec ;	/* species vector	*/	
Real*	spec1 ;	/* species 1 vector	*/	
Real*	spec2 ;	/* species 2 vector	*/	
Real*	usrVals ;	/* user values	*/	
Real*	xBodyF ;	/* x-component of body force	*/	
Real*	yBodyF ;	/* y-component of body force	*/	
Real*	zBodyF ;	/* z-component of body force	*/	
udfCheckNumUsrVals( udfHd, 2 ) ;	/* check for error	*/
usrVals 	= udfGetUsrVals( udfHd ) ;	/* get the user vals	*/
coef1 	= usrVals[0] ;	/* get coef. 1	*/
coef2 	= usrVals[1] ;	/* get coef. 2	*/
        spec          = udfGetElmData( udfHd, UDF_ELM_SPECIES ) ;
		/* get the user vals	*/
spec1	= &spec[0*nItems] ;	/* localize species1	*/
spec2	= &spec[1*nItems] ;	/* localize species2	*/
xBodyF	= &outVec[0*nItems] ;	/* localized xBodyF	*/
yBodyF	= &outVec[1*nItems] ;	/* localized yBodyF	*/
zBodyF	= &outVec[2*nItems] ;	/* localized zBodyF	*/
for ( elem = 0 ; elem < nItems ; elem++ ) {
xBodyF[elem] = coef1 * spec1[elem] + coef2 * spec2[elem] ;
yBodyF[elem] = 0 ;
zBodyF[elem] = 0 ;
}
} /* end of usrBodyForceExample() */

The dimension of the returned momentum source vector, outVec, is the number of elements times three.

The multiplier_function parameter may be used to uniformly scale all momentum source values. The value of this parameter refers to the user-given name of a MULTIPLIER_FUNCTION command in the input file; see the MULTIPLIER_FUNCTION command for an example.

Note: For thermal problems with the Boussinesq approximation the momentum source is scaled by the product of the expansivity and the temperature minus reference temperature; see the DENSITY_MODEL command for more details.