cs_user_source_terms.c File Reference

Additional source terms for variable equations. More...

#include "cs_defs.h"
#include <assert.h>
#include <math.h>
#include <ple_coupling.h>
#include "bft_mem.h"
#include "bft_error.h"
#include "bft_printf.h"
#include "cs_base.h"
#include "cs_field.h"
#include "cs_field_pointer.h"
#include "cs_field_operator.h"
#include "cs_mesh.h"
#include "cs_mesh_quantities.h"
#include "cs_halo.h"
#include "cs_halo_perio.h"
#include "cs_log.h"
#include "cs_notebook.h"
#include "cs_parameters.h"
#include "cs_prototypes.h"
#include "cs_rotation.h"
#include "cs_time_moment.h"
#include "cs_time_step.h"
#include "cs_turbomachinery.h"
#include "cs_selector.h"
#include "cs_post.h"

Include dependency graph for cs_user_source_terms.c:

Functions
void	cs_user_source_terms (cs_domain_t *domain, int f_id, cs_real_t *st_exp, cs_real_t *st_imp)
	Additional user-defined source terms for variable equations (momentum, scalars, turbulence...). More...

Detailed Description

Additional source terms for variable equations.

See Examples of data settings for source terms (cs_user_source_terms.c) for examples.

Function Documentation

cs_user_source_terms()

void cs_user_source_terms	(	cs_domain_t *	domain,
		int	f_id,
		cs_real_t *	st_exp,
		cs_real_t *	st_imp
	)

Additional user-defined source terms for variable equations (momentum, scalars, turbulence...).

This function is called at each time step, for each relevant field. It is therefore necessary to test the value of the field id or name to separate the treatments of the different variables.

The additional source term is decomposed into an explicit part (st_exp) and an implicit part (st_imp) that must be provided here. The resulting equation solved by the code for a scalar f is:

Note that st_exp and st_imp are defined after the Finite Volume integration over the cells, so they include the "volume" term. More precisely:

• st_exp is expressed in kg.[var]/s, where [var] is the unit of the variable. Its dimension is the one of the variable (3 for vectors)
• st_imp is expressed in kg/s. Its dimension is 1 for scalars, 3x3 for vectors.

The st_exp and st_imp arrays are already initialized to 0 (or a value defined through the GUI or defined by a model) before entering the function. It is generally not useful to reset them here.

For stability reasons, Code_Saturne will not add -st_imp directly to the diagonal of the matrix, but Max(-st_imp,0). This way, the st_imp term is treated implicitely only if it strengthens the diagonal of the matrix. However, when using the second-order in time scheme, this limitation cannot be done anymore and -st_imp is added directly. The user should therefore check for the negativity of st_imp.

When using the second-order in time scheme, one should supply:

• st_exp at time n
• st_imp at time n+1/2

Warning

If the variable is a temperature, the resulting equation solved is:

rho*Cp*volume*dT/dt + .... = st_imp*T + st_exp

Note that st_exp and st_imp are defined after the Finite Volume integration over the cells, so they include the "volume" term. More precisely:

• st_exp is expressed in W
• st_imp is expressed in W/K

Steep source terms

In case of a complex, non-linear source term, say F(f), for variable f, the easiest method is to implement the source term explicitly.

df/dt = .... + F(f(n)) where f(n) is the value of f at time tn, the beginning of the time step.

This yields: st_exp = volume*F(f(n)) st_imp = 0

However, if the source term is potentially steep, this fully explicit method will probably generate instabilities. It is therefore wiser to partially implicit the term by writing:

df/dt = .... + dF/df*f(n+1) - dF/df*f(n) + F(f(n))

This yields: st_exp = volume*( F(f(n)) - dF/df*f(n) ) st_imp = volume*dF/df

Parameters

[in,out]	domain	pointer to a cs_domain_t structure
[in]	f_id	field id of the variable
[out]	st_exp	explicit source term
[out]	st_imp	implicit part of the source term