# FLASH4.6 API

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## [Functions] source/physics/Diffuse/Diffuse_solveCoupledScalar

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NAME

```  Diffuse_solveCoupledScalar
```

SYNOPSIS

```  call Diffuse_solveCoupledScalar (
integer(IN), dimension(VARDESC_SIZE) :: unkVarsDesc,
integer(IN), dimension(VARDESC_SIZE), OPTIONAL :: xtraVarDesc,
integer(IN), dimension(VARDESC_SIZE) :: diffCoeffDesc,
integer(IN), dimension(VARDESC_SIZE), OPTIONAL :: absorpCoeffDesc,
integer(IN)  :: bcTypes(6),
real(IN)     :: bcValues(2,6),
real(IN)     :: dt,
real(IN)     :: scaleFact,
real(IN)     :: theta,
integer(IN), OPTIONAL :: pass,
integer(IN), dimension(VARDESC_SIZE), OPTIONAL :: iFactorD,
integer(IN), dimension(VARDESC_SIZE) :: iFactorA)
```

DESCRIPTION

```    Advance a diffusion problem for multiple coupled scalar variables.

This routine advances a generalized diffusion operator of the form

A*(df/dt) + C*f = div(B*grad(f)) + D ,

where
f = f(x,t) is the  Variable to be diffused (x=1D..3D position);

A,B,C,D are optional given scalar factors/terms that may depend
on position; they are either physcially constant in time, or at
least considered time-independent for the purpose of the operation
implemented here (typically by computing their values from the
solution state reached by the previous time step).

Presently it is used to do multigroup diffusion coupled to electron energy
via absorption and emission.
```

ARGUMENTS

```   unkVarsDesc    : describes the main unknown variables on which the diffusion operation is performed.
NumGroups variables.
xtraVarDesc    : describes an additional unknown variable coupled to the main unknown variables.
One variable.
iFactorA       : unused
diffCoeffDesc  : Sometimes referred to as "Factor B".
Represents the diffusion coefficient in MGD.
NumGroups variables.
absorpCoeffDesc: Represents absorption in MGD.
NumGroups variables.
iFactorD       : Represents emission in MGD.
Currently ignored, absorpCoeffDesc is used for emission as well as absorption.
NumGroups variables.
bcTypes        : Presently OUTFLOW, VACUUM is supported, DIRICHLET is untested.
bcValues       : Values of unkVars,diffCoeffDesc on boundary (DIRICHLET).
dt             : The time step.
scaleFact      : Factor by which the end solution is scaled (not used).
theta          : varies scheme (0-> Explicit, 1-> backward Euler, 0.5 -> Crank Nicholson
pass           : Ignored in unsplit solver.
pass=1 order of directional sweep X-Y-Z,
pass=2 order of directional sweep Z-Y-X.
```

SIDE EFFECTS

```
```

NOTES

```  Stub implementation.

VARDESC_SIZE is defined in constants.h.
```