```
PHOTON USE
p

msg             Numerical solution  contours
msg                     Time=1.
con C1 Z 1 fi;0.001
msg Press Enter to continue
PAUSE
CLEAR
con EXAC Z 1 fi;0.001
msg               Exact solution  contours
msg                      Time=1.
msg Press e to END
ENDUSE

GROUP 1. Run title and other preliminaries
#cls
TITLE
LIBREF
DISPLAY
PLANT sets up and solves a 2D unsteady diffusion problem with
time-dependent source and boundary conditions and compares the
results with the exact solution.

The single dependent variable is C1

The "PLANTed" formulae are:
* at the west wall the value   = TIM*YG2D**2
* at the east wall the value   = TIM*(1.0+YG2D**2)
* at the south wall the value  = TIM*XG2D**2
* at the north wall the value  = TIM*(1.0+XG2D**2)
* within the volume the source = XG2D**2+YG2D**2-4.*TIM

* the exact solution           = TIM*(XG2D**2+YG2D**2)
ENDDIS
NX=10;NY=10
GROUP 2. Transience; time-step specification
GROUP 3. X-direction grid specification
GRDPWR(X,NX,1,1.0)
GROUP 4. Y-direction grid specification
GRDPWR(Y,NY,1,1.0)
GROUP 7. Variables stored, solved & named
SOLVE(C1)
GROUP 8. Terms (in differential equations) & devices
TERMS(C1,N,N,Y,Y,Y,N)
GROUP 9. Properties of the medium (or media)
RHO1=1.0;PRNDTL(C1)=1.0;ENUL=1.0
GROUP 11.
FIINIT(C1)=0.0
GROUP 13. Boundary conditions and special sources

PLANTBEGIN
PATCH(WEST,WWALL,1,1,1,NY,1,NZ,1,LSTEP)
VAL=TIM*YG2D**2
COVAL(WEST,C1,1.,GRND)
PATCH(EAST,EWALL,NX,NX,1,NY,1,NZ,1,LSTEP)
VAL=TIM*(1.0+YG2D**2)
COVAL(EAST,C1,1.,GRND)
PATCH(SOUTH,SWALL,1,NX,1,1,1,NZ,1,LSTEP)
VAL=TIM*XG2D**2
COVAL(SOUTH,C1,1.,GRND)
PATCH(NORTH,NWALL,1,NX,NY,NY,1,NZ,1,LSTEP)
VAL=TIM*(1.0+XG2D**2)
COVAL(NORTH,C1,1.,GRND)
PATCH(SOURC,VOLUME,1,NX,1,NY,1,NZ,1,LSTEP)
VAL=XG2D**2+YG2D**2-4.*TIM
COVAL(SOURC,C1,FIXFLU,GRND)
STORE(EXAC)
EXAC=TIM*(XG2D**2+YG2D**2)
PLANTEND

GROUP 15  Termination of sweeps
LSWEEP=10
GROUP 19.
NAMSAT=MOSG
SPEDAT(SET,GXMONI,TRANSIENT,L,F)
GROUP 20. Preliminary print-out
GROUP 21. Print-out of variables
tstswp=-1
dmpstk=t
DISTIL=T
EX(C1)=6.626E-01; EX(EXAC)=6.650E-01
LIBREF=119
STOP
```