photon use
  p;parphi;;;;;
 
  mesg horizontal direction (z) represents time
  gr y 1
 
  msg first-phase volume-fraction contours
  con r1 y 1 fi;0.05;pause
  msg first-phase velocity contours
  con u1 y 1 fi;0.05;pause
  msg pressure contours
  con p1 y 1 fi;0.05
  enduse
 
    GROUP 1. Run title and other preliminaries
TEXT(fluids moving in opposite directions:423
TITLE
  DISPLAY
 
  The initial condition is that two fluids of equal density are
  moving at equal but opposite velocity within a duct of uniform
  cross-section; and each has a volume fraction of 0.5.
 
  Because the ends of the duct are closed, each fluid tends to
  accumulate at the end towards which it is moving.
 
  Fluid-to-fluid and fluid-to-wall friction are neglected. One
  consequence is the growth of an instability in the interface,
  exhibited by a reduction in the volume fraction of each fluid
  just ahead of the point at which it rises to unity.
 
  Instabilities of Kelvin-Helmholtz type are known to arise in
  such flows; but the one just mentioned is more likely to be a
  consequence of the fact that no provision has been made for the
  enhanced fluid-to-fluid momentum transfer which occurs at an
  abrupt chnage of vulume fraction, which is akin to a 'hydraulic
  jump'.
 
  A PHOTON USE file is provided for display purposes.
 
  ENDDIS
    GROUP 2. Transience; time-step specification
STEADY=F;GRDPWR(T,60,1.5,1.0)
    GROUP 3. X-direction grid specification
GRDPWR(X,30,0.5,1.0)
    GROUP 7. Variables stored, solved & named
ONEPHS=F;SOLVE(P1,U1,U2,R1,R2)
    GROUP 8. Terms (in differential equations) & devices
TERMS(R1,P,P,N,P,P,P);TERMS(R2,P,P,N,P,P,P)
    GROUP 9. Properties of the medium (or media)
RHO1=1.0;RHO2=1.0;enul=0.0
    GROUP 10. Inter-phase-transfer processes and properties
CFIPS=0.0
    GROUP 11. Initialization of variable or porosity fields
FIINIT(U1)=1.0e-1;FIINIT(U2)=-1.0e-1;FIINIT(R1)=0.5;FIINIT(R2)=0.5
    GROUP 13. Boundary conditions and special sources
  ** Relief valve
PATCH(RELIEF,CELL,NX/2,NX/2,1,1,1,1,1,LSTEP)
COVAL(RELIEF,P1,FIXP,0.0);COVAL(RELIEF,U1,ONLYMS,0.0)
    GROUP 15. Termination of sweeps
LSWEEP=20
    GROUP 16. Termination of iterations
RESREF(P1)=1.E-05;RESREF(U1)=1.E-5;RESREF(U2)=1.E-5
    GROUP 17. Under-relaxation devices
VARMIN(R1)=1.E-10;VARMIN(R2)=1.E-10
SPEDAT(SET,GXMONI,TRANSIENT,L,F)
    GROUP 21. Print-out of variables
OUTPUT(R1,Y,Y,Y,Y,Y,Y);OUTPUT(R2,Y,Y,Y,Y,Y,Y)
    GROUP 23. Field print-out and plot control
PATCH(IXPLOT,PROFIL,1,NX,1,1,1,1,1,LSTEP);PLOT(IXPLOT,R1,0.0,1.0)
IDISPA=lstep/15;TSTSWP=LSWEEP-1;UWATCH=T;ITABL=1