```

AUTOPLOT USE
file
phi 5

da 1
1u
screen
msg 1st-phase (gas) velocity at t=1s
pl 1
msg press  to continue
pause
cl
da 1
2u
screen
msg 2nd-phase (liquid) velocity at t=1s
pl 1
msg press  to continue
pause
cl
da 1
gas
screen
msg 1st-phase (gas) volume fraction at t=1s
pl 1
msg press  to continue
pause
cl
da 1
liq
screen
msg 2nd-phase (liquid) volume fraction at t=1s
pl 1
msg press  to continue
pause
cl
da 1
p1
screen
msg pressure at t=1s
pl 1
msg press e to END
enduse

GROUP 1. Run title
TEXT(STRATIFIED FLOW, CASE 1 CHANNEL :   W900
TITLE
DISPLAY
NUMERICAL BENCHMARK PROBLEM 2.7: STRATIFIED FLOW
A horizontal duct is divided into two equal-length parts
by a diaphragm, each of which contains both water and air,
the former lying below the latter. The depth of the water
is somewhat greater on the left of the diaphragm than it
is on the right.

The diaphragm is supposed suddenly to break; the task is
then to calculate what happens during the next few
seconds, during which the water and air are set in
motion, in opposite direction, as gravity waves travel
from the diaphragm-rupture point towards the two ends.

Two cases are to be considered, differing in respect of
the postulated cross-section of the duct. In case 1 it
is square, whereas in case 2 it is circular.
_________________________________________________
i                        :--->diaphragm          i
i                        :                       i
i------------------------:                       i
i =    =   =   =  =  =   :                       i
i   =    =   =   =       :-----------------------i
i =   =    water    =    :      air              i
i   =  =    =     =    = :                       i
_________________________________________________
|                        |                       |
|<------- 5 m ---------->|                       |
|<---------------- 10 m ------------------------>|
ENDDIS

GROUP 2. Transience; time-step specification
LSTEP=100;TFRAC(1)=-100.0;TFRAC(2)=0.01
GROUP 3. X-direction grid specification
GRDPWR(X,100,10.0,1.0)
GROUP 7. Variables stored, solved & named
ONEPHS=F;SOLVE(P1,U1,U2,R2,R1)
NAME(U1)=1U;NAME(U2)=2U
NAME(R1)=GAS;NAME(R2)=LIQ
GROUP 8. Terms (in differential equations) & devices
** Cut off built-in sources and diffusion terms
TERMS(GAS,Y,Y,N,Y,Y,Y);TERMS(LIQ,Y,Y,N,Y,N,Y)
TERMS(1U,Y,Y,N,Y,Y,Y);TERMS(2U,Y,Y,N,Y,N,Y)
GROUP 9. Properties of the medium (or media)
RHO2=1.0E3
GROUP 11. Initialization of variable or porosity fields
FIINIT(LIQ)=0.51;FIINIT(GAS)=0.49
FIINIT(1U)=0.0;FIINIT(2U)=0.0
PATCH(START,INIVAL,NX/2+1,NX,1,1,1,1,1,1)
INIT(START,LIQ,0.0,-0.02);INIT(START,GAS,0.0,0.02)
GROUP 13. Boundary conditions and special sources
Because the compressibility of the air is neglected in
this calculation, it is necessary to introduce an imaginary
aperture connecting the air space with a fixed-pressure
region; for otherwise the pressure is not determined.
PATCH(RELIEF,CELL,NX/2,NX/2,1,1,1,1,1,LSTEP)
COVAL(RELIEF,P1,FIXP,0.0)
The following two lines activate the momentum-source
term which represents the effect of the gravitational
acceleration lateral to the channel.
COVAL(LATG,2U,0.0,9.81*(RHO2-RHO1))
GROUP 15. Termination of sweeps
LSWEEP=30
GROUP 16. Termination of iterations
RESREF(P1)=1.E-6;RESREF(1U)=1.E-4;RESREF(2U)=1.E-4
RESREF(GAS)=1.E-6;RESREF(LIQ)=1.E-6
GROUP 17. Under-relaxation devices
RELAX(1U,FALSDT,0.01);RELAX(2U,FALSDT,0.01)
RELAX(GAS,LINRLX,0.3);RELAX(LIQ,LINRLX,0.3)
SPEDAT(SET,GXMONI,TRANSIENT,L,F)
GROUP 21. Print-out of variables
NTPRIN=LSTEP/2;NXPRIN=NX/10
GROUP 22. Spot-value print-out
IPLTL=LSWEEP;ITABL=1;NXPRIN=NX/20
TSTSWP=-1;IXMON=NX/2+1;NPLT=1
GROUP 23. Field print-out and plot control
PATCH(LONGPLOT,PROFIL,1,NX,1,1,1,1,1,LSTEP)
COVAL(LONGPLOT,LIQ,0.0,0.0);COVAL(LONGPLOT,2U,-1.0,-1.0)
COVAL(LONGPLOT,1U,-1.0,-1.0)
PATCH(TIMEPLOT,PROFIL,NX/4,NX/4,1,1,1,1,1,LSTEP)
COVAL(TIMEPLOT,2U,-1.0,-1.0);COVAL(TIMEPLOT,LIQ,0.0,0.0)
COVAL(TIMEPLOT,1U,-1.0,-1.0)
ORSIZ=0.4
GROUP 24. Dumps for restarts
```