Does anyone know how to calculate the mass flow rate through a given surface, over some specified time interval? I know that the mass flux can be plotted for a single cell for some time period, but what I need is to get the mass flux over a specified surface over sometime period.

You could use a boundary monitor for this - assuming the surface is aligned with cell faces, then apply an Internal Boundary.

Ping, Thanks. But the boundary surface that I'm interested in is of Attach boundary type.

Try putting this into your posdat.f subroutine.

DO i = 1, NBMAXU

IRNB = IRN(IB)

if (irnb.eq."boundary region number of interest") then

ic=LX(i)/7

ip=ICLMAP(ic)

indx=MOD(LX(IB),7)

ibnd=LQ(indx,ip)

IDIR = NDIN(INDX)

IF ( IDIR.GT.0 ) THEN

IBS = IP

ISENS = 1

ELSE

IBS = IBND

IDIR =-IDIR

ISENS =-1

ENDIF

flux = flux+(F(IDIR,IBS)*ISENS) ! kg/s

ENDIF ENDDO

This will loop though all of the boundaries and calculate the mass flux in kg/s across any boundary number of interest.