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-   -   Dimensions of laplacian in PISO loop (http://www.cfd-online.com/Forums/openfoam-solving/60134-dimensions-laplacian-piso-loop.html)

 kumar2 July 1, 2006 21:47

Hello Friends 1. In the pr

Hello Friends

1.
In the present PISO loop in say interFoam we have

laplacian(rUAf,pd)=div(phi) say , eqn A

both rUAf , phi are evaluated at the cell face.

dimension of rUAf = (m^3 . S)/kg
dimension of phi = m^3/S
dimension of pd = kg/(m.S^2)

since pd is at the cell centre . i found the laplacian of (rUAf,pd) as

div( [m^3 .S /kg] * [m^2] * [kg/(m.S^2)]*[1/m])

and this works out to be div ( m^3 / S ) . which equals the RHS eqn A. first question is IS THIS CORRECT ??

2.
when i multiply both sides with density as shown in eqn B below why is it dimensionally incompatible ?

phi=phi*rhof

rhof is the density at the face of cell with dimensions [kg/m^3 ]

rUAf_rhof = rUAf*rhof

laplacian( rUAf_rhof , pd)=div(phi) equation (B) .

But now OpenFOAM gives the following error

--> FOAM FATAL ERROR : Different dimensions for =
dimensions : [0 3 -1 0 0 0 0] = [1 0 -1 0 0 0 0]

From function dimensionSet::operator=(const dimensionSet& ds) const
in file dimensionSet/dimensionSet.C at line 158.

FOAM aborting

This is really strange. phi has changed its dimensions on expected lines , BUT NOT the LHS of eqn B .

Kumar

 kumar2 July 3, 2006 06:08

Dear Friends

Any comments on the above. I am really stuck with my work

kumar

 kumar2 July 3, 2006 14:34