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-   -   Confused by dimensions of Presure (http://www.cfd-online.com/Forums/openfoam-solving/59658-confused-dimensions-presure.html)

jack2000 June 4, 2007 23:16

Could anybody explain to me wh
 
Could anybody explain to me why in the tutorial (pitzDaily)of SimpleFoam solver, the dimensions of pressure is:
dimensions [0 2 -2 0 0 0 0];

But based on Henry's explanation
________________________________________
By Henry Weller on Wednesday, June 01, 2005 - 07:49 am: Edit Post
> Excuse me, Henry, isn't pd = p - g*h ?

No pd = p - rho*(g.h) and you can calculate p from pd using this expression. Notice the dimension of pd:

dimensions [1 -1 -2 0 0 0 0];

h is the position vectors of the cell centres and g is the gravitational force vector.

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I am confused by that.
If I need to use pressure as my inlet boundary, should I use Pd/rho? Thanks.

Jack

jackdaniels83 June 5, 2007 03:51

the p in that case is p/rho.
 
the p in that case is p/rho.

formula: p [kg/(m*s) / rho [kg/m] = p' [m/s]

Daniel

zakifoam June 5, 2007 03:56

Hi Jack, There are two type
 
Hi Jack,

There are two types of pressure used in OF. The first is dynamic pressure, the dimension of which is kg/(ms2) or [1 -1 -2 0 0 0 0]. The second is kinematic pressure [0 2 -2 0 0 0 0], which is simply dynamic pressure divided by density. If you are not sure which pressure you are dealing with, you can always refer to *Foam.cfg file in directory FoamX. There you can find which pressure is used, under the entry 'fields'.

zaki

jack2000 June 5, 2007 10:24

Thank you very much, Daniel an
 
Thank you very much, Daniel and Zaki for your clear answer.

I have another question. Is it possible to input the triangular pressure distribution as the inlet BC? It appears that FoamX could handle uniform and no-uniform pressure distribution. But how pressure relate to the change of the position (water depth) is not clear. FoamX has only "item" relates to "pressure value". Could anybody give me an example to show how I can enter such kind of BC?

Basically, I try to verify a simple test case: when a pipe connects to two different water level reservoirs in both ends, what would be the mass flux relationship to the hydrostatic head differential of the reservoirs based on OF simulation result.

Thank you very much in advance!

Jack


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