# a question about icoFoam equations

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 July 2, 2013, 06:15 a question about icoFoam equations #1 Member   Yu CAO Join Date: May 2013 Posts: 76 Rep Power: 5 Hi I found that the pressure term in the momentum equation in icoFoam does not divided by the denssity rho. Isn't it magnified the speed U by rho times in this way? Any one who could tell the reason？ Thanks.

 July 2, 2013, 14:16 #2 Senior Member     Mohsen KiaMansouri Join Date: Jan 2010 Location: CFD Lab Posts: 118 Rep Power: 8 Dear Yu If you open the p in icoFoam/cavity/0/p you can see that: Code: `dimensions [0 2 -2 0 0 0 0];` which is the dimension for p/rho. it means that for incompressible solvers the p equals to kinematic pressure p/rho. but it differs for compressible solvers for example if you see the tutorials/compressible/rhoPimpleFoam/ras/cavity/0/p you can see that: Code: `dimensions [1 -1 -2 0 0 0 0];` which is the correct dimension that you want! So in conclusion,p differs in compressible & incompressible solvers. Good Luck sharonyue and ScarFace like this. __________________ “If you have an apple and I have an apple and we exchange these apples then you and I will still each have one apple. But if you have an idea and I have an idea and we exchange these ideas, then each of us will have two ideas.”

July 2, 2013, 14:47
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Yu CAO
Join Date: May 2013
Posts: 76
Rep Power: 5
Quote:
 Originally Posted by cfdonline2mohsen Dear Yu If you open the p in icoFoam/cavity/0/p you can see that: Code: `dimensions [0 2 -2 0 0 0 0];` which is the dimension for p/rho. it means that for incompressible solvers the p equals to kinematic pressure p/rho. but it differs for compressible solvers for example if you see the tutorials/compressible/rhoPimpleFoam/ras/cavity/0/p you can see that: Code: `dimensions [1 -1 -2 0 0 0 0];` which is the correct dimension that you want! So in conclusion,p differs in compressible & incompressible solvers. Good Luck
Thank you. I have noticed this. However, I am wondering if the result would be the same when I use these two ways. I mean, if in the p file, there are only two boundary conditions, fixedvalue zero and zerogradience. and the result of solving equation with rho and without the rho would be the same. In other words, would boundary conditions fixedvalue zero and zerogradience would stay unchanged in both p and p/rho files. the only change would only be the dimension in this way. Thank you.

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