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August 30, 2010, 16:45 
Adding U*U as a source term

#1 
New Member
Ozgur Kirlangic
Join Date: May 2009
Location: Istanbul
Posts: 16
Rep Power: 10 
Hi Foamers,
I am trying to solve the following equation: dU/dt = grad(p)  0.5*(f/L)*(U*U) here, U(velocity vector), p (pressure), and L is length, (f is dimensionless scalar). In fact, I made an attempt and it seems working for 1D .. However, I can not be very sure about the approach that I use (especially for 2D and 3D problems), and I need some comments from more experienced people. I constructed my equations as follows: Code:
... vector identity(1,1,1); ... volVectorField gradp = fvc::grad(p); ... fvVectorMatrix UEqn ( fvm::ddt(U) + fvm::Sp((U & identity)/2*(1/L+f/D), U) ); ... solve(UEqn == fvc::grad(p)); ... Instead, assuming that only the i component of the vectors will be used while solving Ux, I wrote "U & identity" to obtain the value of the x component of U and put it in to the first parameter place of Sp function. This works for 1D but I cannot be sure about my assumption about the behavior of U & identity expression in 2D and 3D problems.. In short, say that U(1, 2, 3) then will "U & identity" produce 1+2+3=6 in ALL Ux, Uy and Uz solutions OR will it produce 1 in Ux solution, 2 in Uy solution and 3 in Uz solution?? Kind regards, Ozgur 

August 31, 2010, 11:05 

#2 
New Member
Ozgur Kirlangic
Join Date: May 2009
Location: Istanbul
Posts: 16
Rep Power: 10 
Hi,
PS: The above is not a realistic CFD problem, but I am just playing a little with the code so to get some understanding... Anyway, I think I got the answer of my question by constructing a 2D case. And, I saw that unfortunately (in fact fortunately) my assumption was wrong, i.e. use of "U & identity" produced Ux+Uy+Uz. So, in this problem, what is needed is something like scale(U,U) function, which gives the product of corresponding components of vectors U and U; so it will produce another vector (not a matrix which only the main diagonal is useful). I couldn't still make scale(U,U) work. But asked about it under another title. Ozgur 

July 20, 2011, 11:22 

#3 
New Member
Amin
Join Date: Oct 2010
Location: Notre Dame, US
Posts: 6
Rep Power: 9 
scale(U,U) doesn't work and unfortunately it has not been updated in programmers guide. You should use cmptMultiply(U,U) instead.


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