Equation implementation in IcoFoam

rol97 · April 13, 2021, 12:08

Hi everyone, I have recently started working with OpenFoam for my master's thesis. I am also new to C++ (I have already read the documentation up to the classes and done some exercises but obviously there are a lot of things I don't know). It is now quite clear to me how OpenFoam works (especially IcoFoam, that is the solver I have to work with). My problem is that I am not able to find any documentation related to the syntax of complex equations.
For example I am currently trying to substitute the standard momentum equation with the Stokes equation (not a complex one yet):

$\nabla \cdot [-pI + \eta (\nabla \vec{u} + \nabla \vec{u}^T) ] = 0$

however I don't know how to do that. I would write:

- fvm::laplacian(nu, U) - fvm::laplacian(nu, U.T())

(the minus is just so that I don't have to modify anything in the PISO loop) and obviously this does not compile. Is there any good guide/documentation that I can refer to in order to learn the syntax for the equations?
I have also already performed the tutorials about adding the energy equation or the scalar passive transport equation but they were useless (strictly talking about the syntax learning).

Thank you in advance.

brunomramoa · April 13, 2021, 12:24

Quote:

Originally Posted by rol97

Hi everyone, I have recently started working with OpenFoam for my master's thesis. I am also new to C++ (I have already read the documentation up to the classes and done some exercises but obviously there are a lot of things I don't know). It is now quite clear to me how OpenFoam works (especially IcoFoam, that is the solver I have to work with). My problem is that I am not able to find any documentation related to the syntax of complex equations.
For example I am currently trying to substitute the standard momentum equation with the Stokes equation (not a complex one yet):

$\nabla \cdot [-pI + \eta (\nabla \vec{u} + \nabla \vec{u}^T) ] = 0$

however I don't know how to do that. I would write:

- fvm::laplacian(nu, U) - fvm::laplacian(nu, U.T())

(the minus is just so that I don't have to modify anything in the PISO loop) and obviously this does not compile. Is there any good guide/documentation that I can refer to in order to learn the syntax for the equations?
I have also already performed the tutorials about adding the energy equation or the scalar passive transport equation but they were useless (strictly talking about the syntax learning).

Thank you in advance.

Hi,

I asked something similar to this recently @ Accessing and clarificaiton of the terms in turbulence.divDevRhoReff(U). However, I did not get any reply.

I would say that you should implement this as:

Code:

      +( - fvc::div(nu* fvc::grad(U)().T()  )
          - fvm::laplacian(nu, U)
        )

Best regards

rol97 · April 14, 2021, 04:32

Thank you very much for you answer, the implementation that you suggested works.

I am now trying to modify that to understand what I can and cannot do and it looks like that whenever I have to perform some kind of operation concatenation (for example transposing a vector inside another operator) I must write it as "fvc" despite it being an unknown, and to rely on the convergence of the overall solving routine.

April 13, 2021, 12:08	Equation implementation in IcoFoam	#1
rol97 New Member Raoul Join Date: Apr 2021 Posts: 16 Rep Power: 5	Hi everyone, I have recently started working with OpenFoam for my master's thesis. I am also new to C++ (I have already read the documentation up to the classes and done some exercises but obviously there are a lot of things I don't know). It is now quite clear to me how OpenFoam works (especially IcoFoam, that is the solver I have to work with). My problem is that I am not able to find any documentation related to the syntax of complex equations. For example I am currently trying to substitute the standard momentum equation with the Stokes equation (not a complex one yet): $\nabla \cdot [-pI + \eta (\nabla \vec{u} + \nabla \vec{u}^T) ] = 0$ however I don't know how to do that. I would write: - fvm::laplacian(nu, U) - fvm::laplacian(nu, U.T()) (the minus is just so that I don't have to modify anything in the PISO loop) and obviously this does not compile. Is there any good guide/documentation that I can refer to in order to learn the syntax for the equations? I have also already performed the tutorials about adding the energy equation or the scalar passive transport equation but they were useless (strictly talking about the syntax learning). Thank you in advance.

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April 14, 2021, 04:32		#3
rol97 New Member Raoul Join Date: Apr 2021 Posts: 16 Rep Power: 5	Thank you very much for you answer, the implementation that you suggested works. I am now trying to modify that to understand what I can and cannot do and it looks like that whenever I have to perform some kind of operation concatenation (for example transposing a vector inside another operator) I must write it as "fvc" despite it being an unknown, and to rely on the convergence of the overall solving routine.