[rapierrz]

Hi friends,

I want to equal a variable with a pde :

U is calculated from navier stokes equation and I want to have :

a=ddt(U)+div(phi.U)-g;

How I can define this equation in openfoam solver???

[chriss85]

First you need to define your new variable. This is usually done in createFields.H in the solver.
Then you will have to assign it. You're not solving a PDE here, you're just assigning a value. For this you want to use the explicit schemes, namely

a = fvc::ddt(U) + fvc::div(phi, U) - g;

[rapierrz]

Thank u very much Chriss,

I have another question.

How I can solve equation x*mag(x)=a that x is a vector and a is the

same a=fvc::ddt(U)+fvc::div(U)-g

[chriss85]

Can't you just rewrite that to x=sqrt(mag(a))*sign(a) ?
Edit: Nevermind, I skipped the vector part. So you're coupling between the vector components by using the magnitude? I think you might have to use a fixed point procedure then, such as Newton's method

[rapierrz]

thank you friend,

my target is that calculate the drift velocity then use it in navier stokes

equations but in my equation there is V*mag(V)=a and V is relative velocity

How I can solve V?what is initial guess for V in createField?

thank u

[chriss85]

You don't have a differential equation there but a normal equation that you need to invert numerically. Please read up and see if Newton's method (or some similar iterative method) is suitable for this. Equation solving capability of OpenFOAM is not going to help you here, you will have to program this yourself.

[rapierrz]

Thank u chriss,

I will search about it.