CFD Online Discussion Forums - Old and new values of a field

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Old and new values of a field

Hi, I have a problem with using old value of a field.

I have A and B - dimensionless volScalarFields
I also have

Code:

dimensionedScalar c ("c", dimensionSet(0, 0, 1, 0, 0, 0 ,0), 1.0);

In the beginning, I know

and

I want to solve two equations below in OpenFOAM (n - number of time layer):

1) $A_i^{n+1} = 2*B_i^n$ ;

2) $\frac{A_i^{n+1}B_i^{n+1} - A_i^nB_i^n}{\tau} + B_i^{n+1} = 0$

(I want to find $A_i^{n+1}$ from the first equation and then $B_i^{n+1}$ from the second one)

I used this code with Euler scheme:

Code:

A = 2*B;



fvScalarMatrix BEqn

(

c*fvm::ddt(A,B) + fvm::Sp(1,B)

);

BEqn.solve();

The problem here with row

Code:

A = 2*B

This row let me find $A_i^{n+1}$ but it also deleted old

values because OpenFOAM took the second equation as

$\frac{A_i^{n+1}B_i^{n+1} - A_i^{n+1}B_i^n}{\tau} + B_i^{n+1} = 0$

And I got the wrong decision!

What code will solve $A_i^{n+1} = 2*B_i^n$ equation and won't delete old

values?
Thank you very much for any help ...

So, what will be the best way to solve the equations without any derivatives in OpenFOAM?
For example look at the equation

(A and B - volScalarFields).

Do you use this code to solve it:

Code:

A=2*B;

??
Is that correct code?
Please help!

Are you going to use always that kind of temporal discretization? If the answer is yes, you can access runTime delta T:

Code:

dimensionedScalar mydeltaT1("mydeltaT",dimensionSet(0, 0, 1, 0, 0, 0, 0),scalar(runTime.deltaTValue()));

And then solve manually the set of equations.