[idefix]

Hello,

I am using the interFoam-solver. Alpha1 is characterizing the volume fraction (alpha1= 0 means: there is only gas in the cell, alpha1=1 means: there is only liquid in the cell).
Now I´ve got the problem, that alpha1 is getting negative or greater than 1 which is not realistic.

My idea is to "correct" the alpha1 after every time step.
I need something which goes into every cell: If alpha1 is negative or nearly 0 (for example 0.1e-6), alpha1 should be put to 0. If alpha1 is greater than or nearly 1, alpha1 should be put to 1.

I would put this function in tha alphaEn.H - file after the for loop.

Can anybody help me?

Thanks a lot

[Phicau]

Hi,

I would not suggest doing this, as MULES handles itself the boundedness, continuity and conservation of alpha1. Although you may obtain vales slightly lower than 0 or larger than 1, if your BCs are correctly set, these will only be on the order of machine precision.

However, to do what you asked:

threshold = 0.01; // You name it
alpha1 = alpha1 * pos(alpha1-threshold) * neg(alpha1-1+threshold) + pos(alpha1-1+threshold)

Best,

Pablo

[sophie_l]

Hi Pablo,

I am using interFoam/waveFoam as well. I remember in the code, the value of alpha1 is restricted between 0 and 1. However, the case I am running at the moment gives out results like this, where Max(alpha1) = 1.57337. But the case is still running even though.

Courant Number mean: 0.00685244 max: 0.558313
Interface Courant Number mean: 6.62914e-05 max: 0.558313
deltaT = 0.00132421
Time = 0.186161

MULES: Solving for alpha1
Liquid phase volume fraction = 0.666668 Min(alpha1) = 0 Max(alpha1) = 1.16009
MULES: Solving for alpha1
Liquid phase volume fraction = 0.666668 Min(alpha1) = 0 Max(alpha1) = 1.57337
GAMG: Solving for p_rgh, Initial residual = 5.1474e-05, Final residual = 6.30767e-06, No Iterations 1
time step continuity errors : sum local = 1.13037e-05, global = -3.39547e-08, cumulative = -6.81206e-06
DILUPBiCG: Solving for epsilon, Initial residual = 0.00528157, Final residual = 7.39422e-09, No Iterations 7
DILUPBiCG: Solving for k, Initial residual = 0.00656354, Final residual = 3.16796e-09, No Iterations 8
ExecutionTime = 1408.43 s ClockTime = 1465 s

I am thinking it should be due to the input boundary conditions, however, I haven't got a clue where the check should start with. Do you some suggestions on this please?

Thanks a lot!
Sophie

[Phicau]

Hi

it is very likely that your alpha1 unboundedness is caused by the BCs. I suggest you take a look at this post for BC reference:

Waves2Foam Related Topics

if that does not solve your problems, explain your case there and I am sure they will be able to help you.

Best,

Pablo

[ngj]

Hi Sophie,

From the snap-shot of the log file, it is clear that:

1. You are not solving the momentum equation.

2. You are only solving the pressure equation one (1) time per time step.

If you relate that with the fact that interFoam/waveFoam is based on a PIMPLE-algorithm, then you will have no chance what so ever in obtaining mass conservation on the cell level; in your case the error in mass conservation is considerable. This is most probably the reason for the unboundedness of alpha1.

So: start solving for the momentum equation and have 3-4 iterations in the pressure equation per time step. This will greatly improve on your mass conservation problems and therefore improve on the boundedness of alpha1.

Kind regards,

Niels

[sophie_l]

Hi Niels,

Thank you so much for your reply. Really appreciate your help so far!

That's quite inspiring really. I'll resort to the code to work it out. By the way, I am using OF 1.7.1 based on PISO algorithm. However, in the cases I ran earlier, I didn't see alpha1 could be larger than 1.5, at most 1.0*.

Many thanks,
Sophie

[ngj]

Hi Sophie,

If you are using the PISO algorithm, the problem is at least as bad as using PIMPLE (though, if your relaxing factors are 1, then the results should be identical).

You do not need to modify/read the code, but merely change some keywords in fvSolution.

Furthermore, I can recommend that you read the section on the PISO algortihm in

@book { Ferziger2002,
Author = {Ferziger, J. H. and Peric, M},
Title = {Computational Methods for Fluid Dynamics},
Publisher = {Springer},
Year = {2002},
Edition = {3rd},
}

which will aid you in understand the importance of the settings for the PISO-algorithm.

Kind regards,

Niels