[Aurelien Thinat]

Hi everyone,

I'm stuck on a problem about pressure type boundaries with buoyantSimpleFoam.

My case is a simple tank with an inlet and an outlet. I try to force the convection movement by a difference of pressure between the inlet and the outlet :
- inlet : pressure : fixedValue = 150 kPa; U : pressureInletOutletVelocity
- outlet : pressure : fixedValue = 100 KPa; same for U.

The error message I got is :

Continuity error cannot be removed by adjusting the outflow.
Please check the velocity boundary conditions and/or run potentialFoam to initialise the outflow.
Total Flux : 0.00198728
Specified mass inflow : 3.98..e-06
Specified mass outflow : 1.75...e-05
Adjustable mass outflow : 0

I may have something wrong with p and p_rgh. Why these 2 are needed in the BC ?

If someone has any idea, I'd take it.

Aurélien

[robbirobocop]

Originally Posted by Aurelien Thinat

I try to force the convection movement by a difference of pressure between the inlet and the outlet :
- inlet : pressure : fixedValue = 150 kPa; U : pressureInletOutletVelocity
- outlet : pressure : fixedValue = 100 KPa; same for U.

I would try zeroGradient for U at the outlet.

pressureInletOutletVelocity calculates the velocity by the pressure you assigned.
Based on the mass inflow let's say with 150 kPa you get 3.98E-06 kg/s.
If you now choose a different pressure at the outlet and choose pressureInletOutletVelocity once again you'll get a mass flow according to your pressure. Since the inlet and outlet pressure differ, your mass flows are different as well and that is probably your problem because your "mass balance" is not equal to zero.

[Aurelien Thinat]

Hi Rob,

Yes, I guess you are right and I thought the same thing. So I changed my BCs into walls everywhere. Which is working.
And then, I changed the inlet to :

pressure p :
{
type fixedValue;
value uniform 1e5;
}
pressure p_rgh
{
type buoyantPressure;
value uniform 1e5;
}
U
{
type pressureInletVelocity;
value uniform (0 0 0);
}

and the outlet to :

pressure p :
{
type fixedValue;
value uniform 1e5;
}
pressure p_rgh
{
type buoyantPressure;
value uniform 1e5;
}
U
{
type zeroGradient;
}

It's the same pressure value in inlet and outlet.
I got the same kind of error message :

Total flux : 0.00189...
Specified mass inflow : 3.12...e-10
Specified mass outflow : 0
Adjustable mass outflow : 0

EDIT :
When I make a difference between the intlet pressure and outlet pressure (105000Pa and 95000 Pa). The solver iterates few times and then stop with the same error mesage :

Total flux : 1.23259
Specified mass inflow : 0.000621281
Specified mass outflow : 0.00104133
Adjustable mass outflow : 0

[robbirobocop]

I only gave buoyantSimpleFoam a quick try back then so I am not an expert there.

But usually you only fix the pressure at the outlet and use zeroGradient for the inlet.

I at least do it this way anytime. I do not know if you can use pressureInletVelocity then as a BC.

What is the purpose or the goal of your simulation or what do you want to simulate?

[Aurelien Thinat]

It should be a pretty easy case (it is with other cfd codes at least) :
I want to simulate the forced convection movement created by a difference of pressure between the inlet and the outlet.

EDIT :
I did what you suggested : a fixedValue of 90kPa at the outlet. zeroGradient pressure at the inlet. For U : pressureInletOutletVelocity at the inlet and zeeroGradient at the outlet. It computes. I'll keep you updated of the results. Thank you.

[Aurelien Thinat]

The case is now close to what it should be with fixedValue pressure and free velocity in inlet and outlet.

I still have a problem with the wall boundaries in pressure. In the tutorials, they are using buoyantPressure for walls. When I use it, the case is running during something like 100 iterations. And when I use zeroGradient, it explodes after 3 iterations.

If someone has any idea to solve this problem. Thank you.

Aurelien

[robbirobocop]

Originally Posted by Aurelien Thinat

The case is now close to what it should be with fixedValue pressure and free velocity in inlet and outlet.

That's cool.

Originally Posted by Aurelien Thinat

I still have a problem with the wall boundaries in pressure. In the tutorials, they are using buoyantPressure for walls. When I use it, the case is running during something like 100 iterations. And when I use zeroGradient, it explodes after 3 iterations.

Does this mean your case explodes with buoyantPressure for walls as well? Or does your case converge within the 100 iterations?

[Aurelien Thinat]

It explodes after 100 or 200 iterations (depends on the relaxation factor I used).

[robbirobocop]

Maybe you should post your schemes/solution files and of course your BC's.
Maybe the reason for the simulation blowing up is something else.

[Aurelien Thinat]

It seems that buoyantSimpleFoam wasn't able to deal with this type of study. I switched to rhoSimplecFoam and it's starting. But now I have some problems to keep the computation stable.

EDIT :
It's now working well. If someone could list the limits of the different "buoyant*" solvers, or confirm they are not suitable for high pressure gradients. Thank you.