## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Tank filling time

• javat33489
Subscriber

Hi all.

My task is to calculate how long it will take for the container to fill through a small tube and the pressure to become the same. The only known pressure at the inlet is 10 MPa, and the pressure in the tank is atmospheric.

1.Can I calculate this in a non-stationary form, and the resulting time/pressure graph be considered real physical time?

2.At the start of the task, the pressure graph will jump strongly, and then begin to calm down. This will affect the fill time measurement. How can you increase blood pressure without surges? To get real time?

• Federico Alzamora Previtali
Ansys Employee

Hello,

yes this can be done in Fluent. For tank filling modeling, you will need to enable Floating Operating Pressure. You can read more on this in the User Guide, section 9.4.4 -> 9.4. Compressible Flows (ansys.com)

• javat33489
Subscriber

OK. I have studied this section.

This model:

I only have inlet and wall

inlet 8 MPa.

___

I turned energy ON.

I turned on the floating operation pressure 101325 Pa.

I included viscous heating in the turbulence models.

I used transient analysis and got the following results:

Can I say that the cylinder was filled with water and the pressure equalized as at the inlet in 0.015 seconds?

• Federico Alzamora Previtali
Ansys Employee

What is the expression that you are using here? I would suggest monitoring the mass flow rate at the inlet. You can say it is filled once the mass flow rate tends to zero.

• javat33489
Subscriber

Good idea! I will try!

I use expression - this is the outer surface of the cylinder

• javat33489
Subscriber

Ok I got these results

I added a mass flow chart for inlets. It turns out that the cylinder was filled in 0.15 seconds. Can I say that this is real physical filling time?

• Federico Alzamora Previtali
Ansys Employee

This is the most I can say without seeing your setup.

The reasoning is that if your mass flow rate converges to a value of 0kg/s, you could say that the tank is filled. This assumes that your case is properly set up.

For example, the oscillations in sign (direction) of the mass flow at the beginning would raise some questions for me.

• javat33489
Subscriber

>>For example, the oscillations in sign (direction) of the mass flow at the beginning would raise some questions for me.

The jump is visible and at pressure it is a transitional process.

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All settings are very simple, here is everything that differs from the basic settings:

• Federico Alzamora Previtali
Ansys Employee

You have not enabled the Floating operating pressure, as I suggested previously.

Are you working with a compressible fluid?

• javat33489
Subscriber

That’s right. Sorry sir. I added UDF for compressible water but forgot to connect it in the water material.

I did this, I took the UDF for compressible water from the tutorial and connected it.

But now I get the solution stopping almost immediately due to errors FLOATING POINT EXEPTION #f

1. I tried to play with the URF coefficients
2. Make the time step smaller
3. I checked the grid, one is very good

This may be due to the rapid change in water density. We need to limit this somehow. There was a mention of this in the tutorial, but it was not indicated how to do it.

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What’s the matter? Thank you.

• Federico Alzamora Previtali
Ansys Employee

You do not need a UDF for compressible liquid. You can find it from the Fluent Material database

• javat33489
Subscriber

Yes, I found water. Set the density to compressible. But it didn't work.

• javat33489
Subscriber

Yes, sir. I did everything again and selected compressible water in the fluent database, in the density settings. And I got this result:

• javat33489
Subscriber

What do you say?

• javat33489
Subscriber

What do you say?

• Federico Alzamora Previtali
Ansys Employee

Hello,

the residuals plot does not look right to me. You should try to reduce the time step size until you find good convergence within a time step.

• javat33489
Subscriber

Thank you! I will improve the convergence