Learning Forum

[scabo]

Hi

I am trying to simulate air-water flow in a straight pipe with periodic left and right boundaries with Volume of Fluid. After making the boundaries periodic, there is no option to specify mass flow rate, only pressure gradient option is there. I want to specify the mass flow rate at the inlet. How will I initialise the problem? Will it be through normal Initialisation? I am slightly confused-any help is greatly appreciated..thanks!

AB

[Rob]

Have a look in the documentation - there's a section on limitations. 

[scabo]

Hi documentation means user guide right which is available online?

 

[scabo]

 

https://www.#######/project/neptunius/docs/fluent/html/ug/node252.htm

this one??

 

[Rob]

Not the third party and very out of date one, no. If you click on Help in the solver you'll get the current set. Or  Public Ansys Help tends to work equally well. 

[scabo]

This is the one https://ansyshelp.ansys.com/public/account/secured?returnurl=//Views/Secured/corp/v242/en/flu_ug/flu_ug_sec_periodic.html?q=periodic%20boundary%20conditions%20fluent

Here it is written multiphase flow can be modeled, that means i can use VOF models also right?

[Rob]

Correct, but read that line carefully relative to your original question. 

[scabo]

Yes i can only use a pressure gradient with VoF. Then will i just hybrid initialise? Because in standard initialisation we have to provide velocities which are not known on the periodic face..

[Rob]

No, you need to use standard. Then patch the liquid. Do NOT patch a non 1 or 0 volume fraction of phase: read the VOF model approach to see why. 

[scabo]

Hi, What is meant by your line: Do NOT patch a non 1 or 0 volume fraction of phase. Does that mean i should only patch a surface with volume fraction of either 1 or 0?

[Rob]

Cell zone, but yes VOF needs to be 1 or 0 as it's an interface tracking model. Eulerian/Mixture are fine with values between zero and one depending on other factors. 

[scabo]

Hi, I have checked, after standard initialisation i have applied patch and selected volume fraction>air=0 on the periodic face. It is correct right?

[scabo]

Also, i have left the standard initialise panel as it is without giving any X, Y velocities-just initialised and applied the patch..i hope it is fine?

[Rob]

You need to patch into the cell (volume) zone. 

[scabo]

I created a region through cell register and then patched water into it after initialisation. Is that correct?

[Rob]

Yes, assuming the patched region gives you the correct liquid volume in the domain. 

[scabo]

Thanks-I will solve and see what happens

[scabo]

But one thing is there: i have specified pressure gradient on the periodic faces and then created a 3D region near the periodic face and patched it with water. So the pressure gradient is not acting on the water right? but instead it is acting on the inlet face. but still i am getting movement of water in the results.

[Rob]

I'd patch to a set depth rather than from a boundary. Pressure drop is for the domain, it's not phase related. 

[scabo]

What is meant by patch to a set depth? did you mean just patch anywhere in the pipe with a certain depth? I have attached my patched region as a picture. The patch is near the periodic face and is a 3D box type. Is it fine?

[Rob]

It'll work, but take time to find a peridic flow level. I'd generally patch so the liquid is in about the right place - ie start as near to the expected solution as possible. 

[scabo]

okay thanks

[scabo]

I was just asking after standard initialisation i do not need to provide any X,Y velocities right? just patch and apply a pressure gradient on the periodic face..

[Rob]

You don't. However, the better the initialisation data the more stable the solution. For a transient that also means a faster overall solution as it takes less time to reach equilibrium.