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Vadim Baines-Jones August 13, 2007 05:37

Modelling a Fluid Jet
 
Hi all,

I've seen in a few posts on forums that people often simply state to people with regards some problems of jet modelling that "look at a garden hose for example, that breaks up".

What I am interested in is actually modelling fluid jets. It amases me that there isn't a wealth of data on this subject (or maybe I'm just looking in the wrong places [all advice on reference sources welcome please]).

I am really sturglling to find simulations of exit jets. I have at present a few papers on similar things such as fire supression in tunnels using lots of jets positioned round a circle. The author however seems just happy to present the results but doesnt take into account the fact that his results are changing whenever he changes the mesh (width of spread etc).

>>>>>>> On to my Model.

I am Modelling a jet emrging from a nozzle into air with gravity. Imagine holding a fire hose and pointing it at a burning fire, thats the kind of simulation I want (withought the fire of course).

I use a multiphase simulation with a nozzle in a surrounding box with one end set as a pressure outlet, all the others set as entrainment boundaries, and the inlet as water vol. frac. 1 air vol. frac. 0. I use free surface flow models with bouyancy. I just cannot seem to get the model to perform. With large jets and lower velocities it does ok (9mm orrifice with 5 m/s) but if I go anywhere near the smalle rnozzles I use (2.5mm) and higher velocities (30 m/s) the model simply spreads far to rapidly compared with experimentation.

I really am at my wits end and would apreciate any help or advice, or any pointers to sources of reference.

Yours Hopefully

Vadim

Have a great day all :)

Glenn Horrocks August 13, 2007 07:02

Re: Modelling a Fluid Jet
 
Hi,

Try the Journal of Fluid Mechanics. It will certainly have many articles on this type of topic, and anything published in JFM is of the highest quality.

Glenn

Q August 13, 2007 08:01

Re: Modelling a Fluid Jet
 
try this... http://www.flow3d.com/apps/ink/app_ink_bubb.html

regards


Vadim Baines-Jones August 13, 2007 09:29

Re: Modelling a Fluid Jet
 
Thanks for the input so far guys.

The simulation of a bubble looks very interesting and their simulation seems to predict the bubble flow quite well. I will contact the providers and see if they have any further information.

With regards the Int J. Fluid I have open access to post reference sources through the university and have checked this reference source many times. Maybe it is a keyowrd thing, but I just cannot understand why no clear comparisons or investigations are available on this as so much work focused on this before the advent of CFD. It seems a simple technique to me but evidently not as Im having great dificulties.

A more basic question to the users if I may:

When runnign the simulation, I know fluctuating residuals are bad, often due to poor mesh construction. What I find is that when I adapt the mesh to the water volume fraction (at the edges of the jet) the residuals although decreasing fluctuate even more. IS this a case of poor meshing from the start? Or is the solver finding it too difficult for the multiphase free surface case?

thanks in advance

Vadim

Glenn Horrocks August 13, 2007 18:24

Re: Modelling a Fluid Jet
 
Hi,

Don't forget the surface tension model will make convergence more difficult.

The documentation discussed obtaining convergence in some detail. Have a look at the residuals to find where the region of poor residuals is. If it is away from the important region it may not be important and can be ignored. However if you have a free surface with surface tension I but you have waves on the surface which are not going to steady state. Try a transient model and see what the waves are doing.

Glenn Horrocks

Vadim Baines-Jones August 15, 2007 09:19

Re: Modelling a Fluid Jet
 
Hi Glenn,

thanks very much for the input, its really apreciated.

Ive now got access to the Journal of Fluid Mechanics and am currently scouring them to find any similar information.

With regards steady state and transient. I have never run a transient simulation and no little about this. Do you have any vital tips of conditions I may need to change etc or how to set up a transient simulation?

All information welcome. On inspection the residual errors do occour as exspected at the free surface. This is the area Im struggling with most. A transient studyt would actually be very suitable to my work but I have never completed one. Do you have any tutorials you could recomend?

thanks in advance

Vadim

Glenn Horrocks August 15, 2007 18:42

Re: Modelling a Fluid Jet
 
Hi,

A number of the included tutorials cover transient flows. For the general idea that is a good starting point.

Remember that with a transient simulation you have an additional thing to validate, that is the time step size. A good starting point for the timestep size is the recommendation in the documentation - between 2 and 5 iterations per time step.

Regards, Glenn Horrocks


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