
[Sponsors] 
November 22, 2012, 13:46 
Free surface in mixing vessel

#1 
New Member
Germany
Join Date: Aug 2012
Posts: 3
Rep Power: 4 
Hi all,
I'm quite new to CFX. I want to simulate the flow in a mixing vessel. It's a steady state simulation. The vessel has no inlet and no outlet. I only set boundary conditions for the walls and an opening boundary condition. And here is my problem. It's a free surface problem and I don't know which opening boundary condition setting is the best for my problem. The baffled vessel only contains water. The aim of the simulation is to determine the mixing time, so I want to include a scalar tracer after simulating the steady state flow. Later I also want to include a particle tracking. I'm very thankful for every hints. 

November 22, 2012, 14:01 

#2 
Senior Member
Chris DeGroot
Join Date: Nov 2011
Location: Canada
Posts: 387
Rep Power: 6 
Hi. It sounds like an interesting problem. Have you gone through the relevant CFX tutorials? There are some covering free surface flows, scalar transport, and particle tracking. If you haven't given those a try, you should. If you have some specific problems, post them here.


November 22, 2012, 15:33 

#3 
New Member
Germany
Join Date: Aug 2012
Posts: 3
Rep Power: 4 
Thank you for your reply.
I already have done some tutorials concerning this problem... My problem is that I don't reach convergence. I already read about this problem in relation with free surfaces. I hope I can get a hint which mass and momentum options for the opening condition I should use to avoid this problem as much as possible. I think the air don't affect the fluid flow very much. On the other side I think it is not possible to model the liquid surface as wall because in this case the fluid flow in the vessel would definitly not be the real one. 

November 22, 2012, 15:49 

#4 
Senior Member
Chris DeGroot
Join Date: Nov 2011
Location: Canada
Posts: 387
Rep Power: 6 
The easiest thing to converge will be if you use a homogeneous model (both water and air share the same velocity field). Start with this. At the top of the domain use an Opening boundary condition with a relative pressure of 0 Pa. Set the volume fraction of air to 1 and the volume fraction of water to 0.


November 22, 2012, 15:50 

#5 
Senior Member
Bruno
Join Date: Mar 2009
Location: Brazil
Posts: 236
Rep Power: 12 
Where is this opening boundary condition? Is it at the top of the vessel?
Also, you said you have a free surface problem but your vessel has water only. Do you intend to calculate the free surface position from the pressure level at the top? This might be valid only if the swirl in the fluid isn't too strong, otherwise the free surface height might be important and this simplification might deliver incorrect results. If swirl is not strong, them this can be a good first analysis simplification, but in this case it would be better to use a freeslip wall at the top boundary condition. After you get your results, you can estimate the actual free surface height from the pressure levels. This simplification is only a good idea if you plan on doing this type of simulation several times and want to cut your total time, in which case I recommend you also do at least one simulation with an actual free surface to check how good your results are and tune your model. If you're only doing one simulation, than just use an actual free surface with the homogeneous model and the free surface algorithm turned on. The computational cost won't be that much higher and your results will be better. Cheers 

November 22, 2012, 16:00 

#6  
Senior Member
Bruno
Join Date: Mar 2009
Location: Brazil
Posts: 236
Rep Power: 12 
Quote:
But I agree that an actual free surface simulation with the homogeneous model is the best option. 

November 22, 2012, 16:02 

#7 
Senior Member
Chris DeGroot
Join Date: Nov 2011
Location: Canada
Posts: 387
Rep Power: 6 
Yes definitely do a real free surface simulation! There really shouldn't be any great difficulties getting this to converge.


November 22, 2012, 17:04 

#8 
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,809
Rep Power: 85 
Surface waves can often make steady state free surface simulations hard to converge. This simulation does not have inlets or outlets (which are a frequent source of small numerical noise which then become surface waves which cause convergence problems) so that might help a bit.
But in my experience you often have to do free surface simulations in transient simulations and run them out to steady state, rather than doing a steady state simulation. 

November 28, 2012, 11:16 

#9 
New Member
Germany
Join Date: Aug 2012
Posts: 3
Rep Power: 4 
Hi,
thank you for your advices. At the moment I have made a free surface simulation as you described and it works. I have reached convergence. Now I want to include an additional variable to simulate mixing times. Therefore I definied an expression for the injection of the tracer: Tracerinjection=0.1*step((t0.1[sec])/1 [sec]) [kg s^1] Because I had no inlet in my vessel I have defined a source point where I include the tracer. I run this simulation as transient and set 'solve fluids', 'solve ernergy' and 'solve tubulence' to false. I use the result file from the steady state simulation as initial value. Is that right? I have defined some monitor points. For this points I get a pulse response curve which I should analyse to get the mixing time. But the results are not these I predicted. There is a first peak and some smaller ones after that but it doesn't reaches a constant value at the end. Are there any advices concering this problem with the additional tracer. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Thermocapillary free surface flow  zakifoam  OpenFOAM Running, Solving & CFD  6  August 24, 2014 03:24 
free surface model  sjtusyc  CFX  3  September 5, 2012 18:33 
governing equations for liquid mixing with free surface?  phsieh2005  Main CFD Forum  3  October 6, 2009 20:37 
curve of vortex of free surface in mixing tank  bioman66  CFX  1  June 30, 2006 06:19 
Modeling Free Surface Flows  Elliot Schwartz  Main CFD Forum  5  August 25, 1998 21:03 