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March 30, 2012, 14:39 
QUICK scheme oscillation

#1 
New Member
kong
Join Date: Sep 2011
Posts: 21
Rep Power: 6 
i am running gassolid flow in ee.
all the discretization are selected to second order upwind, except volume fraction. When i use first order upwind scheme of volume fraction, the result got converged. However, when i change the volume fraction to QUICK, it can not get converged. I plotted the volume fraction, there is a big oscillation along the height. Can any one give me some suggestions? what i should improve to solve this oscillation under QUICK scheme thank you 

March 31, 2012, 16:02 

#2  
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,291
Rep Power: 20 
Quote:
Have you tried lowering our underrelaxation factors? Doing so may allow the solution to change more gradually. I do not recommend this method myself, but I am throwing it up as an option to try. Fluent 14 and (possibly 13 also) has a higherorder term relaxation option that is essentially an underrelaxation term on your higher order discretization. I recommend trying this first if you have it available. I do not recommend lowering the base underrelaxation factors, especially if you have HOD available. There are few things you can do for an easy solution. The oscillation may be enhanced by a inherent instability in the flow or one of the models used. Even a very fine mesh can produce these oscillations, it all depends on the flow. The NS equations inherently lead to instabilities (turbulence!) so it is really not too surprising. A brute force method is to run an unsteady simulation and start collecting statistics, but that is just avoiding having to figure out what is the meaning of the oscillation (in the steady sense). Doing the unsteady simulation might help you find the reason for the oscillation. 

April 4, 2012, 15:36 

#3  
New Member
kong
Join Date: Sep 2011
Posts: 21
Rep Power: 6 
Quote:
Actually, i am now using unsteady simulation. For under relaxation number , do you have any suggestion how to choose or lower how much would be ok? the iteration can got converged each time step, only the solution is oscillating in a large range. thank you so much for your reply again 

April 4, 2012, 16:05 

#4  
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,291
Rep Power: 20 
Quote:
For unsteady simulation, rather than lowering URF, it is easier & better to just make the timestep smaller without touching URF. The unsteady simulation will not make the oscillation (in time) go away obviously, but per timestep you should be able to get converged results. Last edited by LuckyTran; April 4, 2012 at 16:27. 

April 4, 2012, 16:22 

#5  
New Member
kong
Join Date: Sep 2011
Posts: 21
Rep Power: 6 
Quote:
I will try smaller time step next. and will let you know whether it works. 

Tags 
oscillation, scheme 
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