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kongl1986 March 30, 2012 15:39

QUICK scheme oscillation
 
i am running gas-solid flow in e-e.
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

LuckyTran March 31, 2012 17:02

Quote:

Originally Posted by kongl1986 (Post 352355)
i am running gas-solid flow in e-e.
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

kong, this is a typical and expected behavior of higher order schemes (2nd order and up). You trade stability for improved accuracy. Not much you can do except to investigate the reason for the oscillation.

Have you tried lowering our under-relaxation 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 higher-order term relaxation option that is essentially an under-relaxation term on your higher order discretization. I recommend trying this first if you have it available. I do not recommend lowering the base under-relaxation 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.

kongl1986 April 4, 2012 16:36

Quote:

Originally Posted by LuckyTran (Post 352461)
kong, this is a typical and expected behavior of higher order schemes (2nd order and up). You trade stability for improved accuracy. Not much you can do except to investigate the reason for the oscillation.

Have you tried lowering our under-relaxation 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 higher-order term relaxation option that is essentially an under-relaxation term on your higher order discretization. I recommend trying this first if you have it available. I do not recommend lowering the base under-relaxation 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.

thank you for ur reply LuckyTran.
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

LuckyTran April 4, 2012 17:05

Quote:

Originally Posted by kongl1986 (Post 353182)
thank you for ur reply LuckyTran.
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

lowering under relaxation factors by a factor of 1/2 or 1/3 is common when running into issues such as you are facing. If possible, it is better to not change URF at all.

For unsteady simulation, rather than lowering URF, it is easier & better to just make the time-step smaller without touching URF.

The unsteady simulation will not make the oscillation (in time) go away obviously, but per time-step you should be able to get converged results.

kongl1986 April 4, 2012 17:22

Quote:

Originally Posted by LuckyTran (Post 353183)
lowering under relaxation factors by a factor of 1/2 or 1/3 is common when running into issues such as you are facing. If possible, it is better to not change URF at all.

For unsteady simulation, rather than lowering URF, it is easier & better to just make the time-step smaller.

The unsteady simulation will not make the oscillation (in time) go away obviously, but per time-step you should be able to get converged results.

thank you so much LuckyTran.
I will try smaller time step next. and will let you know whether it works.


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