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March 5, 2009, 04:44 
Bouyancydriven flows and convergence

#1 
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Dear CFD users,
I am dealing with simulating a bouyancydriven flow in a room with Fluent. When I set up the case and lauch the simulations, the residuals drop down of two orders of magnitude in the space of onetwo hundreds of iterations, depending on the boundary conditions I impose. After that, residuals stop to drop down and begin to fluctuate slightly, without increasing or decreasing any more. Changing the underrelaxation factors for pressure, energy and/or turbulence variables makes the residuals drop down some few iterations, but after that they begin again to fluctuate around another value. At the same time, I monitor the value of some variables I am interested to, and they oscillate very slightly too. To give an idea, a volume flow rate at a surface of interest have fluctuations of 0.1% around its absolute value. My question is: can I consider converged the solution, even though residuals have drop down only of two orders? If yes, is it normal that residuals and solutions fluctuates slightly when reached convergence? Could this mean that a steadystate solution does not exist? Thank you in advance. Best regards, Marcello Caciolo phD student CEP  Center for Energy and Processes of Ecole de Mines de Paris 

March 5, 2009, 08:57 
Re: Bouyancydriven flows and convergence

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Hi,
I think you have 2 options here. It seems you will not get steady state but I think that if you average over enough iterations you will have a decent solution given that your residuals have dropped sufficiently and that your oscillations are very small. If you run for long enough you will see the solution oscillating with a certain period and average over that. You can also run it a transient simulation but you must be careful selecting timesteps and I would not say you need a time accurate solution. I maybe wrong so see what others say. Tom 

March 5, 2009, 12:18 
Re: Bouyancydriven flows and convergence

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I have had similar experience with these flows at larger Rayleigh numbers using my own code. This leveling off of the residual can be reduced by going to even more underrelaxation. If the relaxation parameter is small enough, there is no leveling of the residual down to the level of numerical roundoff, but who is to say that it still would not level off with higher numerical precision.
Probably your solution is not changing much (to engineering accuracy) as you increase underrelaxation, and you can accept the remaining residual. Though it is a stretch to accept this analogy, there is the case of evaluation of asymptotic series. As you add terms to the partial sums, the error gets smaller, up to a point where the error increases and the series diverges. The accepted procedure is to sum until the error stops decreasing and stop summation there. That is the best you can get out of this series. 

March 5, 2009, 14:27 
Re: Bouyancydriven flows and convergence

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try increasing the mesh density


March 6, 2009, 06:30 
Re: Bouyancydriven flows and convergence

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Nah, try decreasing mesh density. It's likely that you're resolving transient flow features (big wobbly plumes). A good old bit of numerical diffusion will help to settle things down


March 6, 2009, 11:42 
Re: Bouyancydriven flows and convergence

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My experience with my code: this behavior is largely independent of mesh density.


March 6, 2009, 12:31 
Re: Bouyancydriven flows and convergence

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not mine


March 6, 2009, 14:43 
Re: Bouyancydriven flows and convergence

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Jonas Holdman wrote "this behavior is largely independent of mesh density." (I guess you refer to the stationary oscillations of the residuals).
The left hand side of the Navier's Stokes equation is hyperbolic in nature, that means the advection of scalar and vectorial quantities is carried out by wave like phenomenun. Translate that to the computational grid, and a discrete perturbation analysis will show you the effect of the so called "cell Peclet Number". Good Luck 

March 6, 2009, 15:45 
Re: Bouyancydriven flows and convergence

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Jonas, sorry I did not have this reference at hand, when I wrote my previous comment, Computational Fluid Dynamics For Engineers by Klaus A Hoffmann has some nice figures showing the nature of error.


March 6, 2009, 16:56 
Re: Bouyancydriven flows and convergence

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if you stick a radiator in a middle of a room and pour some smoke to visualize air movement from buoyancy, you are not going to see some sort of a nice steadystate hot air rising, you will see a blob of warm stuff accumulating over the thing and then puff, it goes up, and then the deal repeats itself, in other words, there is no steadystate and most of the time buoyancy stuff is transient. Now, all commercial codes are full of diffusion and won't just blow up, they will converge a bit and then keep oscillating and that's what you are seeing in the residuals. So, take them and be happy, just like Jonas suggested. And, btw why would you add more diffusion with coarser mesh, to get as far as possible from an already remotely correct solution and dispersion has nothing to do with buoyancy driven stuff either....


March 7, 2009, 07:20 
Re: Bouyancydriven flows and convergence

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how many grid sytem did you use and laminar flow or tubulent flow


March 8, 2009, 04:34 
Re: Bouyancydriven flows and convergence

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"you will see a blob of warm stuff accumulating over the thing and then puff"
Are u sure about this? I know the buoyant plume is transient, i.e. it will meander/wobble like people say, but what you are suggesting is different to that. You seem to be saying that the plume stops and starts, which I don't think could be physical. Unless you have seen the plume to loop right around before rising? Maybe if you could point me to the appropriate literature that proves this. 

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