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October 19, 2000, 07:40 
Convergence in 3D simulation

#1 
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I'm using the FLUENT 5 to simulate turbulent airflow with mixed convection in a threedimensional room, in my case the heat sources are two persons, two computers and six fluorescent lamps. I'm not reaching the convergence for the steadystate solution (this solution exist according to the paper used to comparison), the residuals begin to oscilate and stop to decrease. I already tried change the values of underrelaxation and use a timedependent approach, but the problem continues. Could anybody please help me?
Jarbas 

October 19, 2000, 08:25 
Re: Convergence in 3D simulation

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(1). I must say that if you need reliable results you have to run the test. This is the only approach, and in my case, we are also going to run test because commercial cfd solutions are simply not reliable nor accurate at the present time. (2). This is consistent with my previous messages. It is nothing new. After having said that, it is possible to get some information from the cfd results, if you are highly trained in your field. But the interpretation of the cfd results is very tricky, and requires a lot of luck. (3). This is the reason why I made the prediction sometimes ago that a company using only commercial cfd codes will eventually die. (in many cases, the company is already nearly dead.) Many years ago, a friend of mine showed me that his simulation of the flow in a room would change directions if he looked at it next day on the worstation. (4). If you think cfd is just like finite element analysis (structure), you are just like one of our project manager, he was thinking that with the CAD geometry, the cfd solution could be done right away. (this is the reason why his project will fail next year, when these cfd results finally are used in the design. actually, in many cases, only a rough guess were used, not even the cfd results,because the number keeps changing as a function of mesh.) (5). My suggestion is: write your own code, and if you still have problem, do something else. Even in my case, the answer is to run the test if the solution is important.


October 19, 2000, 12:19 
Re: Convergence in 3D simulation

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You need to determine if you have a properly posed problem that won't converge or an improperly posed problem that can't converge. Don't automatically look to the solver first.
In your problem, for example, buoyancy effects will be significant. Check your applied gravity and your density vs. temperature data. Do not use the Boussinesq approximation for problems with nonnegligible density gradients. This is just an example of the things to check, before turning to the solution controls. If everything look right with your inupts, converge the solution as far as possible. Do the velocity contours change a lot, or oscillate, every few iterations? This could indicate that the global solution is being overwhelmed by the local effects. Or does the solution appear to be approaching one result? Maybe the mesh isn't fine enough. I hope this helps you somehow. Problems with strong buoyancy forces, like your probelm, require extra attention to the interaction of global and local effects. 

October 20, 2000, 08:00 
Re: Convergence in 3D simulation

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Well, I did have that problem with my 3D simulations. A dynamic solution took in the order of a day for a time period, but some errors were still occuring. (mainly pressure) This could be due to some error on part as Evan has mentioned. However, I have accomplished my simulation. Problem is I can't exactly remember how. I know that increasing the minimum residual sum does reduce solution times. Probably can tighten this once it starts converging. I did play around with the relaxation parameters and the number of sweeps. Should read up on ways of solving problem in manual. Thats your best bet. Then try each way and note down what happens. That way you can repeat your method when you do solve your problem.
Cheers 

October 23, 2000, 10:01 
Re: Convergence in 3D simulation

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I just try every suggestions of the manual, but the problem continue.
Jarbas 

October 23, 2000, 11:33 
Re: Convergence in 3D simulation

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(1). If you are still interested in solving your problem, you need to do it stepbystep. (2). You can do this with one object in the room, and try to obtain the converged solution. Once you have done this, then you can add one more object in the room. And continue the exercise. (3). So, don't try the complex problem right away. You need to find out whether the code can solve the simple problem you are interested in. If you still have problem with one object in the room, try to replace it by a simple block to make it easier to generate the mesh. (I don't know what's wrong with your problem or the code, but with this approach, you should know where you stand.)


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