I am working on a CFD analysis of a 80m2 greenhouse used for drying seeds. The space has 8 shelves stacked at 4 different heights and openable windows for excess moisture and heat to be removed. I use both '2-k-e' and 'constant effective velocity' solvers and used them separately as well as in succession to generate results. I vary Relaxation Factor on x/y/z velocities, mass and temperature and I also vary Termination Residuals to direct the solver towards convergence.
My question is to anybody with experience of none-symmetrical CFD work at scales close to room/building level; what is a reasonable number of iterations before ditching the simulation as non-convergent? I have had convergence at up to 100,000 iterations when my PC was running over the weekend, so my inclination is to wait, but this particular CFD task leads to a lot of unstable, osculating patterns on Normalised Residuals (see attached) and I wonder if it is worth giving up as soon as the residuals begins to follow similar patterns and ‘repeat’ themselves?
After viewing the iteration file. The x-velocity value is fluctuating in the range of [-0.07 to -0.08 (m/s)]. Try selecting laminar flow assumption during setting up (Neglect all model of turbulence). May be you have a laminar case study. Good Luck
Use this forum it may be helpful. Good luck.
https://www.cfd-online.com/
Dear Mohammad.
Firstly you need to evaluate characteristic time of the process. It will be a minimal cumulant for time steps.
Then you need to evaluate minimal step. The last is in accordance with the stability of the method.
Some solution are transient in nature. For example convective flows. Tolerance level about 10-3 .. 10-4 is normal.
In some cases try to fine grid.
Best regard,
Serge
Hello,
You can have good guidelines on cfd-online about convergence issues. Meanwhile, you can
1. Decrease time step (you can adjust the number of iterations as per your requirement of the simulation time)
2. Increase inner iterations
3. Refine mesh appropriately
4. If you can deal your problem in non-dimensional numbers ( ex. Re ) , using scaled down geometry does improve the convergence.
Hope it helps !
You are solving in-compressible flow problem (with energy equation). Iteration is not only factor to get converged solution. You have to couple properly momentum with continuity equation in conjunction with energy equation since the flow is in-compressible. Check the stability, algorithm, and explicit or implicit scheme. If you are using any commercial code go through its manual to find out the convergence criteria. You can watch the flow pattern. It may have re-circulation region.
I solve many problems about the heat and mass transfer, for instance, the CFD for Heat pipe which is work by phase change phenomena the mesh size of 14,175 Quad is selected for good simulation analysis. For more information read this paper (Experimental and numerical investigation of thermosyphone performance in HVAC system applications)
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