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February 15, 2011, 16:44 
Realizable kepsilon model kresidual

#1 
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Verene Martin
Join Date: Feb 2011
Location: Seattle, WA, USA
Posts: 1
Rep Power: 0 
Hello,
I am modeling an atmospheric boundary layer (on a 2000 m by 300 m grid where the cells are all 1 m by 1 m, uniform), over flat terrain (roughness lenght = 2 cm), and with a forest of height H=25 m starts at 5H from the inflow boundary, and is 10H long. The forest is not a part of the mesh in any way; instead it is represented by source and sink terms (in the form of userdefined functions) added to the momentum, TKE, TDR equations. I'm using the Realizable kepsilon closure model with the pressurebased solver, and the "coupled" pressurevelocity scheme with a Courant number of 200 and explicit relaxation factors both set to 0.7. My spatial discretization settings are: Gradient: Leastsquares, Pressure: PRESTO!, and Momentum, TKE, and TDR: 2nd order upwind. I'm getting a really strange result in my residuals that I'm hoping someone has seen before and knows how to remedy. The residuals fluctuate slowly in the first few hundred iterations, but show a strong downward trend after that (other than the kresidual). After 14500 iterations, the continuity residual is just less than 10E6 and the xvelocity, yvelocity, and epsilon residuals are all less than 10E10. However, even though the kresidual got down to less than 10^3 in the first couple hundred iterations, at around 200 iterations it goes up and stays at exactly "4.0000E01". This value (0.4) is exactly equal to the value I set for the underrelaxation factor for k. If I change the underrelaxation factor to 1.0, the residual changes to 1.0 in one iteration, and stays there. Similarly, if I change the underrelaxation factor to 0.001, the underrelaxation factor changes to 0.001 in one iteration and stays there. This only happens when using the realizable kepsilon closure model. I've tried using the standard and RNG kepsilon models, but so far I haven't been able to get convergence with those (I wouldn't expect the those to do as well anyway). I've looked at countour plots of k (and many other parameters) at various points in my simulation after 14000 iterations, and I can't see any change in the countour plot of k (or any other parameters). The plots all look like I'd expect them to. I know I could change the underrelaxation factor to be equal to the tolerance I set for the residuals and attain what FLUENT would call "convergence" that way, but I think that would be an artificial convergence. I need to know that my solution has truly converged. (I really need to be able to defend my results!) Has anyone ever seen similar behavior from the kresidual (or any other residual) before? If so, is there a fix? Also, is there any way to tell if my solution (for k in particular) is essentially converged, even though the residual is only as low as the associated underrelaxation factor?? Any help or advice is appreciated! Thanks! Verene 

May 12, 2017, 11:15 

#2  
Senior Member
Yuehan
Join Date: Nov 2012
Posts: 142
Rep Power: 10 
Quote:
I am not faced with the same problem, although I am using OpenFOAM. The k residual drops around 1E12 and the maximum value of k is as high as 0.2. Here is the picture of the residual plot. 

Tags 
convergence, kepsilon, kepsilon model, realizable, residual 
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