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A question about warning:flux scaling factor in film evap

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Old   March 8, 2020, 07:24
Default A question about warning:flux scaling factor in film evap
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HAORAN LI
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Hello everyone,

I`m running my case(GDI engine) with CONVERGE2.3, the dt turn into dt_min(in my case, dt_min=1E-08) near to the end of injection, and there are many warnings like "flux scaling factor in film evap should be greater than 0 and less than 1.1, current value is -0.000850". After this, my case is still running but dt is so small that the running time will be unacceptable.Can anyone tell me how to fix it?
Best regards!
L.H.R.
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Old   March 9, 2020, 13:22
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Kislaya Srivastava
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Hello Haoran,

Firstly, please ensure you are using our recommended settings for such case setups by comparing it with the TumbleGDI example case. You have stated that your time-step is dt_min, but could you elaborate on what's forcing it to dt_min? (dt_cfl, temperature recoveries, etc)

If your simulation is plagued by a lot of (temperature_extrapolation, etc) recoveries, prior to seeing those flux_scaling_factor warning messages, CONVERGE might be having some problems arriving at a solution. This could lead to unphysical temperatures at certain locations and if there is spray/film in these regions, it could be affecting spray/film evaporation and temperatures, and might be the cause for the warnings reported. Make sure you aren't using too large time-steps before dt_min was forced. Try decreasing your cfl limits to offer more stability. Also, ensure that you have adequate grid resolution for your simulation.

However, typically those warnings arise depending on whether or not you have turned the boiling flag ON in spray.in. Turn ON the boiling flag in spray.in to see it if helps. This will limit the parcel temperatures to the boiling temperature. We always recommend you move to a more recent version of CONVERGE as there have been many improvements to the code that eliminate such warnings/issues.

Hope this helps,

Sincerely,
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Old   March 11, 2020, 12:02
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Hello,

After those warnings show up, dt become dt_min and "time-step limit =dt_min forced by dt_recover". Today I try lower CFL limits but dt turn into dt_min even earlier (about-320, 40 degree before injction). In both two cases, I see dt is higher and higher and than "time-step limited by max_residual_passive, recovering ......" than dt=dt_min, dt is limit by dt_recover.

This problem has been bothering me for three weeks. Could you please help me check my case? I really don't know what to do. If you are willing to help me check my case, please give me your email, and I will send you my case file and the calculation log file. My email address is "lihaoran18@tju.edu.cn".

Best regards!
L.H.R.
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Old   March 11, 2020, 12:07
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Kislaya Srivastava
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Hello Haoran,

We would be happy to go over your case setup and guide you through your issues. Please contact support@convergecfd.com . Please use your official email for all correspondence with Convergent Science. Please mention the issues you were facing, attach the case setup, and the cfd-online thread, as reference.

Sincerely,
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