Negative granular temp at wall set to 1e-8

MFiX How to
#1

Hello, everyone,

When i used JJ boundary in TFM-KTGF simulation, the following error was encountered. Can you tell me how to solve this problem? Thank you!

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ERROR calc_grbdry.f:1029
CALC_GRBDRY => CALC_MU_SV_BDRY Warning: Negative granular temp at wall set to 1e-8

#2

Hi. Thanks for the report.

In the upcoming 21.4 release we will change this message from ERROR to WARNING.

It just means that the computation of granular temp yielded a value below zero, which is non-physical, and the solver adjusted the value to an enforced minumum value (1e-8). It should not be treated as a fatal error.

– Charles

#3

Thank you for your reply, Charles. Can you please tell me how to set the JJ boundary condition correctly?

#4

Hi, Charles

According to your advice, i updated MFIX to the newest version. However, the same error occurred and was not changed to the form of WARNING. Is it due to my incorrect setting of JJ boundary condition? Can you please tell how to set this kind of boundary correctly? I will try it again. Thank you.

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#5

Please attach your input files so we can test this. Thanks.

– Charles

#6

Hi, Charles

The source file i created has been attached here. You can check it. Thank you.

mfix.dat (5.8 KB)

#7

Hi.

Sorry that this problem is still happening - as you noted the change from ERROR to WARNING did not get into the 21.4 release, it will be in the next update.

I found that changing the values of IC_THETA_M and BC_THETA_M from 0 to a small value (1e-8) gets around this problem.

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for both IC_1 and IC_2, as well as the mass inflow BC_1

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We will try to address this issue in the next release. Thanks for the report!

– Charles

#8

Hi, Charles

It seems not convergent by adjusting these parameters, though the errors were got round. So, what is the correct setting of JJ boundary condition? When the NSW condition was used, the simulation ran smoothly.

#9

I think the JJ settings are OK. You are starting with a packed bed (ep_g=ep_star=0.4) which is the max you can have. It runs fine if I fluffed the bed a bit (set ep_s =0.55 i.e. ep_g =0.45 in IC1, keeping initial granular temp=1e-8), or if I set the initial granular temperature to 1E-4 with a packed bed (ep_g=0.4).

#10

Hi, Dr. Dietiker

Can you please help me check the attached mfix.dat file? According your advice, close pack state was not used again and the initial granular temp was set to 1e-8. However, it was still not converged after many attempts. It may be not necessary to note that the version of MFIX i used is 20.2.1, for the update of model of TFM is much slower than that of other methods.
mfix.dat (6.0 KB)

#11

Either try with ideal gas instead of constant density, or increase the number of iterations for the gas pressure eq. say leq_it(1) = 200.