Velocity exceeds limit

MFiX Bug report
#1

Hi,
I am running the self-contained biomass case in MFIX 21.4. The solver report many warnings and they are all about ‘Velocity exceeds limit: 47.400’. How to solve this problem?

image

#2

As the message indicates, you can try increasing MAX_INLET_VEL_FAC.

shot-2021-12-19_07-38-10
shot-2021-12-19_07-38-101344×931 222 KB

#3

I have increased this factor to 2 and do not find any improvements. I am not sure whether it can be assigned a larger number such as 10 or even larger.

#4

Did you modify anything in the tutorial? I ran this case locally and did not see any “velocity exceeds limit” warnings. Please submit a full bug report (zip file) with all the inputs and logs. Thanks.

– Charles

#5

I run this case on Win10 with 10 threads (12 threads in all). I change the gas mol. weight to mixture and its density to ideal gas law. Then, NORM_G is assigned to zero. Now, max_inlet_vet_fac = 5, the solver still shows this warnings with elapsed time only 10min.

image
image818×524 19.2 KB

The attached files:
Biomass.mfx (19.5 KB)
usr_rates.f (4.2 KB)

Thank you

#6

I’m not sure why changing the molecular weight and density models results in increased inlet velocities - this is someting to investigate.

I note that changing MAX_INLET_VEL_FAC to 10 supresses the warning messages.

There is no limit to how high you can set this factor. If there are limits on settings, the GUI will tell you. Did you try setting a value higher than 2?

– Charles

#7

Hi @cgw

Yes, I changed MAX_INLET_VEL_FAC to 5. After iterating whole night, the solver prompts 841 warnings about ‘Velocity exceeds limit: 47.400’. By the way, the elapsed time and ‘time remaining (est.)’ shown on the pane seem to be strange. I will change MAX_INLET_VEL_FAC to 10 and have a try.

image

image
image584×521 12.7 KB

I also have run the case under the default settings. ‘Velocity exceeds limit: 47.400’ warns again with 155 warnings within 7 elapsed time. Then, an error appear, showing DT<DT_MIN.

image

I am not sure whether ‘Velocity exceeds limit: 47.400’ is important and may have effects on results.

#8

Hi,@Fei @cgw
I’m suffering the same question. I also try to give a bigger value of MAX_INLET_VEL_FAC, and there are lots of warnings and the calculation elapsed. and I find that it happens in lab size apparatus(D<1cm), when the inlet velocity is small, such as 0.18 m/s, and when v > 1m/s (for example), nothing wrong. I’m not sure if there is something wrong in MFIX when the inlet velocity is small in small size model.

#9

Thanks @ChenYuan . I think we can maybe left this bug until we meet this problem in our projects. I have changed this TFM case to a DEM case according to a public reference, not finding this problem. If we have learned and understood the settings from this case, it is enough.