backflow in-between some transient time-steps

soumitra2102 · September 23, 2019, 22:33

I am facing backflow issue in my transient simulation only for some time-steps.
It occurs in periodically in-between the entire transient solving process only for some random timesteps. It vanishes for sometime and later reappears.

What could this be indicating about the flow?

P.S. I am dealing with condensation along a tube flow.
The backflow does not appear for the same geomtery when run for single phase.

BlnPhoenix · October 30, 2019, 08:33

Backflow from vortices do not have to be stationary since the simulation itself is not stationary. In multiphase simulations it should be avoided since the backflow is usually not well described. Either your volume fraction or the backflow velocity can cause stability and accuracy issues very rapidly.
On the other hand if you concider the influence on accuracy and stability as very minor, steps to avoid this backflow may not be needed.

soumitra2102 · October 30, 2019, 10:43

Thank you.

Yes, backflow conditions are not well know and thइस can seriously deviate the results at outlet.

I resolved the problem. It was bad material properties.
The main issue was that I had not used negative sign for vapor phase standard state enthalpy.

BlnPhoenix · October 30, 2019, 10:51

Quote:

Originally Posted by soumitra2102

Thank you.

Yes, backflow conditions are not well know and thइस can seriously deviate the results at outlet.

I resolved the problem. It was bad material properties.
The main issue was that I had not used negative sign for vapor phase standard state enthalpy.

Ok, good job!

Bodo1993 · December 28, 2019, 11:26

Quote:

Originally Posted by BlnPhoenix

Backflow from vortices do not have to be stationary since the simulation itself is not stationary. In multiphase simulations it should be avoided since the backflow is usually not well described. Either your volume fraction or the backflow velocity can cause stability and accuracy issues very rapidly.
On the other hand if you concider the influence on accuracy and stability as very minor, steps to avoid this backflow may not be needed.

Hi,

I have a problem with single inlet and two outlets. I use multiphase solver for that. However, I get vortices at the outlet in the direction perpendicular to the flow outlet. I am wondering how would I solve this issue.

Thanks in advance. Looking forward to hearing from you.

September 23, 2019, 22:33	backflow in-between some transient time-steps	#1
soumitra2102 Member Soumitra Vadnerkar Join Date: Aug 2018 Posts: 70 Rep Power: 8	I am facing backflow issue in my transient simulation only for some time-steps. It occurs in periodically in-between the entire transient solving process only for some random timesteps. It vanishes for sometime and later reappears. What could this be indicating about the flow? P.S. I am dealing with condensation along a tube flow. The backflow does not appear for the same geomtery when run for single phase.

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October 30, 2019, 08:33		#2
BlnPhoenix Senior Member Join Date: Aug 2014 Location: Germany Posts: 292 Rep Power: 14	Backflow from vortices do not have to be stationary since the simulation itself is not stationary. In multiphase simulations it should be avoided since the backflow is usually not well described. Either your volume fraction or the backflow velocity can cause stability and accuracy issues very rapidly. On the other hand if you concider the influence on accuracy and stability as very minor, steps to avoid this backflow may not be needed.

October 30, 2019, 10:43		#3
soumitra2102 Member Soumitra Vadnerkar Join Date: Aug 2018 Posts: 70 Rep Power: 8	Thank you. Yes, backflow conditions are not well know and thइस can seriously deviate the results at outlet. I resolved the problem. It was bad material properties. The main issue was that I had not used negative sign for vapor phase standard state enthalpy.