Mass flow imbalance for a specific mesh position

Sasquatch · October 11, 2016, 13:31

Dear CFX Users,

I am currently performing a steady-state RANS simulation of a small-scale, 3-stage ROT turbine.

The pitch of the first stage for now equals to 1.

When I position the meshes manually, see picture below:
https://i.imgsafe.org/d20bf73950.png

The mass flow relative error is around 0.01% - very good.

BUT, when I use only one rotor domain, like in the figure below:
https://i.imgsafe.org/d21127adfd.png

the relative error in mass flow is 100%.

The field quantities are clearly comparable. Stage interface is a mixing plane approach (circumferential averaging). I cannot find the source of my error here though. Why there is a difference?

Any help is appreciated. Regards!

Sasquatch · October 12, 2016, 12:36

If anybody has an idea, I would really appreciate the explanation for this discrepancy.

Regards,

ghorrocks · October 12, 2016, 18:21

Are the simulation results equivalent?

Sasquatch · October 12, 2016, 18:47

Yes, quantitatively, there is only a small difference (i didn't calculate exactly but smaller than other sources of error I think). It is hardly visible with the naked eye on the contours.

This small difference in e.g. mach contours makes sense due to the idea of mixing plane where the averaged fluxes (from stator outlet) are transferred to the downstream component (here 1st rotor inlet). Hence I am puzzled why the massflow is actually, entirely lost.

When i match the domains, the only lost massflow is probably due to small unmatched regions due to doing it manually. As I mentioned, in such case it is a negligible discrepancy.

ghorrocks · October 12, 2016, 21:58

I suspect this is just a case where the imbalance calculation does not take into account the interface properly. There is going to be twice the mass flow into the modelled region for the inner domain compared to the outer region, and if the imbalance calculation does not take into account the interface this would appear as mass loss.

So I suspect this is nothing to worry about and is a case where the imbalances are not a suitable measure of convergence. That is why imbalances is not used by default as a convergence criteria, they are not appropriate to all classes of flows.

October 11, 2016, 13:31	Mass flow imbalance for a specific mesh position	#1
Sasquatch Member Join Date: Jul 2016 Posts: 33 Rep Power: 9	Dear CFX Users, I am currently performing a steady-state RANS simulation of a small-scale, 3-stage ROT turbine. The pitch of the first stage for now equals to 1. When I position the meshes manually, see picture below: https://i.imgsafe.org/d20bf73950.png The mass flow relative error is around 0.01% - very good. BUT, when I use only one rotor domain, like in the figure below: https://i.imgsafe.org/d21127adfd.png the relative error in mass flow is 100%. The field quantities are clearly comparable. Stage interface is a mixing plane approach (circumferential averaging). I cannot find the source of my error here though. Why there is a difference? Any help is appreciated. Regards!

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October 12, 2016, 12:36		#2
Sasquatch Member Join Date: Jul 2016 Posts: 33 Rep Power: 9	If anybody has an idea, I would really appreciate the explanation for this discrepancy. Regards,

October 12, 2016, 18:21		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Are the simulation results equivalent?

October 12, 2016, 18:47		#4
Sasquatch Member Join Date: Jul 2016 Posts: 33 Rep Power: 9	Yes, quantitatively, there is only a small difference (i didn't calculate exactly but smaller than other sources of error I think). It is hardly visible with the naked eye on the contours. This small difference in e.g. mach contours makes sense due to the idea of mixing plane where the averaged fluxes (from stator outlet) are transferred to the downstream component (here 1st rotor inlet). Hence I am puzzled why the massflow is actually, entirely lost. When i match the domains, the only lost massflow is probably due to small unmatched regions due to doing it manually. As I mentioned, in such case it is a negligible discrepancy.

October 12, 2016, 21:58		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	I suspect this is just a case where the imbalance calculation does not take into account the interface properly. There is going to be twice the mass flow into the modelled region for the inner domain compared to the outer region, and if the imbalance calculation does not take into account the interface this would appear as mass loss. So I suspect this is nothing to worry about and is a case where the imbalances are not a suitable measure of convergence. That is why imbalances is not used by default as a convergence criteria, they are not appropriate to all classes of flows.