Recirculating region not shown

ArathoN · March 24, 2014, 15:13

Hi foamers,

I'm simulating the backward facing step and I'm facing a strange problem, if i plot the wallshearstress the variable varies its sign meaning there is a recirculating region but if I use the stream tracer option on paraFoam it shows me no recirculating region.

I changed the BCs, schemes/fvsolution and mesh (I'm considering uniform mesh with various grid size from coarse to fine), but still the problem persist.

Am I using the wrong option to see the stream lines and so the recirculating region? I didn't modify any option in the 'stream tracer' menu.

Please can someone help me here, I don't know what to do anymore. If it is needed i'll post my dropbox link of teh folder case.

linnemann · March 24, 2014, 16:00

Hi

You just need to initialize the streamlines just after the backwards facing step.

Think about it, for a particle of zero mass it will by virtually impossible to be "dragged" into the re-circulation zone as the velocity gradient is simply too steep.

The particle will only "see" the upper part of the re-circulation zone if released at the inlet. At the re-circulation area it simply "glides" on top of it.

So whenever looking at streamlines think of them as particles released into the flow. That also means that when you are looking for specific flow-patterns it is of great importance to release the streamlines at proper locations.

ArathoN · March 24, 2014, 16:25

Quote:

Originally Posted by linnemann

Hi

You just need to initialize the streamlines just after the backwards facing step.

Think about it, for a particle of zero mass it will by virtually impossible to be "dragged" into the re-circulation zone as the velocity gradient is simply too steep.

The particle will only "see" the upper part of the re-circulation zone if released at the inlet. At the re-circulation area it simply "glides" on top of it.

So whenever looking at streamlines think of them as particles released into the flow. That also means that when you are looking for specific flow-patterns it is of great importance to release the streamlines at proper locations.

I thought paraFoam would automatically do it and trace all the streamline even in the recirculating region, I didn't know that it works from the point of view of the particle. If i remember correctly in the pitzaily tutorial there was an object-function for the streamLines, is that the initialization needed for parafoam? or should i do something else?

P.S Even though the recirculating region can be considered an 'isolated' region from the flow it should be the Entrainment, can paraFoam "trace" it?

ArathoN · March 25, 2014, 13:13

No one has an idea how to resolve this?

ArathoN · March 25, 2014, 13:39

Ok problem solved, the problem was in the settings of the streamtracer in fact if i plot the velocity vectors using cellcenters/glyph I can see the recirculating region. In particular the problem was the seed line, in fact I have to consider a line parallel to the section of the channel after the step otherwise it will not plot the recirculatin region.

Do any of you know why? And how can i specify the other options with regard of my geometry?

March 24, 2014, 15:13	Recirculating region not shown	#1
ArathoN Senior Member ArathoN Join Date: Jul 2011 Posts: 137 Rep Power: 16	Hi foamers, I'm simulating the backward facing step and I'm facing a strange problem, if i plot the wallshearstress the variable varies its sign meaning there is a recirculating region but if I use the stream tracer option on paraFoam it shows me no recirculating region. I changed the BCs, schemes/fvsolution and mesh (I'm considering uniform mesh with various grid size from coarse to fine), but still the problem persist. Am I using the wrong option to see the stream lines and so the recirculating region? I didn't modify any option in the 'stream tracer' menu. Please can someone help me here, I don't know what to do anymore. If it is needed i'll post my dropbox link of teh folder case.

March 24, 2014, 16:00		#2
linnemann Senior Member Niels Nielsen Join Date: Mar 2009 Location: NJ - Denmark Posts: 556 Rep Power: 27	Hi You just need to initialize the streamlines just after the backwards facing step. Think about it, for a particle of zero mass it will by virtually impossible to be "dragged" into the re-circulation zone as the velocity gradient is simply too steep. The particle will only "see" the upper part of the re-circulation zone if released at the inlet. At the re-circulation area it simply "glides" on top of it. So whenever looking at streamlines think of them as particles released into the flow. That also means that when you are looking for specific flow-patterns it is of great importance to release the streamlines at proper locations. __________________ Linnemann PS. I do not do personal support, so please post in the forums.

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March 25, 2014, 13:13		#4
ArathoN Senior Member ArathoN Join Date: Jul 2011 Posts: 137 Rep Power: 16	No one has an idea how to resolve this?

March 25, 2014, 13:39		#5
ArathoN Senior Member ArathoN Join Date: Jul 2011 Posts: 137 Rep Power: 16	Ok problem solved, the problem was in the settings of the streamtracer in fact if i plot the velocity vectors using cellcenters/glyph I can see the recirculating region. In particular the problem was the seed line, in fact I have to consider a line parallel to the section of the channel after the step otherwise it will not plot the recirculatin region. Do any of you know why? And how can i specify the other options with regard of my geometry?