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Strange results for jet dispersion into ambient

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Old   October 16, 2015, 06:17
Default Strange results for jet dispersion into ambient
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Alireza Shardi
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Hi all,

I have simulated a jet dispersion case with two flowrates. One of the flowrates is around 5 times higher than the other (800 m3/hr vs 150 m3/hr).

My question is about the secondary flows in the ambient domain that are almost identical in these two cases. I would expect lower intensity of the ambient currents in the case with lower flow rate but this is not seen in the results.

I have attached the results link. The top surface is free water surface, the sides are presure opening and the opening is placed in a no-slip surface BC.

https://www.dropbox.com/s/90aqn1wawig46na/CFD.PNG?dl=0

Why do I see the identical secondary flows?

I do appreciate your help. Thanks.

Alireza,
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Old   October 16, 2015, 07:59
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Filippo Maria Denaro
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Quote:
Originally Posted by shardiali View Post
Hi all,

I have simulated a jet dispersion case with two flowrates. One of the flowrates is around 5 times higher than the other (800 m3/hr vs 150 m3/hr).

My question is about the secondary flows in the ambient domain that are almost identical in these two cases. I would expect lower intensity of the ambient currents in the case with lower flow rate but this is not seen in the results.

I have attached the results link. The top surface is free water surface, the sides are presure opening and the opening is placed in a no-slip surface BC.

https://www.dropbox.com/s/90aqn1wawig46na/CFD.PNG?dl=0

Why do I see the identical secondary flows?

I do appreciate your help. Thanks.

Alireza,

In my opinion if you do not specify much better your problem (geometry, setting of the formulation, method, etc.) the figures you posted cannot say nothing...
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Old   October 16, 2015, 09:06
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Alireza Shardi
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Hi FMDenaro,


Thanks for the reply. You are right, here is the info, hopefully it makes it clear:

1- Geometry: I have two pipes entering a cylindrical domain. However the CFD domain uses a symmetry line which is in the middle of two jets (not seen in my 2D picture). Therefor the modeled domain is a half cylinder with one jet. The dimensions are: diameter of cylinder=12 m, height of cylinder= 6 meter and opening of the jet is 240 mm

2- settings: I have used shear stress turbulence model and used steady state.

3- BC: I have specified mass flux at the jet opening. Free water surface on the top side of my domain, zero pressure opening on the sides and no-slip wall on the BC with the jet.

As shown in the picture, I have high and low flowrate cases, but the ambient currents for entrainment of the jet (shown by arrows on a plane in the middle of jet) seem to be identical, but why would it be?. You can see the currents come from the zero pressure BC towards the jet and bend it downward.

I also edited the picture:

https://www.dropbox.com/s/90aqn1wawig46na/CFD.PNG?dl=0

Thanks again and look forward to hearing from you.
Alireza,
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Old   October 16, 2015, 09:20
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Filippo Maria Denaro
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well, you are working with a 2D RANS simulation... From the two pictures I see relevant different in the solutions, consider that you are solving for the statistical steady state.

What is not clear to me is the real geometry, I suspect that you fixed pressure BC.S to close to the jet... why you have 2 sides with pressure?

Are you solving the incompressible flow model?

could you sketch the real global geometry?
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Old   October 16, 2015, 11:34
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Alireza Shardi
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Hi again.

Thanks for the reply. I actually have a 3D domain but I have no access to my case at this moment, so I tried to sketch a similar domain to mine with all the boundaries. Please have a look and see if it makes things more clear. I also tried with a bigger domain, roughly twice as big, and by changing the distance of pressure boundaries to the jet, I still get the similar secondary currents. My main doubt is why I get the same ambient currents in low jet flowrates as I get in high jet flowrates!?

Here is my global domain with BCs:
https://www.dropbox.com/s/z8itqt455f...CFD-2.PNG?dl=0


Thanks again.

Alireza,
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Old   October 16, 2015, 15:41
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Filippo Maria Denaro
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Quote:
Originally Posted by shardiali View Post
Hi again.

Thanks for the reply. I actually have a 3D domain but I have no access to my case at this moment, so I tried to sketch a similar domain to mine with all the boundaries. Please have a look and see if it makes things more clear. I also tried with a bigger domain, roughly twice as big, and by changing the distance of pressure boundaries to the jet, I still get the similar secondary currents. My main doubt is why I get the same ambient currents in low jet flowrates as I get in high jet flowrates!?

Here is my global domain with BCs:
https://www.dropbox.com/s/z8itqt455f...CFD-2.PNG?dl=0


Thanks again.

Alireza,
to tell the truth, I find strange both solutions for your problem...however, if you have a 3D case, what you are posting is just a vector field over a plane.
Any analysis you want to do between differences in the two inflow setting, requires to do quantitative checking, not a snapshot of the single-plane vector field
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