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Pressure results different from Function Calculator and Contour View

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Old   May 20, 2014, 22:32
Default Pressure results different from Function Calculator and Contour View
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Hi!

Just wondering if you've encountered such problems like this and hope you could help me out.

I'm trying to check the pressure difference in an orifice with two nozzle designs. Made two cases wherein the setup nozzles are Convergent and Divergent designs. Normally I've seen high pressure on Divergent nozzles compared to the Convergent ones. But in my latest simulation, I got opposite results based on the calculator - massflowave(total pressure)@inlet. I investigated further and cut a plane and view pressure contour on both models and results where different from the calculator. What seems to be the problem?

Geometry:
Model domain = 120mm in diameter and 750mm in total length
Nozzle Convergent: D1 = 2mm; D2 = 1mm ; Length = 3 mm
Nozzle Divergent: D1 = 1mm; D2 = 2mm ; Length = 3 mm
(Circular array of nozzles and domain pls refer to file attachement)

BCs:
Inlet: Normal Vel = 4m/s about 200mm from orifice plate
Outlet: Opening Pressure = 0 about 500mm from orifice plate (opposite side)

Calculator Results: (total pressure)
Convergent = 17 kPA
Divergent = 13 kPA

Pressure Contour (please refer to attachement)

Thanks everyone!
Attached Images
File Type: jpg domain.jpg (82.1 KB, 11 views)
File Type: jpg nozzle_array.jpg (34.7 KB, 12 views)
File Type: jpg convergent.jpg (91.8 KB, 12 views)
File Type: jpg divergent.jpg (92.9 KB, 13 views)

Last edited by mariconeagles96; May 20, 2014 at 22:34. Reason: Clearer Problem Title
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Old   May 20, 2014, 23:29
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Im sorry got mixed up...the simulation results shown awhile ago was correct on both calculator and contour view. These results were just different from my old simulation wherein i've got bigger nozzle geometry. Convergent should have lower pressure drop than the Divergent nozzle in an orifice problem because of the smaller opening area.

Can anyone shed light on the matter. Confusing cause I have to different results using different nozzle sizes.

Old Simulation with big nozzles and low speed:
Calculator Results: (total pressure)
Convergent = 1955 Pa
Divergent = 2803 Pa
Attached Images
File Type: jpg convergent_big.jpg (101.1 KB, 7 views)
File Type: jpg divergent_big.jpg (100.2 KB, 8 views)

Last edited by mariconeagles96; May 20, 2014 at 23:38. Reason: with file attachement
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Old   May 21, 2014, 03:25
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Frank Weise
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...this is normal bahavior. More can be found under: Torsten Gerlach "Microdiffusers as dynamic passive valves for micropump application" 1998 (http://www.sciencedirect.com/science...24424798000569).



Last edited by FrankW; May 21, 2014 at 03:25. Reason: insert a word
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Old   May 21, 2014, 07:31
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Glenn Horrocks
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Why are you modelling zillions of these nozzles? From what I can see, the flow forms an essentially stationary high pressure region on the inlet side, and a stationary low pressure region on the outlet side. This means that each individual nozzle sees the same pressure drop, and is flowing 1/(count of nozzles) of the flow.

So why not model a single nozzle with 1/(count of nozzles) flow rate? Then this is a very simple model which you can do proper mesh sensitivity studies and get a really accurate result.
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Old   May 27, 2014, 21:09
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Hi!

@ghorrocks, actually already did single nozzle simulation and just wanted to know the outcome with many nozzles around it. It was different results depending on my flow velocity input and divergent/convergent nozzles angle. Very interesting.

@FrankW...thanks. Got some references too wherein for subsonic flows, divergent nozzles produces high pressure. But if the flow is too low, then it produces low pressure than the convergent nozzles.

Thanks again guys! Will do more simulation studies about this. =)
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