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flow accelerates for unknown reason

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06-05-2012 06:01 AM

Hi all,

 

I'm a rookie in flow analysis using Autodesk simulation,

I've done a simple simulation of a fluid that flows through a header & saw that the velocity has increased.

I figured that it might be because of gravity's influence (although in the parameters gravity was not checked) or becuase the flow area has decreased so I made another analysis of a flow through a plain tube (no change in the flow area) and saw that the velocity also increased there, instead of decreased - again for unknown reason.

 

can anyone think of a possible reason?

 

inlet nodal velocity was 0.823m/sec

outlet velocity as attached

 

Thanks a lot.





 

 

Product Support
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Registered: ‎11-12-2009

Re: flow accelerates for unknown reason

06-05-2012 12:44 PM in reply to: omriv

It may be more appropriate to view the velocities in specific vector directions.  Choose to view the velocities in the direction of the applied velocity. I suspect that tangential velocities may be participating in the magnitude.  Also, fully developed fluid flow produces a parabolic velocity profile, where velocities are zero at the walls and their largest in the center of the flow field (for circular cross sections).  Please let me know if you have further questions.

Pat Tessaro, P.E.
Customer Support Engineer

Autodesk, Inc.
285 Kappa Drive
Suite 250
Pittsburgh, PA 15238
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Re: flow accelerates for unknown reason

06-06-2012 01:52 PM in reply to: PipePakPat

Thanks a lot Pat for your response.

I''ve checked the velocity in the direction of the applied velocity and it did made a little difference,

and I also totaly accept the parabolic velocity profile issue, which explains why some of the flow will have higher velocity values.

is there a way for me to check a certain surface (a section) mean velocity (not by hand calculation it from the mass flow rate)? 

Thanks again

 

 

Product Support
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Registered: ‎11-12-2009

Re: flow accelerates for unknown reason

06-06-2012 02:12 PM in reply to: omriv

Set the displayed results type for velocities in the axial direction, then select the inlet or outlet surface then right-click and choose to select sub-entities, then nodes.  One can then right-click and choose to inquire on results.  A window will appear.  Change the "Summary" type to "Mean".  Please let me know if you have further questions.

 

(And if you suspect your original question has been solved, right-click on the post and mark it as solved for the benefit of other viewers.)

Pat Tessaro, P.E.
Customer Support Engineer

Autodesk, Inc.
285 Kappa Drive
Suite 250
Pittsburgh, PA 15238
Employee
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Registered: ‎03-25-2010

Re: flow accelerates for unknown reason

06-06-2012 03:30 PM in reply to: omriv

Hi,

 

I think you should not be calculating the mean velocity at a surface if your goal is to check that the "flow" makes sense.  The mean velocity depends on the mesh size at each node. Instead, you should look at the "Flow Rate through Face", turn "Results Contours > Smooth Results" off, then select the surface and "Results Inquire > Inquire > Current Results" and then sum the flow through all of the faces.

 

Sincerely,
John Holtz, P.E.
Senior User Experience Designer, Simulation
Autodesk, Inc.

Current version of Mechanical & Multiphysics: 2013 SP1 (2013.01.00.0012 28-Jun-2012)
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Re: flow accelerates for unknown reason

06-12-2012 02:07 PM in reply to: PipePakPat

thanks John & Pat for your elaborating responses.

with the inforamtion that you've provided I managed to learn more about the velocities in the header's outlet.

I'd appreciate if you'll be able to help me with another issue:

 

I'm trying to find out if a certain header structure will cause a smaller pressure drop than the other - how can I find out what is the pressure loss that occurs while the fluid runs through the header (meaning, how can I find out what is the pressure in the header's outlet)?

 

how should I approach this problem? from your experience, what are the the setup conditions that should be defined?  

 

Thanks a lot.