Total Pressure problem in a duct

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Discussion Overview

The discussion revolves around a problem related to total pressure in a duct, specifically a 2D S-duct configuration. Participants explore the physical implications of pressure changes observed in the duct, including flow separation and the behavior of static and total pressures in viscous flow conditions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes a 2D S-duct with specific intake velocity and gauge pressure, noting unexpected changes in total pressure despite expectations of constancy.
  • Another participant questions whether the flow is inviscid or viscous, leading to a clarification that the flow is viscous and laminar.
  • It is noted that in viscous flow, total pressure is not conserved, which aligns with the observed results.
  • A participant points out the presence of negative static pressure, suggesting it may be due to an offset in the CFD code, while another argues that negative fluid pressures are not physically possible.
  • Concerns are raised about the flow separation occurring only at one curve of the duct, prompting a suggestion for preliminary hand calculations to verify results.
  • Discussion includes references to pressure recovery in ducts, with some participants providing insights on expected performance metrics for pressure recovery in duct designs.

Areas of Agreement / Disagreement

Participants generally agree that total pressure is not conserved in viscous flows, but there is disagreement regarding the interpretation of negative static pressure and its implications. The discussion remains unresolved regarding the specific causes of the observed pressure changes and the validity of the CFD results.

Contextual Notes

There are limitations regarding the assumptions made in the CFD analysis, particularly concerning the treatment of static pressure and the potential effects of boundary layers on total pressure. The discussion also highlights the complexity of flow behavior in duct geometries.

physixlover
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Hello
Please help me, a physical relation which my friend is facing, I'm unable to get my head around..
I created a 2D S-duct, the contours are as follows
1.Static pressure
2.Velocity contours
3. Total pressure gradient graph

intake Velocity 20m/s
Guage pressure is taken as '0' pascals

there is flow separation and reversal at + cruvature
created a rake through the duct to find out the pressure gradient

but the total pressure should be constant in duct, only static pressure changes but i was unable to find out what physical problem is causing such as change as shown in graph

Thanks in Advance
 

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sorry for got to attach geometry, rake view
 

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Inviscid or viscous?
 
viscous, Laminar.

Thanks
 
In a viscous flow, total pressure won't be conserved, so it shouldn't have constant total pressure.

I also notice you have a negative static pressure, which makes no sense. That isn't physically possible.
 
there is probably an offset used in the CFD code for static pressure, like 101325 Pa. There are good numerical reasons for doing this. 0 Pa would then actually be 101325 Pa.

Also, the Casimir effect can cause negative pressures, but that's quantum physics, so naturally nothing works as it's supposed to.
 
The Casimir effect still doesn't cause negative fluid pressures. For starters, it is a quantum electrodynamic effect that occurs canonically in a vacuum. That isn't relevant in the least.
 
Yeah as boneh3ad said total pressure isn't conserved in viscus flows. Did you do any hand calculations as a preliminary check of your results? I don't have much CFD experience personally, but it seems strange that the flow separates only at your first curve and not the second.
 
ever heard "Pressure Recovery"...

a duct with 98% Pressure recovery is nice...

an "S" duct with a 96% recovery (a length-diameter ratio of say 4'ish) is good too...

if all is perfect... you will loose total pressure from boundary layer effects...

incorporate diffusion and see how messy it gets! :O
 

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