Total Pressure problem in a duct

physixlover

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

physixlover

sorry for got to attach geometry, rake view

boneh3ad

Inviscid or viscous?

physixlover

viscous, Laminar.

Thanks

boneh3ad

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.

bigfooted

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.

boneh3ad

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.

Aero51

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.

Mithrandir13

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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physixlover	sorry for got to attach geometry, rake view Attached Thumbnails

physixlover	Total Pressure problem in a duct viscous, Laminar. Thanks

boneh3ad	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.

bigfooted	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.

Aero51	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.

Mithrandir13	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