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Reversed flow in XXX faces on pressure outlet

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Old   September 5, 2012, 17:05
Default Reversed flow in XXX faces on pressure outlet
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Hi,

I'm trying to model two static mixers in a serial configuration with helical elements. The length of the geometric model is around 72 inches from the inlet to the outlet with helicals running the first 34 inches with a 3 inch break then another 34 inches of helicals.

I have my axis's set so that the Z is the direction of the fluid flow. I then put a magnitude of 0.00025 m/s in the direction normal to the boundary (i'm assuming this is going to give me flow along the Z axis and when I look at my contours it does).

The inlet is set as a velocity boundary condition (where I inputted the magnitude of the flow) and the outlet as a pressure outlet condition that I left as default.

I'm using a non-newtonian fluid in the flow domain as well.

I also am initializing my solution via the standard method.

When I run my simulation I get the error as in the title. Is this something that I need to be worried about or can I just leave it as it is?

How do I get rid of this as well.
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Old   September 6, 2012, 08:27
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Hi,

Reversed flow means, your flow comes from the pressure outlet into your domain instead of going out through it. Sometimes it happens, when your initial guess is bad... or if strong vorticies (with backflow) go out through the outlet. Does the message vanish after some iterations? Are you running a time dependent simulation? Did you try to use a different initialization?
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Old   September 6, 2012, 14:20
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The message does not go away so far however I'm going to run a lot more iterations and see if it does. The number decreases then increases again.

I'm also going to try initializing the solution with a different velocity and pressure to see if that changes anything.

Besides this anything else that I should do?
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Old   September 6, 2012, 14:35
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Again: Are you running a time dependent simulation?
If your steady state solution seems to "swing" it could be a hint, that your problem is actually time dependent and the solver has problems to "force it" to a steady solution.
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Old   September 7, 2012, 04:05
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The FAQ for Ansys CFX covers the same situation in Fluent:

http://www.cfd-online.com/Wiki/Ansys...f_an_OUTLET.22
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Old   September 7, 2012, 12:08
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Try to extend the outlet and see if that helps to get rid of the reverse flow
The message does not mean something is wrong. It just gives you an indication of flow coming in and you may want to review your setup if that is something you are not expecting.
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