Reversed flow in XXX faces on pressure outlet

parradux · September 5, 2012, 16:05

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.

RodriguezFatz · September 6, 2012, 07:27

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?

parradux · September 6, 2012, 13:20

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?

RodriguezFatz · September 6, 2012, 13:35

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.

flotus1 · September 7, 2012, 03:05

The FAQ for Ansys CFX covers the same situation in Fluent:

http://www.cfd-online.com/Wiki/Ansys...f_an_OUTLET.22

nikhil · September 7, 2012, 11:08

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.

deldan · May 2, 2015, 04:26

Sir
I working on earth air heat exchanger where air is pass through buried pipe underneath. while iteration i am also facing reverse flow at pressure outlet till end of iteration and solution does not converge.
so what could causes of reverse flow since there is no reverse flow physically occurring in my study.

September 5, 2012, 16:05	Reversed flow in XXX faces on pressure outlet	#1
parradux New Member asdasdasd Join Date: Aug 2012 Posts: 15 Rep Power: 13	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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September 6, 2012, 07:27		#2
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 26	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?

September 6, 2012, 13:20		#3
parradux New Member asdasdasd Join Date: Aug 2012 Posts: 15 Rep Power: 13	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?

September 6, 2012, 13:35		#4
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 26	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.

September 7, 2012, 03:05		#5
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,399 Rep Power: 46	The FAQ for Ansys CFX covers the same situation in Fluent: http://www.cfd-online.com/Wiki/Ansys...f_an_OUTLET.22

September 7, 2012, 11:08		#6
nikhil New Member Nik Join Date: Mar 2010 Location: Michigan Posts: 25 Rep Power: 16	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.

May 2, 2015, 04:26		#7
deldan New Member Deldan Namgial Join Date: Mar 2014 Posts: 5 Rep Power: 12	Sir I working on earth air heat exchanger where air is pass through buried pipe underneath. while iteration i am also facing reverse flow at pressure outlet till end of iteration and solution does not converge. so what could causes of reverse flow since there is no reverse flow physically occurring in my study.