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Old   January 26, 2011, 06:51
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Rhodri Jones
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Hi all,
I am running a turbulent simulation with regions of swirling, this is causing reverse flow at my outlet. My solution however is converging. Wil the reverse flow affect my results?

thanks in advance
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Old   January 31, 2011, 15:18
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Tom Keheley
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It depends on where you are looking for data. If you are measuring something far from the exit, then no it will not affect it. Try using a pressure boundary for the exit. That should fix it.
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Old   March 30, 2011, 14:43
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Hi. I am solving a steady state laminar problem (flow through microchannel) with velocity inlet and pressure outlet. Although my solution converges (and the mass fluxes are well balanced at convergence); I get reverse flow on pressure outlet (in almost 50% of the total number of faces). That is definitely not acceptable as a solution even though it is converged !! What options do I have?
Note: If I give the outlet as "outflow" instead of "pressure-outlet" ; the number of reverse flow faces significantly decreases!! Am I doing something wrong here ?
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Old   April 1, 2011, 18:28
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if you are getting reverse flow, you should change the temperature on the outlet to the local temperature, to reduce instability. Initial set up is 300k.
what this means is, if you get reverse flow the flow comes back to the region through the outlet boundary with temp value of 300K

if you dont want to see any reverse flow, change turbulence specification to "Intensity + length scare" and change the length scale to be 10% of your outlet diameter.

modeling outlets with flow split outlet is not an accurate way of modeling.

sometimes, for numerical reasons, you need to extent the outlet to be further downstream. in ccm+ you can choose the extruder in mesh models and then select the boundary to be extruded and by how much.

Hope this is all helpful.
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Old   April 5, 2011, 14:53
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Hi,

Thanks for the response. The thing is, so far I am just solving the flow equations, and getting reverse flow. I havent even turned on the energy equation yet. So, Temperature may not be an issue. My flow is laminar, so turbulent specifications should not really matter, if I'm not wrong.

I will probably try extending the domain though. that might wokr. I did not understand the last part of your post.. about split model not being a good way of modeling. Could you please explain what is meant by that?

Thanks!
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Old   May 4, 2012, 05:28
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Quote:
Originally Posted by ara1362 View Post
if you are getting reverse flow, you should change the temperature on the outlet to the local temperature, to reduce instability. Initial set up is 300k.
what this means is, if you get reverse flow the flow comes back to the region through the outlet boundary with temp value of 300K

if you dont want to see any reverse flow, change turbulence specification to "Intensity + length scare" and change the length scale to be 10% of your outlet diameter.

modeling outlets with flow split outlet is not an accurate way of modeling.

sometimes, for numerical reasons, you need to extent the outlet to be further downstream. in ccm+ you can choose the extruder in mesh models and then select the boundary to be extruded and by how much.

Hope this is all helpful.
Hi,

I'm wondering which outlet boundary to use.
I have set up two similar cases (a flow streaming through a pipes). One is set up with "pressure outlet" and the other with "flow-split outlet". The results show slight differences in the calculated velocity values.

What could be the reason for that or rather which boundary is the best to use? I'm afraid I have no data to validate yet.

Best Regards,
tH3f0rC3
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Old   September 4, 2012, 10:55
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Hi,
I may have an answer for you, if you're still alive...
Regarding my system (jet in cavity) it seems that the "pressure outlet" boundary condition give a symetric flat velovity profile.
Using the "Flow-split" boundary condition give an unsymetric velovity profile which is strongly influenced by the air motion in the cavity.
Using this second boundary conditions the solution need quite less iterations to converge (x6 in my case).

Regards,
Djack

Last edited by djack; September 12, 2012 at 08:46.
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Old   January 29, 2015, 01:15
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IreneLing
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Quote:
Originally Posted by djack View Post
Hi,
I may have an answer for you, if you're still alive...
Regarding my system (jet in cavity) it seems that the "pressure outlet" boundary condition give a symetric flat velovity profile.
Using the "Flow-split" boundary condition give an unsymetric velovity profile which is strongly influenced by the air motion in the cavity.
Using this second boundary conditions the solution need quite less iterations to converge (x6 in my case).

Regards,
Djack
Hi Jack, I hope u can help me. I am facing a problem now.. I am simulating water and oil flow in a valve like element, and keep facing the problem of reverse flow at the outlet. isit normal to have reverse flow when the flow is split due to hindrances?
p/s I had extent the outlet to be longer then the 7-8x longer then the inlet. but still it shows reverse flow.

Please help.
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