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-   -   Review: Reversed flow (http://www.cfd-online.com/Forums/fluent/105019-review-reversed-flow.html)

CRT July 20, 2012 14:03

Review: Reversed flow
 
As a lot of user I've encountered with the reversed flow at the outlet. I would like to explain briefly the available outlets: "outflow" and "pressure outlet" and discuss about the solution than some users have given.

1. Outflow --> Extrapolates the required information from the interior. (used to model flow exits where the details of the flow velocity and pressure are not known prior to solving the flow problem)

Conditions:

-An overall mass balance correction.

-A zero diffusion flux for all flow variables. " Ansys updates the outflow velocity and pressure in a manner that is consistent with a fully-developed flow assumption" = flow in which the flow velocity (and/or profiles of other properties such as temperature) is unchanging in the flow direction.

2. Pressure Outlet --> (Pressure user input) = The static pressure of the fluid at the outlet plane. Ansys extrapolates all other conditions from the interior of the domain.
There are several options such as:

-average Pressure: should be use when the flow of the boundary exit is subsonic.

-Target mass flow rate: Iīm not sure how useful is this option, I've got better results whit this option disabled.

Reversed flow problems & solutions:

1- if we are using outflow as a BC, we canīt accept to have this reversed flow. some users suggest to enlarge your outlet or to switch your BC to pressure outlet.

2- if we are using pressure outlet as a BC, we should ask ourselves.
does it have physical sense? It could be just a recirculation area, then for a affirmative answer we just have to take care to give suitable values to the reversed flow properties ( such as T, .. ).
if it doesn't have physical sense, then we should try to fix it, as I said before, enlarging the outlet or with procedure strategies to modify solution methods and/or solution controls :to decrease the under-relaxation factors...

*
What's not clear for me, is where is the limit to use outflow? I used to do some calculations and it tends to create this fully developed flow path, the theory says that i doesn't have influence upstream but for it seems to be really strange, to have a fully developed flow just at the entrance of an outlet pipe.
the same simulation with outflow and pressure outlet are quite different, pressure outlet seems to be more real, in spite of the reversed flow warning is painful to me eyes..

Why does the manual say that outflow is used "where the details.. are not know .." ? I understood that i should be use when there is fully-developed flow or similar conditions. or?

If one decides to enlarge the outlet, should expect to have a unrealistic results ? since we're using not a real model. Or shouldn't it have a great impact on the results ?

I hope we could get a clearer idea about this "problem" and the best way to deal with it. thanks! i wait for your answers!


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