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Old   September 7, 2009, 09:43
Default outflow boundary condition
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Dear All,

I am using FLUENT.
I am not understanding when to use outflow boundary conditions at the outlet.
I am giving velocity & temperature at the inlet.
I want to know the pressure at outlet which is below atmospheric pressure.

Please explain in brief about the out-flow boundary conditions.

Thanks in advance.
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Old   September 7, 2009, 09:50
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From User Guide:

7.10 Outflow Boundary Conditions



Outflow boundary conditions in FLUENT are used to model flow exits where the details of the flow velocity and pressure are not known prior to solution of the flow problem. You do not define any conditions at outflow boundaries (unless you are modeling radiative heat transfer, a discrete phase of particles, or split mass flow): FLUENT extrapolates the required information from the interior. It is important, however, to understand the limitations of this boundary type.


Note that outflow boundaries cannot be used in the following cases:


  • If a problem includes pressure inlet boundaries; use pressure outlet boundary conditions (see Section 7.8) instead.
  • If you are modeling compressible flow.
  • If you are modeling unsteady flows with varying density, even if the flow is incompressible.
  • With the multiphase models (Eulerian, mixture, and VOF (except when modeling open channel flow, as described in Section 23.3.9).
For an overview of flow boundaries, see Section 7.2.

--------------------------
7.10.1 FLUENT's Treatment at Outflow Boundaries




The boundary conditions used by FLUENT at outflow boundaries are as follows:
  • A zero diffusion flux for all flow variables.
  • An overall mass balance correction.
The zero diffusion flux condition applied at outflow cells means that the conditions of the outflow plane are extrapolated from within the domain and have no impact on the upstream flow. The extrapolation procedure used by FLUENT updates the outflow velocity and pressure in a manner that is consistent with a fully-developed flow assumption, as noted below, when there is no area change at the outflow boundary.
The zero diffusion flux condition applied by FLUENT at outflow boundaries is approached physically in fully-developed flows. Fully-developed flows are flows in which the flow velocity profile (and/or profiles of other properties such as temperature) is unchanging in the flow direction. It is important to note that gradients in the cross-stream direction may exist at an outflow boundary. Only the diffusion fluxes in the direction normal to the exit plane are assumed to be zero
--------------------------
7.10.2 Using Outflow Boundaries

As noted in Section 7.10.1, the outflow boundary condition is obeyed in fully-developed flows where the diffusion flux for all flow variables in the exit direction are zero. However, you may also define outflow boundaries at physical boundaries where the flow is not fully developed--and you can do so with confidence if the assumption of a zero diffusion flux at the exit is expected to have a small impact on your flow solution

Note that convergence may be affected if there is recirculation through the outflow boundary at any point during the calculation, even if the final solution is not expected to have any flow reentering the domain. This is particularly true of turbulent flow simulations
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Old   September 7, 2009, 10:42
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ThanQ MAX,

I am using mixtures model so, i should not use the outflow boundary condition.

i will explain my problem.
I am going to simulate the flow of mixture model which is entering through 3 pipes (which are having different lengths and bends) into a mixing chamber which would be driven out further.
Total system is operating below the atmospheric pressure.
My problem is i should get the same pressure at the inlet of the mixing chamber so that i can drive out the equal amount of mass flow rate from each pipe.
what type of boundary conditions i have to give ?
on what basis i have to change the dimensions of the pipes so that the pressure will be same at the inlet of the mixing chamber ?
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Old   September 8, 2009, 02:13
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fix the pressure inlets and set the pressure outlet.
your computation will give your the massflow rates for each pipe
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Old   September 8, 2009, 09:17
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Thanq MAX,

But i have to match the fluent results with practical data for that, i should give the velocity at inlet and i should not give pressure inlets & outlets.


Now,

I want to findout the pressure drop in each pipe(curved like L-shaped).
I am specifying velocity at inlet and zero pressure at outlet so that i will get the overall pressure drop in the pipe.
but fluent is giving the negative pressure values at inlet.
If i model a straight pipe it is giving the possitive pressure values at inlet.
Why is it happening like this ?
Should i take the negative values of fluent as possitive ?
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Old   September 8, 2009, 09:34
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it depends on the value you give at the outlet.
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Old   August 21, 2012, 07:26
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dear friends,
i am trying to simulate 3D wing ie external aerodynamics, i draw in gambit and for solving i am using fluent, for my 3D wing [ laminar incompressible flow, velocity=8m/s, area of the wing is 0.0102 sq m, length is 0.065m]

please find the image, i give my boundary condition in that 1 refers velocity inlet, 2 refers pressure outlet, i am considering wing as wall, remaining faces in my domain is symmerty its correct or wrong? please give your feed back.
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Old   March 26, 2014, 18:35
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Hi Dear friend
Could you please describe what the outflow boundary condition is?

regards
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