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Old   May 27, 2010, 11:08
Default Specify both velocity AND total pressure
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Ian Helms
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Hi all,

Does anyone know how to specify both the inlet velocity and the total pressure at the boundary inlet? None of the options seem possible. I am running a compressible flow simulation so velocity inlet BC is not appropriate. Thanks
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Old   May 27, 2010, 14:43
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I have the same problem. If anyone has an idea, please share ...
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Old   June 2, 2010, 05:24
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Quote:
Originally Posted by OxIan View Post
Hi all,

Does anyone know how to specify both the inlet velocity and the total pressure at the boundary inlet? None of the options seem possible. I am running a compressible flow simulation so velocity inlet BC is not appropriate. Thanks
Defining a total pressure at an inlet implies that you "know" something about the velocity there (see attachment). Thus, if you give correct static pressure, Ma-Number and total-temperature at the inlet, the velocity obtained from the solution should match with reality.

cheers
[IMG]file:///tmp/moz-screenshot.jpg[/IMG][IMG]file:///tmp/moz-screenshot-1.jpg[/IMG]
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Old   June 2, 2010, 06:21
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Hi Coglione,

Thank you for your reply, I actually realized what my problem was. The original geometry has some streamtube variation in the axial direction, and since I was initially trying to utilize a 2D simulation the absence of the area ratio was essentially creating my problem. Turns out if you properly define your problem you tend to get a better solution .

Jpo I'm sot sure what type of problem you are modeling, so I don't know if that information helps you at all, good luck.
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Old   June 2, 2010, 10:01
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I am trying to achieve supersonic flow in a pipe. Because the flow is compressible, I cannot set supersonic velocity at inlet. Why? Because the available boundary conditions for compressible flow (pressure-inlet, mass-flow-inlet) do not accept velocity values. They accept mass flux or pressure values.

I tried setting "large" total inlet pressure and "small" (skip the numbers here) static inlet pressure. This produces high velocity at inlet. Still, as soon as calculation starts to converge, the flow chokes and drops down to subsonic.

Tried pressure-far-field as inlet. Same story.
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Old   June 2, 2010, 10:10
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coglione,

Thank you for suggesting the formula p_total / p_static
Same statement is made by the Fluent manual sec. 7.3.1:

"If you choose to initialize the solution based on the pressure-inlet conditions, the Supersonic/Initial Gauge Pressure will be used in conjunction with the specified stagnation pressure to compute initial values according to the isentropic relations (for compressible flow) or Bernoulli's equation (for incompressible flow)"

It seems that even if the flow is initialized as supesonic at inlet, it still reverts back to subsonic after a number of iterations. This is my problem - how to keep the flow supersonic in a pipe.

If anyone has ideas, please share ....
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Old   March 1, 2017, 12:15
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Jose Luis Xancopinca T.
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I have the same problem, the Mach number at the inlet decreases to subsonic after a number of iterations.

Have you solved the problem??

Thanks

Quote:
Originally Posted by jpo View Post
coglione,

Thank you for suggesting the formula p_total / p_static
Same statement is made by the Fluent manual sec. 7.3.1:

"If you choose to initialize the solution based on the pressure-inlet conditions, the Supersonic/Initial Gauge Pressure will be used in conjunction with the specified stagnation pressure to compute initial values according to the isentropic relations (for compressible flow) or Bernoulli's equation (for incompressible flow)"

It seems that even if the flow is initialized as supesonic at inlet, it still reverts back to subsonic after a number of iterations. This is my problem - how to keep the flow supersonic in a pipe.

If anyone has ideas, please share ....
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Old   March 1, 2017, 13:01
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Lucky Tran
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Quote:
Originally Posted by Xancopinca View Post
I have the same problem, the Mach number at the inlet decreases to subsonic after a number of iterations.

Have you solved the problem??

Thanks
Just check your boundary conditions (inlet and outlet). Often people set the stagnation and static pressure at the inlet and forget that the downstream pressure also matters.

Btw for flow in a constant area pipe with wall friction and any pressure outlet, so-called Fanno flows, the flow can't be supersonic in the pipe.

Last edited by LuckyTran; March 1, 2017 at 14:14.
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Old   March 1, 2017, 14:11
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This was long time ago... But remember I was able to achieve supersonic flow by setting the walls to 'slip walls'.
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Old   March 1, 2017, 15:30
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Jose Luis Xancopinca T.
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Do you remember what kind of B.C. you were using?? Both in the inlet an outlet???

Thanks!


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This was long time ago... But remember I was able to achieve supersonic flow by setting the walls to 'slip walls'.
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Old   March 1, 2017, 15:32
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Jose Luis Xancopinca T.
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Sorry for not mentioning, but i'm trying to model a 2D supersonic diffuser with inviscid flow.

But I will check the downstream pressure anyway thanks!

Quote:
Originally Posted by LuckyTran View Post
Just check your boundary conditions (inlet and outlet). Often people set the stagnation and static pressure at the inlet and forget that the downstream pressure also matters.

Btw for flow in a constant area pipe with wall friction and any pressure outlet, so-called Fanno flows, the flow can't be supersonic in the pipe.
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Old   March 1, 2017, 16:28
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Quote:
Originally Posted by Xancopinca View Post
Sorry for not mentioning, but i'm trying to model a 2D supersonic diffuser with inviscid flow.

But I will check the downstream pressure anyway thanks!
Try a laminar flow model with slip walls. The inviscid model can be tricky because it more-or-less skips the energy equation.

Also be aware that for the pressure based solver you might need to go to define/models/energy and turn on the kinetic energy terms. You can also turn on the pressure work term while you're at it.

Last edited by LuckyTran; March 29, 2017 at 09:23.
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Old   March 29, 2017, 08:56
Default Similar problem
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Hi all:
I have a problem similar to yours: I'm simulating a complicated geometry with a complex flow inside it witn FLUENT. With complex I mean that it has a complicated path to follow and in some places i could be separated, so there could be some reversed flow. The inlet Mach N░ is 1.13 and I have a pressure outlet at the exit. My BC's area as follow: for inlet: Mass-flow-inlet, a total temperature and I set a static gauge pressure for the firs iteration, and for the outlet a static pressure with target mass flow option enabled. I'm using a Density based formulation, air as an ideal gas, SST-kw for turbulence, 2nd order for the spa┐tial discretizacion, coupled solver with pseudo transient formulation (I have tried to lower the timescale factor but it didn't work)
My problem is that I can't reach the specified Mach N░ at the inlet and the convergence for the continuity is not as good as I wish (5e-3), and I don't know why. Can someone guide me on what I can do to solve this problem? Thanks in advance
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