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May 27, 2010, 11:08 
Specify both velocity AND total pressure

#1 
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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 

May 27, 2010, 14:43 

#2 
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I have the same problem. If anyone has an idea, please share ...


June 2, 2010, 05:24 

#3  
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Max
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Quote:
cheers [IMG]file:///tmp/mozscreenshot.jpg[/IMG][IMG]file:///tmp/mozscreenshot1.jpg[/IMG] 

June 2, 2010, 06:21 

#4 
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Ian Helms
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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. 

June 2, 2010, 10:01 

#5 
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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 (pressureinlet, massflowinlet) 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 pressurefarfield as inlet. Same story. 

June 2, 2010, 10:10 

#6 
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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 pressureinlet 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 .... 

March 1, 2017, 12:15 

#7  
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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:


March 1, 2017, 13:01 

#8  
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Lucky Tran
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Quote:
Btw for flow in a constant area pipe with wall friction and any pressure outlet, socalled Fanno flows, the flow can't be supersonic in the pipe. Last edited by LuckyTran; March 1, 2017 at 14:14. 

March 1, 2017, 14:11 

#9 
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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'.


March 1, 2017, 15:30 

#10 
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Jose Luis Xancopinca T.
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March 1, 2017, 15:32 

#11  
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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:


March 1, 2017, 16:28 

#12  
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Lucky Tran
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Quote:
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. 

March 29, 2017, 08:56 
Similar problem

#13 
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Gonzalo
Join Date: Mar 2011
Location: Argentina
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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: Massflowinlet, 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, SSTkw 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 (5e3), 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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