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Old   April 15, 2008, 18:34
Default flow separation problem
  #1
bob
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Hi

I am running a simulation of a 3d diffuser of a 4 degree angle.

The geometry has no sharp angles, the inlet BC is a velocity profile with suitable turbulence parameters. The outlet is area averaged static pressure of 0 relative to atmospheric pressure

I have run this with both SST and Reynolds stress models and i allways get flow separation when it should be attached.

Is thee any kind of setting in CFX (i am fairly new) that i may have missed

(have also tried total pressure at inlet with mass flow boundary at outlet and still separation)

Thanks bob
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Old   April 15, 2008, 19:12
Default Re: flow separation problem
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Glenn Horrocks
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Hi,

How far are you modelling upstream and downstream of the diffuser? What Reynolds number and Mach number?

Glenn Horrocks

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Old   April 16, 2008, 00:43
Default Re: flow separation problem
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bob
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It's just the diffuser, no upstream or downstream modeling. The inlet initially has a very shallow expansion angle so is practically a pipe for the first 1/5th of the length then blends smoothly towards the outlet.

Re= 30,000 M=0.1

bob
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Old   April 16, 2008, 19:53
Default Re: flow separation problem
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Glenn Horrocks
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Hi,

You should model further upstream and downstream. If you don't know what is upstream make sure your inlet velocity and turbulence profile is correct. You will definitely need a domain or a pipe or something for the flow to go into downstream of the diffuser.

Glenn Horrocks
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Old   April 25, 2008, 09:24
Default Re: flow separation problem
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Felix
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My experience with this kind of calculation is that the inlet b.c. has to be specified very carefully. If the flow isn't fully developped there is a probably a radial component to the inlet velocity. This would bring energy towards the boundary layer and give it robustness.

If you want to make a quick check, run the k-epsilon turbulence model with wall functions. In this case the wall function is likely to inaccurately rise the near-wall velocity and the flow will then stay attached longer. If it does so, it means that the k-epsilon sort of "hides" the fact that your inlet b.c. is not well defined and that you should have a radial velocity towards the wall in the inlet plane.

Then you'll know that you have to model further upstream, as Glenn pointed out.

Regards,

Felix
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