# flow separation problem

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 April 15, 2008, 18:34 flow separation problem #1 bob Guest   Posts: n/a 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

 April 15, 2008, 19:12 Re: flow separation problem #2 Glenn Horrocks Guest   Posts: n/a Hi, How far are you modelling upstream and downstream of the diffuser? What Reynolds number and Mach number? Glenn Horrocks

 April 16, 2008, 00:43 Re: flow separation problem #3 bob Guest   Posts: n/a 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

 April 16, 2008, 19:53 Re: flow separation problem #4 Glenn Horrocks Guest   Posts: n/a 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

 April 25, 2008, 09:24 Re: flow separation problem #5 Felix Guest   Posts: n/a 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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