# Simulation of a 2D diffuser

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 August 10, 2021, 04:03 Simulation of a 2D diffuser #1 New Member   Join Date: Aug 2021 Posts: 4 Rep Power: 2 Hi CFD users, for my CFD cource at the university I have to simulate flow into a 2D diffuser. Section of the diffuser is reptangular. Fluid can be treated as incompressibile. As a data, I know only Reynolds number based on hydraulic diameter, at the outlet, and the exit of the diffuser is at the atmosferic pressure. I calculate the mean velocity at the outlet using the Reynolds number given me as a data, and then the mass flow rate. As boundary conditions, since I know nothing about the inlet, I calculate the velocity at the inlet based on the mass flow rate,because the fluid can be treated as incompressibile. Then I imposed velocity inlet as a boundary condition at the inlet, and pressure outlet(gauge pressure equal to zero) at as boundary condition at the outlet. I simulate the flow for two different Reynolds numbers: 50 and 8200. However, a strange thing appears: in both cases I have flow separation, but the reattach point in case Re=8200(turbulent flow) is much more far away from the diffuser outlet than in case Re=50 (laminar flow). Is something wrong with my boundary condition? Someone can help me? Is very important Last edited by Sam6789; August 10, 2021 at 07:03.

August 10, 2021, 12:35
#2
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Filippo Maria Denaro
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Quote:
 Originally Posted by Sam6789 Hi CFD users, for my CFD cource at the university I have to simulate flow into a 2D diffuser. Section of the diffuser is reptangular. Fluid can be treated as incompressibile. As a data, I know only Reynolds number based on hydraulic diameter, at the outlet, and the exit of the diffuser is at the atmosferic pressure. I calculate the mean velocity at the outlet using the Reynolds number given me as a data, and then the mass flow rate. As boundary conditions, since I know nothing about the inlet, I calculate the velocity at the inlet based on the mass flow rate,because the fluid can be treated as incompressibile. Then I imposed velocity inlet as a boundary condition at the inlet, and pressure outlet(gauge pressure equal to zero) at as boundary condition at the outlet. I simulate the flow for two different Reynolds numbers: 50 and 8200. However, a strange thing appears: in both cases I have flow separation, but the reattach point in case Re=8200(turbulent flow) is much more far away from the diffuser outlet than in case Re=50 (laminar flow). Is something wrong with my boundary condition? Someone can help me? Is very important

If you say "strange thing" I immagine you already know the solution of your flow problem. What references are you using?

Owing to the 2D model, you are using for sure a RANS formulation, your solution is therefore strongly dependendent on the model parameters.

August 10, 2021, 14:32
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 Originally Posted by FMDenaro If you say "strange thing" I immagine you already know the solution of your flow problem. What references are you using? Owing to the 2D model, you are using for sure a RANS formulation, your solution is therefore strongly dependendent on the model parameters.
I don't have any references about this case, I'm assuming that the reattach point in case Re=50 should be much more far away than in case Re=8200, because turbulent flow gives more resistance to separation than laminar flow.
I'm using a k-omega SST model

August 10, 2021, 14:38
#4
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Filippo Maria Denaro
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Quote:
 Originally Posted by Sam6789 I don't have any references about this case, I'm assuming that the reattach point in case Re=50 should be much more far away than in case Re=8200, because turbulent flow gives more resistance to separation than laminar flow. I'm using a k-omega SST model
Have you ever studied the test case of the backward facing step in laminar and turbulent regimes?

August 10, 2021, 14:50
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 Originally Posted by FMDenaro Have you ever studied the test case of the backward facing step in laminar and turbulent regimes?
No, unfortunatly.

 August 10, 2021, 18:01 #6 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 5,905 Rep Power: 63 Several papers can be found by searching on internet, for example https://www.google.com/url?sa=t&rct=...PFMiBHZkJQCi07

 Tags boundary condition, diffuser pipe flow, separate region