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Old   October 8, 2019, 03:58
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Hello, everybody,
I'm about to simulate a Coanda nozzle. However, speeds of up to 1600 m/s always occur in the narrow gap, which does not agree with the corresponding literature. On the left side is the inlet with a boundary condition of 0 bar pressure, on the right the outlet with a boundary condition of 0 bar and from above the air flows in with a pressure of 6 bar.
Does anyone have any idea what I can change in the simulation that the velocity significantly decreses?
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Henrike
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Old   October 8, 2019, 04:06
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What kind of speeds are you expecting?
What kind of solver are you using (transient or steady state)?
Is the solver crashing?
What about mesh resolution in the narrow gap?
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Old   October 8, 2019, 04:10
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I expect speeds of about 50-70 m/s.
I am using a steady state solver SST.
No, the calculation converges.
The mesh in the narrow gap should be okay as I have 5 Inflation layers.
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Old   October 8, 2019, 04:28
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Ok. Can you use a velocity inlet instead of pressure inlet?
Have you checked that 6 bar inlet pressure is correct? It looks like your inlet pressure may be too high for your expected speeds.
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Old   October 8, 2019, 04:30
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Unfortuatley, I have to use a pressure inlet.
Even with 2 bar the velocity is about 500 m/s which is way too high.
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Old   October 8, 2019, 04:33
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Can you give rough estimate of mass flow and gap diameter?
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Old   October 8, 2019, 04:42
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The gap has a width of 40 Ám and the mass flow should be around 0.25 m^3/s.
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Old   October 8, 2019, 06:16
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If you look in standard literature you will find that when the pressure ratio Pin/Pout >2 (absolute pressures, for air, atmospheric conditions) the velocity will be around Mach=1=340m/s. So for a ratio of 6, your results look fine. Provided you have compressible conditions.

If you expect 50-60 m/s then your pressures are incorrect, or you are dealing with a multiphase system.
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Old   October 10, 2019, 04:22
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Thank you for your reply. Can you name the literature that you are referring to?
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Old   October 10, 2019, 04:45
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Which fluid are you trying to simulate? Air? Water? Something else?
Are you using compressible or incompressible fluid in the solver?
How did you ensure that the results we are discussing here are actually from a converged solution?
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Old   October 10, 2019, 04:47
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Hello I am simulating air that is incompressible. I also carried out simulations were the air was modelled as compressible but also there the velocities are a lot higher than expected (800 m/s).
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Old   October 10, 2019, 06:57
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So that answers the first part of my question. For air you absolutely need a compressible formulation for this type of flow. E.g. the compressible ideal gas model.
Now back to the ignored part of my question: are you absolutely certain that your solutions are converged? How did you check for convergence?

Edit: and what's the speed of sound in the fluid you are simulating?
maybe related: Supercritical outflow from an orifice
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