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November 4, 2013, 13:11 |
Boundary conditions for 'wind tunnel'
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#1 |
Member
Olie
Join Date: Oct 2013
Posts: 51
Rep Power: 12 |
Hi all,
I'm trying to set up a simulation that acts as a 'wind tunnel', with a backward facing step in the test section. I'm using a RANS solver (pisoFoam) with k-epsilon turbulence model. I've got it all to solve but the results aren't believable - so I think it's to do with my boundary/initial conditions - so here's what I've got at the moment: Inlet: U - uniform 30m/s field: Code:
inlet { type fixedValue; value uniform (30 0 0); } Code:
inlet { type zeroGradient; } U - fully developed, so dU/dx = 0: Code:
outlet { type zeroGradient; } Code:
outlet { type fixedValue; value uniform 0; } Initial k and epsilon for the inlet were calculated, and outlet set to zeroGradient. Fixed walls use k and epsilon wall functions. Now when the flow reaches steady state I'd expect there to be a higher pressure at the inlet than outlet in order to drive the flow, but a significant drop in pressure just aft of the step in the recirculation region - in the first image I've posted you can see the velocity contours/stream tracer looks as you'd expect - 30m/s at the inlet, about 15m/s at the outlet, and there's the recirculation region just aft of the step. However in the second image the pressure distribution looks all wrong.. The high pressure is at the outlet, not the inlet, and there doesn't seem to be any drop in pressure in the recirculation region. Please help! Thanks a lot, Olie |
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November 5, 2013, 03:41 |
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#2 |
Senior Member
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Hi Odellar,
I think you should keep pressure indeed at zeroGradient for the walls, but for the velocity you should use a fixedValue with a velocity of uniform (0 0 0). Otherwise there is no reason why your pressure would drop (no friction). Regards. Tom |
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November 5, 2013, 07:10 |
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#3 |
Member
Olie
Join Date: Oct 2013
Posts: 51
Rep Power: 12 |
[QUOTE=tomf;460581]Hi Odellar,
I think you should keep pressure indeed at zeroGradient for the walls, but for the velocity you should use a fixedValue with a velocity of uniform (0 0 0). Otherwise there is no reason why your pressure would drop (no friction). Regards. Tom[/QUOTE Hi Tom, I'd actually made a mistake typing that - yes I have set the walls to U (0 0 0) (no-slip boundary condition). Thanks anyway!! |
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November 5, 2013, 08:31 |
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#4 |
Senior Member
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Hi,
Ah, I've looked at it more closely now. You are plotting static pressure, which will drop with the velocity (Bernoulli's principle). If you calculate total pressure (p+0.5*mag(U)^2) you will probably see the drop you expect. Regards, Tom |
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November 5, 2013, 09:08 |
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#5 | |
Member
Olie
Join Date: Oct 2013
Posts: 51
Rep Power: 12 |
Quote:
Thanks, Olie |
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November 5, 2013, 10:32 |
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#6 |
Senior Member
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Hi,
You can either use the ptot OpenFOAM utility or use the calculator in paraView to get the total pressure. Than you have this as an additional variable in paraView. Regards, Tom |
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November 5, 2013, 11:31 |
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#7 |
Member
Olie
Join Date: Oct 2013
Posts: 51
Rep Power: 12 |
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