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where is the error? 3D Fluent

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Old   March 14, 2005, 09:43
Default where is the error? 3D Fluent
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pieizquierdo (miguel)
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Hi:

I have tested and validated in 2D for bluff bodies such as ahmed vehicle. The 2-D worked perfectly.

I used the same work method including modelling and meshing in gambit and creating boundary properties. Back to Fluent, I set up the experiment as I used to do in 2D. but.... I got outlet velocity=0m/S

Inlet velocity was high enough 60-100m/s to have outlet velocity in the ahmed vehicle test. Outlet boundary condition was Outlet pressure.

Where can the error be? It seems as if there was a very high viscosity, but the fluid is air.

Thanks:

Miguel
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Old   March 14, 2005, 12:53
Default Re: where is the error? 3D Fluent
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Jason
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If you have a velocity inlet, but the flow isn't exiting through your outlet, then where is it going?!

Calculate the mass flux through all of the surfaces (Report->Fluxes) to try and find out if you have mass entering, and if so where it's going (do a hand calc on what you think the mass flow should be as a sanity check). Even if there was a very high viscosity there should still be a velocity at the outlet (massin = massout, assuming you're doing a steady state solution of course).

If there is a mass flow in, then there has to be a mass flow out. Check your operating pressure, your gauge pressure at the outlet, and the direction and magnitude of your velocity inlet.

If there isn't a mass flow through your inlet (or if the mass flow is much lower than you expect) then check the direction of flow at the inlet. Double check your boundary conditions to make sure that there is a way for the flow to leave the system (in other words, make sure your outlet is really defined as a pressure outlet and that the pressure isn't some rediculous value).

Good luck, Jason

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Old   March 14, 2005, 18:43
Default Re: where is the error? 3D Fluent
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pieizquierdo (miguel)
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I think I got it!!!

It had to be a very simple thing. I checked the mass flux and it was the same at the entrance and at the exit. I calculated it by hand and the flux value was correct. So some things where discarted.

I tried to visualize path lines and colour organised them by velocity magnitude and.... there was velocity!!!

But then I tried to see the velocity magnitud on the body surface and it was.... 0!!!!

I guess this is due to boundary layer properties.

So if this is correct, the question is how can I visualize the velocity iso-values over the body?

thanks

Miguel
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Old   March 15, 2005, 07:18
Default Re: where is the error? 3D Fluent
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dirk
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I suppose you defined your body as a wall, so there will of course be no velocity because of the no slip condition. You should visualize the pressure instead.

Dirk
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Old   March 15, 2005, 07:38
Default Re: where is the error? 3D Fluent
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miguel
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Dirk, you are right, The body had a wall condition and I could see the pressure distribution. The way I have found easier to visualize the velocity is via path lines and planes coluored with velocity magnitude
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Old   March 15, 2005, 08:35
Default Re: where is the error? 3D Fluent
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Jason
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Dirk's right... since it's a wall, there's no velocity at the boundary. If you're trying to visualize the flow, you can do plots of Mach contours. Also, plotting vectors on the surface can create some pretty interesting flow viz (it'll color the vectors based on cell center values instead of node values, you can get the same color plot by plotting contours and turning off node values).

Hope this helps, and goodluck, Jason
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