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Rupp Carriveau June 13, 2001 15:12

Free Surface Gravity Driven Flow
I am trying to solve a simple gravity driven funnel flow in Fluent. My mesh geometry is a simple cone, (upside down lampshade). I have specified that both the inlet and outlet have zero gauge pressure (both open to atmosphere)---(this flow is purely gravity driven). As this funnel drains I wish the water level to remain constant as if exactly what is drained is re-fed to the top. I have set the top and bottom of my cone as pressure inlet and outlets respectively, and set my cone structure simply as a wall. When I solve this flow the pressures vary so minutely from zero in between the 2 boundary conditions, and, velocity at the outlet is not even close to V=(2gh)^0.5, which it should at least be in the neighbourhood of. I have gravity turned on in the model, I checked most other things, can anyone suggest what I am missing in this simple problem?

Thanks so much,


rafat June 13, 2001 21:40

Re: Free Surface Gravity Driven Flow
Have you tried to apply body weighted coupling.

Rupp June 14, 2001 15:03

Re: Free Surface Gravity Driven Flow

Results unfortunately remain unchanged.

Thanks for the response,


Scott Whitney June 14, 2001 16:53

Re: Free Surface Gravity Driven Flow
There is a small mistake in your logic. You forgot the pressure from the water on top of the bottom exit. For example if the funnel is 1m high, and the top inlet is set to 0 guage pressure, then the bottom outlet must be set to: density*gravity*height=-(1000kg/m^3)*(-9.81m/s^2)*(1m)=-9810 Pa guage

Note that gravity is negative in this case. I tried the solution and it works perfectly when the water pressure is included, but fails if both openings are set to the same pressure.

Scott Whitney June 14, 2001 16:54

Re: Free Surface Gravity Driven Flow
Oops, I put an extra negative sign accidently at the front of the formula. It should read:

density*gravity*height=(1000)*(-9.81)*1=-9810 Pa guage

Sorry about the typo.

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