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Shankar November 15, 2003 23:47

gravity driven flow- boundary conditions?

I am studying flow through a U-tube like structure. The water level on the left hand side is higher than the water level on the right and this height difference alone causes flow. So for boundary conditions, I specify the left-side water level as "pressure-inlet" and the right-side one as "pressure-outlet".(can't have "outflow" if u have "pressure-inlet")And I turn the gravity on with a value of -9.8 m/s2 in the Y-direction.

For the pressure values to be specified FLUENT manual says that the hydrostatic pressure differences should not be included. So, I specified 101325 Pa for both inlet and outlet and got good convergence of the equations, but later I realised we can only input gauge pressure, which is Pabs-Patm which becomes zero in my case. When I change the pressure value to zero at both ends (with gravity on), I get divergence of the continuity equation.

Can anybody tell me what I am doing wrong?

Thanks, Shankar.

ashish November 17, 2003 02:54

Re: gravity driven flow- boundary conditions?
Hello Shankar,

Static pressure will not be zero. static pressure on the left limb is the static pressure of the water column i.e. rho x g x ht. ans this pressure is causing the flow. I think you have to sepcify inlet gauge pressure as rho x g x ht. If you specify zero pressure on either side there will be problem in continuity that is very correct. There is a difficulty in calculating driving force for the flow (which is zero in your case), and thus you are getting divergence.

Bye, ashish

Shankar November 18, 2003 13:54

Re: gravity driven flow- boundary conditions?
Hello Ashish,

I tried the flow with a simple vertical pipe for which we know the analytical solution.

Pressure at the bottom=0Pa Pressure at top=rho*g*H. gravity is off. reference pressure location is (0,0,0) located at the centre of the bottom surface and operating pressure=101325 Pa.

The solution converges rapidly to 10E-6.

But still the results are way off the analytical results.


co2 November 26, 2003 15:43

Re: gravity driven flow- boundary conditions?
pressure at the bottom should be rho*g*H

Shankar November 26, 2003 16:45

Re: gravity driven flow- boundary conditions?
When the gravity is turned off in the case of a straight pipe flow, I guess it doesnt matter which end you give the rho*g*H pressure as long as it is with respect to the reference pressure location?


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