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Mike L June 5, 2008 08:45

using gravity to generate flow
I am having a hard time with a project. I've contact SolidWorks and the software supplier and they don't have any answers for me. My assembly is very simple. Essentially, it is just a tank in the shape of a simple cylinder with a hole in the top and a hole in the bottom. If I can make this work, I will add a down tube, hoses and other components to the bottom. I am trying to find a volumetric flow rate and/or a mass flow rate. The only thing responsible for draining the fluid out is gravity. I set up the analysis with ambient air pressure at each opening. When I run this set up, I get 0 for flow and it shows a distribution of pressure throughout the tank. I am testing for a volumetric/mass flow rate so I cannot specify that in the setup. With equal pressures at each end, I get no flow. The fluid drains from this tank, through a hose into another tank, and the air vents out of the second tank. The pressure initially at each end of the line is atmospheric. What am I doing wrong?

Bill McEachern June 5, 2008 22:21

Re: using gravity to generate flow
Where to may think this is a simple problem but from a CFD code perspective fairly advanced capabilities that Flowroks does not have are required to execute this problem as you described.

Gravity in Floworks is there to account for density variations so that things like natural convection can be handled. You can also add gravity to assist in trajectory tracking of particles with mass (Lagragian particle tracking function). To get a tank to drain in the way you suggest you need to have the capability to have a free surface amongst other things which are fairly advanced capabilites, not to mention hydrostaticly induced flows.

If you just want to get the inital flow rate you can just adjust the upper surface pressure BC to be equal rho*g*H - the head. If you want to simulate the whole tank drain thing it might be possible by making the top BC so that it is reduced based on the discharged volume.......not sure how to do this off the top of my head but it might be possible. If you phrase the question this way back to solidworks you may get some help. However, you need to keep in mind that tank will always be full but the driving pressure and times would be more of less right if it can be done. It would be a transient analysis.

Most people handle this type of problem by doing a nozzle calc. Just look in any thermodynamics text book (say Sontag). It is not a completely trivial calculation to do and may require a spreadsheet, MathCAD or a VB program depending on your skill set. Good luck.

If you want to see some simulations with hydrostaticly induced flows go to the Flow3D web site. Other codes do it as well - this isn't a plug for Flwo3D but there is some pretty cool animations on their site.

Richard Ozaki June 6, 2008 07:21

Re: using gravity to generate flow
Hi Mike,

As Bill mentions, Floworks cannot simulate a free surface but if you want to drive the flow by gravity, ensure "pressure potential" is ticked on in both pressure boundary conditions.


billmce June 6, 2008 07:53

Re: using gravity to generate flow
nice tip Richard.

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