# Tank Discharging (Boundary Condition)

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 December 17, 2009, 18:11 Tank Discharging (Boundary Condition) #1 New Member   Join Date: Mar 2009 Posts: 18 Rep Power: 10 Hi there! I'm trying to simulate a simple "tank discharging" problem. There is a hole at the bottom of the tank as an outlet. I wonder what the best Boundary Condition is for outlet! The hydrostatic pressure can be a good prediction, but it deponds on water height in the tank that is decreasing by time! How can I set such a B.C that varies with time and need to be updated every timestep??? Thank you very much

 December 17, 2009, 21:46 #2 Member   Tristan Burton Join Date: Mar 2009 Posts: 43 Rep Power: 10 If the tank is discharging then you should use the ambient pressure that it is discharging to as the outlet boundary condition. The hydrostatic pressure doesn't make sense since the fluid is discharging and therefore not static. Tristan

 December 18, 2009, 07:11 #3 New Member   Join Date: Mar 2009 Posts: 18 Rep Power: 10 Thank you Tristan for your answer! Well, somthing that makes the fluid discharge is the weight of it above the outlet, isn't it? Does CFX understand it when we use ambient pressure? Thanks again

 December 18, 2009, 15:15 #4 Member   Tristan Burton Join Date: Mar 2009 Posts: 43 Rep Power: 10 I'd never noticed that there is no gravity checkbox available in CFX. Can you add a momentum source to introduce the force due to gravity? Tristan

 December 19, 2009, 14:31 #5 New Member   Join Date: Mar 2009 Posts: 18 Rep Power: 10 of course CFX has gravity option! when you activate Buoyancy Force, it means that you are including gravity in your simulation. but the point is: "the pressure calculated by the solver when buoyancy is activated, excludesthe hydrostatic pressure gradient" anyway, can anyone help me set a boundary condition using some variables from transient result?

 December 20, 2009, 21:32 #6 Member   Ali Torbaty Join Date: Jul 2009 Location: Sydney, Australia Posts: 72 Rep Power: 10 I assume you have a tank full of water with an outlet at the bottom. and model is a multiphase Air/Water (buoyancy activated). you need two boundaries. First is an open boundary at the top (tank roof) set to Pres & Dirn, 0 Kpa. Air volume Fraction=1, Water Volume fraction=0. this will ensure air ingress when water leaves the tank. Second boundary is similar to the fist one but at your outlet. This will ensure gravity discharge into atmospheric pressure. Make sure that you have enough mesh refinement around the outlet, say at least 8 diagonal nodes.

 December 21, 2009, 16:02 #7 New Member   Join Date: Mar 2009 Posts: 18 Rep Power: 10 Dear AliTr Thanks for your respond I'm going to check it ,and if it doesnt work out I'll come back sooon! Eli

 December 22, 2009, 09:42 #8 Member     Rui Igreja Join Date: Mar 2009 Location: Aveiro Posts: 68 Blog Entries: 1 Rep Power: 10 Hi, Have a look at these: http://www2.dem.uc.pt/antonio.gameir...SEAS_paper.pdf http://www.wseas.us/e-library/confer...rs/498-704.pdf http://www.ansys.com/magazine/issues...r-products.pdf http://www.ansys.com/industries/cons...oiletvalve.htm ... and though it may sounds simple, the discharge of tank is a difficult numerical problem (transient, two-phases, free-surface), and very CPU time consuming. But I'm sure you'll find it by yourself. __________________ Rui

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