static pressure in water wheel and open ambient

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 November 13, 2014, 03:57 static pressure in water wheel and open ambient #1 Senior Member   Join Date: Oct 2014 Posts: 124 Rep Power: 11 Hi everybody. I am simulating a water wheel in an open channel and open ambient. When there is no water inside the channel yet, the static pressure inside the wheel, where only air is present, should be g*p*z, where g is the gravity, p the density of the air and z the distance between each point and the top of the wheel. The air is lightweight, so the difference of static pressure should be very small. But in my wheel it is 400 Pa at the top of the wheel and 30 Pa at the bottom and it is very strange, both because these values are too high, both because the difference is too high, and because it is the contrary, since the highest value must be on the bottom! The wheel is 2 m in diameter. Then, when the water begins enter the bottom of the wheel (the water level is about 0.1 m, so negligible than the wheel dimensions), the static pressure is -3000Pa at the top and - 5000Pa at the bottom..It seems that the water carries out the air and a depression inside the wheel appears, but it is wrong, since the wheel is open and in the ambient at atmospheric pressure. If it is (I suppose this, it is not sure at 100%), how can I impose the atmospheric pressure on the open surfaces of the wheel where it is in contact with the external ambient at atmospheric pressure?? What boundary conditions? the Outlet is already imposed where the open channel exits..I have used symmetry. And which operative density I have to use? Thank you for your advises.

 Tags open atmosphere, open channel flow, static pressure, turbine, water wheel