Boundary-Condition-Problems with simulating a hydroplant with draft tube
 

Hello everybody!

I'm simulating a small hydroplant with draft tube. The contours are shown in the attached picture.

I have some trouble setting up the right boundary conditions. Values for the volume flow at defined rpm are known.

The geometry is desigend so that the outlet is on the level of the surface of lower water. My assumption is that p_outlet must be atmosphere = 101325Pa

On the inlet i've set a velocity inlet because the mass flow and also the geometrical measures are known, (V/A)=c.

Further I activated gravity and set the operation pressure = 0.
Because of the "gravity" which considers the hydrostatic head, I have set p_outlet=86636Pa (101325Pa - considered hydrostatic head starting from the inlet)

The plot of static pressure is also shown in the picture. At the inlet, the static pressure is around about 98000Pa.
But physically, if I'm not wrong, there must be a pressure about 102666Pa (atmosphere + hydrostatic head from surface upper water to inlet)

What's the reason? If I switch to pressure inlet the values at outlet and inlet for static pressure are ok, but there is a significant difference at torque an massflow.

If there are some questions left about my set up feel free to ask. I would appreciate it if someone could give me some tips! Thank you!