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Model setting
Since equation (![[*]](https://www.cfdsupport.com/wp-content/uploads/2022/02/crossref.png "Node109 7")
) is independent on actual pressure field values, but only on its gradients, we can formally simplify the boundary condition at the outlet by adding a certain constant 
to the right hand side of the prescription (![[*]](https://www.cfdsupport.com/wp-content/uploads/2021/12/crossref.png "Node109 9")
) in order to get mean value of the outlet pressure equal to zero and not to hydrostatic presure at the center of mass of the outlet surface. If we do this, we have to amend the inlet boundary condition () the same way, of course. We can find easily that the constant has following value
) is independent on actual pressure field values, but only on its gradients, we can formally simplify the boundary condition at the outlet by adding a certain constant to the right hand side of the prescription () in order to get mean value of the outlet pressure equal to zero and not to hydrostatic presure at the center of mass of the outlet surface. If we do this, we have to amend the inlet boundary condition () the same way, of course. We can find easily that the constant has following value
where 
is a position vector of the centre of mass of the outlet surface. Hence prescription () changes to
is a position vector of the centre of mass of the outlet surface. Hence prescription () changes to
and prescription () to
) to
A mean value 
of the total pressure at the inlet surface now gives
where 
is a position vector of the center of mass of the inlet surface and 
.
We can see that in this model setting there is no need to know the position of a turbine to the water level. It suffices to know 
and take the measurement of 
.
of the total pressure at the inlet surface now gives
where is a position vector of the center of mass of the inlet surface and .
We can see that in this model setting there is no need to know the position of a turbine to the water level. It suffices to know and take the measurement of .
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