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Model setting without an explicit use of $ \vec{g}$ in the momentum equation
Let us take a look at the equation (![[*]](https://www.cfdsupport.com/wp-content/uploads/2022/02/crossref.png "Node110 7")
) as if its right hand side were known. Then we can introduce an alternative quantity, denoted by 
, to the static pressure 
) as if its right hand side were known. Then we can introduce an alternative quantity, denoted by , to the static pressure
By means of () we can formally substitute equation () by the following one
) we can formally substitute equation () by the following one
for the right hand side takes the same values. This is the consequence of a fact that field () has potential and thus a change in potential energy of an arbitrary element of a fluid is not dependent on its path, but on its initial and final position only.
By introducing 
we formally drop 
out from the momentum equation. However, in order to obtain the same solution as in the previous settings, it is necessary to alter the presrciption () by adding 
to its right hand side8.8. By doing this we obtain a new prescription, but this time for the quantity
) has potential and thus a change in potential energy of an arbitrary element of a fluid is not dependent on its path, but on its initial and final position only.
By introducing we formally drop out from the momentum equation. However, in order to obtain the same solution as in the previous settings, it is necessary to alter the presrciption () by adding to its right hand side8.8. By doing this we obtain a new prescription, but this time for the quantity
If we calculate the mean value of (), we obtain
), we obtain
Prescription at the outlet surface remains formally the same as (![[*]](https://www.cfdsupport.com/wp-content/uploads/2021/12/crossref.png "Node110 27")
), but this time for the quantity 
), but this time for the quantity
and hence its mean value is zero.
We can see that in this setting there is no need to know a position of a turbine with respect to the water level and there is also no need to even take the measurement of 
. It only suffice to know 
, the head.
. It only suffice to know , the head.
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