where L, D2 and A2 are length, diameter and cross-sectional area of
penstock, respectively. In addition, f is friction factor, Pk12 is the
sum of local losses in penstock due to entrance, bends, penstock
fittings and gates, AN is nozzle area at its exit (section 3 in Fig. 1) and
kN is nozzle head loss coefficient, which is given by (e.g., [1]).