The non-uniform flow field at the exit of vanes caused by wakes and
blade loading generate a periodic flow pattern [36]. The induced
pressure wave rotates at the same runner speed and thus the
characteristic pressure oscillation has a frequency\(f_{s}=m\frac{\mathrm{\Omega}}{2\pi}N_{r}\) at every stationary point
and \(f_{r}=n\frac{\mathrm{\Omega}}{2\pi}N_{s}\) at every rotating
point. The common action of the rotating and stationary fields gives
rise to \(N_{r}N_{s}\) pressure impulses per a revolution [31]. The
flow field leaving the distributor vanes (Figure 13) is characterized by
a velocity defect and the pressure field attached to the runner blades
induces to the incoming flow field distortions.