The present study has shown that a rotating runner is a powerful means of i
nvestigating draft tube flows because of the convenience in using the hot w
ire anemometry. A rotating runner acts as a powerful mixing device giving g
ood pressure recovery even in the presence of considerable inlet swirl. Spe
ctrum analysis of the hot wire signal reveals the phenomenon similar to vor
tex breakdown that introduces oscillations outside a narrow range of unit s
peed surrounding the design point. There is every indication that water tur
bine surge and the oscillations observed in the present experiments are one
and the same. There is also some evidence that classical vortex break down
may not be identical to what happens in the draft tube.