AN EXPERIMENTAL INVESTIGATION OF VORTEX SHOCKS

An experimental study of the response of an intake vortex following cu tting by a thin blade traveling normal to the vortex axis has been per formed in order to examine the validity of predictions of ''vortex sho ck'' formation obtained from recent theories of vortex filaments with variable core radius. For the problem of cutting of an intake vortex w ith an ambient axial flow, the vortex filament theories predict format ion of a ''vortex shock'' on the side of the vortex on which the ambie nt axial flow compresses the vortex axis and formation of a ''vortex e xpansion wave'' on the side which is stretched by the axial flow. Both the predicted vortex shock and vortex expansion wave propagate away f rom the blade in opposite directions. In the experiments, the formatio n of a sudden distortion of the vortex just following cutting by the b lade is observed, which forms on the same side of the vortex on which the vortex shock is predicted to occur and propagates away from the bl ade at roughly the same speed as predicted for the vortex shock. The d isturbance appears visually similar to a traveling vortex breakdown. T he form of the disturbance varies with the ambient vortex state and th e blade speed, although forms similar to bubble, spiral, and double-he lix-type breakdowns have been observed. No vortex shock is observed on the opposite side of the vortex, on which the filament theory predict s formation of a vortex expansion wave. Several aspects of the experim ents which are not included in the predictions of vortex filament theo ries are also noted, including the nonaxisymmetric form of the vortex shock and the rapid suction of blade boundary layer fluid into the vor tex core following cutting of the vortex.