Discrete-frequency tones can be generated when objects obstruct the path of an axially symmetric jet. An experimental study of this phenomenon was conducted when the objects were (1) a flat plate perpendicular to the jet axis, there being a hole in the plate of the same diameter as the jet orifice and concentric with the jet, and (2) a ring whose plane was perpendicular to the jet axis and whose diameter was equal to that of the jet orifice. The tone from the former geometry is the hole tone and that from the latter, not previously observed, is the ring tone. The mechanism of tone generation appears to be very similar to that of the edge tone, although presently only qualitative explanations can be offered for certain aspects of the phenomena. The axially symmetric jet systems are sensitive to very weak acoustic reflections.
A descriptive experimental study was made in both air and water of the temporally periodic motion that occurs in the vortex whistle and cyclone separator. The motion can be described in terms of an oscillator that derives its energy from hydrodynamic instability of the steady swirling flow and whose frequency is determined by an angular velocity characteristic of this steady flow. The relevant dynamical parameters are the Rossby number and Reynolds number for the steady flow with the addition of the Strouhal number for the time-dependent flow. The results of this study were compared with the vortex breakdown phenomenon over swept-back wings. Breakdown can be described in the same terms as for the other two cases and it appears that all three motions are basically the same.
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