Experiments Concerning the Sound-Sensitive Jet

Abstract

An experimental study of laminar jets sensitive to sound was conducted both in air and in water at low Reynolds numbers, the water jet permitting visual determination of a condition of neutral stability. Plotted on a Reynolds-Strouhal diagram, the neutral contours show the unstable region to be bounded at high Strouhal numbers and low Reynolds numbers. The boundary is dependent on disturbance amplitude, enclosing a somewhat larger region for larger disturbances. The lower Strouhal number boundary, if it exists, must be extremely close to the Reynolds number axis. A qualitative comparison of the recent results of instability theory for pseudolaminar jets with the experimental results is made on the basis of local parameters, this making allowance for jet spreading which otherwise is a major cause of disparity. However, the velocity profiles, determined in the air jet, do not compare closely with the analytic ones for a slit jet which have been used in recent theoretical studies of laminar-jet instability, even at low Reynolds numbers. It appears that the embryo vortex, formed when the jet is disturbed, begins near the inflection point of the velocity profile and at a phase when the fluctuating velocity at the orifice is near maximum. Very long waves, which become quite unstable after an initially very small special rate of amplification, are considered as a possible cause of transition to turbulence in the jet.