Wave Propagation on Turbulent Jets: II. Growth

Abstract

The growth in amplitude of a sinuous disturbance propagating along a fully turbulent diverging planar air jet is studied experimentally for a range of slit widths, blowing pressures and acoustic excitation frequencies relevant to organ pipes. The wave amplitude is found to increase approximately exponentially ·with distance provided this amplitude remains .small compared with the local jet width. For larger amplitudes the growth becomes linear. In the exponential growth regime the local behaviour is very similar to that- predicted by simple theory for a laminar inviscid non-divergent jet with the same veloCity piofile: Iii-particular, for a. jet with local half-width b and local exponential growth rate μ, the quantity μ b rises to_ a peak value near 0.5 for values of the Strouhal number k b near 0.6, then falls towards zero, or may even become negative, for k b greater than about 2. The significance of these ·results for the sound generation mechanism in organ pipes is noted.