Krasheninnikov S., Mironov A.

In accordance to Lighthill’s acoustic analogy the total acoustic power of turbulent isothermal jet is proportional to mechanical energy flux and fifth power of Mach number (M). Coefficient of proportionality (C) is almost constant up to M<2 for axisymmetric jets with uniform profiles of velocity and temperature at nozzle exit. In that way acoustic-to-mechanic efficiency of a jet, as a first approximation, can be expressed as C multiplied by fifth power of M. In presented work on the base of experimental tests and analysis of known data the influence of different factors, which can change the jet structure and correspondingly change its acoustic efficiency was investigated. The influences of initial boundary layer thickness, heating of a jet were analyzed. The co-axial jets with different velocity and temperature initial distributions and various by-pass ratios were considered also. Analysis showed that in wide range of axisymmetric jet exhaust conditions (including co-axial jets) correlation between acoustic and mechanical energy stays the same as for Lighthill’s analogy. In particular in the case of co-axial jets the interchange of energy between internal and external jets leads to the strong spectra changing. In spite of that the relation between mechanical energy flux and total acoustic power remains valid. Discrepancy in experimental data and Lighthill’s theory was been marked only at low density single jets for M<0.2 and co-axial jets with low external to internal velocity ratio. Known devices for jet noise reduction (chevrons, lobed nozzles) change the acoustic efficiency of jet, as well as structure of acoustic field and jet noise spectra.