Acoustic control of a coaxial jet

Abstract

Measurements in the near field of a coaxial jet under unperturbed and controlled conditions are reported; the Reynolds number (defined with diameter and exit velocity of the inner-jet) is 5600, while the ratios of the velocities and of the diameters of the two jets are 1.8 and 2, respectively. The unperturbed flow, which issues in a nominally laminar state from the contraction nozzles, develops a shear-layer mode instability in the outer-shear layer of the outer-jet, followed by vortex pairing. A control perturbation, composed of sinusoidal acoustic waves at the frequency of the instability and its half, is applied to the flow. The control parameter is the phase difference Δ φ between the two sinusoidal waves, and is initially varied in the entire range of phases. Then, two values of Δ φ, providing the maximum attenuation and enhancement of the turbulence intensity, are chosen and, for these, detailed measurements in the axial and radial directions are performed. The data show that the control affects the mean and turbulent fields and is able to redistribute the kinetic energy between them. As a result, the potential core of the inner-jet can be shortened or lengthened. The turbulence intensity can also be modified, thus altering the ratio of the streamwise to the radial velocity components, and the ratio of the fundamental to the subharmonic spectral components.

Keywords:

• Coaxial jet
• Flow control
• Turbulent flows

Acknowledgments

This work is part of the PhD Thesis of PB conducted at the Dipartimento di Ingegneria Aerospaziale, Università di Pisa, and supervised by Professor Guido Buresti. His comments on a first draft of the manuscript are gratefully acknowledged. PB is also thankful to Manfredo Manfredi, Massimo Pisani and Marco Ciofini for their help in setting up the experiment.