AZIMUTHAL MODE MEASUREMENTS OF ELLIPTIC JETS

A procedure to quantitatively measure the relative amplitudes of azimu thal modes in the acoustic field of an elliptic jet is presented. The work describes how the azimuthal modes in an elliptic jet can be repre sented by a linear combination of Mathieu function modes and how the a mplitude coefficients of each individual mode can be determined throug h an orthogonal decomposition based on Mathieu functions. The modal de composition is performed in an elliptic cylindrical coordinate system natural to the elliptic jet geometry. The procedure is first tested on an artificially excited, perfectly expanded Mach 1.5 elliptic Set wit h preferential varicose and flapping mode excitation of discrete frequ encies, The excitation was provided with a four electrode glow dischar ge system with phase control of the individual electrodes. Following t hat, the procedure was applied to naturally excited Mach 1.5 jets with both air and a helium/air mixture as the jet working gas. The helium/ air jets simulate the higher jet velocity and lower jet density of hea ted jets. The modal decomposition technique is verified by experiment, allowing significant differences to be identified in the azimuthal mo dal content as the jet simulated temperature is increased. (C) 1997 Am erican Institute of Physics.