Experimental investigation of near-field pressure fluctuations generated by supersonic jets

This paper describes an experimental investigation of near-field pressure fluctuations generated by round supersonic jets. Previous investigations have shown that there are two pressure components that exist in the near field of a subsonic jet. These are the hydrodynamic pressure fluctuations and the acoustic pressure perturbation components that form two distinct humps in the pressure-wavenumber spectrum (when the wavenumber is calculated directly from the radian frequency and the ambient acoustic velocity and is non-dimensionalized by the radial distance from the middle of the jet shear layer). Microphone arrays have been used to measure the near-field pressure and auto and cross spectra between all microphone pairs. The double humped pressure-wavenumber spectra are not present in the near field of a highly supersonic jet where the acoustic Mach number is M_a = 1.85 and the spectra are dominated by the acoustic perturbations that radiate directly to the far field. The spectra in the intermediate subsonic range exhibit a reduced double humped behavior. The processing technique, Empirical Mode Decomposition, can assist in the identification of the components of the near field. Fourier mode decomposition is performed, including both + and - helical modes, including contributions from all 64 components of the cross spectral matrix obtained from the 8 microphone azimuthal array. This shows that the composition of the lower order modes did not change substantially between the subsonic and high supersonic jets. Proper Orthogonal Decomposition is also used to produce mode spectra only slightly different from those obtained with the Fourier azimuthal decomposition.