Novel method for investigating broadband velocity fluctuations in axisymmetric screeching jets

The noise spectrum of screeching jets consists of broadband and discrete components referred to as turbulent mixing noise and screech tones, respectively. The screech tones make the application of classical noise models to screeching jets difficult for modern problems. These problems involve an additional treatment of the near-field dynamics before the broadband noise prediction schemes can be implemented. This paper proposes an a posteriori approach for studying the velocity fluctuations associated with the broadband turbulent mixing noise in screeching jets. The broadband and screeching fluctuations associated with the broadband and discrete noise components are calculated using proper orthogonal decomposition to decouple the velocity variables. The methodology is demonstrated on velocity fields of screeching jets measured with high-resolution particle image velocimetry. Jets dominated by the precessing flapping (B) and helical (C) instability screech modes are considered, which correspond to ideally expanded Mach numbers of Mj 1.25, 1.45, and 1.59, respectively. Axially averaged spatial wavelength spectra of the broadband velocity fluctuations are compared to illustrate the decomposition. The proper-orthogonaldecomposition-based approach is shown to be highly dependent upon the dominant instability regime of the jets. Further analysis shows that the integral length scales of the broadband velocity fluctuations normalized by the incompressible momentum thickness in the screeching jets considered agree well with values reported in studies on nonscreeching jets despite the absolute values differing.