A novel method for decoupling the velocity fluctuations in screeching axisymmetric jets

The noise spectrum of screeching jets consists of discrete and broadband components that are usually referred to as screech tones and turbulent mixing noise, respectively. The turbulent mixing noise is always present, while the screech tones are generated through an acoustic feedback mechanism. The screech tones are often eliminated as they are predominantly observed in laboratory scale models. The current work explores an a posteriori approach to the elimination of the screech tones. This elimination is typically achieved through modifications to the nozzle with devices, such as notches and tabs, to disrupt the feedback process by distorting the flow at the lip of the nozzle. The intrusive nature of these devices introduces additional uncertainties when analysing the broadband noise component. The approach uses proper orthogonal decomposition to decouple the coherent and random fluctuations that can be linked to the discrete and broadband components of noise. The fluctuations are calculated from velocity measurements acquired using high-resolution particle image velocime-try to image a screeching jet issuing from a convergent nozzle at ideally expanded Mach numbers of 1.45 and 1.59. At these conditions, the jet is dominated by the same helical C instability mode. To characterise the turbulence in the jet flows, the integral length scales relevant to aeroacoustic prediction schemes are calculated from the decoupled velocity variables. The length scales of the random fluctuating velocity are shown to agree well with with results reported in previous studies where the screech tones were eliminated a priori.