Identification of temporal and spatial signatures of broadband shock-associated noise

Broadband shock-associated noise (BBSAN) is a particular high-frequency noise that is generated in imperfectly expanded jets. BBSAN results from the interaction of turbulent structures and the series of expansion and compression waves which appears downstream of the convergent nozzle exit of moderately under-expanded jets. This paper focuses on the impact of the pressure waves generated by BBSAN from a large eddy simulation of a non-screeching supersonic round jet in the near-field. The flow is under-expanded and is characterized by a high Reynolds number $$\mathrm{Re}_\mathrm{j} = 1.25\times 10^6$$Rej=1.25×106 and a transonic Mach number $$M_\mathrm{j}=1.15$$Mj=1.15. It is shown that BBSAN propagates upstream outside the jet and enters the supersonic region leaving a characteristic pattern in the physical plane. This pattern, also called signature, travels upstream through the shock-cell system with a group velocity between the acoustic speed $$U_{\mathrm{c}}-a_\infty $$Uc-a∞ and the sound speed $$a_\infty $$a∞ in the frequency–wavenumber domain $$(U_\mathrm{c}$$(Uc is the convective jet velocity). To investigate these characteristic patterns, the pressure signals in the jet and the near-field are decomposed into waves traveling downstream ($$p^+$$p+) and waves traveling upstream ($$p^-$$p-). A novel study based on a wavelet technique is finally applied on such signals in order to extract the BBSAN signatures generated by the most energetic events of the supersonic jet.