Resolvent analysis of planar supersonic impinging jet*

Supersonic jets impinging on a ground surface are important in vertical takeoff/landing operations. A major determinant of the dynamics is a resonant feedback loop between the ground and the nozzle, producing intense noise tones. An examination of these dynamics in the context of input-output characteristics has the potential to guide noise mitigation strategies. To this end, we employ resolvent analysis on an LES time-mean flow of an under-expanded Mach 1.27 planar jet at Reynolds number of 579,320, impinging on the ground surface 4 diameters downstream. The resolvent analysis is performed over frequency (St), and spanwise wavenumber (β) ranges of 0.1≤ St ≤ 1.05 and 0 ≤ β ≤ 10π, respectively. The gain-peak resides in the 0.35 ≤ St ≤ 0.5 and 2π ≤ β ≤ 4π ranges. Key conclusions are that the energy amplification mechanics is dominated by Kelvin–Helmholtz type of response. The corresponding forcing is essentially white noise around the nozzle. At St~0.45, the KH types of response exhibit symmetric structures at β = 0, π, 3π, 4π, 5π and 6π with higher energy amplification compared to the antisymmetric counterparts at other cases of β. The physical insights provided by the resolvent analysis aid flow control that targets noise mitigation.