Upstream-travelling acoustic jet modes as a closure mechanism for screech

International audienceExperimental evidence is provided to demonstrate that the upstream-travelling waves in two jets screeching in the A1 & A2 modes are not freestream acoustic waves but rather waves with support within the jet. Proper Orthogonal Decomposition is used to educe the coherent fluctuations associated with jet screech from a set of randomly sampled velocity fields. A streamwise Fourier transform is then used to isolate components with positive and negative phase speeds. The component with negative phase speed is shown, by comparison with a vortex-sheet model, to resemble the upstream-travelling jet wave first studied by Tam & Hu (1989). It is further demonstrated that screech tones are only observed over the frequency range where this upstream-travelling wave is propagative