On the waves underpinning screech in rectangular jets

A study of the different waves involved in the screech resonance loop in rectangular jets is undertaken in this work. Using the linearised compressible Navier-Stokes equations, the 2D linear stability analysis eigenvalue problem is solved numerically for a range of conditions, from which the characteristics of the Kelvin-Helmholtz and guided jet modes can be obtained. Using a modelled analytical mean flow, the dominant symmetries of the modes are readily identified, and the effect of aspect ratio is studied. In line with the literature, it is observed that disturbances associated with a flapping in the minor axis direction have the largest growth rates for aspect ratio AR = 2, 4 jets, and can be expected to be dominant in experiments if the necessary screech wavenumber relationship holds for this symmetry. The associated flapping guided jet mode is also found to be neutral at frequencies close to screech. The wavenumber matching condition required to close screech is satisfied for the flapping mode at this condition, leading to predictions close to the ones observed experimentally. Several features from the AR = 2 schlieren data are captured by the LSA-based model, extending the validity of this approach to non-axisymmetric jets.