Effect of axis switching on the coherent structures of an elliptical jet

The dynamics of wavepackets in an elliptical jets are studied using direct numerical simulation (DNS) data of an AR = 2 incompressible elliptical jet at Reynolds number Re = 400. Analysis of the numerical data displays several complex features, the most striking being axis switching, a phenomenon that strongly affects the development of coherent structures in the flow. By applying spectral proper orthogonal decomposition (SPOD), it was found that modes in the SA symmetry were dominated by the Se1 flapping geometry in the upstream region. After the axis-switching point, the mode structure becomes more complex and multi-modal at mid-frequencies, while the mode structure remains largely unchanged for very low frequencies. Linear parabolised stability equations (PSE) are also used to evaluate the development of the different Kelvin-Helmholtz wavepackets in this elliptical jet showing excellent agreement with the SPOD modes.