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The dynamics of a confined thick orifice plate jet, at Mach 0.4, are studied with dynamic mode decomposition (DMD), of the velocity from a large eddy simulation (LES). The jet exhibits strong periodic structures, due to an initially laminar shear layer, and a non-deterministic switching is observed between an axisymmetric and an azimuthal jet mode. The DMD captures the shape of these structures as different dynamic modes, but (by definition) not their true time-evolution. In order to study the time-evolution of semi-periodic structures in the flow, such as the jet modes that come and go in time, it is suggested to use DMD for identifying the shape of the structures and then calculate time-coefficients for them, by expressing the velocity field as a linear combination of the most important dynamic modes. These time-coefficients are then shown to capture the physics of the flow; they oscillate at the frequency of the corresponding mode, within an envelope with a non-deterministically varying period, representing the mode switching. Additionally, a time variation of the strength of the jet, represented by mode zero, is found to be related to this switching. |
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