A numerical analysis is presented to identify key flow and its flow scale for generation of a vortex in a turbulent flow, in terms of the local flow topology. The swirlity that indicates the unidirectionality and intensity of the azimuthal flow in a plane specifies the flow transition process into a vortex, and swirl plane after the transition is predicted. The velocity gradient tensor components in a coordinate system associated with the predicted swirl plane are analysed to identify key flow as a trigger for vortex generation, where the key flow is expressed as an invariant flow. Furthermore, the key flow is decomposed into several flow scales, which enables to identify the predominant flow scale of the key flow.

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