A method of identification and estimation of swirling motion in complicated flow and its application are presented. Classification of flow can be performed with velocity gradient tensor and its eigenvalue, and complex eigenvalue indicates that flow is swirling motion. Here the complex number of the eigenvalue is defined as swirling function, and the local maximum point of swirling function is assumed to be the axis of swirling. This method enables to identify the swirling motion hidden in complicated flow, which is impossible to identify with velocity field or streamline. This definition prevents misunderstanding between swirling flow and non-swirling flow with (non-zero) vorticity, and the intensity of swirling can be estimated.

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