Eduction of swirling structure using the velocity gradient tensor

We propose a technique for locating swirling regions of a flowfield. The technique is based on the eigenvalues of the velocity gradient tensor. We show that regions of swirling flow are characterized by complex eigenvalues for a constant velocity gradient tensor. Using results obtained in this analysis, we define an approximate parameter to indicate the tendency for the fluid to swirl about the point in question. The technique is illustrated by application to several two- and three-dimensional flowfields. In addition, the basic ideas contained here suggest the existence of a fluid property that we have termed intrinsic swirl