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Vortices are important features in many research and engineering fields. Visualization is an important step in gaining more understanding and control of vortices. Vortex detection criteria fall into two categories: local scalar quantities, calculated at single points, and regional geometric / kinematic criteria, calculated for e.g. streamlines. The first category is easy to compute, but does not work in all cases. The second category is more intuitive and should work in all cases, but is more difficult to compute. We try to link the two categories with our selective visualization techniques [10]. By first selecting candidate areas based on the sclar quantities, the way is paved for applying a geometric criterion. Some early applications of this approach to hydrodynamic flows are shown.

In this paper we explore a novel combined application of two of our existing visualisation techniques to thetracking of 3D vortex tubes in an unsteady flow. The applied techniques are the winding‐angle vortex extractiontechnique based on streamline geometry, and the attribute‐based feature tracking technique. We have applied theseto the well‐known case of an unsteady 3D flow past a tapered cylinder. First, 2D vortices are detected in a number of horizontal slices for each time step, by means of the winding‐anglevortex extraction method. For each 2D vortex a number of attributes are calculated and stored. These vorticesare visualised by a special type of ellipse icons, showing the position, shape and rotational direction and speed ineach slice. Next, for each time step, 3D vortex tubes are constructed from the 2D vortices by applying the feature trackingprocedure in a spatial dimension to connect the corresponding vortices in adjacent slices. The result is a graphattribute set with the 2D vortex attributes in the nodes and the spatial correspondences as edges. Finally, the 3D vortex tubes are tracked in time using the same tracking procedure, for finding the correspondingtubes in successive time steps. The result is a description of the evolution of the 3D vortices. An interactive, time‐dependentvisualisation is generated using the temporal correspondences of each vortex tube. This analysis revealsa number of interesting patterns. ACM CSS: I.3.8 Computer Graphics—Applications

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