Vortex tracking and visualisation in a flow past a tapered cylinder

In this paper we explore a novel combined application of two of our existing visualisation techniques to the tracking of 3D vortex tubes in an unsteady flow. The applied techniques are the winding-angle vortex extraction technique based on streamline geometry, and the attribute-based feature tracking technique. We have applied these to 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-angle vortex extraction method. For each 2D vortex a number of attributes are calculated and stored. These vortices are visualised by a special type of ellipse icons, showing the position, shape and rotational direction and speed in each slice. Next, for each time step, 3D vortex tubes are constructed from the 2D vortices by applying the feature tracking procedure in a spatial dimension to connect the corresponding vortices in adjacent slices. The result is a graph attribute 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 corresponding tubes in successive time steps. The result is a description of the evolution of the 3D vortices. An interactive, time-dependent visualisation is generated using the temporal correspondences of each vortex tube. This analysis reveals a number of interesting patterns.