PROCESSING TOOLS TO TRACK AND CHARACTERIZE SURFACE SWIRLS

The present work introduces a new combination of technical tools developed to track and study surface swirls and associated "source vortices." These new tools are based on the detection of the contour of the surface swirl on high speed PIV frames using image processing and on the detection of the physical contour of swirling strength levels (S = -det J, J = [Ji,j] is the Jacobian defined by Ji,j = ∂ui/∂xj) calculated from the PIV velocity fields. The image processing allows one to track and to measure the surface swirl's characteristics. The processing of PIV velocity data permits description of the local flow associated with the surface swirl before its appearance within the laser plane. Experiments are performed in an open channel flow where a free surface shear flow is combined with a downward flow to induce occurrences of surface swirls and gas entrainment. The surface swirls, the source vortices, and their trajectories are identified in the horizontal plane at 10 mm under the free surface. This permits one to investigate the appearance and disappearance of surface swirls with the quantification of associated flows, e.g., swirls' diameters and flow circulation. The tracking tool is applied to the case where a 12.5-mm-radius half cylinder is added to the channel inlet. It highlights that the surface swirl presents an elliptical shape and describes a spiral trajectory moving from upstream to downstream compared to the suction nozzle center. The circulation increases from the source vortex to the surface swirl occurrence. It then reaches a maximum and decreases down to the circulation level of the source vortex before the disappearance of the surface swirls in the measurement plane.