A robust technique for tracking particles over long image sequences

A particle tracking technique at high particle concentration for the evaluation of flow fields beneath water waves is described. A 1-4 cm thick light sheet parallel to the main wave propagation direction was used to illuminate small polystyrol seeding particles. The depth of the light sheet was chosen such that the particles stay long enough in the illuminated area to enable tracking. An area of up to 14.0 x 9.0 cm2 is observed by a digital CCD camera. The recording of continuous image sequences at up to 60 fields/s allow an extensive study of the flow field. An automatic tracking algorithm is described allowing particles to be followed over more than 200 images at particle concentrations up to 800 particles/image (image resolution 480 x 512 pixel), yielding both Lagrangian and Eulerian vector field. The error of the computation of a velocity vector is usually below 3%.