Measurement of wave-induced turbulent flow structure using digital image sequence analysis

The turbulent flow in water close to the wavy wind-stressed water surface has been examined quantitatively in а large circular wind-water tunnel with а flow visualization method using а special image sequence analysis. The flow field beneath the moving air-water interface is visualized by means of small polystyrol particles. These are illuminated by а thin light sheet, oriented along wind direction and parallel to the main flow field. The mie-scattered light of the particles is observed perpendicularly by а CCD Video camera. First measurements, using two low cost image processing systems, allow а determination of velocity vectors in image sequences of up to 32 binary images. The tracking algorithm solves the correspondence problem of а particle’s path in subsequent images by use of the particle’s orientation and the video interlaced mode. Since every particle is traced along its full path within the light sheet, both the Eulerian and the Lagrangian velocity fields can be calculated. This provides access of а new kind to the examination of transport and turbulence below the surface, as demonstrated by means of different evaluation methods for the calculation of spatially dependent velocity distributions, the Stokes drift and friction velocity.