Combined stereoscopic wave mapping and particle image velocimetry

We describe a technique of image analysis providing the time resolved shape of a water surface simultaneously with the three-component velocity field on this surface. The method relies on a combination of stereoscopic surface mapping and stereoscopic three-component Particle Imaging Velocimetry (PIV), using a seeding of the free surface by small polystyrene particles. The method is applied to an experiment of `weak turbulence' in which random gravity waves interact in a weakly non-linear regime. Time resolved fields of the water elevation are compared to the velocity fields on the water surface, providing direct access to the non-linear advective effects. The precision of the method is evaluated by different criteria: tests on synthetic images, consistency between several camera pairs, comparison with capacity probes. The typical r.m.s. error corresponds to 0.3 pixel in surface elevation and time displacement for PIV, which corresponds to about 0.3 mm or 1 \% in relative precision for our field of view 2 x 1.5 m$^2$.