Free surface measurement by stereo-refraction

An optical method for the measurement of the surface topography, height and normal, and the velocity in free surface flows is presented. This method of surface reconstruction is based on the analysis of the apparent displacement between a reference and refracted images of a laser sheet through an interface viewed from two viewpoints. First, the validation of the method is performed in a wave tank by comparison with resistive wave probe measurements. This technique is then applied in a towing tank to measure the flow in the wake of a ship model. The results highlight correspondences between the free surface shape and the 2D velocity field beneath surface in the wake of the ship model. The instantaneous measurement of free surface allows a reconstruction of the global wave field pattern behind the ship. These results, obtained with a non-intrusive technique and with a high spatial resolution, allow an accurate analysis of the flow behind the ship. Tests were conducted for different Froude numbers in a range between 0.24 and 0.52. The results highlight the effect of the Froude number on established waves system.

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