A novel technique for interface reconstruction in transparent media

Abstract

A novel technique for measuring capillary/gravity waves travelling on air/water interfaces is investigated. The main motivation is to measure the free surface deformations of an air cavity used for air lubrication of ship hulls. Due to limitations in the available techniques, two novel methods are proposed, out of which the Stereo Particle Image Velocimetry synthetic Schlieren (Stereo-PIV-SS) technique is found to be more promising. This technique uses the input of random dot pattern images captured with and without an interface by two cameras mounted in a Stereo configuration. These images are processed to generate "pseudo" particle motion vectors using a Stereo-PIV code. An analytical study is conducted to find a simple linear relation between the pseudo vectors and interface height. Two sets of experiments are conducted to test this technique. One with defined transparent solid Plexiglas and glass profiles and another on small amplitude and slope water waves. For the experiments on water waves, the Free Surface Synthetic Schlieren (FS-SS) technique is used as validation data. In addition to experiments, a synthetic image reconstruction study is conducted to test the quality of the images generated by an in-house code.

The investigation reveals that the Stereo-PIV-SS technique is capable of capturing the flat interface height with sub millimeter precision for all interface heights tested in the range of 5 to 60 mm. Nevertheless, the technique can capture a wave profile with sub millimeter precision only for a limited range of wave parameters. The exact limits can only be defined by understanding more about the interplay of various wave parameters, which include the wave orientation, influencing the reconstruction. However, this technique can capture the profiles of waves having wavelength of the order of 1 cm and amplitude of the order of 1 mm, without requiring information about the mean interface height. Moreover, it is low cost, easy to use and apply since it requires only a standard PIV setting. The first attempt to study this new technique has been promising and it can be applied to specified wave regimes. The method can be developed further by understanding how to remove the effect of wave orientation on the reconstruction and including other wave parameters in the interface height correlation.