Optical measurement of the instantaneous wavy surface structure on a water plane jet

• Published in: Journal of physics. Conference series 2009-02, Vol.147 (1), p.012046
• Main Authors: Itoh, K, Maruwaka, D, Kumamaru, H
• Format: Article
• Language: English
• Subjects: Correlation coefficients; Cross correlation; Evaluation; Free surfaces; Hydraulic jets; Laser beams; Lasers; Light spots; Nozzles; Optical measurement; Physics; Semiconductor lasers; Slopes; Surface structure; Surface waves; Time lag; Velocity; Wave breaking; Wave height
• ISSN: 1742-6596; 1742-6588; 1742-6596
• DOI: 10.1088/1742-6596/147/1/012046

Laser beam refraction technique for measurement of free surface waves on a high-speed water jet has been developed in the present work. The new method employs two pulse laser diodes and the single light-spot detector. Through detecting the two dimensional trajectory of laser beams refracted at free surface, the local slope-angle fluctuation at two measuring locations 1.2 mm apart on a jet free surface is evaluated. The experiments are conducted for several locations along the jet center axis within the range of average velocity up to 5 m/s. The wave speed is evaluated from dominant time lag of cross-correlation coefficient for individual waves, which are discriminated by applying the zero-up-crossing method to the slope-angle time series data. The shape of waves is also calculated by integrating the free-surface slope angle data numerically. The ensemble averaged wavelength increases with the distance from nozzle exit almost monotonically. On the other hand, the mean wave height first increase with distance, reaches a maximum and then decreases. The maximum value of wave height is less than 0.23 mm for the tested velocity range. The steepness of free surface wave, that is the ratio of wave height and wavelength, takes a maximum at 20 ∼ 30 mm downstream nozzle exit. This suggests that the wave breaking occurs on the jet free surface for higher velocity case.