Stereoscopic measurement of a fluctuating free surface with discontinuities

This paper presents a new innovative method for measuring two-dimensional water surface configurations. This method uses a pair of sequential images captured by two high-resolution CCD cameras arranged in a stereo position. The idea of the method is to make the water colour white so that the instantaneous water surface appears like a solid surface with a clear pattern when an irregular pattern of light is projected onto it. It should be noted that this method is suitable to measure waves with small amplitude accurately and at the same time a discontinuous surface robustly. First, we explain the measurement principle and the experimental procedure for obtaining stereo images. Secondly, we examine the measurement accuracy by applying our technique to a still water surface and calibrated wavy plates. Finally, we present applications of the technique applied to actual flow fields such as surface waves and ripples generated in a shallow water box. The result shows that we can trace the propagation of small waves as small as I mm. In another application, we measure periodic surface fluctuation generated at an asymmetric cavity installed in an open-channel flow, demonstrating robustness of the technique applicable even when a discontinuous surface with wave breaking occurs in the course of the measurement.