A real-time free surface slope mapping technique

Abstract

Recently, there has been an increased interest in the interaction of vortices and turbulence with free surfaces. A central issue in understanding the free surface turbulence is to relate the surface elevation to the near surface flow field. In that respect, the Jack of a global surface mapping technique which could reveal the temporal evolution of the surface elevation has prevented the progress of viable research. Thus, it is the purpose of this work to present a new technique integrating optics, colorimetry, digital image processing, to measure the 3-D surface elevation for a time-evolving flow. It is highly undesirable to use a probe which extends through the water surface to study surface structures due to the fact that the probe interacts with the free surface. It is therefore desirable to use optical methods since these techniques do not interfere with the free surface. As suggested by Cox, it is convenient to measure the water-surface slope instead of the free surface elevation, since the higher frequency waves have lower amplitudes while they maintain higher slopes. Therefore, Cox (1958) measured the surface slope by the refraction of light through a free surface. A light source of spatially linearly-varying intensity was placed at the bottom of a water tunnel, and a telescope imaged one point on the water surface into a photocell. Therefore, the brightness of the light seen by the photocell was proportioned to one component of the slope of the free surface. This technique is referred to as the "refraction mode" since the observed light rays are refracted through the surface. Likewise, one can perform the experiment by illuminating the surface of the water from air and observing the reflections. This technique is referred to as the "reflective mode" since the observed light rays are reflected from the surface. Using a photo diode array and a laser as a single point light source, Long and Huang (1976) were able to measure both components of the surface slope at one point. Further work done by Keller and Gotwols (1983), Jahne and Waas (1989), and Jahne and Riemer (1990), progressed the technique to allow for two slope component measurements, one slope at a time. Zhang and Cox (1992) extended this work by allowing the measurement of two slope components simultaneously, throughout a two-dimensional spatial domain. Since the technique is photographically-based, it allowed data acquisition at only one point in time. In this paper, we report on the development of a real-time version of this technique that facilitates studies of time evolving flows.

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