OPTICAL MAPPING OF FLUID DENSITY INTERFACES - CONCEPTS AND IMPLEMENTATIONS

Several ideas of color encoding for surface slope measurements are sys tematically explored and reviewed to develop a new set of fundamental concepts. It is shown that different systems, such as shadowgraphs, Sc hlieren optics, and our water surface gradient detectors, can also be universally described through the concepts of sun glitter functions, i ncident light-source encoding, and observer encoding. These concepts p rovide a more precise way of mathematically formulating and physically interpreting the flow visualization images, thereby providing quantit ative results. It is this new system of concepts that uncover the quan titative potential of these optical methods. The measurement abilities of various existing optical systems are thus enhanced from qualitativ e observation or visualization to the well-defined quantitative measur ement. This is a critical step forward. The concepts can also be furth er extended to measure fluid flows with multiple density layers or how s with continuous density variations. As an example of implementation, the method of measuring a water-surface gradient is extended into a r eflective approach of detecting small changes of surface slope at an a ir-water interface. In this process, fluid surface slopes (surface gra dients) are first optically mapped into color space. An array of lense s is used to transform the rays of an optical light source into a seri es of colored parallel light beams by passing the light through a grou p of two-dimensional color palettes at the focal planes of the lens ar ray. This system of parallel light beams is used to illuminate a free surface of water. The reflected rays from the free surface are capture d by a charge-coupled device color camera located above the surface. T he slopes are derived from the color images after the calibration, and surface elevations are obtained by integrating the slopes. This techn ique is then applied to observe free-surface deformations caused by ne ar-surface turbulence interacting with the free surface. (C) 1996 Amer ican Institute of Physics.