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Optical mapping of fluid density interfaces: Concepts and implementations

Zhang, X. and Dabiri, D. and Gharib, M. (1996) Optical mapping of fluid density interfaces: Concepts and implementations. Review of Scientific Instruments, 67 (5). pp. 1858-1868. ISSN 0034-6748. doi:10.1063/1.1146990.

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Several ideas of color encoding for surface slope measurements are systematically explored and reviewed to develop a new set of fundamental concepts. It is shown that different systems, such as shadowgraphs, Schlieren optics, and our water surface gradient detectors, can also be universally described through the concepts of sun glitter functions, incident light‐source encoding, and observer encoding. These concepts provide a more precise way of mathematically formulating and physically interpreting the flow visualization images, thereby providing quantitative results. It is this new system of concepts that uncover the quantitative potential of these optical methods. The measurement abilities of various existing optical systems are thus enhanced from qualitative observation or visualization to the well‐defined quantitative measurement. This is a critical step forward. The concepts can also be further extended to measure fluid flows with multiple density layers or flows with continuous density variations. As an example of implementation, the method of measuring a water‐surface gradient is extended into a reflective approach of detecting small changes of surface slope at an air–water interface. In this process, fluid surface slopes (surface gradients) are first optically mapped into color space. An array of lenses is used to transform the rays of an optical light source into a series of colored parallel light beams by passing the light through a group of two‐dimensional color palettes at the focal planes of the lens array. This system of parallel light beams is used to illuminate a free surface of water. The reflected rays from the free surface are captured by a charge‐coupled device color camera located above the surface. The slopes are derived from the color images after the calibration, and surface elevations are obtained by integrating the slopes. This technique is then applied to observe free‐surface deformations caused by near‐surface turbulence interacting with the free surface.

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Additional Information:© 1996 American Institute of Physics. Received 23 October 1995; accepted for publication 19 February 1996. This work has been supported by the Office of Naval Research Fluid Dynamics Program (ONR-URI, Grant No. N00014-92-J-1610). X.Z. thanks Professor C. S. Cox for many suggestions and discussions.
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Office of Naval Research (ONR-URI)N00014-92-J-1610
Issue or Number:5
Classification Code:PACS: 47.80.-v; 42.30.Va; 92.10.-c
Record Number:CaltechAUTHORS:20130913-092536085
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:41311
Deposited By: Tony Diaz
Deposited On:17 Sep 2013 20:06
Last Modified:10 Nov 2021 04:28

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