Soap film thickness imaging by infrared methods

Citation

Mackey, Ryan M. E. (1997) Soap film thickness imaging by infrared methods. Engineer's thesis, California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-01182008-085524

Abstract

A new method of studying soap film flows is introduced and discussed from several viewpoints. Using a commercial infrared camera and a cold background as an infrared light source, one can recover nonintrusively a measurement of the film thickness distribution. Once the thickness is known it is easy to compute the two-dimensional pressure in the film, allowing one to compute other film properties as well.

Blackbody and infrared detector theory are covered in brief and a simple theory of operation is introduced to explain the connection between emissivity and thickness. This theory is demonstrated to behave similarly to the physical system, predicting detected temperature values as a function of thickness of the same approximate magnitude as observed, but is too simple to provide a perfect match. An empirical calibration routine is demonstrated allowing one to calculate the film thickness to a high degree of precision.

The infrared method is applied to a number of familiar test problems as a demonstration. A gravity driven soap film tunnel has been constructed, and infrared images of the film surface are provided for unobstructed flow, separated flow past a cylinder held at one wall, and turbulent flow through a grid. These images are presented in raw and processed formats and the phenomena observed are discussed.

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