Optical methods for the study of transparent media

• Main Author: Johansson, Eva-Lena
• Format: Others
• Language: English
• Published: Luleå 2003
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-17242

In the fields of combustion and fluid mechanics, temperature-, density- and velocity fields are of interest to measure. Optical methods may be used to study transparent media, even transient events. Flames, gases and liquids are examples on transparent media, often called phase objects. Phase objects often have non-uniform refractive index distributions. Variations in density, temperature or pressure affect the refractive index. The phase of the optical wave is affected by the refractive index. Therefore measurement of the phase of the optical wave gives useful information. In this thesis, three different optical measurement methods are presented. The first one is defocused digital speckle photography, which is a method to obtain quantitative data about phase objects with non-uniform refractive index distributions. A phase object inserted in a laser speckle field introduces speckle displacement, from which information about the object may be extracted. The speckle displacements are used to determine the phase gradients and the position of phase objects. For the study of transient events, like rapid changes of refractive index in gases and liquids, pulsed TV holography is combined with digital speckle photography. Holograms are recorded, with and without the phase object present. The speckle patterns are reconstructed and numerically transformed to different imaging planes, where speckle displacement fields are calculated. At least two displacement fields are needed to obtain data about the phase object. The third measurement method is particle image velocimetry (PIV) for measurement of the instantaneous velocity field in a fluid. PIV is a whole field measurement method that measures the two-dimensional (2D-2C) or three- dimensional (2D-3C) velocity field in a plane. In this thesis, the two methods using the speckle photography technique are applicable on phase objects that can be treated as thin sheets. A flow of helium gas and weak lenses are used to demonstrate and evaluate the techniques. The PIV technique is used to determine velocity fields in two planes outside the opening at a blown organ pipe labium. The results of the measurements show that the methods are suitable for the study of transparent media.