The development of laser speckle metrology for the study of vibration and fluid flow

Abstract

Two original applications of laser speckle metrology are presented. First, a portable Laser Speckle Interferometer is described which allows the Engineer to simply point a laser beam at target surface in order to measure its vibration velocity amplitude and phase. This non-contacting capability complements an accelerometer where use of the latter is precluded; i.e. hot, light or rotating surfaces. The mechanisms which determine the noise floor of the interferometer are examined and minimisation procedures are defined. A prototype instrument is described which has a dynamic range of 60 dB over a frequency range of d.c. - 20 kHz which is adequate for general purpose use. Three applications are reported which exploit the non-contact capability of the instrument. Second, the application of Laser Speckle Photography to fluid flows is described and the development of an entirely new technique is introduced which is more appropriately named Particle Image Velocimetry (PIV). In PIV, the instantaneous two-dimensional velocity of a fluid flow is measured by taking a double-exposure photograph of a thin sheet of light within a seeded flow. Pointwise processing of the resulting transparency with a laser beam produces Young's fringes in the far field diffraction pattern whose spacing and orientation are measured to find the local fluid velocity within the illuminated region. It is shown both theoretically and by experiment, that the signal to noise ratio of these fringes and hence measurement accuracy, is significantly improved by using a two-step photographic process. In addition, it is shown that this process allows the use of more sensitive photographic films which reduce the laser power requirement and hence the cost of a PIV system. Two methods of fringe analysis are examined and it is shown that, by preprocessing the fringe pattern to remove the low frequency pedestal component, fringe spacing errors of less than 1% are achieved. It is also shown that this method significantly decreases the time required to measure fringe spacing when compared with conventional Fourier methods. In addition, a method of decreasing the overall time required to analyse an entire PIV transparency based on detection of the presence of particle images within the illuminated region is described.

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