Theoretical and experimental aspects of image shifting by means of a rotating mirror system at particle image velocimetry

Abstract

One possibility of remove the ambiguity of the direction (i.e. sign) of the velocity vectors at the utilization of Particle Image Velocimetry is to employ the technique of image shifting when recording the images of the tracer particles. The increasing number of applications of the PIV technique expected in near future requires a flexible image shifting system, which can be easily adapted to most different problems and experimental set-ups. Such a system, which must able to provide very high and easily adjustable shift velocities, can be attained by utilizing a rotating mirror for image shifting. In contrast to other image shifting methods no additional demands have to made on the light source and the scattering behaviour of the tracer particles. Problems as e.g. generating and controlling the rotation of the mirror and synchronizing the laser pulses with the angular position of the mirror have been solved successfully. A detailed theoretical model of the imaging of tracer particles by means of a rotating mirror system, which is required to obtain information about the accuracy of the method, is presented here for the first time. Further capabilities of the image shifting technique, which - in general - increase the applicability of the PIV-method for measuring complex flow fields are also described. The efficiency of the rotating mirror system is demonstrated by referring to the results of different wind tunnel measurements.