THEORETICAL AND EXPERIMENTAL ASPECTS OF IMAGE-SHIFTING BY MEANS OF A ROTATING MIRROR SYSTEM FOR PARTICLE IMAGE VELOCIMETRY

One way to remove the directional ambiguity (of sign) of the velocity vectors inherent to particle image velocimetry is to employ the technique of image shifting while recording the images of the tracer particles. The ever increasing number of applications of this technique requires a flexible image-shifting system that can be easily adapted to a wide variety of problems and experimental set-ups. Such a system must be able to provide very high, easily adjustable shift velocities. This can be achieved through the use of a rotating mirror. In contrast to other image-shifting techniques no additional demands are imposed on the light source and the scattering behaviour of the tracer particles. Successful solutions to problems such as the angular control of the rotating mirror in synchronization with the laser pulses are described. A detailed theoretical model of the imaging of tracer particles by means of a rotating mirror system is required to obtain information about the accuracy of the shifting method. Some of the capabilities of the described image-shifting technique, which, in general, increases the range of applicability of the particle image velocimetry method, are demonstrated by referring to the results of different wind tunnel measurements.