Measurement of the velocity gradient of a rotating body by means of a speckle method

摘要

Abstract: The second order statistical properties of coherent radiation scattered from rough surface in the focal plane of an optical system are analyzed. Two types of motion of the scattering surface are considered: arbitrary movement in his own plane without gradient of velocity. The expressions for the space-time intensity correlation function and the correspondent power spectrum are derived and the speckle motion characteristics are analyzed. It is shown that under rotational motion conditions speckle pattern also rotates in the focal plane around certain axis leading thus to characteristics dependence of power spectrum width on spatial position in the focal plane. At the same time arbitrary movement of scattering surface without gradient across laser spot results in no spatial dependence of spectrum width or correlation time in the focal plane. So analysis of spatial dependence of second order statistical properties of scattered radiation gives one the possibility to measure the velocity gradient across the laser spot of the surface under investigation. An experiment has been carried out by using a He-Ne laser at $lambda $EQ 0.63 $mu@m to validate the mathematical model suggested. The scattering in the turbulent flow of a liquid has shown the similar behavior for spectrum width dependence on spatial position of observation point giving thus possibility for vortex monitoring in a liquid. !9