Laser speckle methods in fluid dynamics applications

A novel flow visualization and velocity measurement technique is described. It permits the simultaneous visualization of instantaneous two-dimensional streamline patterns in unsteady flows, and the quantification of the velocity field. The technique involves seeding the flow with small particles and lighting the plane of interest with a sheet of pulsed laser light to obtain a multiple exposure photograph of the particles as they travel with the flow. The multiple particle images provide the streamlines. The local velocity information is retrieved using an interferometric method. The photographic negative is analyzed by passing a probe laser beam through each point of interest and allowing the diffraction produced by the multiple particle images to generate fringes in the Fourier plane of a lens. These fringes have an orientation which is perpendicular to the direction of the local velocity vector and a spacing which is inversely proportional to its magnitude. The parameters involved in obtaining the multiple exposure photograph are discussed and a relationship is established between these parameters. The accuracy of the measurement technique is also discussed.