A NEW LASER-BASED METHOD FOR STRAIN-RATE AND VORTICITY MEASUREMENTS

A novel laser-based technique for the measurement of strain rate and v orticity in transparent fluid flows is presented, The technique is bas ed on the collection and direct heterodyning of coherent light scatter ed from individual seed particles in two adjacent locations and, there by, allowing the determination of a component of the strain rate tense r. The beat frequency of the heterodyned light, which is proportional to the velocity difference of the two points, is analysed using a conv entional LDA processor. The angle between the laser beam and the direc tion of the scattered light determines the measured component of veloc ity, Therefore, a component of the vorticity vector can be measured by using two sets of transmitting and collecting optics, focused at a si ngle location, along with two LDA processors. The basic principle of t he technique is verified through measurements on a thin rotating wire and in a laminar flow between two concentric cylinders with excellent results. In the latter experiment, the measured velocity differences b etween two radial locations in the flow agree very well with the exact solution.