A new laser-based method for strain rate and vorticity measurements

A novel laser-based technique for the measurement of strain rate and vorticity in transparent fluid flows is presented, The technique is based on the collection and direct heterodyning of coherent light scattered from individual seed particles in two adjacent locations and, thereby, allowing the determination of a component of the strain rate tenser. The beat frequency of the heterodyned light, which is proportional to the velocity difference of the two points, is analysed using a conventional LDA processor. The angle between the laser beam and the direction of the scattered light determines the measured component of velocity, Therefore, a component of the vorticity vector can be measured by using two sets of transmitting and collecting optics, focused at a single location, along with two LDA processors. The basic principle of the 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 between two radial locations in the flow agree very well with the exact solution.