This technique uses small tracer particles added to the flow (particles with diameter below 1 micrometer, e.g. oil). These particles scatter light projected by two laser beams into a small measurement volume. In this volume these laser-beams interfere producing interference planes (tiny planes of light one behind the other). The particle reflects the light when flying through these planes; the frequency of these light flashes allows the calculation of the velocity if the distance between the interference planes is known.
For our work we use a commercially available LDV system together with four other institutes at the Faculty of Mechanical Engineering. This system consists of a Coherent Innova 90/6, a two-dimensional DANTEC fiber flow probe system, two DANTEC burst spectrum analyzers for real time Fourier processing of the signal and a DANTEC 590x590x690mm computer-controlled traverse. Additionally this system can be used with a DANTEC PDA Processor for particle sizing. A small DANTEC Flow Lite system is used during the student’s lab and outdoor measurements (for details on the systems please visit DANTEC website).
Together with flow visualization and interferometric techniques LDV measurements are used for precise velocity measurements in transonic flow in a linear cascade as well as in rotating machinery. The picture shows the result of a LDV measurement in the linear turbomachinery cascade of the institute.