Doppler global velocimetry with laser frequency modulation for the analysis of complex turbulent flows

A. Fischer, L. Büttner, J. Czarske, M. Gottschall, R. Mailach, K. Vogeler

abstract:

Abstract A Doppler global velocimetry technique using laser frequency modulation (FM-DGV) is presented for velocity field measurements with measurement rates up to 100 kHz. Hence, it is appropriate for investigating fast, unsteady, turbulent flows, e.g. occurring in turbo machines. The measurement principle is based on measuring the Doppler frequency shift of light scattered on particles by using a cesium absorption cell for frequency to intensity conversion. The intensity signals are measured spatially resolved by an avalanche photo diode array, which finally provides simultaneous velocity measurements at multiple points with high temporal resolution. These unique capabilities compared with conventional DGV systems are proven by measurements in an unsteady free jet flow up to about 100 m/s, a wake flow of an infinite cylinder (von Kármán vortex street) resolving turbulence spectra up to 10 kHz and a tip leakage flow in a linear cascade resolving the tip clearance vortex and its turbulence intensity. Despite a high incidence of the inflow, the measurements show that the cascade flow continues along the passage formed by the profiles, especially at the pressure side of the blade. At the suction side, the tip clearance vortex results in deviations. The complex structure of the tip clearance vortex can be analyzed, because three component velocity measurements in a plane (2d3c) were conducted with the FM-DGV measurement system, which are presented for the first time.

reference:

Fischer, A. Büttner, L., Czarske, J. Gottschall, M., Mailach, R., and Vogeler, K., 2010: “Doppler-Global-Velozimeter With Laser Frequency Modulation for the Analysis of Complex Turbolent Flows”, 15th International Sympossium on Applications of Laser Techniques to Fluid Mechanics, Paper No. 1557, July 5-8, 2010, Lisbon, Portugal.