Investigation of time-resolved single detector Doppler global velocimetry using sinusoidal laser frequency modulation

Abstract

We present a new Doppler global velocimetry (DGV) technique that is based on sinusoidal laser frequency modulation: FM-DGV. DGV is an optical method to measure two-dimensional velocity fields in fluid flows. The measurement principle is based on measuring the Doppler shift of the laser frequency with the use of an absorption cell converting frequency variations into intensity variations. In order to measure the scattered light intensity, conventional DGV systems use a signal and a reference camera. Using the FM-DGV technique, no additional reference camera is required. Thus significant error sources can be eliminated, e.g. image misalignment errors, which can limit the performance of conventional DGV. In this paper, a comparison between both DGV systems and an error analysis of the FM-DGV system is given. The parameter optimization of the FM-DGV is discussed as well. A minimum measured standard deviation of 0.02 m s−1 was achieved. Measurement results of applications to a rotating glass disc as a calibration object, a free jet flow and a laminar boundary layer flow are presented to demonstrate the ability of FM-DGV also for low-speed and unsteady flow conditions.