Doppler Global Velocimetry with sinusoidal laser frequency modulation and a molecular absorption cell: Error investigation

We present a novel Doppler global velocimetry (DGV) system using a single-frequency laser with sinusoidal laser frequency modulation for measuring three component velocity fields in a plane of a flow. Here, the velocity dependent Doppler frequency shift of laser light scattered at particles contained in the flow is evaluated. In order to measure these changes in light frequency, the edge of a molecular absorption line is used for frequency-power conversion. The velocity corresponding changes in light power can be measured by e.g. a camera. Conventionally, a beam splitter and a second reference camera are also necessary to normalise the signals with respect to the scattered light power. As consequence, image misalignment errors and beam split errors occur, which can amount up to 20 m/s and 7 m/s, respectively.