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Reduction of the measurement uncertainty in Doppler Global Velocimetry


Owing to new procedures, the measurement uncertainty in Doppler Global Velocimetry for the detection of flow velocity fields could be significantly reduced.

Optical procedures are used in numerous fields of application of flow metrology. One of these procedures is Doppler Global Velocimetry (DGV), which enables the measurement of 3-component flow velocity fields. For this purpose, a measurement plane is illuminated from different directions by means of laser lightsheets, so the velocity can be derived from the Doppler-induced frequency shift of light scattered from particles in the flow. The frequency shift is thereby converted into intensity changes of the scattered light by means of an absorption cell which is used as a frequency-dependent filter (Fig. 1). Conventional systems use 2 cameras to compare the intensity in front of and behind the absorption cell. A different response of the two cameras, as well as changes in the filter characteristics, can lead to systematic measurement errors of up to 5 m/s.

Principle of Doppler Global Velocimetry (DGV)

Figure 1: Principle of Doppler Global Velocimetry (DGV)

Within the scope of a thesis, a self-calibrating DGV procedure has been developed which does not record the light intensity in front of and behind the cell with two cameras, but which captures sequences of images at different laser frequencies by means of just one single camera behind the absorption cell. By comparing the intensities which have been successively recorded, it is not only possible to omit the reference camera, but also to derive the filter characteristics from the recorded images (self calibration). By using this procedure, it has been possible to reduce systematic measurement deviations to less than 0.2 m/s and to record the velocity field in a pipe flow with a swirl caused by a double elbow (Fig. 2).

Pipe flow after double bend

Figure 2: Pipe flow after double bend

Especially when measurements with low scattered light intensities and short exposure times are performed, the conventional DGV technique shows a higher susceptibility to the different characteristics of the two cameras which cannot be reduced by averaging over a large number of images. The main advantage of the new procedure is that the systematic uncertainty is independent of the recorded image intensities. This opens up the possibility of performing measurements at low intensities over a high number of image cycles. With this procedure, for the first time phase-resolved DGV measurements of the turbulent flow field behind a finite cylinder have been performed by means of a high-speed CMOS camera.

It was thus possible to show that the newly developed frequency-shift-keying procedure enables 3-component DGV measurements with low systematic measurement error, also in applications with particularly low scattered light intensity, especially when high temporal resolutions have to be achieved. Such measurements can support the analysis and the understanding of complex flow processes.

Contact person:

Michael Eggert, Dept. 1.4, WG 1.41, E-mail: michael.eggert@ptb.de