Fast Three-dimensional Measurement of Flow Fields

A detailed characterisation of flow fields in pipes is of high economic importance, e. g., for the measurement of natural gas delivered and charged to customers. Often, the uncertainty of the gas flow meter arrangement depends on perturbations of the flow field at the inlet of the gas meter caused by installation effects. In unfavourable cases, the uncertainty can be as high as several per cent.

PTB has developed a test facility for Doppler global velocimetry in collaboration with DLR, Cologne, the German centre of aerospace research. Laser Doppler techniques evaluate the Doppler frequency shift of laser light scattered by tracer particles embedded in the flow. In contrast to conventional laser Doppler anemometer systems which measure the Doppler shift by optically heterodyning of the scattered light, Doppler global velocimetry converts the Doppler shift on the steep slope of an absorption line into an intensity variation. By imaging the whole field of view through an absorption cell onto a CCD camera, an image is obtained the local intensity of which corresponds to the spatially resolved Doppler shift. A frequency-to-velocity calibration of the absorption allows one to directly map out the velocity components of the total field.

The new facility allows to perform fast routine measurements of a complete three-dimensional velocity field including the radial velocity component with a local resolution limited only by the number of pixels of the CCD camera. With the new facility, the influence of perturbations caused by installed elements on the reading of gas flow meters can be investigated and modelled. Potential future applications include accurate routine gas flow measurements independent of flow perturbations.