Development and accuracy determination of a two-component Doppler Global Velocimeter (DGV)

A two-component Doppler Global Velocimeter (DGV) system was constructed and tested to research problems associated with the accuracy of this unique system. The uniqueness of the system lies in its ability to simultaneously and non-intrusively measure velocities in a laser illuminated plane. A key component of the system is a frequency discriminating optical filter containing iodine vapor which allows direct measurement of the Doppler frequency shift caused by particle motion. Corrections for optical distortions and non-uniform intensities as well as the conversions from intensity data to velocity data are performed by an extensive image processing algorithm. Measurements were made of a 12″ diameter rotating wheel and turbulent pipe/jet flow. Both RMS deviations and velocity range measurement errors from a single component for the rotating wheel with a maximum velocity of 58 m/s were less than 2%, better than most published results, to date, for similar systems. Pipe/jet flow profiles agreed very well with the shape of pitot probe measurements. RMS errors were on the order of 5--10%, but velocity offset error was as much as 10--15% of the 42 m/s velocity range. DGV measured turbulence intensities at the center of the pipe, 4 diameters downstream agreed with hot wire data, with some reservations. Several factors such as repeatability of calibrations, precision of wheel/pipe speed measurement, measurement of viewing angles, and 8-bit camera digitization contributed to the errors in DGV velocity data. Proper techniques for preparing and acquiring correction images are also critical steps toward the goal of producing accurate velocity data