Some consequences of bias effects in laser-Doppler velocimetry

The results of three dynamic flow experiments using laser-Doppler velocimetry confirm that velocity bias is present except when the time scale of sampling is larger than both the time scale corresponding to the particle arrival rate and to that of the integral turbulence time scale. Firstly, measurements using a 2 W argon laser were obtained on the center-line of a 30 mm diameter pulsating air jet centrally enclosed in a 60 mm diameter tube. Secondly, round-jet measurements were obtained on the center-line of a 10 mm nozzle diameter jet with a 5 mW helium-neon laser, a beam-splitter without frequency shift, and a 100 mm focusing lense. Thirdly, measurements in water flows were taken in a 74 mm diameter perspex pipe with a 54 mm disc centrally located in it. Results are presented for the effects of sampling time and Vcut in the velocity probability distributions and for the effect of discrimination level. The choice of discrimination level is demonstrated to be crucial for the removal of amplitude bias and to be the dominant factor in high trigger level values. Cases where velocity bias inflences measured mean and rms velocities are summarized.