Some consequences of bias effects in laserDoppler velocimetry
Abstract
The results of three dynamic flow experiments using laserDoppler 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 centerline of a 30 mm diameter pulsating air jet centrally enclosed in a 60 mm diameter tube. Secondly, roundjet measurements were obtained on the centerline of a 10 mm nozzle diameter jet with a 5 mW heliumneon laser, a beamsplitter 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.
 Publication:

Applications of LaserDoppler Anemometry to Fluid Mechanics
 Pub Date:
 1982
 Bibcode:
 1982alaf.symp...16D
 Keywords:

 Bias;
 Flow Measurement;
 Instrument Errors;
 Laser Doppler Velocimeters;
 Turbulent Flow;
 Beam Splitters;
 Flow Velocity;
 Frequency Shift;
 Probability Distribution Functions;
 Pulse Amplitude;
 Sampling;
 Signal Processing;
 Fluid Mechanics and Heat Transfer