Effects of sample size and concentration of seeding in LDA measurements on the velocity bias in open channel flow

A Laser Doppler Anemometer was used to measure the mean flow and turbulence in fluid experiments despite it having a problem with the mean velocity bias when a LDA is used to measure turbulent flows. It is generally considered that given a sufficiently large sample size, LDA will produce measurements free of bias, even with high turbulence intensity, but there is no relative experimental validation to demonstrate how the sample size affects the mean velocity bias. This paper first tries to find the reasonable sample size that ensures a mean velocity calculation free of bias. Furthermore, the effects of particle seeding concentration on the measurements of velocity were also considered. Throughout the experimental process the particle velocities were measured using a Dantec 2-D LDA system. To describe the effects of sample size and particle seeding concentration, this paper will also address the reasonable sample size and range of concentration in the design of water flow prior to any experimental application.