Influence of sampling frequency and sample size on turbulence statistics in open-channel flow measurements

The influences of sampling frequency and sample size on the accuracy of turbulence measurements are experimen tally investigated.A set of 340535 velocity data are first sampled at a frequency of 600Hz with the particle image velocimetry to measure the turbulent parameters of the flow.A series of statistical samples corresponding to various sampling frequencies and sample sizes are then achieved by extracting data from the original data set.Comparison of the mean velocity,turbulent in tensity,and Reynolds stress obtained from these samples reveals that sampling frequency and sample size both show a threshold value above which the measured turbulence characteristics become relatively stable,and that,to achieve the same level of the experimental accuracy,the required sampling frequencies and sample sizes increase progressively in evaluating mean velocity,turbulence intensity and Reynolds stress.