Effect of Duration and Frequency of Acoustic Doppler Velocimetry Measurement on Calculation of Turbulent Flow Characteristics

Acoustic Doppler velocimetry (ADV) is one of the most suitable devices for measuring flow characteristics. Determination of measurement frequency and duration, in a way that the results are calculated with the lowest error, is very important. The goal of this study was to determine the optimum measurement frequency and duration to save money and time. 3D instantaneous subcritical flocharacteristicsts are measured at 200, 100, 25, and 5Hz frequencies for a duration of 3 minutes, in a laboratory flume with an aspect ratio of less than 5. Then, 3D averaged velocities, shear velocity, turbulence intensity, and Reynolds shear stress are calculated. Results show that the reduction of error is independent of the number of measured data and its dependence is on the data collection duration and frequency. For measurements of 3D averaged velocity components, the appropriate measurement frequency and duration are 1Hz and 50 seconds, respectively. To determine the shear velocity, using logarithmic law, reducing the frequency and duration, results in a maximum error of 13%. For calculation of turbulence flow characteristics, like turbulence intensity and Reynolds shear stress, the measurement frequency, and duration of up to 25Hz and 50-70sec, respectively, results in an error of less than 10%.