A SCHEME FOR RESAMPLING, FILTERING, AND SUBSAMPLING UNEVENLY SPACED LASER-DOPPLER ANEMOMETER DATA

Loser Doppler Anemometry (LDA) has proved a powerful tool for quantify ing fluid turbulence and is increasingly being applied in fields such as fluvial sedimentology and geomorphology. When operated in the burst -signal processing mode, high-frequency velocity fluctuations are meas ured at irregular time intervals. In many situations, there is a need to transform these data to obtain evenly spaced velocity values but at a lower frequency. However, clear guidelines for this type of data pr ocessing are lacking. Three steps are necessary in order to transform the original files into evenly spaced data: (1) resampling at the aver age sampling rate, (2) low-pass filtering with half-power frequency ad justed to the final sampling frequency, and (3) decimating al the desi red frequency. The decision taken at each step will affect the resulti ng signal and may cause, if nor assessed carefully, severe problems in the signal such as aliasing errors. This paper examines each stage of data processing and details the advantages and drawbacks of different techniques in relation to the effects on turbulence statistics (varia nce, instantaneous shear stress, etc.). A standard method and specific guidelines are finally proposed.