Time and spatial correlation measurements in the turbulent wake of a circular cylinder using Laser Doppler Anemometry

The present paper reports Laser Doppler Anemometry (LDA) measurements in the turbulent (R d =1300) wake of a circular cylinder (d=2 mm). On several typical locations in the wake, at cross-section x/d=125, temporal and lateral space velocity correlations have been measured. Temporal autocorrelation functions were determined from the randomly sampled LDA data by means of the “Slotting Technique”. The autocorrelation functions which were obtained were too low, the discrepancy giving an indication of the influence of noise in the measurements. The influence of noise generated by the photomultiplier tubes, which appeared to be the most important noise source, could be reduced by a cross-correlation technique. A partly new LDA system for two-point velocity correlation measurements was developed, consisting of an elongated measuring volume and a two-point detection system including a semireflecting mirror to divide the scattered light. The results of this investigation clearly demonstrate that LDA is a powerful tool for measuring time and spatial correlations and related properties of turbulence.