A digital image processing approach to large-scale turbulence studies.

An image processing approach to turbulence studies has been developed. The approach employs a structure tracking technique to quantify the movement of coherent, large-scale turbulent structures. The 'structure tracking' technique has been applied to the shear layer of a low speed jet issuing into a low speed crossflow. A study of the characteristics of the turbulent flow within this region involved comparative measurements with hot-wire anemometry measurements within the same flow regime and fractal analysis of the flow visualisation images used by the tracking routine. Fractal analysis was applied to flow visualisation images to educe a range of length scales made apparent by the flow visualisation equipment The results obtained with the structure tracking technique included the instantaneous velocity of the structures and a measure of their length scales. The instantaneous velocity measurements were used to calculate a turbulence characteristic associated with the structures. Further analysis revealed subsets of this turbulence characteristic involving the variation in average velocity of individual structures as well as variations in the instantaneous velocity of individual structures. Where possible, the results of the structure tracking technique were compared to those achieved by hot wire anemometry and good correspondence was found between the mean flow characteristics measured by both techniques. The results of the two techniques began to diverge in the regions of the flow where conventional hot-wire anemometry was unable to discriminate between the flow associated with the jet and that associated with the crossflow. In such regions, time-averaged hot-wire anemometry produced results which combined the measurements in both flow regimes and therefore attenuated any characteristics of the jet which were significantly different from those of the crossflow. In the same flow regions the structure tracking technique was able to measure those characteristics specifically associated with the jet, producing results, which reflected the behaviour of the jet more accurately