A COMPARISON OF DIFFERENT ANALYTICAL TECHNIQUES FOR IDENTIFYING STRUCTURES IN TURBULENCE

Vortical structures play an important role in the kinematics and dynam ics of turbulence, but in order to understand this role we require tec hniques to identify and classify them. Proper Orthogonal Decomposition (POD), conditional sampling with ensemble statistics, and conditional sampling with conditional statistics are applied to a simple test fun ction and the results are compared to determine the strengths and weak ness of each approach. The second method gives the closest approximati on to the test signal and is the easiest to use, although it is sensit ive to the choice of conditions. None of these techniques can give muc h insight into the dynamics of turbulence, or into the organisation of eddies with complex, fine-scale structure. New methods for investigat ing complex (self-similar) structures based on fractal and wavelet ana lyses are presented. Methods of distinguishing between locally (accumu lating) and globally (fractal) self-similar structures are suggested.