Wavelet quadrature methods for detecting coherent structures in fluid turbulence
Jon Harald Kaspersen, Lonnie H. Hudgins · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 1996
The energy cascade found in fully developed fluid turbulence is known to originate as large-scale organized motions called coherent structures. The process of detecting, locating, and tracking these coherent structures is therefore of central importance to the continued study of turbulence. For certain types of flow, these structures can be associated with quadrature events between the streamwise- and transverse-instantaneous velocities. A number of researchers have begun applying wavelet-based methods to the problem of coherent structure detection. Significant performance improvements over other existing methods have already been reported. In this paper, we describe several variations of the standard wavelet cross-transform suitable for use as quadrature detectors. One adaptation simultaneously employs two different analyzing wavelets: one with odd symmetry and one with even symmetry. With these wavelets, the resulting cross-transform can be made to detect quadrature events in the time-scale plane, and thus isolate coherent structures very effectively. The method is described, and demonstrated for cylinder wake flow data. Its performance is then quantitatively compared to some of the popular alternative techniques for detecting coherent structures.