THE USE OF PATTERN-RECOGNITION AND PROPER ORTHOGONAL DECOMPOSITION INIDENTIFYING THE STRUCTURE OF FULLY-DEVELOPED FREE TURBULENCE

The eigenvectors obtained front proper orthogonal decomposition (POD) are used as the selection criteria to classify the individual events c ontained in data files of two-component velocity signals recorded in t he non-homogeneous (vertical) and homogeneous (horizontal) planes of a fully-developed turbulent cylinder wake. This procedure uses the domi nant eigenvectors from POD as initial templates to perform a pattern r ecognition (PR) analysis of the signals so that the individual coheren t events appearing randomly in the signals can be educed more objectiv ely. It is found that the prototype or ensemble average of the group o f events classified as the large-scale structure in the vertical plane has a circulatory motion with strong negative streamwise and outward lateral velocity fluctuations. In the horizontal plane, the average st ructure is a double roller with negative streamwise velocity fluctuati ons in its centerplane. The class of instantaneous events selected con tribute significantly to the variance in the outer intermittent region , but much less in the fully turbulent core. While the first eigenvect or from POD captures some of the features shown by this prototype, it is not enough to depict by itself all the characteristics shown by the ensemble average of the repetitive three-dimensional instantaneous ev ents that occur in the fully-developed turbulent wake.