Controllability and Evolution of Coherent Structures in Turbulent Wakes.

The main objectives were to answer some of the questions of evolution of large scale structure in turbulent wakes and to develop new techniques of experimentation and data reduction to identify and document the organized structures in turbulent flows. The techniques of thermal tagging, polychromatic signal decomposition, local and convective flow visualization, digital image processing and pattern recognition, two-dimensional spectral decomposition of velocity fields, as well as spatial reconstruction are applied to the bluff body wake problem. The results of these experiments have clarified the picture of wake evolution over a wide range of Reynolds numbers, from laminar to turbulent. These techniques demonstrate that an approach as elaborate as the orthogonal decomposition scheme in not necessary to objectively bring out the important large scale mechanisms. The study also clearly refutes the pairing mechanism active in shear layers as that responsible for the growth of the large scale.