WIT Press

Techniques Of Research And Results In The Field Of Coherent Structures Of Wall-bounded Turbulence


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G. Alfonsi


Coherent structures of turbulence represent a widely-used viewpoint in describing turbulence in which categories like coherency and intermittency (associated in this context with the process of evolution of the coherent structures) are implied. In the present work the issue of the coherent structures developing in wall-bounded turbulent flows is considered. After a short historical synthesis, some basic concepts and various research methods and techniques for the scientific investigation of turbulent flows are reviewed. Some emphasis is given to the description of the available approaches to the numerical simulation of turbulent flows and to the problem of the construction of a turbulent-flow database. Then the phenomena occurring in the inner- and in the outer region of the turbulent boundary-layer are considered, mainly with reference to the large amount of experimental research existing on the subject. The flow phenomena are described in terms of: i) events occurring in the inner region, ii) large-scale motions developing in the outer region and, iii) dynamics of vortical structures. The method of the Proper Orthogonal Decomposition for the eduction of the coherent structures of turbulence is then presented. This technique permits the analysis of a turbulent-flow database in terms of dynamics of mathematically-defined coherent structures, allowing the calculation of properties of turbulent flows with precise physical meaning. 1 Introduction A still unresolved problem in fluid sciences is turbulence. In the last decades a particularly intense effort has been produced by researchers in this field and several new concepts have been generated. Nevertheless, still there is a lack of a general theory of turbulence. New concepts based on results obtained with the use of continuously evolving research techniques of both numerical and experimental nature are often