Coherent structures and turbulence

The characteristic measures of coherent structures are discussed, emphasizing coherent vorticity as the crucial property. Presented here is a general scheme for educing structures in any transitional or fully turbulent flow. From smoothed vorticity maps in convenient flow planes, this scheme recognizes patterns of the same mode and parameter size, and then phase-aligns and ensemble-averages them to obtain coherent structure measures. The departure of individual realizations from the ensemble average denotes incoherent turbulence. This robust scheme has been used to educe structures from velocity data using a rake of hot wires as well as direct numerical simulations and can educe structures using newer measurement techniques such as digital image processing. Recent studies of coherent structures in several free shear flows are briefly reviewed. Also discussed is the role of coherent structures in aerodynamic noise generation and the authors argue that the structure breakdown process, rather than vortex pairing, is the dominant mechanism of noise generation. The coherent-structures approach to turbulence, apart from explaining flow physics, has also enabled turbulence management via control of structure evolution and interactions.