A coherent structure is a large scale vortex structure which conserves its spatial and temporal features during a long time in comparison with eddy timescales (typiccaly the eddy turn over time). If eddies are the core of the academic studies in turbulence, because of their role in turbulence fundamental processes as energy cascade and/or small scale intermittency (the local departure to isotropy), the coherent structures form the keystone of mechanical engineering, because they are application-dependant (aero or hydrodynamics, flow stability, large scale mixing, all these problems proceed from a better understanding of coherent structures). Coherent structure control the mass, momentum and heat transfer at the larger scale of the cascade towards dissipation and diffusion ones. You can consider also that eddies are all embedded or intricated into themselves leading to a continuous complex medium whereas coherent structures separate the flow in distinct dynamic regions.
A coherent structure is a large scale vortex structure which conserves its spatial and temporal features during a long time in comparison with eddy timescales (typiccaly the eddy turn over time). If eddies are the core of the academic studies in turbulence, because of their role in turbulence fundamental processes as energy cascade and/or small scale intermittency (the local departure to isotropy), the coherent structures form the keystone of mechanical engineering, because they are application-dependant (aero or hydrodynamics, flow stability, large scale mixing, all these problems proceed from a better understanding of coherent structures). Coherent structure control the mass, momentum and heat transfer at the larger scale of the cascade towards dissipation and diffusion ones. You can consider also that eddies are all embedded or intricated into themselves leading to a continuous complex medium whereas coherent structures separate the flow in distinct dynamic regions.
Anything is misleading? Sorry, if I was.
For understanding what coherent structures are, you have to be familiar with fundamentals in turbulence. I encourage you to read the manuscript from Tennekes and Lumley, A first Course of turbulence, 1976. If you read more classical textbooks on the subject, with a strong orientation on the statistical approach, if you dont have the related mathematical background, you could be bored in a first reading.
That will give you tools for understanding the classical approach from the Kolmogorov's 1941 (K41) and what eddies are.
Ideally, then, you could read modern approaches, as described in
Turbulence: The Legacy of A. N. Kolmogorov, by Uriel Frisch, where departures to K41 are described, with a special attention to small scale intermittency (observed in experiments but beyond the scope of the K41 theory).
In another words, eddies are common to every turbulent flow. They form the intimate structure of the flow in turbulent regime and their complex non linear interactions explain the mass, momentum and energy transport in every turbulent flow: this is THE challenging question of academical research.
That does not really address what coherent structures are.
Coherent structures rather proceed from peculiar aspects of real flow (hydrodynamic instability, wake behind solid bodies, acoustic interaction etc etc..). They are the aim of mechanical engineering (propulsion, engine, hydro aero dynamics, medicine, etc etc...).
As already explained perfect by xavier i dont think so further explanation need and for further gain in sight of the perfect definition you may follow some standard books or standard articles.
Abhay , go through the paper " Large scale structure in the mixing layer of a round jet" by Yule (1978).
They are two different things, but somehow related. Eddy is a hypothetical way of defining different fluid entities in a complex flow, such as turbulent. Eddy can be as small as the Kolmogrov scale to as large as the mean flow largest scale. Distribution of eddies define the 3 important layers (inner, inertial, and outer). However, when we talk about coherency in turbulent flows, we want to say that even though turbulence has random nature, but some defined structures can be found in it, e.g. the particular shape of a turbulent boundary layer (look at the edges of the turbulent B.L. which have a certain form).
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