Our thanks go to Martin Landahl for his stimulating expository lectures on recent development in "lab scale" turbulent flow. This subject was covered by him and other staff members from the experimental, analytic, and numerical point of view. The seminars on two-dimensional coherent structures provided a nice connecting link for subsequent lectures on large scale ocean eddy dynamics (e.g. warm core rings detaching from the Gulf Stream).The purpose of these notes is to indicate the range of the topics covered, and the degree of beneficial comunication.It is something of a wonder, with all these lectures going on, that our GFD Fellows are still able to come up with a (sometimes) admirable research project! For this our thanks go to those staff members who have given so generously of their time and spirit.We acknowledge with sincere appreciation the financial support provided by the Office of Naval Research and the National Science Foundation. WHAT IS A COHERENT STRUCTURE
Melvin E. Stern -v -
Some early findingsWe wish to examine the physical nature of so called coherent structures and what they imply regarding turbulence. There is a great deal of information but not much is known in the sense well understood. This is because a great deal of the 'knowledge' is not reliable (due to lack of theoretical and experimental understanding) and because the theoretical and conceptual models have to be oversimplified (due to the complexity involved).There is no precise, or agreed upon, definition of a coherent strucure. Generally it is meant to describe a localized region of enhanced velocity fluctuations, usually containing a vorticity anomaly. Simple examples of coherent structures (not found in turbulent shear flows) are a vortex ring (i.e. a limited region of enhanced vorticity propagating through the flow) and a Hill's spherical vortex (which has bipolar axisyrnmetry). In the literature the terms eddy and typical eddy are often used. The term coherent structure, however, usually implies a typical structure one which recurs at (usually) random intervals. This typical shape will tend to dominate the entire structure. A liberal definition is: any structure educed by a suitably chosen sampling technique. Townsend (1976) states: "Since the experimental data is always incomplete, the identification of eddy type must be informed guesswork followed by measurements designed to confirm the guess, and then to fit the inferred structure into a coherent dynamical account of the matter". In that respect, turbulence, like pornography, is in the eyes of the beholder ... or rather -there is a personal bias in interperating the observations. The first account of turbulence is perhaps a drawing by Leonardo DaVinci, portraying the outlet of a sewer in Milan. The wealth of eddy sizes in that drawing is remarkable. Early ideas on structures where formed by Richardson (1922), who observed the cascade of "whorls n , paraphrasing on Swift's hierarchy of fish. Prandtl (1925) used the term "lumps n for structures that carry and propagate some pr...