Untitled

THEORY OF TURBULENCE 33"For boundary-free flow, the Kelvin and Helmholtz theorems implythat an initially smooth, inviscid flow remains smooth so long as vortexlines are stretched only a finite amount. Indeed, the restriction of theflow to two space dimensions precludes vortex-line stretching so globalregularity follows. However, in three dimensions, vortex lines can twist,tangle, turn, and stretch. It is conceivable that flow velocities remainbounded and, still, a singularity of the flow appears spontaneously aftera finite time in the interior of the flow. Segments of vortex lines coulddevelop infinite length by becoming intricately wound up and twistedwithout the end points of the segment being separated by an infinitedistance."The evidence presented in both these papers, and discussed at theLaJolla conference, raises a series of deeper questions about theimplications of the experimental evidence of the central role played bycoherent structure, and the theoretical difficulties:(1) Are these singularities "real?" That is, are there physicalphenomena (intermittency, etc.) to which these singularities correspond,in the same sense that the singularities in the equations of gasdynamics indicate the onset of a new set of laws governing shockwaves?(2) If this conjecture is correct, then there must be something like a"characteristic" equation—as there is for shock waves—that providesan invariant for the overall evolution of turbulence. This invariantwould provide the deeper explanation for the appearance and dynamicsof vortex phenomena as well as for the ubiquitous nature of coherentstructure in turbulent fluids.(3) What is the broader implication of an invariant specification ofthe qualitative nature of time evolution in a nonlinear system? That is,such an invariant would describe the seemingly nonenlropic nature ofstructure formation in "geometrical" terms. Can the same be done forother continuum systems?REFERENCES1. For an extensive bibliography on these vortex structures, see Lamb'sHydrodynamics.2. For a comprehensive account of the modern statistical theory of turbulence,see Orszag's Lecture Notes on the Statistical Theory of Turbulence.3. Saffman has an early critique of these theories.4. Norman Zabusky describes the "supra-nova-like experience which revealedkey links and exposed new analytical and computational directions uponseeing the set of computer-generated films on the formation of vortices.5. See J.H. Ferziger, Phys. Fluids 23, 1 (1980), for a discussion.6. H. Grad, "Reconnection of Magnetic Field Lines in an Ideal Fluid,"unpublished report.