Analysis of a Symmetry leading to an Inertial Range Similarity Theory for Isotropic Turbulence

Abstract

We present a theoretical attack on the classical problem of intermittency and anomalous scaling in turbulence. Our focus is on an ideal situation: high Reynolds number isotropic turbulence driven by steady large scale forcing. Moreover, the fluid is incompressible and no confining boundaries are present. We start from a good set of basis functions for the velocity field. These are real and divergence-free. To each wave-vector k in Fourier space there is one pair of basis functions with respectively left and right-handed polarity. Isotropy makes all k on the shell of constant |k| statistically equivalent. Consequently, the coefficients, X+ and X-, to the basis functions in that shell become two random variables whose joint pdf describes the statistics at scale L =2*pi/|k|. Moreover, (X+)**2+(X-)**2 becomes a random variable for the energy. Switching to polar coordinates, the joint pdf expands in azimuthal modes. We focus on the axisymmetric mode which is itself a pdf and characterized by it radial profile P(r;L). Observations from both shell model and DNS data indicate that (1) the moments of P(r;L) scale as power laws in L, and (2) the profile obeys an affine symmetry P(r;L)=C(L)*f((lnr-mu(L))/sigma(L)). We raise the question: What statistics agree with both observation? The answer is pleasing. We find the functions f, mu, sigma C analytically in terms of a few constants. Moreover, we obtain closed form expressions for both scaling exponents and coefficients in the power laws. A virtual origin also emerges as an intrinsic length scale L0 for the inertial range.