TECHNIQUE FOR THE EXPERIMENTAL ESTIMATION OF NONLINEAR ENERGY-TRANSFER IN FULLY-DEVELOPED TURBULENCE

A new procedure for calculating the nonlinear energy transfer and line ar growth/damping rate of fully developed turbulence is derived. It av oids the unphysically large damping rates typically obtained using the predecessor method of Ritz [Ch. P. Ritz, E. J. Powers, and R. D. Beng tson, Phys. Fluids B 1, 153 (1989)]. It enforces stationarity of the t urbulence to reduce the effects of noise and fluctuations not describe d by the basic governing equation, and includes the fourth-order momen t to avoid the closure approximation. The new procedure has been imple mented and tested on simulated, fully developed two-dimensional (2-D) turbulence data from a 2-D trapped-particle fluid code, and has been s hown to give excellent reconstructions of the input growth rate and no nlinear coupling coefficients with good noise rejection. However, in t he experimentally important case where only a one-dimensional (1-D) av eraged representation of the underlying 2-D turbulence is available, t his technique does not, in general, give acceptable results. A new 1-D algorithm has thus been developed for analysis of 2-D measurements of intrinsically 2-D turbulence. This new I-D algorithm includes the non resonant wave numbers in calculating the bispectra, and generally give s useful results when the width of the radial wave number spectrum is comparable to or less than that of the poloidal spectrum. (C) 1996 Ame rican Institute of Physics.