ON THE SPECTRUM OF A STRETCHED SPIRAL VORTEX

Corrections are found to the k(-5/3) spectrum of Lundgren [Phys. Fluids 25, 2193 (1982)] for a stretched spiral vortex model (a is the stretching strain rate and k the scalar wave number) of turbulent fine scales. These take the form of additional terms arising from the early time evolution, when the stretching of vortex lines is small. For the special case when the spiral takes the form of a rolled-up shear layer, it is shown that the composite spectrum is divergent, thus requiring the introduction of a finite early cutoff time tau(1) in the time integral for the nonaxisymmetric contribution. The identity nu omega(2) $($) over bar$$ = 2 nu integral(0)(x)k(2)E(k)dk which gives the dissipation is then satisfied self-consistently. Direct numerical calculation of the energy spectrum from the approximate vorticity field for a special choice of spiral structure nevertheless indicates that the one-term k(-5/3)-spectrum result is asymptotically valid in the inertial range provided a tau(1) is O(1) but that the numerically calculated dissipation spectrum appears to lie somewhere between an exp(-B(1)k(2)) and an exp(-B(2)k) farm. It is also shown that the stretched, roiled-up shear-layer model predicts asymptotic shell-summed spectra of the energy dissipation and of the square of the vorticity, each asymptotically constant, with no power-law dependence, for k smaller than the Kolmogorov wave number. The corresponding one-dimensional spectra each show -log(k(1)) behavior for small k(1). The extension of the model given by Pullin and Saffman [Phys. Fluids A 5, 126 (1993)] is reformulated by the introduction of a long-time cutoff in the vortex lifetime and an additional requirement that the vortex structures be approximately space filling. This gives a reduction in the number of model free-parameters but introduces a dependence of the calculated Kolmogorov constant and skewness on the ratio of the initial vortex radius to the equivalent Burgers-vortex radius. A scaling for this ratio in terms of the Taylor microscale Reynolds number is proposed in which the stretching strain is assumed to be provided by the large scales with spatial coherence limited to the maximum stretched length of the structures. Postdictions of the fourth-order fatness factor and of higher moments of the longitudinal velocity gradient statistics are compared with numerical simulation.