Energy spectra power laws and structures

Direct numerical simulations (DNS) of two inviscid flows, the Taylor-Green flow and two orthogonal interacting Lamb dipoles, together with the DNS of forced isotropic turbulence, were performed to generate data for a comparative study. The isotropic turbulent field was considered after the transient and, in particular, when the velocity derivative skewness oscillates around -0.5. At this time, R(lambda) approximate to 257 and a one decade wide k(-5/3) range was present in the energy spectrum. For the inviscid flows the fields were considered when a wide k(-3) range was achieved. This power law spectral decay corresponds to infinite enstrophy and is considered one of the requirements to demonstrate that the Euler equations lead to a finite time singularity (FTS). Flow visualizations and statistics of the strain rate tensor and vorticity components in the principal axes of the strain rate tensor ((S) over tilde (lambda), (omega) over tilde (lambda)) were used to classify structures. The key role of the intermediate component (S) over tilde (2) is demonstrated by its good correlation with enstrophy production. Filtering of the fields shows that the slope of the power law is directly connected to self-similar structures, whose radius of curvature is smaller the steeper the spectrum.