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Zusammenfassung:
An essential ingredient of turbulent flows is the vortex stretching mechanism, whichemanates from the nonlinear interaction of vorticity and strain-rate tensor and leads toformation of extreme events. We analyze the statistical correlations between vorticity andstrain rate by using a massive database generated from very well-resolved direct numericalsimulations of forced isotropic turbulence in periodic domains. The grid resolution is upto 12 288^3, and the Taylor-scale Reynolds number is in the range 140-1300. In orderto understand the formation and structure of extreme vorticity fluctuations, we obtainstatistics conditioned on enstrophy (vorticity-squared). The magnitude of strain, as well asits eigenvalues, is approximately constant when conditioned on weak enstrophy; whereasthey grow approximately as power laws for strong enstrophy, which become steeper withincreasing Rλ. We find that the well-known preferential alignment between vorticity andthe intermediate eigenvector of strain tensor is even stronger for large enstrophy, whereasvorticity shows a tendency to be weakly orthogonal to the most extensive eigenvector (for large enstrophy). Yet the dominant contribution to the production of large enstro-phy events arises from the most extensive eigendirection, the more so as Rλ increases. Nevertheless, the stretching in intense vorticity regions is significantly depleted, consistentwith the kinematic properties of weakly curved tubes in which they are organized. Furtheranalysis reveals that intense enstrophy is primarily depleted via viscous diffusion, thoughviscous dissipation is also significant. Implications for modeling are nominally addressedas appropriate.
