A Preliminary Study on the Formation of Elongated Vortices in Turbulence

Abstract

It has recently been observed that intense elongated vortices (‘worms’) are present among the small scales of many turbulent flows (Siggia, 1981), (Vincent & Meneguzzi, 1991), (Jiménez et al., 1993). Jiménez et al. (1993) have shown that worms are natural features of both decaying and forced turbulence and, even when they are artificially removed, they reform from the background vorticity. Following the explanation given in Jiménez & Wray (1994) for the formation of elongated vortices in isotropic turbulence, three numerical experiments are presented to show that axial waves exist and form vorticity filaments in turbulent flows. In the first experiment angled short vortices (‘sticks’) are considered showing that waves homogenize the core also for not aligned vortices, which is most likely the case of turbulence. In the second, to balance the viscous diffusion, a uniform strain has been superimposed and a Burgers vortex has been obtained. However the model with a uniform strain is in contrast with the statistical turbulence properties. A non uniform strain with zero spatial mean is compatible with the strain field along worm axes observed in numerically simulated isotropic turbulence. This strain has been used in the third experiment producing axial waves and forming an elongated vortex. In this case the equilibrium solution was independent of the initial conditions, in agreement with real worms whose properties depend only on Re _Г and Re _λ.