Statistical analysis of inhomogeneous fluctuation fields. Scalar transport in shearless turbulent mixing, effects of stratification, solar wind and solar wind-interstellar medium interaction.

Turbulence is a condition that can occur in a broad range of fluids, which may belong to very different physical environments, each with their own unique characteristics. Mathematical and analytical studies are generally limited by the high degree of complexity of the system, therefore, numerical/laboratory experiments and in-situ measurements play a fundamental role in the study of these phenomena. An analysis on two different anisotropic fluctuating fluid fields has been performed: both flows, while belonging to different physical contexts, are characterized by the presence of multiscale inhomogeneous fluctuations, to which is associated a strong anisotropy, and by the presence of effects related to stratification / mixing. The first is one of the most simple anisotropic turbulent flow, namely the shearless turbulent mixing, and it has been studied by means of direct numerical simulation of Navier-Stokes equations, with the aim of characterize the passive scalar transport and the effects related to the presence of a thermal stratification. The second is a more complex fluid field, that is the solar wind, which belong to magnetohydrodynamic flows; the analysis on solar wind have been performed taking advantage of in-situ measurement of the Voyager 2 spacecraft, trying to provide a statistical and spectral characterization despite the presence of gaps in the recorded time-series.