Fronts in passive scalar turbulence

18 pages, 27 figuresThe evolution of scalar fields transported by turbulent flow is characterized by the presence of fronts, which rule the small-scale statistics of scalar fluctuations. With the aid of numerical simulations, it is shown that: isotropy is not recovered, in the classical sense, at small scales; scaling exponents are universal with respect to the scalar injection mechanisms; high-order exponents saturate to a constant value; non-mature fronts dominate the statistics of intense fluctuations. Results on the statistics inside the plateaux, where fluctuations are weak, are also presented. Finally, we analyze the statistics of scalar dissipation and scalar fluxes