Linear and nonlinear optimal growth mechanisms for generating turbulent bands

Linear and nonlinear energy optimizations in a tilted domain are used to unveil the main mechanisms allowing creation of turbulent band channel flow. optimization predicts an optimal growth for streamwse spanwise wavenumbers $k_x = 1.2$, $k_z - 1.75$, corresponding peak values premultiplied spectra direct numerical simulations. At target time, linear perturbation is composed by oblique streaks, which, sufficiently large initial energy, induce turbulence whole domain, due lack spatial localization. When localization achieved adding effects optimization, or artificially confining localized region direction, large-scale flow created, which leads generation localised band. These results suggest that inducing transition towards bands two elements needed: mechanism such as lift-up generating large-amplitude structures produce inflection points; vortices ensuring localisation. Remarkably, these alone able generate isolated also large, non-tilted domain.