Patterns in transitional shear turbulence. Part 2. Emergence and optimal wavelength

Low Reynolds number turbulence in wall-bounded shear flows en route to laminar flow takes the form of oblique, spatially intermittent turbulent structures. In plane Couette flow, these emerge from uniform via a spatio-temporal process which localised quasi-laminar gaps randomly nucleate and disappear. For slightly lower numbers, periodic approximately stationary turbulent–laminar patterns predominate. The statistics regions, including distributions space time scales their Reynolds-number dependence, are analysed. A smooth, but marked transition is observed between with gaps, whereas regular more gradual. Wavelength selection analysed numerical simulations oblique domains various sizes. Via lifetime measurements minimal domains, wavelet-based analysis wavelength predominance large domain, we quantify existence nonlinear stability pattern as function number. We report that preferred maximises energy dissipation large-scale along laminar–turbulent interfaces. This optimal behaviour due primarily advective nature fluctuations playing only secondary role.