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Large-scale motions, important in turbulent shear flows, are frequently attributed to the interaction of structures at smaller scales. Here we show that, in a turbulent channel at Re{τ}≈550, large-scale motions can self-sustain even when smaller-scale structures populating the near-wall and logarithmic regions are artificially quenched. This large-scale(More)
We collect and discuss the results of our recent studies which show evidence of the existence of a whole family of self-sustaining motions in wall-bounded turbulent shear flows with scales ranging from those of buffer-layer streaks to those of large-scale and very-large-scale motions in the outer layer. The statistical and dynamical features of this family(More)
The nature of dynamo action in shear flows prone to magnetohydrodynamc instabilities is investigated using the magnetorotational dynamo in Keplerian shear flow as a prototype problem. Using direct numerical simulations and Newton's method, we compute an exact time-periodic magnetorotational dynamo solution to three-dimensional dissipative incompressible(More)
Reducing skin friction is important in nature and in many technological applications. This reduction may be achieved by reducing stresses in turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the boundary layer laminar as long as possible by forcing small optimal perturbations.(More)
The rst instability of a spring-mounted, damped, rigid circular cylinder immersed in a viscous ow and free to move in a direction orthogonal to the unperturbed ow, is investigated by a global stability analysis. The ow is modeled by the full Navier-Stokes equations. For low ratios of the uid density to the structure density, the von Karman mode is always(More)
This study is concerned with the numerical calculation of the maximum spatial growth of GG ortler vortices on a concave wall. The method is based on the direct computation of a discrete approximation to the spatial propagator that relates the downstream response to the inlet perturbation. The optimization problem is then solved directly by making use of the(More)