Claudio Viotti

Streamwise-travelling waves of spanwise wall velocity for turbulent drag reduction

Waves of spanwise velocity imposed at the walls of a plane turbulent channel flow are studied by direct numerical simulations. We consider sinusoidal waves of spanwise velocity which vary in time and are modulated in space along the streamwise direction. The phase speed may be null, positive or negative, so that the waves may be either stationary or travelling forward or backward in the direction of the mean flow. Such a forcing includes as particular cases two known techniques for reducing friction drag: the oscillating wall technique (a travelling wave with infinite phase speed) and the recently proposed steady distribution of spanwise velocity (a wave with zero phase speed). The travelling waves alter the friction drag significantly. Waves which slowly travel forward produce a large reduction of drag that can relaminarize the flow at low values of the Reynolds number. Faster waves yield a totally different outcome, i.e. drag increase (DI). Even faster waves produce a drag reduction (DR) effect again. Backward-travelling waves instead lead to DR at any speed. The travelling waves, when they reduce drag, operate in similar fashion to the oscillating wall, with an improved energetic efficiency. DI is observed when the waves travel at a speed comparable with that of the convecting near-wall turbulence structures. A diagram illustrating the different flow behaviours is presented.

Streamwise oscillation of spanwise velocity at the wall of a channel for turbulent drag reduction

Steady forcing at the wall of a channel flow is studied via direct numerical simulation to assess its ability of yielding reductions in turbulent friction drag. The wall forcing consists of a stationary distribution of spanwise velocity that alternates in the streamwise direction. The idea behind the forcing builds on the existing technique of the spanwise wall oscillation and exploits the convective nature of the flow to achieve an unsteady interaction with turbulence. The analysis takes advantage of the equivalent laminar flow, which is solved analytically to show that the energetic cost of the forcing is unaffected by turbulence. In a turbulent flow, the alternate forcing is found to behave similarly to the oscillating wall; in particular an optimal wavelength is found which yields a maximal reduction in turbulent drag. The energetic performance is significantly improved, with more than 50% of maximum friction saving at large intensities of the forcing, and a net energetic saving of 23% for smaller int...

Skin-Friction Drag Reduction Via Steady Streamwise Oscillations of Spanwise Velocity

Reducing the skin-friction drag in turbulent wall flows has seen a growing interest in recent years, owing to potential energetic and environmental advantages. Passive techniques (like riblets) are not yet in widespread use, notwithstanding their applicative appeal; most of the strategies currently under investigation are active techniques. One of the simplest and most interesting amongst active approaches is the oscillating-wall technique [1], where the wall moves according to:

Numerical simulations and experimental measurements of dense-core vortex rings in a sharply stratified environment

We present three-dimensional direct numerical simulations of a vortex ring settling in sharply stratified miscible ambient fluids for near two-layer configurations, and comparisons of these simulations with the results from laboratory experiments. The core fluid of the vortex rings has density higher than both the top and the bottom layers of the ambient fluid, and is fully miscible in both layers. This setup ensures a rich parameter space that we partially explore in this study. In particular, a critical (bifurcation) phenomenon is identified that distinguishes the long-time behavior of the settling vortex ring as either being fully trapped at the ambient density layer or continuing through the layer in its downward motion. This critical behavior is determined by the initial conditions (e.g. the size and speed of the vortex ring, the initial distance to the layer, etc). The numerical simulations are able to provide evidence for this in qualitative agreement with an experimental phase diagram. Our setup isolates essential elements of mixing, trapping and escape through stratified fluids in a variety of situations, such as the mixing and dispersion of pollutants and plankton in the ocean.

Analysis of passive scalar advection in parallel shear flows: Sorting of modes at intermediate time scales

The time evolution of a passive scalar advected by parallel shear flows is studied for a class of rapidly varying initial data. Such situations are of practical importance in a wide range of applications from microfluidics to geophysics. In these contexts, it is well-known that the long-time evolution of the tracer concentration is governed by Taylor’s asymptotic theory of dispersion. In contrast, we focus here on the evolution of the tracer at intermediate time scales. We show how intermediate regimes can be identified before Taylor’s, and in particular, how the Taylor regime can be delayed indefinitely by properly manufactured initial data. A complete characterization of the sorting of these time scales and their associated spatial structures is presented. These analytical predictions are compared with highly resolved numerical simulations. Specifically, this comparison is carried out for the case of periodic variations in the streamwise direction on the short scale with envelope modulations on the long...