Publications by Francesco Viola

Subharmonic parametric instability in nearly brimful circular cylinders: a weakly nonlinear analysis

François Gallaire, Francesco Viola, Alessandro Bongarzone

In labscale Faraday experiments, meniscus waves respond harmonically to small-amplitude forcing without threshold, hence potentially cloaking the instability onset of parametric waves. Their suppression can be achieved by imposing a contact line pinned at ...

Cambridge University Press2022

Relaxation of capillary-gravity waves due to contact line nonlinearity: A projection method

François Gallaire, Francesco Viola, Alessandro Bongarzone

We present a physics-inspired mathematical model based on successive linear eigenmode projections to solve the relaxation of small-amplitude and two-dimensional viscous capillary-gravity waves with a phenomenological nonlinear contact line model. We show t ...

American Institute of Physics2021

Flow dynamics of a dandelion pappus: A linear stability approach

François Gallaire, Francesco Viola, Lorenzo Siconolfi, Pier Giuseppe Ledda

The study and control of flow instabilities is a key problem in aerodynamics. Aircrafts are designed not only to generate the lift force needed to balance their weight but, more importantly, to be stable and reasonably steady when in cruise conditions. Sim ...

2019

Capillary hysteresis in sloshing dynamics: aweakly nonlinear analysis

François Gallaire, Francesco Viola, Pierre-Thomas Paul Brun

Cambridge University Press2018

Parabolic RANS solver for low-computational-cost simulations of wind turbine wakes

Giacomo Valerio Iungo, Francesco Viola

2018

Suppression of von Karman vortex streets past porous rectangular cylinders

François Gallaire, Francesco Viola, Lorenzo Siconolfi, Pier Giuseppe Ledda

Although the stability properties of the wake past impervious bluff bodies have been widely examined in the literature, similar analyses regarding the flow around and through porous ones are still lacking. In this work, the effect of the porosity and perme ...

2018

Theoretical framework to analyze the combined effect of surface tension and viscosity on the damping rate of sloshing waves

François Gallaire, Francesco Viola

Gravity force, surface tension, oscillating boundary layers, viscous dissipation, static meniscus, contact angle hysteresis, and moving contact lines are typical features of sloshing dynamics. All these ingredients influence capillary-gravity waves in a co ...

AMER PHYSICAL SOC2018

Flow control of weakly non-parallel flows: application to trailing vortices

François Gallaire, Giacomo Valerio Iungo, Francesco Viola, Erica Pezzica

A general formulation is proposed to control the integral amplification factor of harmonic disturbances in weakly non-parallel amplifier flows. The sensitivity of the local spatial stability spectrum to a base-flow modification is first determined, general ...

Cambridge University Press2017

The viscous torsional pendulum

François Gallaire, Francesco Viola

A disk that is free to rotate about its axis and connected to a torsional spring behaves as a damped oscillator when twisted and released. The initial elastic energy is periodically turned to kinetic energy and it gets progressively dissipated by the visco ...

Academic Press Ltd- Elsevier Science Ltd2017

Sloshing in a Hele-Shaw cell: experiments and theory

François Gallaire, Francesco Viola, Benjamin Jérémy Dollet

The response of the free liquid surface in a Hele-Shaw cell subjected to a horizontal oscillation is investigated. We study the low-oscillation-amplitude regime and we show, by varying the fluid viscosity, nu, and the forcing frequency, omega, that the rat ...

Cambridge University Press2017

Hub vortex instability within wind turbine wakes: Effects of wind turbulence, loading conditions, and blade aerodynamics

François Gallaire, Giacomo Valerio Iungo, Francesco Viola

The near wake of wind turbines is characterized by the presence of the hub vortex, which is a coherent vorticity structure generated from the interaction between the root vortices and the boundary layer evolving over the turbine nacelle. Bymoving downstrea ...

Amer Physical Soc2016

Mode selection in trailing vortices: harmonic response of the non-parallel Batchelor vortex

François Gallaire, Cristobal Manuel Arratia Martinez, Francesco Viola

In the present study, the response of model trailing vortices subjected to a harmonic forcing is studied. To this purpose, a globally stable non-parallel Batchelor vortex is considered as the baseflow. Direct numerical simulations (DNS) show that a large v ...

Cambridge University Press2016

Foam on troubled water: Capillary induced finite-time arrest of sloshing waves

François Gallaire, Francesco Viola, Pierre-Thomas Paul Brun, Benjamin Jérémy Dollet

Interfacial forces exceed gravitational forces on a scale small relative to the capillary length-two millimeters in the case of an air-water interface-and therefore dominate the physics of sub-millimetric systems. They are of paramount importance for vario ...

Amer Inst Physics2016

Resonance in swirling wakes and sloshing waves : non-normal and sublinear effects

Francesco Viola

Similarly to mechanical structures, stable flows can exhibit resonance when perturbed by an impulsive or harmonic forcing. Swirling wakes and sloshing waves belong to this kind of flows and manifest large energy response when excited close to their natural ...

EPFL2016

Effects of incoming wind condition and wind turbine aerodynamics on the hub vortex instability

François Gallaire, Giacomo Valerio Iungo, Francesco Viola

Dynamics and instabilities occurring in the near-wake of wind turbines have a crucial role for the wake downstream evolution, and for the onset of far-wake instabilities. Furthermore, wake dynamics significantly affect the intra-wind farm wake flow, wake i ...

Iop Publishing Ltd2015

Data-driven RANS for simulations of large wind farms

Giacomo Valerio Iungo, Francesco Viola

In the wind energy industry there is a growing need for real-time predictions of wind turbine wake flows in order to optimize power plant control and inhibit detrimental wake interactions. To this aim, a data-driven RANS approach is proposed in order to ac ...

Iop Publishing Ltd2015

Instability of wind turbine wakes immersed in the atmospheric boundary layer

Fernando Porté Agel, François Gallaire, Giacomo Valerio Iungo, Francesco Viola

In this work a technique capable to investigate the near-wake stability properties of a wind turbine immersed in the atmospheric boundary layer is presented. Specifically, a 2D local spatial stability analysis is developed in order to take into account typ ...

Iop Publishing Ltd2015

Prediction of the hub vortex instability in a wind turbine wake: stability analysis with eddy-viscosity models calibrated on wind tunnel data

Fernando Porté Agel, François Gallaire, Giacomo Valerio Iungo, Francesco Viola

The instability of the hub vortex observed in wind turbine wakes has recently been studied by Iungo et al. (J. Fluid Mech., vol. 737, 2013, pp. 499–526) via local stability analysis of the mean velocity field measured through wind tunnel experiments. This ...

Cambridge University Press2014

Linear stability analysis of wind turbine wakes performed on wind tunnel measurements

Fernando Porté Agel, François Gallaire, Giacomo Valerio Iungo, Francesco Viola

Wind tunnel measurements were performed for the wake produced by a three-bladed wind turbine immersed in uniform flow. These tests show the presence of a vorticity structure in the near-wake region mainly oriented along the streamwise direction, which is d ...

Cambridge University Press2013