arXiv subjects

Marc Fermigier

Publications and source records attributed to Marc Fermigier.

4 recordsLinked to original sources

arXiv2026

Windsurf-mimetic study about unsteady propulsion

We study experimentally a a three-dimensional reduced model of a sail shape performing pitching oscillations around a mean incidence angle ($\alpha_{m}$) with respect to an incoming flow in a hydrodynamic channel at a constant velocity where the Reynolds number based on the mean chord of the sail is Re$_{c} = \rho U_{\infty} c / \mu = 11900$. The problem is inspired by the "pumping" maneuver used by windsurf athletes. At the start of a race or in light winds, to get or keep the board in foiling mode, for example after a tack change, athletes use intermittent propulsion by "pumping" the sail, i.e. periodically changing the angle of incidence of the sail relative to the wind. The flapping or pitching parameters and position of the sail according to the flow (incidence angle) influence the aerodynamic forces acting on the sail by destabilising the flow and generating unsteady forces. We experimentally characterise the aerodynamic forces of the sail. We compare the sailing ($C_{drive}, \ C_{drift}$) and aerodynamic ($C_{drag}, \ C_{lift}$) coefficients between a static and an oscillating sail for different flapping parameters and different mean incidence angles of the sail and angles of attack of the boat. Thanks to the use of "pumping", we observe that it is possible to generate a drive force greater than the one generated without oscillation. Furthermore, "pumping" increases the range of mean incidence angle in which the drive force is positive. However, this increase inevitably comes with an increase in drift force, which is often detrimental. These data can be used to improve the Velocity Prediction Programme (VPP) associated with windsurfing and to help athletes optimise their "pumping".

physics.flu-dyn↗

arXiv2024

Propulsive performance of a windsurf-inspired pitching foil

We study experimentally a symmetrical rigid foil performing pitching oscillations around a mean incidence angle ($\alpha_{m}$) with respect to an incoming flow in a hydrodynamic channel at a constant velocity where the Reynolds number according to the chord of the foil is, $Re_{c} = \rho U_{\infty} c / \mu = 14400$. The problem is inspired from the pumping maneuver used by athletes on the new hydrofoil-based windsurf boards. The goal of the study is to quantify the forces on this configuration by varying the pitching kinematics characterized by the Strouhal number ($St_{A} = fA/U_{\infty}$), from 0 to 0.27, and the mean incidence angle $\alpha_{m}$, from 0 to 30$^{\circ}$, of the foil. The force measurements show a high lift production and the delay of the stall angle according to $St_A$ which can be linked to previous studies about the generation of vortices at the trailing edge. A general trend of decrease is observed for the drag force coefficient in pitching compare to the static case. For the highest Strouhal numbers tested, drag coefficient can become negative (thrust) in a range of $\alpha_{m}$ up to 15$^{\circ}$ in specific case. We present the various impacts of the amplitude of beating and the frequency of pitching on the aerodynamic forces for small mean incidence angle and high mean incidence angle (above the static stall angle). By using a sport-mimetic approach, we transform the measured lift $\&$ drag forces into a propulsive and drifting force. Doing so allows us to investigate race strategies. We investigate the generation of propulsion in upwind conditions.

physics.flu-dyn↗

arXiv2011

Collective beating of artificial microcilia

We combine technical, experimental and theoretical efforts to investigate the collective dynamics of artificial microcilia in a viscous fluid. We take advantage of soft-lithography and colloidal self-assembly to devise microcapets made of hundreds of slender magnetic rods. This novel experimental setup is used to investigate the dynamics of extended cilia arrays driven by a precessing magnetic field. Whereas the dynamics of an isolated cilium is a rigid body rotation, collective beating results in a symmetry breaking of the precession patterns. The trajectories of the cilia are anisotropic and experience a significant structural evolution as the actuation frequency increases. We present a minimal model to account for our experimental findings and demonstrate how the global geometry of the array imposes the shape of the trajectories via long range hydrodynamic interactions.

cond-mat.soft↗

arXiv2008

Rotational dynamics of a soft filament: wrapping transition and propulsive forces

We analyze experimentally the shape of a long elastic filament rotating in a viscous liquid. We identify a continuous but sharp transition from a straight to an helical shape, resulting from the competition between viscous stresses and elastic forces. This induced helicity generates a propulsive force along the axis of rotation. In addition, we show that the shape transition is associated with an unstable branch in the force-torque relation. A linearized model of the fluid-structure interaction is proposed to account for all the features of the non-linear filament dynamics.

physics.flu-dyn↗