arXiv subjects

Benjamin Thiria

Publications and source records attributed to Benjamin Thiria.

At least 19 recordsLinked to original sources

arXiv2026

Jet creation and wave energy dissipation by a submerged elastic plate

Experiments on a flexible plate horizontally submerged in a wave tank are reported in this paper. The plate is held at a fixed depth at its upstream end, while its downstream end is free to move. The wave field is measured using a synthetic Schlieren technique, giving access to an estimate of the fractions of the incoming wave energy that are transmitted and reflected after encountering the region where the plate is submerged. Part of the incoming wave energy is shown to be dissipated by the plate. Flow field measurements in a vertical plane parallel to the wave propagation are performed to examine the wave-plate interaction. A jet-like flow is produced at the free end of the plate, with strong vortex activity similar to that produced by a flapping fin. The kinetic energy present in this jet-like flow is computed from the velocity field measurements, and the external dissipation produced by jet creation is estimated. Results highlight the efficiency of flexible plates in reflecting and attenuating waves. This study also showcases a change in jet direction, in particular the observation of a jet going upward toward the free surface for certain frequencies. Such a configuration, could be of great interest for wave energy conversion applications, while preventing harmful phenomena such as seabed erosion.

physics.flu-dyn↗

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↗

arXiv2025

Aerodynamic consequences of wing damage in dragonflies

Flapping wings are the primary means by which dragonflies generate forces, but they are susceptible to damage due to their inherent fragility. The damage results in a reduction in wing area and a distortion of the original wing, which in turn leads to a decline in flight ability. Furthermore, the flows of dragonfly fore- and hindwings exhibit an interaction, thus damage to the forewing can also impact the aerodynamic performance of the ipsilateral hindwing. In this study, we examine this problem through CFD (computational fluid dynamics) simulations on a series of damaged dragonfly fore-/hindwing models according to the probability of area loss from the literature. The flow fields and aerodynamic forces for the different damaged wing cases are compared with those for the intact wings. This comparative analysis reveals how the different patterns of wing damage modify the vortex structures around the flapping wings and lead to a drop in aerodynamic force production. The causes behind the diminishing aerodynamic performance are shown to be subtler than the pure area loss and are regulated by the changes in the flow field that result from wing damage. Wing-wing interaction becomes particularly important when forewing damage occurs.

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↗

arXiv2024

Water wave interactions with a horizontal submerged elastic plate

This article explores how a submerged elastic plate, clamped at one edge, interacts with water waves. Submerged elastic plates have been considered as potentially effective design elements in the development of wave energy harvesters but their behavior in a wave field remains largely unexplored, especially experimentally. Positioned at a fixed depth in a wave tank, the flexible plate demonstrates significant wave reflection capabilities, a characteristic absent in rigid plates of identical dimensions. The experiments thus reveal that plate motion is crucial for wave reflection. Sufficiently steep waves are shown to induce a change in the mean position of the plate, with the trailing edge reaching the free surface in some cases. This configuration change is found to be particularly efficient to break water waves. These findings contribute to understanding the potential of elastic plates for wave energy harvesting and wave attenuation scenarios.

physics.flu-dyn↗

arXiv2024

Improving propulsive efficiency using bio-inspired intermittent locomotion

Many swimmers, especially small to medium-sized animals, use intermittent locomotion that differs from continuous swimming of large species. This type of locomotion, called burst-and-coast, is often associated with an energetic advantage. In this work, we investigate the intermittent locomotion inspired by fish locomotion but applied to a propeller. The energy consumption of burst-and-coast cycles is measured and compared to the continuous rotation regime. We show that a substantial drag ratio between the active and passive phases of the motion, as observed in fish, is critical for energy savings. Such a contrast can be obtained using a folding propeller that passively opens and closes as the propeller starts and stops rotating. For this reconfigurable propeller, intermittent propulsion is found to be energetically advantageous, saving up to 24% of the energy required to cruise at a given speed. Using an analytical model, we show that intermittent motion is more efficient than continuous motion when the drag reduction in the coast phase exceeds 65%. For fish-like locomotion, this threshold seems to be closer to 30%. A formal analogy allows us to explain the difference between propeller propulsion and fish locomotion.

physics.flu-dyn↗

arXiv2024

Confinement-driven state transition and bistability in schooling fish

We investigate the impact of confinement density (i.e the number of individuals in a group per unit area of available space) on transitions from polarized to milling state, using groups of rummy-nose tetra fish (\textit{Hemigrammus rhodostomus}) under controlled experimental conditions. We demonstrate a continuous state transition controlled by confinement density in a group of live animals. During this transition, the school exhibits a bistable state, wherein both polarization and milling states coexist, with the group randomly alternating between them. A simple two-state Markov process describes the observed transition remarkably well. The confinement density influences the statistics of this bistability, shaping the distribution of transition times between states. Our findings suggest that confinement plays a crucial role in state transitions for moving animal groups. More generally, they provide an experimental benchmark for active matter models of macroscopic, self-propelled, confined agents.

nlin.AO↗

arXiv2023

Thrust force is tuned by the rigidity distribution in insect-inspired flapping wings

We study the aerodynamics of a flapping flexible wing with a two-vein pattern that mimics the elastic response of insect wings in a simplified manner. The experiments reveal a non-monotonic variation of the thrust force produced by the wings when the angle between the two veins is varied. An optimal configuration is consistently reached when the two veins are spaced at an angle of about 20 degrees. This value is in the range of what has been measured in the literature for several insect species. The deformation of the wings is monitored during the experiment using video recordings, which allows to pinpoint the physical mechanism behind the non-monotonic behaviour of the force curve and the optimal distribution of the vein network in terms of propulsive force.

cond-mat.soft↗

arXiv2023

Illuminance-tuned collective motion in fish

We experimentally investigate the role of illumination on the collective dynamics of a large school (ca. 50 individuals) of Hemigrammus rhodostomus. The structure of the group, defined using two order parameters, is quantified while progressively altering the visual range of the fish through controlled cycles of ambient light intensity. We show that, at low light levels, the individuals within the group are unable to form a cohesive group, while at higher illuminance the degree of alignment of the school correlates with the light intensity. When increasing the illuminance, the school structure is successively characterized by a polarized state followed by a highly regular and stable rotational configuration (milling). Our study shows that vision is necessary to achieve cohesive collective motion for free swimming fish schools, while the short-range lateral line sensing is insufficient in this situation. The present experiment therefore provides new insights into the interaction mechanisms that govern the emergence and intensity of collective motion in biological systems.

nlin.AO↗

arXiv2021

Ocean wave transmission, reflection and absorption by rows of vertical structures along the coastline

Large arrays of wave-absorbing structures could serve the double objective of coastal protection against erosion and clean, renewable electrical power production. In this work, the principle of an artificial canopy is explored, which consists of vertical structures, arranged in rows parallel to the coastline. Sea waves, which propagate towards the shore, interact with the obstacle rows. A part of the wave energy is reflected back towards the ocean, another part is transmitted to the shoreline, while the rest of the energy is, in theory, available for energy production (although losses, due to viscous effects within the fluid, or imperfect efficiency of the power conversion mechanism, will unavoidably take place). First, a simple geometric representation of the reflection/transmission properties of individual, fixed rows is presented. In the case of moving rows, relationships are drawn between the internal stiffness and damping parameters of the devices, on the one hand, and their reflection, transmission and absorption characteristics, on the other hand. Array properties are then examined, depending on both individual row design parameters and row-to-row spacing values, using the wide-spacing approximation. A numerical case study illustrates the capabilities of the proposed modelling framework, with arrays of vertical, oscillating rectangular plates. The transmitted, reflected and absorbed wave spectra are examined, along with their dependencies on individual oscillator control tuning and array design parameters.

physics.ao-ph↗

arXiv2020

Material optimization of flexible blades for wind turbines

Bioinspired flexible blades have been recently shown to significantly improve the versatility of horizontal-axis wind turbines, by widening their working range and increasing their efficiency. The aerodynamic and centrifugal forces bend the blade along its chord, varying the pitch angle by means of non-consuming mechanisms. Here we introduce a general method based on a universal scaling, which finds the optimal soft materials for the blades to maximize the overall turbine efficiency or rotational power, for any required geometry of classical horizontal-axis turbines. The optimization problem, which depends on various parameters, such as the wind velocity, the rotation rate, the density, the rigidity and the geometry of the blade, is reduced to only two dimensionless parameters: the Cauchy number and the centrifugal number. The blade-element momentum theory is coupled to a torsion spring-based model for the blade deformation. Taking into account realistic incoming wind-velocity distributions in the North Sea and a large wind-turbine geometry, we found a significant increase of the total harvested power, up to +35%. In addition, the optimal soft material corresponding to the maximal efficiency over the entire working range for a given wind turbine geometry is, within the limits of small blade deformations, scale-independent. Thus experiments on small wind turbines are a possible way to determine the optimal soft materials for larger ones. These flexible blades are found to be between 5% and 20% lighter than the current rigid blades.

physics.app-ph↗

arXiv2017

Modelling of an actuated elastic swimmer

We study the force production dynamics of undulating elastic plates as a model for fish-like inertial swimmers. Using a beam model coupled with Lighthill's large-amplitude elongated-body theory, we explore different localised actuations at one extremity of the plate (heaving, pitching, and a combination of both) in order to quantify the reactive and resistive contributions to the thrust. The latter has the form of a quadratic drag in large Reynolds number swimmers and has recently been pointed out as a crucial element in the thrust force balance. We validate the output of a weakly nonlinear solution to the fluid--structure model using thrust force measurements from an experiment with flexible plates subjected to the three different actuation types. The model is subsequently used in a self-propelled configuration ---with a skin friction model that balances thrust to produce a constant cruising speed--- to map the reactive versus resistive thrust production in a parameter space defined by the aspect ratio and the actuation frequency. We show that this balance is modified as the frequency of excitation changes and the response of the elastic plate shifts between different resonant modes, the pure heaving case being the most sensitive to the modal response with drastic changes in the reactive/resistive contribution ratio along the frequency axis. We analyse also the role of the phase lag between the heaving and pitching components in the case of combined actuation, showing in particular a non-trivial effect on the propulsive efficiency.

physics.flu-dyn↗

arXiv2015

Resistive thrust production can be as crucial as added mass mechanisms for inertial undulatory swimmers

In this paper, we address a crucial point regarding the description of moderate to high Reynolds numbers aquatic swimmers. For decades, swimming animals have been classified in two different families of propulsive mechanisms based on the Reynolds number: the "resistive" swimmers, using local friction to produce the necessary thrust force for locomotion at low Reynolds number and the "reactive" swimmers, lying in the high Reynolds range, and using added mass acceleration (described by perfect fluid theory). However, inertial swimmers are also systems that dissipate energy, due to their finite size, therefore involving strong resistive contributions, even for high Reynolds numbers. Using a complete model for the hydrodynamic forces, involving both reactive and resistive contributions, we revisit here the physical mechanisms responsible for the thrust production of such swimmers. We show, for instance, that the resistive part of the force balance is as crucial as added mass effects in the modeling of the thrust force, especially for elongated species. The conclusions brought by this work may have significant contributions to the understanding of complex swimming mechanisms, especially for the future design of artificial swimmers.

physics.flu-dyn↗

arXiv2015

On the relation between linear stability analysis and mean flow properties in wakes

In recent studies on wake stability, it has been observed that a simple linear stability analysis applied to the mean flow instead of the basic flow, could give an accurate prediction of the global mode selected frequency, although these phenomena are strongly non-linear. In this letter, we study the transient regime between the stationary (so called basic state) and unstationary solutions of the wake of a circular cylinder at Re=150. We show that the shift of the global frequency as a function of time due to strong non-linear effects, can be interpreted by a continuous mean flow correction induced by the growth of the instability. We show that during this transient regime, the mean state as a function of time plays the role of an instantaneous basic state on which the global frequency can be determined linearly.

physics.flu-dyn↗

arXiv2014

Four-winged flapping flyer in forward flight

We study experimentally a four-winged flapping flyer with chord-wise flexible wings in a self-propelled setup. For a given physical configuration of the flyer (i.e. fixed distance between the forewing and hindwing pairs and fixed wing flexibility), we explore the kinematic parameter space constituted by the flapping frequency and the forewing-hindwing phase lag. Cruising speed and consumed electric power measurements are performed for each point in the $(f,φ)$ parameter space and allow us to discuss the problem of performance and efficiency in four-winged flapping flight. We show that different phase-lags are needed for the system to be optimised for fastest flight or lowest energy consumption. A conjecture of the underlying mechanism is proposed in terms of the coupled dynamics of the forewing-hindwing phase lag and the deformation kinematics of the flexible wings.

physics.flu-dyn↗

arXiv2014

Large-amplitude undulatory swimming near a wall

The propulsive dynamics of a flexible undulating foil in a self-propelled swimming configuration near a wall is studied experimentally. Measurements of the swimming speed and the propulsive force are presented, together with image acquisition of the kinematics of the foil and particle image velocimetry (PIV) in its wake. The presence of the wall enhances the cruising velocity in some cases up to 25% and the thrust by a 45%, for swept angles of 160 and 240 degrees. The physical mechanisms underlying this effect are discussed by studying the vorticity dynamics in the wake of the foil. Proper Orthogonal Decomposition (POD) is applied to the PIV measurements in order to analyse the kinetic energy modal distribution in the flow and to relate it to the propulsion generated by the foil.

physics.flu-dyn↗

arXiv2014

The mechanical response of a creased sheet

We investigate the mechanics of thin sheets decorated by non-interacting creases. The system considered here consists in parallel folds connected by elastic panels. We show that the mechanical response of the creased structure is twofold, depending both on the bending deformation of the panels and the hinge-like intrinsic response of the crease. We show that a characteristic length scale, defined by the ratio of bending to hinge energies, governs whether the structure's response consists in angle opening or panel bending when a small load is applied. The existence of this length scale is a building block for future works on origami mechanics

cond-mat.mtrl-sci↗

arXiv2013

Elastic swimmer on a free surface

We present in this fluid dynamics video a novel experimental setup with self-propelled swimmers on a free surface. The swimmers, modeled as flexible thin filaments, are subjected to external electromagnetic forcing driving a propagating elastic wave that gives rise to self- propulsion. The fluid-structure interaction problem of these passive anguilliform swimmers is analyzed in: S. Ramananarivo, R. Godoy-Diana, and B. Thiria. Passive elastic mechanism to mimic fish-muscles action in anguilliform swimming. J. R. Soc. Interface, 10, 20130667 (2013). DOI:10.1098/rsif.2013.0667

physics.flu-dyn↗