Category: Publications

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Turning without fins: Quantifying the distinct kinematics and vortex dynamics of maneuvering swimming snakes

E. Gregorio, R. Godoy-Diana, A. Herrel
Physical Review E, 114, 024410 (2026)
DOI: 10.1103/c57h-kx57
[PDF file]

The agile turning maneuvers of anguilliform swimmers remain one of the least studied aspects of aquatic locomotion, despite being crucial for survival in underwater environments. To change direction, animals must find a balance between minimizing their moment of inertia and maximizing torque. Anguilliform swimmers, like snakes, are highly maneuverable and accomplish this goal without the aid of limbs or pectoral fins. In this study, we quantify the differences between turning and forward swimming kinematics and hydrodynamics. We present flow field and body deformation measurements taken simultaneously by combining a volumetric three- component defocusing digital particle-tracking velocimetry system with a camera mounted above the tank for swimming kinematics. We determine how snakes manipulate their body position to change swimming direction by analyzing the swimming kinematics of six turns by six individuals as 2D projections (five Natrix maura and one Nerodia rhombifer). Analyses of the body kinematics show that snakes curl their body inwards to shift their center of mass and reduce their moment of inertia to perform faster turns.

Turning without fins (Gregorio e al. Phys. Rev. E 2026) Superimposed swimming kinematics for turns no. 1–6 with 0.071 s between each body position, as demonstrated by the color bar (turns no. 1 and 3–6 Natrix maura; turn no. 2 Nerodia rhombifer)

The volumetric wake measurements allow us to quantify the timing between the turning kinematics and vortex shedding and to compare it with forward swimming measurements. Our results indicate that snakes can generate substantially stronger vortices when turning; for example, the vortex force can be as much as 2.5–8.6× larger when compared to forward swimming. We also use volumetric wake measurements to show that the vortices shed during a turn have the same tubular vortex pair composition as forward swimming. These results help explain how elongated anguilliform swimmers change swimming direction. They widen our understanding of how animals use transient maneuvers to navigate aquatic environments, with implications for both natural swimmers and underwater robots.

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Waviness and self-sustained turbulence in plane Couette-Poiseuille flow

M. Etchevest, P. Dmitruk, S. Karmakar, B. Semin, R. Godoy-Diana, J. E. Wesfreid
Physical Review Fluids, 11, 044603 (2026).
DOI: 10.1103/xcbx-h1kc
[PDF file]

Our previous experimental work on Couette-Poiseuille flow (Liu et al. Phys. Rev. Fluids 2025) measured how the the waviness of streamwise streaks is correlated with the intensity of the streamwise vorticity (the rolls). Here, in collaboration with colleagues from Universidad de Buenos Aires and IRL IFADyFE, we use direct numerical simulations near the transition to turbulence to investigate that nonlinear relationship between streak waviness and rolls. This relationship is a key step in Waleffe’s model for a self-sustaining process. Simulations were conducted for Reynolds numbers ranging from 500 to 940, and a range of initial perturbation amplitudes was used. In these simulations, the streaks, rolls, and streak waviness initially grow. The optimal time for this growth closely matches the linear transient growth period for small perturbations but is much shorter when the initial perturbations are large and highly nonlinear. For higher Reynolds numbers and large initial perturbations, the velocity field reaches a turbulent steady state, while in the remaining cases, the flow relaminarizes. The main result is that the waviness of the streaks is a quadratic function of the rolls, provided that the roll amplitude is sufficiently large.

(a) and (b) Schematic representation of a typical Couette-Poiseuille laminar flow considered. (c) Representative three-dimensional (3D) snapshots of the streamwise velocity field Ux (blue is low speed, red is high speed) for three cases, a decaying laminar flow at Re = 625, a transitioning flow at Re = 806, and a fully turbulent case at Re = 940, from top to bottom, respectively. Rendering using the software VAPOR.
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Aeroelasticity of insect-inspired flapping wings with anisotropic rigidity

C. García-Baena, R. Antier, C. Gutiérrez-Montes, J. I. Jiménez-González, B. Thiria, R. Godoy-Diana. Journal of Bionic Engineering 23, 1464–1475 (2026).
doi: 10.1007/s42235-026-00882-1

In this collaborative work with colleagues from Universidad de Jaén, we studied the aerodynamics of a flexible wing with a two-vein pattern that mimics the structural anisotropy and elastic response of insect wings in a simplified manner. Previous work (Antier et al. J. Fluids Struct. 2024) had revealed a non-monotonic variation in the thrust force produced by the wings as the angle between the two veins varied. An optimal configuration in terms of mean thrust force is systematically achieved when the two veins are spaced at an angle of approximately 20 degrees. This value falls within the range measured in the literature for several insect species. Through simultaneous measurements of 3D deformation and aerodynamic forces (see figure below), we demonstrated that the stiffness distribution, controlled by the angle between the two main veins, allows for passive control of the kinematic phase shift between the leading and trailing edges, maximizing thrust at this 15–20 degree angle.

A: Diagram of the experimental setup: model wings with the vein skeleton marked in black, one vein along the leading edge and the other diagonally with an inter-vein angle β, indicated in blue; the dot pattern used for 3D tracking is shown on the right wing. Three high-speed cameras are used for tracking. B: Front view of the system mounted on the force sensor, where the flapping amplitude ψ₀ is indicated in blue. C: Several superimposed snapshots illustrating the wing flapping motion. D: 3D deformation of the wing at 8 different positions of interest in the flapping cycle (flapping frequency 15 Hz): minimum thrust between upstroke and downstroke (A), minimum lift (B), maximum downward thrust (C), zero downward lift (D), minimum upward thrust (E), maximum lift (F), maximum upward thrust (G), and zero upward lift (H), for inter-vein angles (a) β=10°, (b) β=20°, and (c) β=45°. The color bar indicates the dimensionless displacement of the points in the Z direction perpendicular to the ground, which is zero when the wing is parallel to the ground.

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Aerodynamic consequences of wing damage in dragonfly flapping flight


P. Yu, R. Godoy-Diana, B. Thiria, D. Kolomenskiy, T. Engels
Journal of the Royal Society Interface 23: 20250659 (2026).
doi: 10.1098/rsif.2025.0659

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 forewings 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 computational fluid dynamics simulations on a series of damaged dragonfly forewing/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.

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Propulsive performance of a windsurf-inspired pitching foil

G. Bertrand, T. Aurégan, B. Thiria, R. Godoy-Diana, M. Fermigier
Physical Review Fluids 10, 074401 (2025) * Editor’s suggestion
doi: 10.1103/PhysRevFluids.10.074401
preprint: arXiv:2412.12878

The competitive practice of sailing and windsurfing has seen a recent revolution with the introduction of new appendixes of hydrofoils that generate lift and keep the board or the boat out of the water for a sufficiently high sailing speed (∼3 m/s) . This allows one to increase the speed significantly because the wave drag and the hydrodynamic drag are almost suppressed. These innovations include the new iQFOil class introduced for the 2024 Olympic Games. During race starts or in low wind conditions, particularly after maneuvers like tacking which consist of turning the bow toward and through the wind to go upwind, windsurf athletes employ a technique called pumping to initiate or maintain foiling. This involves rhythmically adjusting the sail’s angle relative to the wind through a movement where the athlete moves their center of mass up and down, causing the sail to oscillate and provide intermittent propulsion to keep the board above water (see figure). We present an experimental study with a symmetrical shape of sail where a pitching movement is applied for various frequencies and amplitudes in a wide range of mean incidence angles to mimic the behavior of a sail under real sailing conditions, especially upwind conditions. This study focuses on the coupled effect of the kinematic parameters of pumping (flapping frequency and amplitude) and the mean incidence angle between the mean chord of the sail and the direction of flow, in a context of application to competitive sailing.

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D-shaped body wake control through flexible filaments

J. C. Muñoz-Hervás, B. Semin, M. Lorite-Díez, G. J. Michon, Juan D’Adamo, J.I. Jiménez-González, R. Godoy-Diana
Physical Review Fluids 10, 063903 (2025)
doi: 10.1103/8jd7-j82q
preprint: arXiv:2411.08556

The wake flow behind blunt, D-shaped objects is an archetype to study recirculation bubbles and velocity deficits that increase drag. This has prompted interest in passive flow control methods using flexible appendages. We investigated how adding series of rigid and flexible plate-like filaments behind a D-shaped body in a water channel could modify wake dynamics and reduce drag. Using Particle Image Velocimetry to measure wake flow patterns and high-speed imaging to track filament motion, we tested various filament stiffnesses and positions across different flow velocities. The flexible filaments demonstrated passive reconfiguration in response to the flow, with tip deflection angles reaching up to 9° and vibration amplitudes of around 4° at higher flow velocities, which corresponded to reduced recirculation bubbles and decreased velocity deficits in the wake compared to cases with no filaments or rigid filaments. Interestingly, pre-curved rigid filaments matched the performance of flexible ones, suggesting that the bending motion rather than the vibration of flexible filaments drives the flow improvement, and highlighting the potential for designing effective passive flow control devices using flexible appendages in the wakes of blunt bodies.

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Water wave interactions with a submerged elastic plate

Experiments on water wave interactions with a horizontal submerged elastic plate
G. Polly, A. Mérigaud, B. Thiria, R. Godoy-Diana
Journal of Fluid Mechanics 1007, R4 (2025)
doi: 10.1017/jfm.2025.90

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. We discuss in this paper how a submerged elastic plate, clamped at one edge, interacts with water waves. 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.

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Subtle frequency matching reveals resonant phenomenon in the flight of Odonata

C. Aracheloff, R. Garrouste, A. Nel, R. Godoy-Diana, B. Thiria
Journal of the Royal Society Interface 21, 20240401 (2024).
doi: 10.1098/rsif.2024.0401

We investigated the connection between the flight flapping frequency and the intrinsic wing properties in Odonata (dragonflies and damselflies). For such large flying insect species, it has been noted that the wingbeat frequency is significantly lower than the structural resonance of the wing itself. However, the structural resonance mechanism is often evoked in the literature for flying and swimming animals as a means to increase locomotion performance. Here, we show that the flight of Odonata is based on a nonlinear mechanism that strongly depends on the wingbeat amplitude. For large flapping amplitudes (as observed in natural flight), the resonant frequency of the wings decreases with respect to its value at low amplitudes to eventually match the wingbeat frequency used in flight. By means of this nonlinear resonance, Odonata keep a strong wing stiffness while benefiting from a passive energy-saving mechanism based on the dynamic softening of the wing.

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Can All Snakes Swim?

A review of the evidence and testing species across phylogeny and morphological diversity
G. Fosseries, A. Herrel, R. Godoy-Diana, P. Gaucher, M. Traimond, A. Joris, K. Daoues, A. Gouygou, O. Chateau, H. Gossuin, P. Banzept, C. Banzept, D. Lefebvre, & X. Bonnet
Zoology 126223 (2024).
doi: 10.1016/j.zool.2024.126223

Whether the ancestors of snakes were burrowing, land-dwelling, or marine creatures has led to questions about whether all modern snakes can swim or if this ability varies among different snake groups. As part of Guillaume Fosseries‘ PhD thesis and of the ANR Dragon2 project, led by Xavier Bonnet at the Centre d’Études Biologiques de Chizé (CEBC UMR7372, CNRS, La Rochelle University), we conducted a comprehensive systematic review following PRISMA guidelines, incorporating published literature, personal communications, and website information to examine swimming abilities across snake species. This review found swimming information for only about 11% of all known snake species, with most data coming from aquatic species and very little from burrowing or early-evolved snake groups. To address these significant phylogenetic gaps in our knowledge, we directly tested the swimming capacity of 103 diverse snake species plus 13 other limbless and limbed reptiles. Every single species we examined demonstrated the ability to swim, confirming that this universal swimming ability appears to be a shared trait among many land vertebrates that move by undulating their bodies from side to side. These results suggest that swimming capacity alone cannot help determine the evolutionary origins of snakes, and that future research should focus on measuring swimming performance, efficiency, and technique rather than simply whether snakes can swim at all.

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Confinement-driven state transition and bistability in schooling fish

B. Lafoux, P. Bernard, B. Thiria, R. Godoy-Diana
Physical Review E, 110(3), 034613 (2024).
doi: 10.1103/PhysRevE.110.034613
arXiv preprint: https://arxiv.org/abs/2401.01850

In this work we have quantified how fish swimming in groups change their behavior based on how crowded they are. We found that fish switch between two main swimming patterns: moving in the same direction or circling like a whirlpool. The amount of space available influences how long the fish stick to each pattern and how often they switch. This research not only helps us understand how fish and other animals behave in groups, but also provides valuable real-world data that can help tuning computer models of group behavior. The findings highlight the importance of considering space limitations when studying how animals move together, which could lead to better understanding of complex group behaviors in nature.

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Form and function of anguilliform swimming: a review

V. Stin, R. Godoy-Diana, X. Bonnet, & A. Herrel
Biological Reviews (2024)
doi: 10.1111/brv.13116

Anguilliform swimmers are long and narrow animals that propel themselves by undulating their bodies. Observations in nature and recent investigations suggest that anguilliform swimming is highly efficient. However, understanding the underlying reasons for the efficiency of this type of locomotion requires interdisciplinary studies spanning from biology to hydrodynamics. Regrettably, these different fields are rarely discussed together, which hinders our ability to understand the repeated evolution of this swimming mode in vertebrates. This review compiles the current knowledge of the anatomical features that drive anguilliform swimming, compares the resulting kinematics across a wide range of anguilliform swimmers, and describes the resulting hydrodynamic interactions using data from both in vivo experiments and computational studies.

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Drag-induced dissipation in wave-structure interaction

In the interaction of water waves with marine structures, the interplay between wave diffraction and drag-induced dissipation is seldom, if ever, considered. In particular, linear hydrodynamic models, and extensions thereof through the addition of a quadratic force term, do not represent the change in amplitude of the waves diffracted and radiated to the far field, which should result from local energy dissipation in the vicinity of the structure. In this work, a series of wave flume experiments is carried out, whereby waves of increasing amplitude impinge upon a vertical barrier, extending partway through the flume width. As the wave amplitude increases, the effect of drag – which is known to increase quadratically with the flow velocity – is enhanced, thus allowing the examination of the far-field effect of drag-induced dissipation, in terms of wave reflection and transmission. A potential flow model is proposed, with a simple quadratic pressure drop condition through a virtual porous surface, located on the sides of the barrier (where dissipation occurs). Experimental results confirm that drag-induced dissipation has a marked effect on the diffracted flow, i.e. on wave reflection and transmission, which is appropriately captured in the proposed model. Conversely, when diffraction becomes dominant as the barrier width becomes comparable to the incoming wavelength, the diffracted flow must be accounted for in predicting drag-induced forces and dissipation.

Modelling the far-field effect of drag-induced dissipation in wave-structure interaction: A numerical and experimental study
A. Mérigaud, B. Thiria, R. Godoy-Diana, G. Perret
Journal of Fluid Mechanics, 987, A24 (2024).
doi: 10.1017/jfm.2024.298

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Lift-up and streak waviness drive the self-sustained process in wall-bounded transition to turbulence

Flow field measurements from a Couette-Poiseuille experiment are used to examine quantitatively certain steps of the self-sustained process (SSP) of wall-bounded transition to turbulence. Although the different parts of the SSP have been discussed at large in the literature, direct measurements from experiment are scarce and the present results show, using a local analysis of the turbulent patterns, that (1) the amplitude of streamwise rolls is related to streak waviness, bringing a quantitative picture to one of the main physical mechanisms of Waleffe’s model of SSP, and (2), at low waviness, direct measurements of the correlation between the streak and roll amplitudes, respectively probed by the streamwise and wall-normal velocity perturbations, quantify the lift-up effect. This analysis method of the SSP does not rely on the specificity of Couette-Poiseuille flows and can be used to investigate this mechanism in other flows.

T. Liu, B. Semin, R. Godoy-Diana, J. E. Wesfreid
Physical Review Fluids, 9, 033901 (2024).
doi: 10.1103/PhysRevFluids.9.033901

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

R. Antier, B. Thiria, & R. Godoy-Diana
Journal of Fluids and Structures, 124, 104043 (2024).
doi: 10.1016/j.jfluidstructs.2023.104043

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.

2-wing flapping system mounted on a force sensor
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Far-field hydrodynamic interaction in a group of swimmers

Three-dimensional schools of hydrodynamically axisymmetric swimmers self-propelling at a constant velocity are studied. We introduce a low-order model for the induced velocity based on the far-field approximation. We inquire if, by holding suitable relative positions in the three-dimensional space, the swimmers can reduce the overall energy consumption of the school in comparison with the same number of isolated individuals at the same velocity. We find a considerable (several per cent) energy saving achievable by chain formations. The benefit increases asymptotically with the number of individuals, towards a finite limit that is a function of the minimum allowed spacing between each pair of neighbours.

G. Li, L. Duan, J. Sesterhenn, R. Godoy-Diana, B. Thiria, & D. Kolomenskiy
Journal of Fluid Mechanics, 974, A34 (2023).
doi: 10.1017/jfm.2023.802

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Volumetric velocimetry of the wake of a swimming snake

We describe a method for measuring the 3D vortical structures produced by an anguilliform swimmer using volumetric velocimetry. The wake of freely swimming dice snakes (Natrix tessellata) was quantified, revealing the creation of multiple vortices along the body of the snake due to its undulation. The 3D structure of the vortices generally consisted of paired vortex tubes, some of which were linked together to form a hairpin structure. The observations match predictions from computational fluid dynamic studies of other anguilliform swimmers. Quantitative measurements allowed us to study vortex circulation and size, and global kinetic energy of the flow, which varied with swimming speed, vortex topology and individual characteristics. Our findings provide a baseline for comparing wake structures of snakes with different morphologies and ecologies and investigating the energetic efficiency of anguilliform swimming.


V. Stin, R. Godoy-Diana, X. Bonnet, & A. Herrel. Measuring the 3D wake of swimming snakes (Natrix tessellata) using volumetric velocimetry.
Journal of Experimental Biology, 226, jeb245929 (2023)
doi: 10.1242/jeb.245929

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Illuminance-tuned collective motion in fish

B. Lafoux, J. Moscatelli, R. Godoy-Diana & B. Thiria
Communications Biology 6, Article number: 585 (2023)
doi: 10.1038/s42003-023-04861-8

We experimentally investigate the role of illumination on the collective dynamics of a large school (ca. 50 individuals) of Hemigrammus rhodostomus.

Experimental setup: shallow swimming arena with illuminance control and infrared video recording

The structure of the group, defined using two order parameters (milling annd polarization), 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. Watch a full experiment here:

Full paper here:
Lafoux, B., Moscatelli, J., Godoy-Diana, R., Thiria, B. Illuminance-tuned collective motion in fish. Commun Biol 6, 585 (2023).
https://doi.org/10.1038/s42003-023-04861-8

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Geometrical framework for hydrodynamics and control of wave energy converters

A. Mérigaud, B. Thiria, & R. Godoy-Diana
PRX Energy 2, 023003 (2023)
doi: 10.1103/PRXEnergy.2.023003

Approaches for converting the energy of sea-surface waves into useful power remain on the fringe of renewable energy solutions despite years of research. Among the hundreds of proposed wave-energy converters, no single wave-energy device provides sufficient power to be an attractive practical solution. Therefore, considering an integrated farm of many devices is an intrinsic part of the problem. Models need to consider the dynamics of many devices simultaneously, and the devices’ interaction with each other and the environment introduces additional complexity. In this work, we propose a conceptually simple analysis framework that uses geometry in the complex plane to understand the basic relations underlying wave-energy conversion in a model farm. Under this framework, a single complex variable encapsulates wave reflection, transmission, and absorption by a row of wave-energy converters and provides guidance for achieving maximal power output under impedance-matching conditions.

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Special issue: Bioinspired fluid-structure interaction

https://iopscience.iop.org/journal/1748-3190/page/Bioinspired-Fluid-Structure-Interaction

Fluid-structure interaction (FSI) studies the interaction between fluid and solid objects. It helps understand how fluid motion affects solid objects and vice versa. FSI research is important in engineering applications such as aerodynamics, hydrodynamics, and structural analysis. It has been used to design efficient systems such as ships, aircraft, and buildings. FSI in biological systems has gained interest in recent years for understanding how organisms interact with their fluidic environment. Our special issue features papers on various biological and bio-inspired FSI problems.

S. Jung & R. Godoy-Diana
Bioinspiration & biomimetics 18, 030401 (2023)
Editorial article: 10.1088/1748-3190/acc778

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Intermittent vs. continuous swimming: An optimization tale

G. Li, D. Kolomenskiy, H. Liu, R. Godoy-Diana & B. Thiria
Physical Review Fluids 8, 013101 (2023)
doi: 10.1103/PhysRevFluids.8.013101
see also in Physics Magazine: Why Fish Swim Intermittently

Intermittent swimming, also termed “burst-and-coast swimming,” has been reported as a strategy for fish to enhance their energetical efficiency. Intermittent swimming involves additional control parameters, which complexifies its understanding by means of quantitative and parametrical analysis, in comparison with continuous swimming. In this study, we used a hybrid computational fluid dynamic (CFD) model to assess the swimming performance in intermittent swimming parametrically and quantitatively. A Navier-Stokes solver is applied to construct a database in the multidimensional space of the control parameters to connect the undulation kinematics to swimming performance. Based on the database, an indirect numerical approach named “gait assembly” is used to generate arbitrary burst-and-coast gaits to explore the parameter space. Our simulations directly measured the hydrodynamics and energetics under the unsteady added-mass effect during burst-and-coast swimming. The results suggest that the instantaneous power of burst is basically determined by undulatory kinematics. The results show that the energetical performance of burst-and-coast swimming can be better than that of continuous swimming, but also that an unoptimized burst-and-coast gait may become very energetically expensive. These results shed light on the mechanisms at play in intermittent swimming, enabling us to better understand fish behavior and to propose design guidelines for fishlike robots.

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Divergence of climbing escape flight performance in Morpho butterflies living in different microhabitats

C. Le Roy, N. J. Silva, R. Godoy-Diana, V. Debat, V. Llaurens, and F. T. Muijres
Journal of Experimental Biology 225 (15), jeb243867 (2022)
doi: 10.1242/jeb.243867

Morpho butterflies are iconic insects of the Amazonian rainforest. Some Morpho species live in the dense vegetation of the understory (e.g. Morpho amathonte, cover photo) while other species inhabit the open canopy. We investigated how the divergent microhabitat specialization influences the evolution of flight performance. Quantification of climbing flight kinematics among closely related butterfly species living in different strata revealed markedly higher climbing ability in canopy species, probably resulting from divergent flight behaviour and morphology. Photo credit: Vincent Debat.

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Wake and aeroelasticity of a flexible pitching foil

The wake of a flapping foil is the basic model representing the propulsive mechanism of swimming and flying animals that use wings, fins, and body oscillations to drive their locomotion. The reverse Bénard-von Kármán vortex street is one of the landmark features of such wakes, since it is associated to the onset of thrust generation. The vortex shedding frequency is clocked by the flapping motion but, in a realistic model the force production dynamics is intimately linked to the elastic response of the flapping structure and the resonance between the different frequencies involved has been invoked in the literature to explain efficient flapping regimes. Here we show, using a wind tunnel experiment and hydrodynamic stability analysis, that thrust peaks occur when the wake resonant frequency is tuned with the foil elastic dynamics.

J. D’Adamo, M. Collaud, R. Sosa & R. Godoy-Diana
Bioinspiration & Biomimetics 17, 045002 (2022)
doi: 10.1088/1748-3190/ac6d96
[pdf file]

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How lateral-line sensing can enable swimming fish to judge their relative position and kinematic synchronization with a neighbor

Flow field computed around a pair of swimming fish. The pressure and shear-stress signals along the sides of the protagonist fish are monitored while changing the relative position and tail-beat phase lag of the companion fish.

For fish, swimming in group may be favorable to individuals. Several works reported that in a fish school, individuals sense and adjust their relative position to prevent collisions and maintain the group formation. Also, from a hydrodynamic perspective, relative-position and kinematic synchronisation between adjacent fish may considerably influence their swimming performance. Fish may sense the relative-position and tail-beat phase difference with their neighbors using both vision and the lateral-line system, however, when swimming in dark or turbid environments, visual information may become unavailable. To understand how lateral-line sensing can enable fish to judge the relative-position and phase-difference with their neighbors, in this study, based on a verified three-dimensional computational fluid dynamics approach, we simulated two fish swimming adjacently with various configurations. The lateral-line signal was obtained by sampling the surface hydrodynamic stress. The sensed signal was processed by Fast Fourier Transform (FFT), which is robust to turbulence and environmental flow. By examining the lateral-line pressure and shear-stress signals in the frequency domain, various states of the neighboring fish were parametrically identified. Our results reveal that the FFT-processed lateral-line signals in one fish may potentially reflect the relative-position, phase-differences, and the tail-beat frequency of its neighbor. Our results shed light on the fluid dynamical aspects of the lateral-line sensing mechanism used by fish. Furthermore, the presented approach based on FFT is especially suitable for applications in bioinspired swimming robotics. We provide suggestions for the design of artificial systems consisting of multiple stress sensors for robotic fish to improve their performance in collective operation.

Hydrodynamical Fingerprint of a Neighbour in a Fish Lateral Line
G. Li, D. Kolomenskiy, H. Liu, B. Thiria & R. Godoy-Diana Frontiers in Robotics and AI: Bio-Inspired Robotics section 9, 825889 (2022)
doi: 10.3389/frobt.2022.825889

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BIOMIM @ EWTEC 2021

Two papers from the team were presented at the 11th European Wave and Tidal Energy Conference in Plymouth, UK!

Ocean wave transmission, reflection and absorption by rows of vertical structures along the coastline
A. Mérigaud, B. Thiria & R. Godoy-Diana
In Proceedings of the 11th European Wave and Tidal Energy Conference, 5-9th Sept 2021, Plymouth, UK.
preprint: arXiv:2111.14816

On the interaction of surface water waves and fully-submerged elastic plates
G. Polly, A. Mérigaud, R. Alhage, B. Thiria & R. Godoy-Diana
In Proceedings of the 11th European Wave and Tidal Energy Conference, 5-9th Sept 2021, Plymouth, UK.
preprint: arXiv:2111.03018

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Revisiting the transmission and reflection of water waves over an array of obstacles

With a view to numerical modelling and optimisation of wave energy farms, a simple recursive formulation is employed to solve for the reflection and transmission of plane water waves by a number of rows of vertical obstacles, under the wide-spacing approximation. The proposed recursive formulation relies on the ‘concatenation’ of any two sets of obstacles, for which the reflection–transmission problem is already resolved. Furthermore, the obstacles are allowed to move in any combination of pitch and surge. The proposed recursive model is validated by means of physical experiments in a small-scale wave flume, whereby waves are reflected and transmitted by one, two and three rows of vertical, flexible blades, taking into account dissipation within the fluid along the wave propagation direction. For the special case of identical, regularly spaced rows, under the adopted formalism, distinct theoretical behaviours are highlighted, depending on whether or not individual obstacles absorb (or dissipate) energy as they interact with incoming waves. In a ‘non-dissipative’ case, the well known fact that discrete values of the row-to-row distance 𝐿 completely cancel reflection is retrieved, as well as the existence of ‘band-gap’ intervals, i.e. intervals for 𝐿 where reflection is high, with maximum reflection occurring away from the Bragg condition. In contrast, when the obstacles dissipate or absorb energy as they interact with the fluid, reflection is always non-zero, and, as the number of rows tends to infinity, forms marked Bragg peaks, reaching unity when 𝐿 is a multiple of half a wavelength.

A wide-spacing approximation model for the reflection and transmission of water waves over an array of vertical obstacles
A. Mérigaud, B. Thiria & R. Godoy-Diana
Journal of Fluid Mechanics 923, A2 (2021)
doi: 10.1017/jfm.2021.532

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Decay of streaks and rolls in plane Couette–Poiseuille flow

We report the results of an experimental investigation into the decay of turbulence in plane Couette–Poiseuille flow using ‘quench’ experiments where the flow laminarises after a sudden reduction in Reynolds number 𝑅𝑒. Specifically, we study the velocity field in the streamwise–spanwise plane. We show that the spanwise velocity containing rolls decays faster than the streamwise velocity, which displays elongated regions of higher or lower velocity called streaks. At final Reynolds numbers above 425, the decay of streaks displays two stages: first a slow decay when rolls are present and secondly a more rapid decay of streaks alone. The difference in behaviour results from the regeneration of streaks by rolls, called the lift-up effect. We define the turbulent fraction as the portion of the flow containing turbulence and this is estimated by thresholding the spanwise velocity component. It decreases linearly with time in the whole range of final 𝑅𝑒. The corresponding decay slope increases linearly with final 𝑅𝑒. The extrapolated value at which this decay slope vanishes is 𝑅𝑒𝑎𝑧≈656±10, close to 𝑅𝑒𝑔≈670 at which turbulence is self-sustained. The decay of the energy computed from the spanwise velocity component is found to be exponential. The corresponding decay rate increases linearly with 𝑅𝑒, with an extrapolated vanishing value at 𝑅𝑒𝐴𝑧≈688±10. This value is also close to the value at which the turbulence is self-sustained, showing that valuable information on the transition can be obtained over a wide range of 𝑅𝑒.

Decay of streaks and rolls in plane Couette-Poiseuille flow
T. Liu, B. Semin, L. Klotz, R. Godoy-Diana, J. E. Wesfreid & T. Mullin
Journal of Fluid Mechanics 915, A65 (2021)
doi:10.1017/jfm.2021.89

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Experiments & numerical modelling of burst-and-coast fish swimming

Body and caudal fin undulations are a widespread locomotion strategy in fish, and their swimming kinematics is usually described by a characteristic frequency and amplitude of the tail-beat oscillation. In some cases, fish use intermittent gaits, where a single frequency is not enough to fully describe their kinematics. Energy efficiency arguments have been invoked in the literature to explain this so-called burst-and-coast regime but well controlled experimental data are scarce. Here we report on an experiment with burst-and-coast swimmers and a numerical model based on the observations to show that: (1) fish modulate a unique intrinsic cycle to sustain the demanded speed by modifying the bursting to coasting ratio while maintaining the duration of the cycle nearly constant; and (2) the chosen kinematics correspond to energy-saving gaits over the range of swimming speeds tested.

Burst-and-coast swimmers optimize gait by adapting unique intrinsic cycle
G. Li, I. Ashraf, B. François, D. Kolomenskiy, F. Lechenault, R. Godoy-Diana & B. Thiria
Communications Biology 4, 40 (2021)
[doi:10.1038/s42003-020-01521-z]
preprint: arXiv:2002.09176
Behind the paper blog post: https://ecoevocommunity.nature.com/posts/on-the-intermittent-tail-beat-kinematics-in-steady-swimming-fish


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Synchronized Side-by-Side Swimmers

On the fluid dynamical effects of synchronization in side-by-side swimmers
R. Godoy-Diana, J. Vacher, V. Raspa & B. Thiria
Biomimetics 4(4), 77 (2019)
https://doi.org/10.3390/biomimetics4040077
(in the Special issue “Fluid Dynamic Interactions in Biological and Bioinspired Propulsion”, Editors K. W. Moored and G. V. Lauder)

https://vimeo.com/378101069

We examined experimentally the in-phase and anti-phase synchronized swimming of two self-propelled independent flexible foils swimming side-by-side in a water tank. The foils are actuated by pitching oscillations at one extremity—the head of the swimmers—and the flow engendered by their undulations is analyzed using two-dimensional particle image velocimetry in their frontal symmetry plane. Following recent observations on the behavior of real fish, we focus on the comparison between in-phase and anti-phase actuation by fixing all other geometric and kinematic parameters. We show that swimming with a neighbor is beneficial for both synchronizations tested, as compared to swimming alone, with an advantage for the anti-phase synchronization. We show that the advantage of anti-phase synchronization in terms of swimming performance for the two-foil “school” results from the emergence of a periodic coherent jet between the two swimmers.

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On the energetics and stability of a minimal fish school

On the energetics and stability of a minimal fish school
G. Li, D. Kolomenskiy, H. Liu, B. Thiria & R. Godoy-Diana
PLoS ONE 14(8): e0215265 (2019)
https://doi.org/10.1371/journal.pone.0215265

See also:
On the interference of vorticity and pressure fields of a minimal fish school
G. Li, D. Kolomenskiy, H. Liu, B. Thiria & R. Godoy-Diana
Journal of Aero Aqua Bio-mechanisms 8 (1), 27-33 (2019)
[doi:10.5226/jabmech.8.27]

The physical basis for fish schooling is examined using three-dimensional numerical simulations of a pair of swimming fish, with kinematics and geometry obtained from experimental data. Energy expenditure and efficiency are evaluated using a cost of transport function, while the effect of schooling on the stability of each swimmer is examined by probing the lateral force and the lateral and longitudinal force fluctuations. We construct full maps of the aforementioned quantities as functions of the spatial pattern of the swimming fish pair and show that both energy expenditure and stability can be invoked as possible reasons for the swimming patterns and tail-beat synchronization observed in real fish. Our results suggest that high cost of transport zones should be avoided by the fish. Wake capture may be energetically unfavorable in the absence of kinematic adjustment. We hereby hypothesize that fish may restrain from wake capturing and, instead, adopt side-to-side configuration as a conservative strategy, when the conditions of wake energy harvesting are not satisfied. To maintain a stable school configuration, compromise between propulsive efficiency and stability, as well as between school members, ought to be considered.

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Interference model for an array of wave-energy-absorbing flexible structures


C. Nové-Josserand, R. Godoy-Diana, & B. Thiria
Physical Review Applied 11, 034054 (2019)
[doi:10.1103/PhysRevApplied.11.034054]

Considerable work has been undertaken for the improvement of wave-energy converters and array design. It has recently been suggested that by extracting wave energy, these farms could also serve to protect shorelines from wave damage. The present work focuses on the local effects of wave-structure interactions within an array of oscillating absorbers to optimize global effects, such as reflection, damping, and energy absorption. We use a model system of flexible blades, subjected to monochromatic waves, and develop a simplified one-dimensional model to predict optimal configurations, depending on various parameters, which include the number of blades, their spacing, and their flexibility. Optimal configurations are found to be close to regular patterns, and the impact of array configurations is shown to be limited regarding wave dissipation, mainly due to a competition between reflection and absorption.

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Hydrodynamics of the frontal strike in aquatic snakes

Hydrodynamics of the frontal strike in aquatic snakes: drag, added mass and the consequences for prey capture success

 

M. Segall, A. Herrel & R. Godoy-Diana
Bioinspiration & Biomimetics 14, 036005 (2019)
[doi:10.1088/1748-3190/ab0316]
bioRxiv preprint: https://doi.org/10.1101/411850

Transient locomotion under water is highly constrained by drag and added mass, yet some aquatic snakes catch their prey using a fast forward acceleration, with the mouth opened. These aquatic snakes show a convergence of their head shape in comparison with closely related species that do not forage under water. As both drag and added mass are related to some extent to the shape of the moving object, we explored how shape impacts the hydrodynamic forces applied to the head of a snake during a prey capture event. We compared two 3D- Continue reading “Hydrodynamics of the frontal strike in aquatic snakes”

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Review paper: Insect and insect-inspired aerodynamics

Image credit: T. Engels (see also Engels et al. Physical Review Fluids 4, 013103, 2019)

 
 
 
 
 
 

Insect and insect-inspired aerodynamics: unsteadiness, structural mechanics and flight control
R. Bomphrey & R. Godoy-Diana
Current Opinion in Insect Science 30, 26–32 (2018)
[doi:10.1016/j.cois.2018.08.003]

Flying insects impress by their versatility and have been a recurrent source of inspiration for engineering devices. A large body of literature has focused on various aspects of insect flight, with an essential part dedicated to the dynamics of flapping wings and their intrinsically unsteady aerodynamic mechanisms. Insect wings flex during flight and a better understanding of structural mechanics and aeroelasticity is emerging. Most recently, insights from solid and fluid mechanics have been integrated with physiological measurements from visual and mechanosensors in the context of flight control in steady airs and through turbulent conditions. We review the key recent advances concerning flight in unsteady environments and how the multi-body mechanics of the insect structure — wings and body — are at the core of the flight control question. The issues herein should be considered when applying bio-informed design principles to robotic flapping wings.

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Forced wakes far from threshold

Forced wakes far from threshold: Stuart–Landau equation applied to experimental data
S. Boury, B. Thiria, R. Godoy-Diana, G. Artana, J. E. Wesfreid, and J. D’Adamo
Physical Review Fluids 3, 091901(R) (2018)
[doi:10.1103/PhysRevFluids.3.091901]

We studied with the Stuart-Landau (SL) amplitude equation, a wake flow control scenario using experimental data from a cylinder wake forced by plasma actuators. Given the formal framework recently discussed by Gallaire et al. [Fluid Dyn. Res. 48, 061401 (2016)] on pushing amplitude equations far from threshold, we analyze experimental data of a forced wake in order to test the SL reduced order model. Linear stability theory and global mode concepts are used to determine the SL parameters. The extension to forced wakes of the SL model had been proposed by Thira and Wesfreid [J. Fluid Mech. 579, 137 (2007)] in the context of their study on stability properties, but its employment still remained an open question. Here, we show that a forced wake at a Reynolds number far from the first threshold can also attain the critical behavior described by the SL model.

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Surface wave energy absorption by a partially submerged bio-inspired canopy

Surface wave energy absorption by a partially submerged bio-inspired canopy
C. Nové-Josserand, F. Castro Hebrero, L.-M. Petit, W. Megill, R. Godoy-Diana & B. Thiria
Bioinspiration & Biomimetics 13 036006 (2018)
[doi:10.1088/1748-3190/aaae8c]
PDF file

Aquatic plants are known to protect coastlines and riverbeds from erosion by damping waves and fluid flow. These flexible structures absorb the fluid-borne energy of an incoming fluid by deforming mechanically. In this paper we focus on the mechanisms involved in these fluid-elasticity interactions, as an efficient energy harvesting system, using an experimental canopy model in a wave tank.We study an array of partially-submerged flexible structures that are subjected to the action of a surface wave field, investigating in particular the role of spacing between the elements of the array on the ability of our system to absorb energy from the flow. The energy absorption potential of the canopy model is examined using global wave height measurements for the wave field and local measurements of the elastic energy based on the kinematics of each element of the canopy. We study different canopy arrays and show in particular that flexibility improves wave damping by around 40%, for which half is potentially harvestable.

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Review paper: Fluid dynamic drag in animal swimming and flying

On the diverse roles of fluid dynamic drag in animal swimming and flying
R. Godoy-Diana & B. Thiria
Journal of the Royal Society Interface 15 20170715 (2018)
[doi:10.1098/rsif.2017.0715]

Questions of energy dissipation or friction appear immediately when addressing the problem of a body moving in a fluid. For the most simple problems, involving a constant steady propulsive force on the body, a straightforward relation can be established balancing this driving force with a skin friction or form drag, depending on the Reynolds number and body geometry. This elementary relation closes the full dynamical problem and sets, for instance, average cruising velocity or energy cost. Continue reading “Review paper: Fluid dynamic drag in animal swimming and flying”

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Flapping elastic plates as a model of fish-like swimmers

Modelling of an actuated elastic swimmer
M. Piñeirua, B. Thiria & R. Godoy-Diana
Journal of Fluid Mechanics 829 731-750 (2017)
[doi:10.1017/jfm.2017.570]PDF file

We studied 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 Continue reading “Flapping elastic plates as a model of fish-like swimmers”

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Synchronisation and pattern formation in fish swimming

Tetrafish2

Simple phalanx pattern leads to energy saving in cohesive fish schooling
I. Ashraf, H. Bradshaw, T. T. Ha, J. Halloy, R. Godoy-Diana, B. Thiria
PNAS 114 (36) 9599-9604 (2017)
[doi:10.1073/pnas.1706503114]PDF file

Synchronisation and collective swimming patterns in Hemigrammus bleheri
I. Ashraf, R. Godoy-Diana, J. Halloy, B. Collignon, B. Thiria
Journal of the Royal Society Interface 13 20160734 (2016)
[doi:10.1098/rsif.2016.0734] PDF file

The question of how individuals in a population organize when living in groups arises for systems as different as a swarm of microorganisms or a flock of seagulls. The different patterns for moving collectively involve a wide spectrum of reasons, such as evading predators or optimizing food prospection. Also, the schooling pattern has often been associated with an advantage in terms of energy consumption. We use a popular aquarium fish, the red nose tetra fish, Hemigrammus bleheri, which is known to swim in highly cohesive groups, to ana- lyze the schooling dynamics. In our experiments, fish swim in a shallow-water tunnel with controlled velocity, Continue reading “Synchronisation and pattern formation in fish swimming”

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Hydrodynamic constraints and evolution of aquatic snakes

snakesDoes aquatic foraging impact head shape evolution in snakes ?
M. Segall, R. Cornette, A-C. Fabre, R. Godoy-Diana & A. Herrel
Proceedings of the Royal Society B 283 20161645 (2016).
[doi:10.1098/rspb.2016.1645] PDF file

Evolutionary trajectories are often biased by developmental and historical factors. However, environmental factors can also impose constraints on the evolutionary trajectories of organisms leading to convergence of morphology in similar ecological contexts. The physical properties of water impose strong constraints on aquatic feeding animals by generating pressure waves that can alert prey and potentially push them away from the mouth. These hydrodynamic constraints have resulted in the independent evolution of suction feeding in most groups of secondarily aquatic tetrapods. Despite the fact that snakes cannot use suction, they have invaded the aquatic milieu many times independently. Here, we test whether the aquatic environment has constrained head Continue reading “Hydrodynamic constraints and evolution of aquatic snakes”

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

resistive_vs_reactiveResistive thrust production can be as crucial as added mass mechanisms for inertial undulatory swimmers
M. Piñeirua, R. Godoy-Diana & B. Thiria
Physical Review E 92 021001(R) (2015).
[doi:10.1103/PhysRevE.92.021001] arXiv

We address here 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). Continue reading “Resistive thrust production can be as crucial as added mass mechanisms for inertial undulatory swimmers”

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Centrifugal instability of Stokes layers in crossflow

centrifugal_instability_cylinderCentrifugal instability of Stokes layers in crossflow: the case of a forced cylinder wake
J. D’Adamo, R. Godoy-Diana & J. E. Wesfreid
Proceedings of the Royal Society A 471: 20150011 (2015).
DOI:10.1098/rspa.2015.0011

A circular cylinder oscillating in a viscous fluid produces an axisymmetric Stokes layer, a fundamental flow susceptible to centrifugal instabilities [see e.g. Seminara & Hall, Proc. Roy. Soc. London A 350, 299 (1976)]. In the present work we study such problem in the wake flow around a circular cylinder at Re = 100 performing rotary oscillations. Continue reading “Centrifugal instability of Stokes layers in crossflow”

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Four-winged flapping flyer in forward flight

mite4ailes Four-winged flapping flyer in forward flight
R. Godoy-Diana, P. Jain, M. Centeno, A. Weinreb & B. Thiria
In Klapp et al. (eds.), Selected Topics of Computational and Experimental Fluid Mechanics, Environmental Science and Engineering, pp. 147-158. Springer (2015).
[doi:10.1007/978-3-319-11487-3_8] PDF file

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. Continue reading “Four-winged flapping flyer in forward flight”

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Undulatory swimming near a wall

wall_effect_visuLarge-amplitude undulatory swimming near a wall
R. Fernández-Prast, V. Raspa, B. Thiria, F. Huera-Huarte & R. Godoy-Diana. Bioinspiration and Biomimetics 10 016003 (2015).

doi:10.1088/1748-3190/10/1/016003

[PDF file]

We study experimentally the propulsive dynamics of flexible undulating foils in a self-propelled swimming configuration near a wall. Measurements of swimming speed and propulsive force are performed, together with full recordings of the elastic wave kinematics and particle image velocimetry. Continue reading “Undulatory swimming near a wall”

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Drag in undulatory swimmers

vortices_foilsVortex-induced drag and the role of aspect ratio in undulatory swimmers
V. Raspa, S. Ramananarivo, B. Thiria & R. Godoy-Diana. Physics of Fluids, 26 : 041701 (2014).

During cruising, the thrust produced by a self-propelled swimmer is balanced by a global drag force. For a given object shape, this drag can involve skin friction or form drag, both being well-documented mechanisms. However, for swimmers whose shape is changing in time, the question of drag is not yet clearly established. Continue reading “Drag in undulatory swimmers”

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Propagating waves in bounded elastic media

fig_beamsPropagating waves in bounded elastic media: an application to the efficiency of bio-inspired swimmers
S. Ramananarivo, R. Godoy-Diana & B. Thiria.
EPL, 105 : 54003 (2014).

Confined geometries usually involve reflected waves interacting together to form a spatially stationary pattern. Our recent study on the locomotion of a self-propelled elastic swimmer on a free surface [Ramananarivo et al. 2013], however, has shown that propagating wave kinematics can naturally emerge in a forced elastic rod, even with boundary conditions involving significant reflections. This particular behavior is observed only in the presence of strong damping. Continue reading “Propagating waves in bounded elastic media”

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Passive elastic mechanism to mimic fish-muscles action in anguilliform swimming

nageur_Ramananarivo_etalPassive elastic mechanism to mimic fish-muscles action in anguilliform swimming
S. Ramananarivo; R. Godoy-Diana & B. Thiria.
Journal of the Royal Society Interface 10 : 20130667 (2013).

Abstract: Swimmers in nature use body undulations to generate propulsive and maneuvering forces. The an- guilliform kinematics is driven by muscular actions all along the body, involving a complex temporal and spatial coordination of all the local actuations. Such swimming kinematics can be reproduced artificially, in a simpler way, by using passively the elasticity of the body. Here we present experiments on self-propelled elastic swimmers at a free surface in the inertial regime. Continue reading “Passive elastic mechanism to mimic fish-muscles action in anguilliform swimming”

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Topology-induced effect in biomimetic propulsive wakes

two_flapsTopology-induced effect in biomimetic propulsive wakes
V. Raspa; R. Godoy-Diana & B. Thiria.
Journal of Fluid Mechanics, 729: 377-387 (2013).

Abstract: It is known that the wake pattern observed in a cross-section behind swimming or flying animals is typically characterized by the presence of periodical vortex shedding. However, depending on species, propulsive wakes can differ according to the spatial ordering of the main vortex structures. We conducted a very precise experiment to analyse the role of the topology of the wake in the generation of propulsion by comparing two prototypical cases in a quasi-two-dimensional view. Continue reading “Topology-induced effect in biomimetic propulsive wakes”

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Force balance in the take-off of a pierid butterfly


montage_Pieris_rapaeForce balance in the take-off of a pierid butterfly: relative importance and timing of leg impulsion and aerodynamic forces

G. Bimbard, D. Kolomenskiy, O. Bouteleux, J. Casas & R. Godoy-Diana.
Journal of Experimental Biology, 216 : 3551-3563 (2013).

Abstract: Up to now, the take-off stage remains an elusive phase of insect flight relatively poorly explored compared to other maneuvers. An overall assessment of the different mechanisms involved in the force production during take-off has never been explored. Focusing on the first downstroke, we have addressed this problem from a force balance perspective in butterflies taking-off from the ground. Continue reading “Force balance in the take-off of a pierid butterfly”

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Stabilizing effect of flexibility in the wake of a flapping foil

flex_vs_rig_jetStabilizing effect of flexibility in the wake of a flapping foil
C. Marais; B Thiria; Wesfreid, J. E. & R. Godoy-Diana.
Journal of Fluid Mechanics, 710 : 659-669 (2012).

Abstract: The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel is used to examine the effect of chordwise foil flexibility in the dynamical features of flapping-based propulsion. Continue reading “Stabilizing effect of flexibility in the wake of a flapping foil”

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Spatiotemporal spectral analysis of a forced cylinder wake

spectral_analysisSpatiotemporal spectral analysis of a forced cylinder wake
J. D’Adamo; R. Godoy-Diana & J. E. Wesfreid.
Physical Review E, 84 : 056308 (2011).

Abstract: The wake of a circular cylinder performing rotary oscillations is studied using hydrodynamic tunnel experiments at $Re=100$. Two-dimensional particle image velocimetry on the mid-plane perpendicular to the axis of cylinder is used to characterize the spatial development of the flow and its stability properties. Continue reading “Spatiotemporal spectral analysis of a forced cylinder wake”

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Behind the performance of flapping wing flyers

flapping_wingRather than resonance, flapping wing flyers may play on aerodynamics to improve performance
S. Ramananarivo; R. Godoy-Diana & B. Thiria.
Proceedings of the National Academy of Sciences (USA), 108 (15): 5964-5969 (2011).

Abstract: Saving energy and enhancing performance are secular preoccupations shared by both nature and human beings. In animal locomotion, flapping flyers or swimmers rely on the flexibility of their wings or body to passively increase their efficiency using an appropriate cycle of storing and releasing elastic energy. Despite the convergence of many observations pointing out this feature, the underlying mechanisms explaining how the elastic nature of the wings is related to propulsive efficiency remain unclear. Here we use an experiment with a self-propelled simplified insect model allowing to show how wing compliance governs the performance of flapping flyers. Continue reading “Behind the performance of flapping wing flyers”

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Impulse response in the subcritical cylinder wake

impulse_cylinder_wakeConvective instability in inhomogeneous media: impulse response in the subcritical cylinder wake
C. Marais; R. Godoy-Diana; D. Barkley & J. E. Wesfreid.
Physics of Fluids, 23 (1): 014104 (2011).

Abstract: We study experimentally the impulse response of a cylinder wake below the critical Reynolds number of the Bénard-von Kármán instability. In this subcritical regime, a localized inhomogeneous region of convective instability exists which causes initial perturbations to be transiently amplified. The aim of this work is to quantify the evolution resulting from this convective instability using two-dimensional particle image velocimetry in a hydrodynamic tunnel experiment. Continue reading “Impulse response in the subcritical cylinder wake”

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Bending to fly

How wing compliance drives the efficiency of self-propelled flapping flyers
B. Thiria & R. Godoy-Diana.
Physical Review E, 82 : 015303(R) (2010).
*arXiv preprint blogged in MIT Technology Review (March 2, 2010)
*Also referenced in Vir. J. Bio. Phys. Res. / Volume 20 / Issue 3 / (August 1, 2010)

 

Abstract: Wing flexibility governs the flying performance of flapping-wing flyers. Here, we use a self-propelled flapping-wing model mounted on a ”merry go roun” to investigate the effect of wing compliance on the propulsive efficiency of the system. Continue reading “Bending to fly”

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Symmetry breaking of the reverse Bénard-von Kármán vortex street

flap_fluo_BKI_ASYM_smallA model for the symmetry breaking of the reverse Bénard-von Kármán vortex street produced by a flapping foil
R. Godoy-Diana; C. Marais; J. L. Aider & J. E. Wesfreid.
Journal of Fluid Mechanics, 622 : 23-32 (2009).

Abstract: The vortex streets produced by a flapping foil of span-to-chord aspect ratio of 4:1 are studied in a hydrodynamic tunnel experiment. In particular, the mechanisms giving rise to the symmetry breaking of the reverse Bénard-von Kármán vortex street that characterizes fish-like swimming and forward flapping flight are examined. Two-dimensional particle image velocimetry measurements in the mid-plane perpendicular to the span axis of the foil are used to characterize the different flow regimes. Continue reading “Symmetry breaking of the reverse Bénard-von Kármán vortex street”

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Transitions in the wake of a flapping foil

transitions_wakeTransitions in the wake of a flapping foil
R. Godoy-Diana; J. L. Aider & J. E. Wesfreid.
Physical Review E, 77 : 016308 (2008).

Abstract: We study experimentally the vortex streets produced by a flapping foil in a hydrodynamic tunnel, using two-dimensional particle image velocimetry. An analysis in terms of a flapping frequency-amplitude phase space allows the identification of (i) the transition from the well-known Bénard-von Kármán (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and (ii) the symmetry breaking of this reverse BvK pattern giving rise to an asymmetric wake. Continue reading “Transitions in the wake of a flapping foil”

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Tuning a wave-energy-driven OWC seawater pump to polychromatic waves

polychromatic_signalsOn the tuning of a wave-energy driven oscillating-water-column seawater pump to polychromatic waves
R. Godoy-Diana & S. P. R. Czitrom.
Ocean Engineering, 34 : 2374-2384 (2007).

Abstract: Performance of wave-energy devices of the oscillating water column (OWC) type is greatly enhanced when a resonant condition with the forcing waves is maintained. The natural frequency of such systems can in general be tuned to resonate with a given wave forcing frequency. In this paper we address the tuning of an OWC sea-water pump to polychromatic waves. We report results of wave tank experiments, which were conducted with a scale model of the pump. Continue reading “Tuning a wave-energy-driven OWC seawater pump to polychromatic waves”

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Internal gravity waves in a dipolar wind

Internal gravity waves in a dipolar wind: a wave–vortex interaction experiment in a stratified fluid
R. Godoy-Diana; J. M. Chomaz & C. Donnadieu.
Journal of Fluid Mechanics, 548 : 281-308 (2006).

 

Abstract: An experimental study on the interaction of the internal wave field generated by oscillating cylinders in a stratified fluid with a pancake dipole is presented. The experiments are carried out in a salt-stratified water tank with constant Brunt–Väisälä frequency ($N$). Experimental observations of the deformation of the wave beams owing to the interaction with the dipole are presented. Continue reading “Internal gravity waves in a dipolar wind”

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Vertical length scale selection for pancake vortices in strongly stratified viscous fluids

conical_diffuser

Vertical length scale selection for pancake vortices in strongly stratified viscous fluids
R. Godoy-Diana; J. M. Chomaz & P. Billant.
Journal of Fluid Mechanics, 504 : 229-238 (2004).

Abstract: The evolution of pancake dipoles of different aspect ratio is studied in a stratified tank experiment. Two cases are reported here for values of the dipole initial aspect ratio $\alpha_0 = L_v/L_h$ (where $L_v$ and $L_h$ are vertical and horizontal length scales, respectively) of $\alpha_0 = 0.4$ (case I) and $\alpha_0 = 1.2$ (case II). Continue reading “Vertical length scale selection for pancake vortices in strongly stratified viscous fluids”

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Diffusion of pancake-like vortices in cyclostrophic balance

conical_diffuserEffect of the Schmidt number on the diffusion of axisymmetric pancake vortices in a stratified fluid
R. Godoy-Diana & J. M. Chomaz.
Physics of Fluids, 15 : 1058-1064 (2003).

Abstract: An asymptotic analysis of the equations for quasi-two-dimensional flow in stratified fluids is conducted, leading to a model for the diffusion of pancake-like vortices in cyclostrophic balance. This analysis permits one to derive formally the model for the diffusion of an axisymmetric monopole proposed by Beckers et al. [J. Fluid Mech. 433, 1 (2001)], and to extend their results. The appropriate parameter for the perturbation analysis is identified as the square of the vertical Froude number Fv=U/(Lv N), where U is the horizontal velocity scale, N is the Brunt–Väisälä frequency, and Lv the vertical length scale. Continue reading “Diffusion of pancake-like vortices in cyclostrophic balance”

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Hydrodynamics of an oscillating water column seawater pump

sibeo_colorHydrodynamics of an oscillating water column seawater pump. Part I: Theoretical Aspects
S. P. R. Czitrom; R. Godoy; E. Prado; P. Pérez & R. Peralta-Fabi.
Ocean Engineering, 27 : 1181-1198 (2000).

Abstract: A wave-driven seawater pump, composed of a resonant and an exhaust duct joined by a variable-volume air compression chamber, is studied. The time dependent form of Bernoulli’s equation, adapted to incorporate losses due to friction, vortex formation at the mouths and radiation damping, describes the pump behaviour. A dimensional analysis of the pump equations shows that a proposed scale-model will perform similar to a full-scale seawater pump. Continue reading “Hydrodynamics of an oscillating water column seawater pump”

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Hydrodynamics of an oscillating water column seawater pump (Part II)

sibeoHydrodynamics of an oscillating water column seawater pump. Part II: Tuning to Monochromatic Waves
S. P. R. Czitrom; R. Godoy; E. Prado; A. Olvera & C. Stern.
Ocean Engineering, 27 : 1199-1219 (2000).

Abstract: Flume experiments with a scale-model of a wave driven seawater pump in monochromatic waves are described. A tuning mechanism optimises the pump performance by keeping it at resonance with the waves. The pumping process itself was found to de-tune the system because of the reduced gravity restoring force due to spilling in the compression chamber. A perturbation analysis of the pump equations shows that performance of the system can be increased by optimising the shape of the pump intake to minimise losses due to vortex formation. Continue reading “Hydrodynamics of an oscillating water column seawater pump (Part II)”

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Vortex suppresion in an oscillating flow

conical_diffuserVortex suppresion in an oscillating flow
C. Stern; S. Czitrom; E. Prado & R. Godoy.
Revista Mexicana de Fisica, 46 : 409-410 (2000).

Abstract: The motivation for this work was the reduction of losses due to vortex formation at the entrance of a wave driven seawater pump. Measurements in a wave tank using a prototype had shown a 10% ¡ncrease in the pumping efficiency when a trumpet like shape was added to the intake. This lead us to search for an inlake that would reduce or completely suppress vortex formation. In this experiment a piston produces an oscil1ating flow inside a partly submerged duct. At the end of the duct four different shapes were tested. Continue reading “Vortex suppresion in an oscillating flow”

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Oscillating Flow through a Funnel

vortex_owc_exitOscillating Flow through a Funnel
C. Stern; S. Czitrom & R. Godoy.
Physics of Fluids, 11 : S3 (Gallery of Fluids) (1999).

Abstract: Our interest in vortex suppression at the entrance of a wave-driven seawater pump leads us to study vortex formation at the exit of a diffuser due to an oscillating flow. In the present experiment, a piston produces an oscillating flow inside a partly submerged duct that ends in a diffuser. The diffuser is designed such that a constant relationship between centripetal and inertial forces is maintained along the profile. The flow in the near field of the mouth is visualized by injecting diluted fluorescent water paint just outside the diffuser. Continue reading “Oscillating Flow through a Funnel”