Biomimetic and Live Medusae Reveal the Mechanistic Advantages of a Flexible Bell Margin

Flexible bell margins are characteristic components of rowing medusan morphologies and are expected to contribute towards their high propulsive efficiency. However, the mechanistic basis of thrust augmentation by flexible propulsors remained unresolved, so the impact of bell margin flexibility on medusan swimming has also remained unresolved. We used biomimetic robotic jellyfish vehicles to elucidate that propulsive thrust enhancement by flexible medusan bell margins relies upon fluid dynamic interactions between entrained flows at the inflexion point of the exumbrella and flows expelled from under the bell. Coalescence of flows from these two regions resulted in enhanced fluid circulation and, therefore, thrust augmentation for flexible margins of both medusan vehicles and living medusae. Using particle image velocimetry (PIV) data we estimated pressure fields to demonstrate a mechanistic basis of enhanced flows associated with the flexible bell margin. Performance of vehicles with flexible margins was further enhanced by vortex interactions that occur during bell expansion. Hydrodynamic and performance similarities between robotic vehicles and live animals demonstrated that the propulsive advantages of flexible margins found in nature can be emulated by human-engineered propulsors. Although medusae are simple animal models for description of this process, these results may contribute towards understanding the performance of flexible margins among other animal lineages

Contribution à l'étude des interactions acoustiques dans les antennes compactes basses fréquences

Thèse de doctorat en Electronique, Université de Valenciennes et du Hainaut-Cambrésis, janvie

Contribution à l'étude des intéractions acoustiques dans les antennes compactes basses fréquences

Acoustic projectors assembled in an array experience an interaction effect as a result of the coupling of their individual radiated powers through the acoustic medium. This interaction impacts the performance and service life of the sonar array system. The effect is frequency dependent and difficult to control partly because of a limited understanding of the physical behavior, particulary for low-frequency volumetric arrays. Flextensional transducers have been proposed to populate these arrays. A class IV flextensional transducer is constructed from an elliptic-cylindrical shell and a piezoelectric stack of ceramic plates joined to the shell at the major axis by end-shanks. Its performance is dictated by the flexural motion of the shell that is induced by electrically driving the stack in a longitudinal motion. A modal analysis of the shell permits the flexural motion to be described in terms of a set of normal modes. The concept addressed in this thesis is to characterize the acoustic interaction in terms of modal-mutual radiation phenomenon, the effect on the individual projector, and the influence on array performance. The manuscript begins by reviewing classical transducer and array modeling techniques. The validity of classical methods to address acoustic interactio effects is investigated. A modal analysis approach is then proposed and developed. A lumped-parameter piece-part euivalent circuit of the transducer is posed. The circuit consist of an assembly of a motional branch for the piezoelectric driver and a set of motional branches corresponding to the flexural modes of the shell. A hybrid modeling technique is presented that combines boundary integral methos with the equivalent circuit model to incorporate modal self - and mutual - radiation impedances. The circuit the produces a set of modal participation factors that govern acoustic peroformance. Results of the bybrid model compare well with measured data and to results of fintie element models of small clos-pack arraysLes projecteurs acoustiques assemblés en antenne sont le siège d'un effet d'interaction qui résulte du couplage, par l'intermédiaire du milieu acoustique, des puissances qu'ils rayonnent individuellement. Cette interaction modifie les performances et la durée de vie des antennes sonar. Cet effet, dépendant de la fréquence, est difficile à contrôler, car le comportement physique des antennes, et plus particulièrement des antennes volumiques basse fréquence, n'est pas bien connu. Les transducteurs flextensionnels ou flextenseurs ont été proposés comme des éléments de base de ces antennes. Un flextenseur de classe IV est construit à partir d'une coque elliptique et d'un pilier de céramiques piézoelectriques connecté à la coque suivant le grand axe par l'intermédiaire d'inserts. Son fonctionnement est déterminé par le mouvement de flexion de la coque qui est induit par l'excitation électrique du mouvement longitudinal du pilier. Une analyse modale de la coque permet de décrire le mouvement de flexion sous la forme d'une somme de modes normaux. Le concept étudié dans cette thèse est la caractérisation des interactions acoustiques en terme d'impédances modales mutuelles de rayonnement entre les projecteurs. Cette approche se prête à une interprétation physique du phénomène d'intéraction, de l'effet du projecteur élémentaire et de son influence sur le fonctionnement de l'antenne. Le manuscrit débute en récapitulant les techniques classiques de modélisation de transducteurs et d'antennes. La capacité de ces méthodes à décrire les effets d'interaction acoustique est évaluée. Ensuite, une approche modale est proposée et développée. Un circuit électrique représentant le transducteur élémentaire est défini à partir d'une description par constantes localisées de chaque partie (pilier, coque). Ce circuit équivalent est constitué de l'assemblage d'une branches motionnelle pour le pilier et d'un ensemble de branches motionnelles correspondant au mode de flexion de la coque. Un modèle hybride combinant méthode d'équations et circuit électrique équivalent est développé pour inclure les impédances de rayonnement (self et mutuelles). le modèle obtenu permet de calculer les facteurs de participations modales qui déterminent le comportement acoustique. Pour de petites antennes compactes, les résultats obtenus sont en bon accord avec les prédictions de calculs numériques par éléments finis et les mesuresVALENCIENNES-BU Sciences Lettres (596062101) / SudocSudocFranceF

Thermal design of high-power active transducers with the Atila finite element code

International audienc

Performance among robots and live medusa.

<p>A. Structural comparison of biological models (top panels) with bio-inspired vehicles (bottom) used for experimental comparisons (both are 16.4 cm diameter). Only flap versions of the robotic vehicles are shown. B. Comparison of the maximum normalized swimming speeds of jellyfish vehicles with representative values for their natural counterparts, <i>Aurelia aurita</i> (14.7 cm diameter) and <i>Cyanea capillata</i> (10 cm diameter). C. Comparison of the normalized swimming speed averaged over the swimming cycle. The Reynolds numbers (Re) shown are averages over 3 swimming cycles. We examined a flap and a no-flap version of both the <i>A. aurita</i> and <i>C. capillata</i> vehicle models (icons illustrate each version). Bars are the mean values over 3 consecutive swimming cycles (± st.dev.). D. Bell kinematics at of no-flap and flap vehicles and live <i>Aurelia</i> during bell contraction. Scale represents 1 cm.</p

Vehicle performance and wake characteristics.

<p>(A) Swimming bell kinematics measured as changes in bell fineness (bell height/diameter) during contraction and relaxation (indicated by small medusa-shaped icons). (B) Corresponding propulsive performance of flap and no-flap vehicles supplied with identical input power. Although all vehicles accelerated forward during bell contraction, only vehicles with flexible margins – flaps – succeeded in making any net progress during bell relaxation. (C) Swimming speed during the second swimming cycle showing that the no-flap vehicles moved backwards (negative velocities) during bell expansion. (D) Circulation values of the starting vortex. Circulation values were normalized by bell area to account for differences in propulsor surface areas among vehicle versions. The increase in circulation occurs during bell contraction as the starting vortex grows. Circulation of the flap versions peaked at higher levels as a result of generating larger starting vortices.</p

Fluid interactions and performance of vehicles with variable flap lengths.

<p>(A) Maximum fluid velocity and (B) peak circulation of the starting vortex for the <i>Aurelia</i> vehicle with variable length flaps. (C) Maximum swimming speed of the <i>Aurelia</i> vehicle with variable length margins. (D) Velocity vectors and vorticity contours of the starting vortex during bell contraction of the different versions of the vehicle.</p

Fluid interactions at the bell margin.

<p>(A) Velocity vector field (from DPIV record) and (B) pressure field of flows around flap and no-flap versions of the <i>Aurelia</i> vehicle. Inset: Area integrated pressure of the pressure field along the bell over time. In the flap version, the greatest fluid velocities and lowest pressure values occurred at the inflexion point of the margin, where the flexible flap joined the more rigid bell. (C) Maximum fluid velocities in the wake were greater for the flap version (red) of the <i>Aurelia</i> vehicle than the no-flap (blue) version although the velocities of the propulsors (bell margins) did not differ. (D) Ratio of the maximum velocities of the fluid entrained (pull) versus expelled (push) by the bell during bell contraction for the flap (red) and no-flap (blue) versions. Velocity and pressure fields of the <i>Aurelia</i> vehicle were representative of the fields for the <i>Cyanea</i> vehicle.</p

Flap length versus vortex diameter.

<p>Maximum diameter of the starting vortex versus the distance measured from the bell margin to the inflexion point (location where the flap joined the rigid actuator) for <i>Aurelia</i> vehicle with variable length flaps.</p

Comparison of margin tip velocities for flap and no-flap versions of the <i>Aurelia</i> vehicle.

<p>Data represent average (error bars - ±1 standard deviation of mean value) velocities taken during three consecutive pulsation cycles for each vehicle type. Insert depicts the average and maximum tip velocities for either vehicle type. Note that the vehicle possessing a flexible marginal flap did not have either higher maximum or average tip velocities than the vehicle without marginal flaps. The frequency of sampling was increased during the portion of the pulsation cycle characterized by maximum marginal tip velocities.</p