Fluid structure interaction in bioinspired locomotion problems

Mención Internacional en el título de doctorNature offers a vast amount of examples of efficient locomotion. Millions of years of evolution have allowed animals —such as fish, insects and birds—, and even plants —such as winged-seeds or dandelions— to achieve outstanding locomotive skills. Therefore, it is not a surprise that scientists and engineers have tried to replicate the flight and swimming capabilities of the former examples in order to develop efficient aerial and nautical robots. In fact, these efforts have led to the design and development of several successful bioinspired robots. However, their performance is still far below their living counterparts. One of the main reasons is that the understanding of the physics underlying biological locomotion is still limited. This is due to the complexity of the problem under consideration: the locomotion of a body through a fluid medium. This can be considered fluid structure interaction (FSI) problem where the dynamics of the specimens is the result from the hydrodynamic interaction with the surrounding fluid, which in turn is modified by the motion of the specimens. Consequently, the resulting problem is highly nonlinear and complex from a mathematical standpoint. This dissertation attempts to contribute to further understand the fluid structure interactions in bioinspired locomotion problems. To that end, direct numerical simulations of several examples of bioinspired FSI problems are performed. These examples include the auto-rotation of a winged-seed, the flow interactions between the wings of a dragonfly, and the schooling patterns that emerge between two fish. In the first part of this dissertation, the algorithm which has been developed to perform part of the aforementioned studies is presented. The proposed algorithm allows the study of the FSI of systems of connected rigid bodies —which serve as a model for the actual specimens— immersed in an incompressible fluid. It is built based on a preexisting flow solver, coupled with a robotic algorithm for the computation of the dynamic equations of the bodies. The use of robotic algorithms endows the proposed methodology with a great fiexibility, allowing to simulate a large variety of problems with different geometries and configurations. The second part of the thesis is devoted to the analysis of the aforementioned examples. In this regard, we first consider the flight of a winged-seed. This is a very interesting, yet complex, problem of fluid-dynamic interaction; in which the auto-rotative motion is the result of a subtle equilibrium between the aerodynamic forces and the inertia properties of the winged-seed. In our study, the dynamics and the flow surrounding the auto-rotating seed are characterized in a range of Reynolds numbers, Re. Specifically, we focus on the study of the leading edge vortex (LEV) that is developed on the upper surface of the seed's wing as it auto-rotates. Our findings suggest that, in the explored range Re = [80 — 240], LEV's stability is not driven by vorticity transport along the spanwise direction nor viscous effects, as reported in the literature of rotating wings. Instead, fictitious accelerations (i.e., Coriolis and centrifugal accelerations) are the most suitable candidates to stabilize the LEV over the seed's wing. In the second example, we study the effect of the three-dimensional (3D) interactions in the performance of two tandem wings, resembling those of a dragonfly. To that end, the wings undergo a two-dimensional (2D) optimum kinematics which is a combination of heaving and pitching. We first analyze the effect of wings' aspect ratio, AR, by comparing the 3D and 2D simulations. The results show that 3D vertical interactions are detrimental for the thrust production of the hindwing, but they do not significantly affect the propulsive efficiency of the tandem arrangement. Next, a more realistic flapping kinematics of the 3D is considered and compared to the previous heaving kinematics. We find a decrease in the propulsive efficiency of the flapping wings compared to their heaving counterparts, which has been linked to a non-desired shedding of vorticity on the inboard region of the wings. The last bioinspired example corresponds to the collective motion of two self-propelled three-dimensional bodies. These bodies are idealized as rectangular, flat plates with flexibility along their chordwise direction, and that self-propels thanks to a prescribed vertical motion of their leading edges. We observe that tandem configurations emerge where both plates swim at a constant mean horizontal velocity and with a mean equilibrium horizontal distance. These configurations can be classified, attending to the resulting flow interactions, into compact and regular configurations. In the former, the performance of the upstream flapper is modified due to the close interaction with the downstream flapper. However, in the regular configurations, the performance of the upstream flapper is similar to that of an isolated flapper. Conversely, the performance of the downstream flapper is affected in both configurations by the interaction with the wake of the upstream flapper. We are able to link the changes in the downstream flapper's performance to its interaction with the vertical jet induced by vortex rings of the upstream flapper's wake. Finally, we propose a model to qualitatively predict the performance of a hypothetical downstream flapper based on the flow field of and isolated flapper, showing good agreement with the actual simulations.La naturaleza ofrece una gran cantidad de ejemplos de locomoción eficiente. Millones de años de evolución han permitido a animales —tales como peces, insectos o pájaros— e incluso plantas —como sainaras o dientes de león— lograr unas habilidades de lomoción excepcionales. Por lo tanto, no es una sorpresa que científicos e ingenieros hayan intentado replicar la capacidades de vuelo y nado de los anteriores ejemplos, con el objetivo de desarrollar robots aéreos y nadadores más eficientes. De hecho, estos esfuerzos han dado lugar al diseño y desarrollo exitoso de varios robots bioinsipirados. Sin embargo, el rendimiento de éstos es todavía muy inferior al de sus referentes biológicos. Una de las principales razones es que la comprensión de la física subyacente de la lomococión de sistemas biológicos es aún limitada. Esto es debido a la complejidad del problema, a saber, el movimiento de un cuerpo a través de un medio fluido. Este se puede considerar como un problema de interacción fluido estructura (FSI) donde la dinámica del espécimen es el resultado de la interacción fluidodinámica con el fluido de alrededor, el cual es a su vez modificado por el movimiento del cuerpo. Consecuentemente, el problema resultante es altamente no lineal y complejo desde un punto de vista matemático. Con esta disertación se pretende contribuir a una mayor comprensión de la interacción fluido estructura en problemas de locomoción bioninspirados. Con tal propósito, se han realizado simulaciones numéricas directas de varios ejemplos bioinspirados de interacción fluido estructura. Estos ejemplos incluyen la autorrotación de una sámara, las interaccionés fluidas entre las alas de una libélula y los patrones de nado que surgen entre dos peces. Durante la primera parte de esta disertación, se describe el algoritmo que ha sido desarrollado con el propósito de simular alguno de los problemas anteriormente citados. El algoritmo propuesto permite el estudio de la interacción fluido estructura de sistemas de cuerpos rígidos conectados —los cuales sirven como modelo de los especímenes reales— que están sumergidos en un fluido incompresible. Está construido sobre un solver fluido pre-existente, acoplado a un algoritmo robótico que se encarga de calcular las ecuaciones dinámicas de los cuerpos. El uso de algoritmos robóticos proporciona a la metodología propuesta una gran flexibilidad, permitiendo simular una gran variedad de problemas con diversas geometrías y configuraciones. La segunda parte de esta tesis está dedicada al análisis de los ejemplos mencionados anteriormente. En este respecto, consideramos primero el vuelo de una sámara, el cual es un problema muy interesante, aunque complejo, de interacción fluido dinámica en el cual el movimiento autorrotativo es el resultado de un sutil equilibrio entre las fuerzas aerodinámicas y las propiedades inerciales de la semilla. En nuestro estudio, caracterizamos la dinámica y el flujo alrededor de la semilla autorrotante en un rango de números de Reynolds, Re. En concreto, nos centramos en el estudio del vórtice del borde de ataque (LEV) que se forma en la parte superior del ala de la sámara cuando ésta autorrota. Nuestros hallazgos sugieren que, en el rango explorado de Re = [80 — 240], la estabilidad del LEV no se debe a un transporte de vorticidad a lo largo de la dirección de la envergadura del ala, ni a efectos viscosos —como se ha mencionado en la literatura de alas rotativas—, sino que las aceleraciones ficticias (es decir, las aceleraciones centrífugas y de Coriolis), son las candidatas más probables responsables de la estabilización del LEV. En el segundo ejemplo, se estudia el efecto de las interacciones tridimensionales (3D) en el rendimiento de dos alas en configuración tándem, basadas en las de una libélula. Para ello, se prescribe que el movimiento de las alas sea una combinación de cabeceo y oscilación vertical que es óptimo en 2 dimensiones (2D). Primero analizamos el efecto de la relación de aspecto de las alas, A% comparando los resultados de las simulaciones en 3D y en 2D. Los resultados revelan que las interacciones vorticales en 3D son perjudiciales para la generación de empuje del ala trasera, pero estas interacciones no afectan de forma significativa a la eficiencia propulsiva del conjunto. Posteriormente, se considera un movimiento de batimiento más realista de las alas, y se compara su eficiencia con la obtenida previamente para las alas en movimiento oscilatorio vertical. Se observa una menor eficiencia de las alas en batimiento en comparación con las mismas alas en movimiento oscilatorio vertical. Este deterioro es asociado a un desprendimiento de estructuras vorticales cerca de los bordes marginales de las alas en batimiento. El último ejemplo bioinspirado es el del movimiento colectivo de dos cuerpos tridimiensionales que se auto propulsan. Estos cuerpos se idealizan como placas planas rectangulares, siendo flexibles a lo largo de su cuerda, y que se auto propulsan gracias a un movimiento vertical impuesto de sus bordes de ataque. Los resultados muestran la aparición de configuraciones tándem donde sendas placas nadan con una velocidad inedia constante y separadas a una distancia de equilibrio que es también constante. Estas configuraciones son clasificadas —atendiendo a las interacciones fluidas— entre compactas y regulares. En las primeras, el rendimiento de la placa que nada aguas arriba (a la que llamaremos líder) se ve afectado por las interacciones cercanas con el cuerpo que nada aguas abajo (al que denominaremos seguidor). En cambio, en las configuraciones regulares el redimiento del líder es el mismo que el de una placa similar nadando de forma aislada. Por el contrario, el rendimiento del seguidor se ve afectado en ambas configuraciones debido a las interacciones con la estela del líder. Se ha podido relacionar estos cambios en la eficiencia del seguidor con la interacción con el chorro inducido por los anillos vorticales de la estela del líder. Finalmente, hemos propuesto un modelo que permite predecir, de forma cualitativa, el rendimiento de un seguidor hipotético basándonos en el campo fluido de una placa aislada. El modelo muestra una buena correlación con los datos obtenidos de las simulaciones numéricas.This thesis has been carried out in the Bioengineering and Aerospace Engineering Department at Universidad Carlos III de Madrid. The financial support has been provided by the Spanish Ministry of Economy and Competitiveness through grant DPI2016-76151-C2-2-R (AEI/FEDER, UE).Programa de Doctorado en Mecánica de Fluidos por la Universidad Carlos III de Madrid; la Universidad de Jaén; la Universidad de Zaragoza; la Universidad Nacional de Educación a Distancia; la Universidad Politécnica de Madrid y la Universidad Rovira i VirgiliPresidente: Francisco Javier Huera-Huarte.- Secretario: Javier Rodríguez Rodríguez.- Vocal: Ignazio María Viol

Development of an unsteady potential model for a flapping wing mav

The main objective of this project is the development of an unsteady potential code that allows to compute the aerodynamic forces of two wings with an arbitrary motion. The code must be able to reasonably predict aerodynamic forces with a low computational cost and in a fast way. In this fashion, the code will allow, in a future, to analyse di erent apping con gurations with di erent parameters, that would be impossible to analyse using high order NS based methods.Ingeniería Aeroespacia

Three-dimensional effects on the aerodynamic performance of flapping wings in tandem configuration

Direct numerical simulations have been performed to analyze how three-dimensional effects influence the performance of wings in tandem configuration undergoing a two-dimensional optimal kinematics. This optimal motion is a combination of heaving and pitching of the airfoils in a uniform free-stream at a Reynolds number and Strouhal number . Wings of two different aspect ratios, 2 and 4, undergoing the 2D motion have been considered. It has been found that the interactions between the vortical structures of the fore- and the hind-wings are qualitatively similar to the two-dimensional case for both . However, the ratio between the mean thrust of the hind-wing and the fore-wing decreases from 80% in 2D to 70% in 3D, implying that the 3D effects are detrimental for the vortical interactions between the wings in terms of thrust production. Nonetheless, the propulsive efficiency remains constant both in 2D and 3D, for both . A more realistic flapping motion has also been analyzed and compared to the heaving motion. It has been found that the aerodynamic forces decrease when the wings are in flapping motion. This detrimental behavior has been linked to a sub-optimal motion of the inboard region of the wings. This sub-optimal region of the wings entails a decrease of the mean thrust and of the propulsive efficiency compared to the heaving case, which are more pronounced for the 4 wings.This work was supported by grant DPI2016-76151-C2-2-R (AEI/FEDER, UE). The computations were partially performed at the supercomputer Tirant from the Red Española de Supercomputación in activity FI-2018-2-0025.Publicad

Numerical simulation of flow over flapping wings in tandem: Wingspan effects

We report direct numerical simulations of a pair of wings in horizontal tandem configuration to analyze the effect of their aspect ratio on the flow and the aerodynamic performance of the system. The wings are immersed in a uniform free stream at the Reynolds number Re=1000, and they undergo heaving and pitching oscillation with the Strouhal number St=0.7. The aspect ratios of forewing and hindwing vary between 2 and 4. The aerodynamic performance of the system is dictated by the interaction between the trailing edge vortex (TEV) shed by the forewing and the induced leading-edge vortex formed on the hindwing. The aerodynamic performance of the forewing is similar to that of an isolated wing irrespective of the aspect ratio of the hindwing, with a small modulating effect produced by the forewing-hindwing interactions. On the other hand, the aerodynamic performance of the hindwing is clearly affected by the interaction with the forewing's TEV. Tandem configurations with a larger aspect ratio on the forewing than on the hindwing result in a quasi-two-dimensional flow structure on the latter. This yields an 8% increase in the time-averaged thrust coefficient of the hindwing, with no change in its propulsive efficiency.This work was partially supported by the State Research Agency of Spain (AEI) under grant DPI2016-76151-C2-2-R including funding from the European Regional Development Fund (ERDF).The computations were performed at the supercomputer Tirant from the Red Espa˜nola de Supercomputaci´on in activity IM-2019-3-0011

Calcifediol is superior to cholecalciferol in improving vitamin D status in postmenopausal women: a randomized trial

Vitamin D has shown to play a role in multiple diseases due to its skeletal and extraskeletal actions. Furthermore, vitamin D deficiency has become a worldwide health issue. Few supplementation guidelines mention calcifediol treatment, despite being the direct precursor of calcitriol and the biomarker of vitamin D status. This 1-year, phase III-IV, double-blind, randomized, controlled, multicenter clinical trial assessed the efficacy and safety of calcifediol 0.266 mg soft capsules in vitamin D-deficient postmenopausal women, compared to cholecalciferol. Results reported here are from a prespecified interim analysis, for the evaluation of the study's primary endpoint: the percentage of patients with serum 25-hydroxyvitamin D (25(OH)D) levels above 30 ng/ml after 4 months. A total of 303 patients were enrolled, of whom 298 were included in the intention-to-treat (ITT) population. Patients with baseline levels of serum 25(OH)D <20 ng/ml were randomized 1:1:1 to calcifediol 0.266 mg/month for 12 months, calcifediol 0.266 mg/month for 4 months followed by placebo for 8 months, and cholecalciferol 25,000 IU/month for 12 months. At month 4, 35.0% of postmenopausal women treated with calcifediol and 8.2% of those treated with cholecalciferol reached serum 25(OH)D levels above 30 ng/ml (p < 0.0001). The most remarkable difference between both drugs in terms of mean change in serum 25(OH)D levels was observed after the first month of treatment (mean ± standard deviation change = 9.7 ± 6.7 and 5.1 ± 3.5 ng/ml in patients treated with calcifediol and cholecalciferol, respectively). No relevant treatment-related safety issues were reported in any of the groups studied. These results thus confirm that calcifediol is effective, faster, and more potent than cholecalciferol in raising serum 25(OH)D levels and is a valuable option for the treatment of vitamin D deficiency

Experimental ovine toxoplasmosis: influence of the gestational stage on the clinical course, lesion development and parasite distribution

P. 1-14The relation between gestational age and foetal death risk in ovine toxoplasmosis is already known, but the mechanisms involved are not yet clear. In order to study how the stage of gestation influences these mechanisms, pregnant sheep of the same age and genetic background were orally dosed with 50 oocysts of Toxoplasma gondii (M4 isolate) at days 40 (G1), 90 (G2) and 120 (G3) of gestation. In each group, four animals were culled on the second, third and fourth week post infection (pi) in order to evaluate parasite load and distribution, and lesions in target organs. Ewes from G1 showed a longer period of hyperthermia than the other groups. Abortions occurred in all groups. While in G2 they were more frequent during the acute phase of the disease, in G3 they mainly occurred after day 20 pi. After challenge, parasite and lesions in the placentas and foetuses were detected from day 19 pi in G3 while in G2 or G1 they were only detected at day 26 pi. However, after initial detection at day 19 pi, parasite burden, measured through RT-PCR, in placenta or foetus of G3 did not increase significantly and, at in the third week pi it was lower than that measured in foetal liver or placenta from G1 to G3 respectively. These results show that the period of gestation clearly influences the parasite multiplication and development of lesions in the placenta and foetus and, as a consequence, the clinical course in ovine toxoplasmosis.S

PhDAY 2020 -FOO (Facultad de Óptica y Optometría)

Por cuarto año consecutivo los doctorandos de la Facultad de Óptica y Optometría de la Universidad Complutense de Madrid cuentan con un congreso propio organizado por y para ellos, el 4º PhDAY- FOO. Se trata de un congreso gratuito abierto en la que estos jóvenes científicos podrán presentar sus investigaciones al resto de sus compañeros predoctorales y a toda la comunidad universitaria que quiera disfrutar de este evento. Apunta en tu agenda: el 15 de octubre de 2020. En esta ocasión será un Congreso On-line para evitar que la incertidumbre asociada a la pandemia Covid-19 pudiera condicionar su celebración

A numerical study of the flow around a model winged seed in auto-rotation

In this study the flow around a winged-seed in auto-rotation is characterized using direct numerical simulations (DNS) at Reynolds number in the range 80-240, based on the descent speed and a characteristic chord length. In this range, the flow is approximately steady when observed from a reference frame fixed to the seed. For all cases, the flow structure consists of a wing tip vortex which describes a helical path, a vortex shed behind the nut of the seed and a stable leading edge vortex above the wing surface which merges with the tip vortex. With increasing Reynolds number, the leading edge vortex becomes more intense and gets closer to the wing surface. The simulation results also show the formation of a spanwise flow on the upper surface of the wing, moving fluid towards the wing tip in a region downstream and beneath the leading edge vortex. This spanwise flow is rather weak inside the core of the leading edge vortex, and the analysis of the streamlines show a very weak transport of vorticity along the vortex for the cases under consideration. The analysis of the flow suggests that the stabilization of the leading edge vortex is mainly due to non-inertial accelerations, although viscous effects may contribute, specially at lower Re. Furthermore, the leading edge vortex has been characterized by analysing the flow variables averaged along cross-sections of the vortex. While some quantities, like the spanwise velocity or the pressure inside the vortex, are rather insensitive to the threshold used to define the leading edge vortex, the same is not true for the circulation of the vortex or its averaged spanwise vorticity, due to the viscous nature of the vortex. Finally, it is observed that the spanwise vorticity scales with the angular rotation of the seed for the different Re.This work was supported by grants TRA2013-41103-P (Mineco/Feder UE) and DPI2016-76151-C2-2-R (AEI/ Feder UE). MGV was partially supported by a grant of the BBVA Foundation. The code was partially developed during a stay of MGV (funded by the Spanish Ministry of Education through the program José Castillejo) at the Karlsruhe Institute of Technology. The simulations were partially performed at the Steinbuch Centre for Computing (SCC), Karlsruhe.Publicad

From flapping to heaving: a numerical study of wings in forward flight

Direct Numerical Simulations of the flow around a pair of flapping wings are presented. The wings are flying in forward flight at a Reynolds number Re=500, flapping at a reduced frequency K=1. Several values of the radius of flapping motion are considered, resulting in a database that shows a smooth transition from the wing rotating with respect to its inboard wingtip (flapping), to a vertical oscillation of the wing (heaving). In this transition from flapping to heaving, the spanwise-averaged effective angle of attack of the wing increases while the effect of the Coriolis and centripetal accelerations becomes weaker. The present database is analyzed in terms of the value and surface distribution of the aerodynamic forces, and in terms of 2D and 3D flow visualizations. While the former allows a decomposition of the force in pressure (i.e., the component of the force normal to the surface of the wing) and skin friction (i.e., tangential to the surface of the wing), the latter allows the identification of specific flow structures with the corresponding forces on the wing. It is found that the aerodynamic forces in the vertical direction (lift) tend to increase for wings moving with larger radius of flapping motion, becoming maximum for the heaving configuration. This is mostly due to the increase of the spanwise-averaged effective angle of attack of the wing with the radius of the flapping motion. Also, the local changes in the effective angle of attack have a strong effect on the structure of the leading edge vortex, resulting in changes in the distribution of suction along the span near the leading edge of the wing. The effect of the apparent accelerations is mostly felt on the spanwise position where the separation of the LEV occurs. On the other hand, the differences in the force in the streamwise direction (thrust/drag) between the configurations with different radius of flapping motion seems to be linked to the position of the stagnation point dividing the suction and pressure side boundary layers, which seems to be controlled by the local effective angle of attack. Finally, the results of the DNS are used to evaluate the performance of an unsteady panel method, and to explain its deficiencies.This work was supported by grants TRA2013-41103-P (MINECO/FEDER, UE) and DPI2016-76151-C2-2-R (AEI/FEDER, UE)

Kinematics and dynamics of the auto-rotation of a model winged seed

Numerical simulations of the auto-rotation of a model winged seed are presented. The calculations are performed by solving simultaneously the Navier-Stokes equations for the flow surrounding the seed and the rigid-body equations for the motion of the seed. The Reynolds number based on the descent speed and a characteristic chord length is varied in the range 80-240. Within this range, the seed attains an asymptotic state with finite amplitude auto-rotation, while for smaller values of the Reynolds number no auto-rotation is observed. The motion of the seed is characterized by the coning and pitch angles, the angular velocity and the horizontal translation of the seed. The values obtained for these quantities are qualitatively similar to those reported in the literature in experiments with real winged seeds. When increasing the Reynolds number, the seed tends to rotate at higher speeds, with less inclination with respect to the horizontal plane, and with a larger translation velocity. With respect to the aerodynamic forces, it is observed that, with increasing Reynolds number, the horizontal components decrease in magnitude while the vertical component increases. The force distribution along the wing span is characterized using both global and local characteristic speeds and chord lengths for the non-dimensionalisation of the force coefficients. It is found that the vertical component does not depend on the Reynolds number when using local scaling, while the chordwise component of the force does.This work was supported by grants TRA2013-41103-P (Mineco/Feder UE) and DPI2016-76151-C2-2-R (AEI/Feder UE). MGV was partially supported by a grant of the BBVA Foundation. The code was partially developed during a stay of MM (funded by BBVA) and MGV (funded by the Spanish Ministry of Education through the program José Castillejo) at the Karlsruhe Institute of Technology. The simulations were partially performed at the Steinbuch Centre for Computing (SCC), Karlsruhe. The computer resources, technical expertise and assistance provided by this center are thankfully acknowledged.Publicad