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Chan, Brian 1980. "Propulsion devices for locomotion at low-Reynolds number." Thesis, Massachusetts Institute of Technology, 2004. http://hdl.handle.net/1721.1/27065.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references (leaves 65-66).
We have designed, built, and tested three novel devices that use low-Reynolds number flows for self propulsion. The three-link swimmer is designed to swim through in a free viscous fluid using cyclic flipping motion of two rigid fins attached to a rigid midsection. Robosnail 1 uses lubrication pressures underneath a flexible, sinusoidally waving boundary to generate thrust, and Robosnail 2 uses five independently controlled translating feet segments to move on a layer of 8 percent Laponite, a shear thinning clay suspension which gives it the ability to adhere to and scale inclines and inverted surfaces. The three link swimmer was found to travel up to 0.034 body lengths per four-stroke cycle, Robosnail 1 was found to move at a speed of roughly half the wave speed of the foot (measured with respect to the snail), a result consistent for wave speeds between 0 and 2 cm/s. Robosnail 2 was able to move forward at all inclines from zero to 180 degrees inverted, with back-slip ranging from 40 to 80 percent.
by Brian Chan.
S.M.
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Ishimoto, Kenta. "Hydrodynamics of squirming locomotion at low Reynolds numbers." 京都大学 (Kyoto University), 2015. http://hdl.handle.net/2433/199079.
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Shehata, Hisham. "Unsteady Aerodynamic/Hydrodynamic Analysis of Bio-inspired Flapping Elements at Low Reynolds Number." Diss., Virginia Tech, 2020. http://hdl.handle.net/10919/97567.
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The impressive kinematic capabilities and structural adaptations presented by bio-locomotion continue to inspire some of the advancements in today's small-scaled flying and swimming vehicles. These vehicles operate in a low Reynolds number flow regime where viscous effects dominate flow interactions, which makes it challenging to generate lift and thrust. Overcoming these challenges means utilizing non-conventional lifting and flow control mechanisms generated by unsteady flapping body motion. Understanding and characterizing the aerodynamic phenomena associated with the unsteady motion is vital to predict the unsteady fluid loads generated, to implement control methodologies, and to assess the dynamic stability and control authority of airborne and underwater vehicles. This dissertation presents experimental results for forced oscillations on multi-element airfoils and hydrofoils for Reynolds numbers between Re=104 and Re=106. The document divides the work into four main sections: The first topic presents wind tunnel measurements of lift forces generated by an oscillating trailing edge flap on a NACA-0012 airfoil to illustrate the effects that frequency and pitching amplitude have on lift enhancement. The results suggest that this dynamic trailing edge flap enhances the mean lift by up to 20% in the stalled flow regime. Using frequency response approach, it is determined that the maximum enhancement in circulatory lift amplitude occurs at stalled angles of attack for lower pitching amplitudes. The second topic presents wind tunnel measurements for lift and drag generated by a sinusoidal and non-sinusoidal oscillations of a NACA-0012 airfoil. The results show that 'trapezoidal' pitching enhances the mean lift and the RMS lift by up to 50% and 35% in the pre-stall flow regime, respectively, whereas the 'reverse sawtooth' and sinusoidal pitching generate the most substantial increase of the lift-to-drag ratio in stall and post-stall flow regimes, respectively. The third topic involves a study on the role of fish-tail flexibility on thrust and propulsive efficiency. Flexible tails enhance thrust production in comparison to a rigid ones of the same size and under the same operating conditions. Further analysis indicates that varying the tail's aspect ratio has a more significant effect on propulsive efficiency and the thrust-to-power ratio at zero freestream flow. On the other hand, changing the material's property has the strongest impact on propulsive efficiency at non-zero freestream flow. The results also show that the maximum thrust peaks correspond to the maximum passive tail amplitudes only for the most flexible case. The final topic aims to assess the unsteady hydrodynamic forces and moments generated by a three-link swimming prototype performing different swimming gaits, swimming speeds, and oscillatory frequencies. We conclude that the active actuation of the tail's first mode bending produces the most significant thrust force in the presence of freestream flow. In contrast, the second mode bending kinematics provides the most significant thrust force in a zero-freestream flow.Doctor of Philosophy
It is by no surprise that animal locomotion continues to inspire the design of flying and swimming vehicles. Although nature produces complex kinematics and highly unsteady flow characteristics, simplified approximations to model bio-inspired locomotion in fluid flows are experimentally achievable using low degrees of freedom motion, such as pitching airfoils and trailing edge flaps. The contributions of this dissertation are divided into four primary foci: (a) wind tunnel force measurements on a flapped NACA-0012 airfoil undergoing forced pitching, (b) wind tunnel measurements of aerodynamic forces generated by sinusoidal and non-sinusoidal pitching of a NACA-0012 airfoil, (c) towing tank measurements of thrust forces and torques generated by a one-link swimming prototype with varying tail flexibilities, and (d) towing tank measurements of hydrodynamic forces and moments generated by active tail actuation of a multi-link swimming prototype. From our wind tunnel measurements, we determine that lift enhancement by a trailing edge flap is achieved under certain flow regimes and oscillating conditions. Additionally, we assess the aerodynamic forces for a sinusoidal and non-sinusoidal pitching of an airfoil and show that 'trapezoidal' pitching produces the largest lift coefficient amplitude whereas the sinusoidal and 'reverse sawtooth' pitching achieve the best lift to drag ratios. From our towing tank experiments, we note that the role of tail flexibility enhances thrust generation on a swimming device. Finally, we conclude that different kinematics on an articulating body strongly affect the hydrodynamic forces and moments. The results of the towing tank measurements are accessible from an online public database to encourage research and contribution in underwater vehicle design through physics-based low-order models that can accommodate hydrodynamic principles and geometric control concepts.
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Jibuti, Levan. "Locomotion et écoulement dans les fluides complexes confinés." Phd thesis, Université de Grenoble, 2011. http://tel.archives-ouvertes.fr/tel-00635980.
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Cette thèse est consacrée à l'étude de la dynamique et de la rhéologie des fluides complexes. Nous utilisons une méthode de simulation numérique à trois dimensions. Les systèmes que nous étudions ici sont des suspensions de micro-nageurs actifs, des suspensions de particules sphériques rigides en présence d'un champ externe auquel elles sont sensibles et de la dynamique de suspensions de particules sphériques et confinées en cisaillement. Les Micro-nageurs sont les objets microscopiques qui se propulsent dans un fluide et ils sont omniprésents dans la nature. Un exemple commun de micro-nageurs est la micro-algue textit{Chlamydomonas} . Un des buts principaux de cette thèse est de comprendre l'effet de la motilité de ces micro-organismes sur les propriétés macroscopiques globales de la suspension, telles que la viscosité effective pour expliquer les observations expérimentales. Nous avons élaboré différents modèles de suspensions de textit{Chlamydomonas} et effectué des simulations numériques utilisant la version 3D de la dynamique des particules fluides (FPD) (méthode expliquée dans cette thèse). Les résultats de nos simulations numériques ont été présentés et discutés à la lumière des observations expérimentales. Un des modèles proposés intègre tous les phénomènes observés expérimentalement et sont applicables à d'autres types de suspensions de micro-nageurs. Cette thèse consacre également un chapitre sur les effets du confinement sur la dynamique de cisaillement des suspensions diluées de particules sphériques. Nous avons constaté que dans la géométrie confinée, la vitesse angulaire des particules diminue par rapport à celle imposée par l'écoulement de cisaillement. La vitesse angulaire des particules diminue également lorsque la particule est proche d'une paroi unique et la vitesse de translation de la particule par rapport à la vitesse de la paroi diminue. Un autre objectif de cette thèse est d'étudier les suspensions à viscosité effective ajustable. Nous avons mené une étude numérique sur des suspensions de particules sphériques en présence d'un couple externe. Nous avons montré que le changement de vitesse angulaire des particules due à l'application d'un couple externe est suffisante pour modifier fortement la viscosité de la suspension. Basée sur des simulations numériques, une formule semi-empirique a été proposée pour la viscosité des suspensions de particules sphériques valables jusqu'à 40% de concentration. Nous avons également montré que la 2ème loi de Faxén peut être étendue par une expression empirique pour de grandes concentrations.
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El, Alaoui-Faris Yacine. "Modélisation et contrôle optimal de micro-nageurs magnétiques." Thesis, Université Côte d'Azur, 2020. http://www.theses.fr/2020COAZ4094.
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Les micro-nageurs robotiques permettent d'effectuer des opérations à petite échelle telles que l'administration ciblée de médicaments et la chirurgie peu invasive. En raison de la difficulté de miniaturiser des sources d'énergie internes, les méthodes d'actionnement externes sont préférables aux sources intégrées, une stratégie populaire étant l'aimantation du nageur ou d'une de ses parties et son actionnement avec des champs magnétiques externes. L'étude qui suit se concentre sur les micro-nageurs magnétiques flexibles qui imitent les cellules flagellées commeles spermatozoïdes dans leur conception et leur mode de locomotion. Le but decette thèse est d'appliquer des outils numériques de modélisation et de contrôle optimal aux nageurs expérimentaux de l'Institut des Systèmes Intelligents et de Robotique (ISIR) afin d’améliorer leur contrôle et de fournir une méthode numérique pour la conception de commandes pour les micro-nageurs flexibles. La première étape de cette thèse a été le développement d'un modèle dynamique simplifié d'un nageur magnétique flexible en trois dimensions, basé sur une approximation des forces hydrodynamiques et sur la discrétisation de la courbure et de l'élasticité du flagelle. Une identification des paramètres hydrodynamiques et élastiques du modèle permet d'avoir un nageur simulé qui présente les mêmes caractéristiques de propulsion (notamment la réponse fréquentielle du nageur) que celles mesurées expérimentalement. La seconde étape a été d'utiliser le modèle développé pour la résolution numérique du problème de contrôle optimal consistant à de trouver le champ magnétique qui maximise la vitesse de propulsion du nageur sous des contraintes sur la commande reflétant les contraintes physiquement imposées au champ magnétique. La dernière étape a été l'implémentation des champ magnétiques calculés dans le dispositif expérimental de l'ISIR et l'étude de leur performances expérimentales ainsi que de la capacité du modèle à prédire la trajectoire du nageurRobotic micro-swimmers are able to perform small-scale operations such astargeted drug delivery, and minimally invasive medical diagnosis and surgery.However, efficient actuation of these robots becomes more challenging as their size decreases. Hence, wireless actuation is preferable over built-in actuation sources, one of the most popular strategies is the magnetization of parts of the swimmer and its actuation with an external magnetic field. In this thesis, we focus on flexible magnetic micro-swimmers that are similar to spermatozoa in their design and flagellar propulsion. Our goal is to use numerical modeling and optimal control tools to improve the performance of existing swimmers made at the ISIR laboratory (Institut des Systèmes Intelligents et de Robotique) and to propose a numerical control design method for experimental flexible micro-swimmers.Firstly, a simplified 3D dynamic model of a flexible swimmer has been developed, based on the approximation of hydrodynamic forces and the discretization of the curvature and elasticity of the tail of the swimmer. By fitting the hydrodynamic and elastic parameters of our model accordingly, we are able to obtain propulsion characteristics (mainly the frequency response of the swimmer) close to those experimentally measured. Secondly, we numerically solve the optimal control problem of finding the actuating magnetic fields that maximize the propulsion speed of the experimental swimmer under constraints on the control that reflect the constraints physically imposed on the magnetic field. The optimal magnetic fields found via numerical optimization are then implemented in the ISIR experimental setup in order to benchmark the experimental performance of the computed controls and the ability of the model to predict the trajectories of the experimental swimmer
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Srinivasa, Murthy P. "Low Reynolds Number Airfoil Aerodynamics." Thesis, Indian Institute of Science, 2000. http://hdl.handle.net/2005/229.
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In this thesis we describe the development of Reynolds- averaged Navier Stokes code for the flow past two- dimensional configuration. Particularly, emphasis has been laid on the study of low Reynolds number airfoil aerodynamics.
The thesis consists of five chapters covering the back ground history, problem formulation, method of solution and discussion of the results and conclusion.
Chapter I deals with a detailed background history of low Reynolds number aerodynamics, problem associated with it, state of the art, its importance in practical applications in aircraft industries.
Chapter II describes the mathematical model of the flow physics and various levels of approximations. Also it gives an account of complexity of the equations at low Reynolds number regarding flow separation, transition and reattachment.
Chapter III describes method of solution, numerical algorithm developed, description of various upwind schemes, grid system, finite volume discrieti-zation of the governing equations described in Chapter II.
Chapter IV describes the application of the newly developed Navier Stokes code for the test cases from GAMM Workshop proceedings. Also it describes validation of the code for Euler solutions, Blasius solution for the flow past flat plate and compressible Navier Stokes solution for the flow past NACA 0012 Airfoil at low Reynolds number.
Chapter V describes the application of the Navier Stokes code for the more test cases of current practical interest . In this chapter laminar separation bubble characteristics are investigated in detail regarding formation, growth and shedding in an unsteady environment.
Finally the conclusion is drawn regarding the robustness of the newly developed code in predicting the airfoil aerodynamic characteristics at low Reynolds number both in steady and unsteady environment.
Lastly, suggestion for future work has been highlighted.
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Tam, Daniel See Wai 1980. "Motion at low Reynolds number." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/49682.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2008.Includes bibliographical references (p. 181-192).
The work described in this thesis centers on inertialess motion at low Reynolds numbers at the crossroad between biofluids and microfluids. Here we address questions regarding locomotion of micro-swimmers, transport of nutrient around micro-organisms as well as mixing and heat exchange inside micro-droplets of water. A general framework for the investigation of optimal locomotion strategies for slender swimmers has been developed and applied to different systems. Here we exclusively study the hydrodynamical aspects of locomotion without further consideration for the swimmers internal dynamics. The first system studied is the "three-link" swimmer, first introduced and discussed by Nobel prize laureate E.M. Purcell in his famous lecture "Life at low Reynolds number" [121]. For this simple swimmer, we find and later discuss optimal stroke kinematics and swimmer geometries. We then further investigate flagellated swimmers and verify the convergence of the optimization procedure in the case of a single flagellum, for which the optimal stroke kinematics are known analytically. Optimal stroke kinematics and geometries for unifiagellates are also computed and found to be relevant in the context of biological microorganisms.
(cont.) We then turn our attention to stroke kinematics of biflagellates and demonstrate that all the different strokes, which are experimentally observed to be performed by biflagellated organisms such as green algae chlamydomonas, are found to be local hydrodynamical optima. These observations strongly suggest the central role of hydrodynamics in the internal dynamical organization of the stroke patterns. Finally, we present experimental results on convective transport and mixing inside small droplets of water sitting on superhydrophobic substrates. We demonstrate by a scaling analysis, that the regular convection pattern is due to a thermocapillary driven Marangoni flow at the surface of the droplet. We develop an analytical solution for the temperature and flow field inside the droplet, which is found to be in agreement with our experimentally recorded data.
by Daniel See-Wai Tam.
Ph.D.
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Erm, Lincoln. "Low-Reynolds-number turbulent boundary layers /." Connect to thesis, 1988. http://eprints.unimelb.edu.au/archive/00000226.
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Stevens, Patrick Robert Robbie James. "Unsteady low Reynolds number aerodynamic forces." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709135.
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Box, Stuart James. "Rotational motion at low Reynolds number." Thesis, University of Bristol, 2015. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.688351.
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Driven cyclic or periodic motion is a recurrent feature of many of the microscopic mechanical systems that support life. For example, fields of cilia, nano-scale hair-like structures, beat together to transport fluids though mammalian tracts. Bacteria and other micro-organisms are able to swim using similar organelles known as flagella, while "molecular motors" provide traction in muscle tissue. These mechanisms rely on the hydrodynamics and statistical mechanics of driven cyclic motion at the micro-scale. The purpose of this thesis is to investigate these principles in an abstract sense, in order to better understand these aspects of biology and to provide a framework for the design of future biomimetic devices. In particular, two aspects of rotational motion at low Reynolds number, the viscous dominated regime occupied by microorganisms and micro-machines,are investigated in this work. First, hydrodynamic synchronisation at low Reynolds number is considered. A model system is created that comprises two colloidal spheres driven along circular paths. The driving forces are applied using optical tweezers, a tool that employs a highly focussed laser beam to exert known forces on micro-particles. Each sphere is driven such that it experiences a given optical force profile, but the net force, and thus the resulting rotation rate, are free to vary. A fluid-mediated interaction force also acts on the spheres, and spontaneously induces synchronisation of their rotational motion. This system is an experimental demonstration of minimal models that were previously proposed to describe the synchronous behaviour of flagella. Synchronisation is only possible under certain conditions at low Reynolds number. In the system employed here, synchronisation can occur either via small deformations of each sphere's circular path, or by modulation of the optical driving force. Synchronisation strength is found to depend on these two mechanisms as predicted by theory. Next, the effect of thermal fluctuations on a rotating system are considered. A micro-rotor that experiences a torque when optically trapped is fabricated using photo-polymerisation. This rotor is used to experimentally demonstrate a rotational Fluctuation Theorem, which describes the probability of observing a trajectory over which the surrounding medium does work on the rotor. In the macro-world, a trajectory of this kind would be said to violate the second law of thermodynamics, but is made possible at the micro-scale because the relevant forces and energies are similar in magnitude to the thermal energy of the system. The probability of observing these trajectories is shown to decrease exponentially with the time over which the rotor is studied, as predicted by the Fluctuation Theorem.
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Chebbi, Brahim. "Experiments in low Reynolds number flows." Thesis, University of Ottawa (Canada), 1989. http://hdl.handle.net/10393/5989.
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Yu, Tony S. (Tony Sheung). "Elastic tail propulsion at low Reynolds number." Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/38700.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 73-76).
A simple way to generate propulsion at low Reynolds number is to periodically oscillate a passive flexible filament. Here we present a macroscopic experimental investigation of such a propulsive mechanism. A robotic swimmer is constructed and both tail shape and propulsive force are measured. Filament characteristics and the actuation are varied and resulting data are quantitatively compared with existing linear and nonlinear theories.
by Tony S. Yu.
S.M.
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Samson, Ophir. "Low Reynolds number swimming in complex environments." Thesis, Imperial College London, 2010. http://hdl.handle.net/10044/1/5723.
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The study of swimming micro-organisms has been of interest not just to biologists, but also to fluid dynamicists for over a century. As they are rarely in isolation, much interest has been focused on the study of the swimmers’ interaction with their environment. By virtue of the typically small sizes of these organisms and their swimming protocols, the characteristic Reynolds number of the motion of the fluid around them is small. Hence they reside in a Stokes flow regime where viscous forces dominate inertial effects and where far-field interactions (e.g. with nearby walls) can have a significant effect on the swimmer’s dynamical evolution. This thesis provides a detailed investigation of idealised models of low Reynolds number swimmers in a variety of wall-bounded fluid domains. Our approach employs a combination of analytical and numerical techniques. A simple two-dimensional point singularity is used to model a swimmer. We first study its dynamics when placed in the half-plane above an infinite no-slip wall and find it to be in qualitative agreement with numerical and experimental studies. The success of the model in this case encourages its use to study the swimmer’s dynamics in more complicated domains. Specifically, we next explore the dynamics of the same swimmer above an infinite straight wall with a single gap, or orifice. Using techniques of complex analysis and conformal mapping theory, a dynamical system governing the swimmer’s motion is explicitly derived. This analysis is then extended to the case in which the swimmer evolves near an infinite straight wall with two gaps. We are also interested in how the presence of background flows can affect the swimmer’s dynamics in these confined geometries. We therefore employ the same techniques of complex analysis and conformal mappings to find analytical expressions for pressure-driven flows near a wall with either one or two gaps. We then extend this to find new solutions for the shear flows and stagnation point flows in the same geometry. The effect of a background shear flow on the swimmer’s dynamics is then explored. Finally, while there have been a number of studies of Stokes flows within domains which are simply connected, the doubly connected analogues are rather rare. By building upon the analytical techniques presented in this thesis, we present numerical solutions to such problems, including that of theWeis-Fogh mechanism in the low Reynolds number regime.
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Calderon, Dario. "Plunging low aspect ratio wings in low Reynolds number flows." Thesis, University of Bath, 2014. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.616645.
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A growing desire exists to develop Micro Air Vehicles (MA Vs) that fall within a 15cm span. Their small scale and low operating flight speeds encourage a low Reynolds Number (Re) regime, in the order of Re - 104 - 105 . Wings under these conditions are highly susceptible to separated flows, posing a significant challenge for the MA V. Natural flyers are able to confront these issues through flapping flight, which has inspired an entire research field on the aerodynamics of oscillating wings. While the number of parameters that govern the problem is exhaustive, studies are required to explain the contribution of each and any phenomena that may ensue. This lends itself to a canonical approach. This thesis presents an experimental study on various wing geometries, undergoing a small amplitude oscillation in the form of a pure plunge. The focus lies on understanding the three-dimensional effects of oscillating a finite wing with a positive geometric angle of attack, to encourage greater lift than that achieved from an unforced wing. This expands on the current research which predominantly focuses on the thrust generating capabilities of a 'flapping' airfoil. Force measurements, hot-film measurements, Particle Image Velocimetry (PIV) and volumetric velocimetry, are used to examine the performance and flow topology that ensues from actuating the various wings. The study presents time-averaged force measurements as a function of Strouhal number (non-dimensionalised plunge frequency) for the various low aspect ratio wings. It is shown that while the finite nature of these wings suppresses lift, significant improvements are nonetheless possible. For example, a semi Aspect Ratio = 2 NACA0012 rectangular wing, is able to achieve 180% more lift than the unforced wing. A phenomenon arises in which peaks are observed in the time-averaged lift curve, for various rectangular and delta wing planforms. This suggests optimal lift conditions at particular Strouhal numbers. In a similar manner to a 2D airfoil the oscillating wing stimulates the formation of both leading and trailing-edge vortices. The trajectory and timing of these vortices, in relation to the plunge cycle, appear to be significantly affected by Strouhal number. At particular frequencies, the vortices interact in such a way that their induced flow generates a significant region of low velocity, recirculating flow near the wing. The size of the recirculating region closely correlates with the shape of the time-averaged lift curve, agreeing well with points of troughs and peaks when this region is maximised and minimised, respectively. It is thought that these Wing/vortex and vortex/vortex interactions contribute to the selection of optimal frequencies, and therefore determine optimal lift for the oscillating wing.
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Nelson, Joshua T. "Practical modification for low Reynolds number propeller applications." Thesis, Wichita State University, 2009. http://hdl.handle.net/10057/2514.
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Applications for Unmanned Aerial Vehicles (UAV) and, specifically, Micro Aerial Vehicles (MAV) are increasing. As is the case for all aircraft, propulsion plays a significant factor in overall vehicle performance.
Most small UAV or MAV propellers are commercial, off-the-shelf products given their availability and low cost. Unfortunately, the off-the-shelf propellers are not tailored to a specific vehicle and/or mission. Only limited technical and performance data is available for the propellers.
A number of modifications have been used on larger (manned) aircraft propellers and rotors in the past to improve performance. Examples of possible modifications include vortex generators, cut tips, and tip sails. An investigation was conducted with the following goals:
1. Experimentally measure the performance impact of various modifications on UAV/MAV-class propellers
2. Study the practical applications for such modifications to UAV/MAV's
3. Evaluate the utility of related propeller performance prediction tools
Experimental and basic analytical investigations address these goals. Special emphasis was placed on studying simple, practical, and cost effective modifications.
This investigation shows that after testing 24 modifications a number of practical modifications to improve propeller performance do exist. 2-Propeller, leading edge notches, and Gurney flap modifications all improve thrust while leading edge notches and Gurney flap modifications also improve cruise efficiency.Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering
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Hall, Jeremiah. "Low Reynolds number aerodynamics for micro aerial vehicles." Connect to online resource, 2007. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1448684.
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Usui, Michiko. "AEROMECHANICS OF LOW REYNOLDS NUMBER INFLATABLE/RIGIDIZABLE WINGS." UKnowledge, 2004. http://uknowledge.uky.edu/gradschool_theses/321.
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Use of an inflatable/rigidizable wing is explored for Mars airplane designs. The BIG BLUE (Baseline Inflatable-wing Glider Balloon Launched Unmanned airplane Experiment) project was developed at the University of Kentucky, with an objective to demonstrate feasibility of this technology with a flight-test of an high-altitude glider with inflatable/rigidizable wings. The focus of this thesis research was to design and analyze the wing for this project. The wings are stowed in the fuselage, inflate during ascent, and rigidize with exposure to UV light. The design of wings was evaluated by using aerodynamic and finite element software and wind tunnel testing. The profile is chosen based upon aerodynamic results and consideration of manufacturability of the inflatable wing structures. Flow over prototypes of inflatable/rigidizable and ideal shaped wings were also examined in the wind tunnel. Flow visualization, lift and drag measurements, and wake survey testing methods were performed. Results from the wind tunnel testing are presented along with suggestions in improving the inflatable/rigidizable wings aerodynamic efficiency and use on a low Reynolds number platform. In addition, high altitude wing deployment tests and low altitude flight tests of the inflatable/rigidizable wing were conducted.
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Gough, Tim. "Low Reynolds number turbulent boundary layers and wakes." Thesis, University of Surrey, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360949.
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Childs, Laura Helen. "Low Reynolds number flows of generalized non-Newtonian fluids." Thesis, University of Bristol, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.633090.
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A Newtonian fluid is one for which the stress induced by flow is proportional to the strain rate, with the constant of proportionality being the fluid's viscosity. Fluids not adhering to this linear relationship are described as non-Newtonian. This thesis considers several problems involving flows of generalized non-Newtonian fluids - where the effective viscosity is a nonlinear function of the strain rate - experiencing negligible inertial effects. First, we consider the flow of a power-law fluid through a rectangular duct. We propose a numerical method of solution, reliant on expressing the governing equations in terms of the stress tensor components, rather than the velocity field. This results in an effective method of determining the flux of fluid through a cross-section of a duct. The calculation is extended to duct flows of regularized Herschel-Bulkley fluids, and the determination of the critical yield stress below which there is no flow. We then consider free-surface dam-break flows of power-law fluids. By utilizing a similarity scaling for the front position of the flow with time, the flux calculation of the preceding work, and laboratory experiments, we propose a method of determining the rheological parameters of a power-law fluid. The settling velocity of a spherical particle through a power-law fluid is next examined. For particles settling within a background shear flow, the settling velocity is found to have two distinct dependencies on the dimensionless flow parameters, corresponding to regimes of dominant background shear or gravitational settling. Finally, we consider the effect that a modified sedimentation law, based upon the results of the settling calculation, has on shear flows of dilute particulate suspensions. The method of characteristics is applied to investigate the sedimentation law's impact on properties such as the concentration profile, and run-out length, of a sustained intrusion of particles suspended in a power-law fluid.
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Howell, R. J. "Wake-separation bubble interactions in low Reynolds number turbomachinery." Thesis, University of Cambridge, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604667.
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There is a continual demand for a reduction in the cost and weight of aero-engines. Suppliers of engines always need to reduce manufacturing costs and airlines want to carry more passengers or cargo. This thesis will show that reducing the number of blades in the low pressure turbine is a potent way of achieving both of these goals. Through a program of experimental and numerical work, it is shown how it is possible to reduce the number of blades in the turbine by approximately 15% relative to the first generation of high lift blading employed in the BMW Rolls-Royce BR715 low pressure turbine. A series of measurements from surface mounted hot films are shown from two full scale BMW Rolls-Royce LP turbines. These measurements define the current state of the art of LP turbine blade design. It is demonstrated that the suction side boundary layer flow is entirely dominated by the passage of wakes from upstream blade rows. Low speed measurements were carried out on a linear cascade of highly loaded low pressure turbine blades (designated TL10) similar in style to those used for the BMW Roll-Royce tests. The rig used for these tests incorporated a moving bar wake generator to simulate the presence of a single upstream blade row. Reductions in loss were measured when wakes were present compared to the case with steady inflow at low Reynolds numbers. A novel technique was used to increase the blade loading to above the levels of the TL10 profile. The loss reduction of a number of suction side pressure distributions were then investigated for cases with and without incoming wakes. Loss reductions were again observed when wakes were present and this reduction in loss increased as the lift of the profiles increased. Hot film measurements indicated that moving the position of boundary layer separation aft will decrease the losses generated. Further measurements proved that aft loaded profiles performed better than forward loaded profiles with unsteady inflow. The key to the loss reductions was the interaction of the wakes (and the turbulent spots that they form) with the separation bubble.
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Jones, Anya Rachel. "Unsteady low Reynolds number aerodynamics of a waving wing." Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608953.
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Curtis, Mark Peter. "Aspects of low Reynolds number microswimming using singularity methods." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:13dcb39b-f5b7-4d46-92d4-21a9afbecd08.
Full textAbstract:
Three different models, relating to the study of microswimmers immersed in a low Reynolds number fluid, are presented. The underlying, mathematical concepts employed in each are developed using singularity methods of Stokes flow. The first topic concerns the motility of an artificial, three-sphere microswimmer with prescribed, non-reciprocal, internal forces. The swimmer progresses through a low Reynolds number, nonlinear, viscoelastic medium. The model developed illustrates that the presence of the viscoelastic rheology, when compared to a Newtonian environment, increases both the net displacement and swimming efficiency of the microswimmer. The second area concerns biological microswimming, modelling a sperm cell with a hyperactive waveform (vigorous, asymmetric beating), bound to the epithelial walls of the female, reproductive tract. Using resistive-force theory, the model concludes that, for certain regions in parameter space, hyperactivated sperm cells can induce mechanical forces that pull the cell away from the wall binding. This appears to occur via the regulation of the beat amplitude, wavenumber and beat asymmetry. The next topic presents a novel generalisation of slender-body theory that is capable of calculating the approximate flow field around a long, thin, slender body with circular cross sections that vary arbitrarily in radius along a curvilinear centre-line. New, permissible, slender-body shapes include a tapered flagellum and those with ribbed, wave-like structures. Finally, the detailed analytics of the generalised, slender-body theory are exploited to develop a numerical implementation capable of simulating a wider range of slender-body geometries compared to previous studies in the field.
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Shirley, Gregory B. (Gregory Brenton) 1975. "An experimental investigation of a low Reynolds number, high Mach number centrifugal compressor." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/9665.
Full textAbstract:
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1998.Includes bibliographical references (p. 119-120).
Experiments were performed on a super-scale test facility to study the performance of very small centrifugal compressors suitable for microfabrication. ThE test facility was 75 times larger than the full-scale device and operated under 17/5 atmospheric pressure conditions to correctly match the Reynolds number, which is about 20,000. The impeller tip speed of the test facility matched those in the design full-scale device to capture the effects of compressibility. Two compressor geometries were tested, one with and one without diffuser vanes; the same impeller was used in both cases. For each of the diffuser designs, speed lines were determined at impeller tip speeds of 400 mis, corresponding to 100% of the design value, and 170 mis, corresponding to 42% of design. Detailed measurements were made at a single operating point on each of the speedlines. The test results show that the total-to-static pressure ratios developed by the facility are in agreement with CFO prediction. The measured mass flow rates are between the values predicted by 2-D and 3-D CFO. The performance of the vaned diffuser (Cp=0.48) is superior to that of the vaneless diffuser (Cp=0.24) in recovering the dynamic pressure of air exiting the impeller. The impeller isentropic efficiencies are about 0.48 and 0.27 for 100% and 42% design impeller speeds, respectively. These values are substantially lower than the predictions of CFO. The difference is thought to be due in part to the interaction of the flow in the impeller with the impeller casing, and to the effects of inlet separation. This study confirms computational predictions for pressure rise, and is consistent with compuational predictions of mass flow rate. However, the measured efficiency is lower than CFD predictions. Additional testing is required to determine the sources of loss in the impeller.
by Gregory B. Shirley.
S.M.
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Sathaye, Sagar Sanjeev. "Lift distributions on low aspect ratio wings at low Reynolds numbers." Link to electronic thesis, 2004. http://www.wpi.edu/Pubs/ETD/Available/etd-0427104-091112.
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Thesis (M.S.)--Worcester Polytechnic Institute.Keywords: Low Reynolds Number; Micro Air Vehicle; Low Aspect Ratio; Spanwise pressure measurements; Spanwise Lift Distributions. Includes bibliographical references (p. 84-85).
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Chen, Suzhen Aerospace & Mechanical Engineering Australian Defence Force Academy UNSW. "An improved low-Reynolds-number k-E [ symbol -dissipation rate]." Awarded by:University of New South Wales - Australian Defence Force Academy. School of Aerospace and Mechanical Engineering, 2000. http://handle.unsw.edu.au/1959.4/38651.
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[Formulae and special characters can only be approximated here. Please see the pdf version of the Abstract for an accurate reproduction.] Since the damping functions employed by most of the low-Reynolds-number models are related to the non-dimensional distance y+[ special character ??? near-wall non-dimensional distance in y direction], which is based on local wall shear stress, these models become invalid for separated flows, because the wall shear stress is zero at the reattachment point. In addition, the pressure-velocity correlation term is neglected in most of these models, although this term is shown in this thesis to be important in the near-wall region for simple flows and large pressure gradient flows. In this thesis, two main efforts are made to improve the k ??? [special character - dissipation rate] model. First, based on Myong and Kasagi???s (1990) low-Reynolds-number model (hereafter referred to as MK model), a more general damping function [special character - turbulent viscosity damping function in LRN turbulent model] is postulated which only depends on the Reynolds numbers [formula ??? near-wall turbulence Reynolds number]. Second, a form for the pressure-velocity correlation term is postulated based on the Poisson equation for pressure fluctuations. This modified model predicts the turbulent flow over a flat plate very well. It is found that the inclusion of the pressure-velocity correlation term leads to significant improvement of the prediction of near-wall turbulence kinetic energy. When the model is applied to turbulent flow over a backward-facing step, it produces better predictions than the traditional k ??? [special character - dissipation rate] model, FLUENT???s two-layer model and the MK model. Again, the pressure-velocity correlation term improves the turbulence kinetic energy prediction in the separated region over that of other models investigated here. The studies of numerical methods concerning computational domain size and grid spacing reveal that a very large domain size is required for accurate flat plate flow computation. They also show that a fine grid distribution in the near-wall region upstream of the step is necessary for acceptable flow prediction accuracy in the downstream separated region.
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Wolgemuth, Charles William. "Theory and experiment on thin life at low Reynolds number." Diss., The University of Arizona, 2000. http://hdl.handle.net/10150/289171.
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Many interesting problems in cellular biophysics involve the dynamics of filamentary elastic objects with bend and twist degrees of freedom, moving in a viscous environment. Motivated by the mysterious macrofiber formation in B. subtilis and the rotational dynamics of bacterial flagella, we have sought to establish a general theoretical structure to deal with elastic filament dynamics, analyze these equations for model systems, and to determine the important physical parameters that set the dynamical scales for these systems. We first studied the novel problem of a rotationally forced elastic filament in a viscous fluid [1] to examine the competition between twist injection, twist diffusion, and writhing motions. Two dynamical regimes separated by a Hopf bifurcation were discovered: (i) diffusion-dominated axial rotation, or twirling, and (ii) steady-state crankshafting motion, or whirling. Next, we extended elasticity theory of filaments to encompass systems, such as bacterial flagella, that display competition between two helical structures of opposite chirality [2]. A general, fully intrinsic formulation of the dynamics of bend and twist degrees of freedom was developed using the natural frame of space curves, spanning from the inviscid limit to the viscously-overdamped regime applicable to cellular biology. To be able to measure the elastic properties of cell-sized objects, such as bacterial fibers [3], we utilized an optical trapping system to study the relaxation of a single fiber of B. subtilis which was bent and then released. By analyzing the relaxation time, the bending modulus of the bacterial cell wall was measured to be 1.6 ± 0.6 x 10⁻¹² erg·cm. This number is important in understanding the scales of forces and torques that are present in macrofiber formation and motion, lending insight into the mechanism behind these phenomena.
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Stoos, James Arthur Leal L. Gary Leal L. Gary Herbolzheimer Eric. "Particle dynamics near fluid interfaces in low-Reynolds number flows /." Diss., Pasadena, Calif. : California Institute of Technology, 1988. http://resolver.caltech.edu/CaltechETD:etd-02022007-110333.
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Mehra, Amitav. "Computational investigation and design of low Reynolds number micro-turbomachinery." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/10516.
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Saha, Nilanjan. "Gap Size Effect on Low Reynolds Number Wind Tunnel Experiments." Thesis, Virginia Tech, 1999. http://hdl.handle.net/10919/35938.
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A system was designed to measure the effect of gap size on semi-span low Reynolds number wind tunnel experiments. The lift forces on NACA 1412, NACA 2412 and NACA 4412 half wings were measured using a strain gauge balance at chord Reynolds numbers of 100,000 and 200,000 and three different gap sizes including sealed gap. Pressure distributions on both airfoil top and bottom surfaces in the chord-wise direction near the gap were recorded for these airfoils. Also recorded was the span wise pressure distribution on both the airfoil surfaces at the quarter chord section. The results revealed that the presence of the gap, however small, affects the measurements. These effects were mainly observed in drop of lift and change in zero lift angle of attack and change in stall angle for the airfoil. The size of the gap is not linearly related to these changes, which also depend on the camber of the airfoil. These changes occur due to the flow through the gap from the lower surface to the upper surface of the model. The wing/end plate gap effect reduces along the span but is not fully restricted to the base of the model and the model behaves more like a full three-dimensional wing than a semi-span model. This study was made possible with the support of Department of Aerospace and Ocean Engineering, Virginia Polytechnic Institute and State University under the supervision of Dr. James MarchmanMaster of Science
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Chen, Jian. "Chemistry and physics in low Reynolds number micro steady streaming devices /." Thesis, Connect to this title online; UW restricted, 2005. http://hdl.handle.net/1773/9928.
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Viieru, Dragos. "Flapping and fixed wing aerodynamics of low Reynolds number flight vehicles." [Gainesville, Fla.] : University of Florida, 2006. http://purl.fcla.edu/fcla/etd/UFE0015704.
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Vargas-Dilaz, Salvador. "Numerical simulations of hydrodynamic particle interactions at low particle Reynolds number." Thesis, University of Edinburgh, 2008. http://hdl.handle.net/1842/11500.
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When solid particles are suspended in the fluid and are not in a jammed state, a fruitful approach to modelling the system can be to describe it as a system of particles interacting both with each other and with an external field. In the specific case when the particles are far enough apart, the dominant interactions between particles are those mediated by the surrounding fluid rather than direct particle-particle interactions, possibly only when the particles are touching. One of the most important phenomena observed in this regime is particle roping – rather than being evenly dispersed throughout the fluid, particles congregate in one or more ‘ropes’ aligned with the flow direction. This can be a serious problem in coal fired power stations, which require coal dust to be evenly distributed to operate at maximum efficiency. This thesis presents a basic numerical study of particle-fluid-particle interactions under conditions characteristic of the roping phenomenon found after bends in the pneumatic transport systems of coal fired power plants. The main objectives of this work are to: 1. Obtain a pair potential hydrodynamic force field from computational fluid dynamics (CFD) simulations of two fixed spherical particles at low particle Reynolds number; 2. Estimate the magnitude of errors introduced by the pair potential approximation by comparing the two fixed spherical particles results with CFD simulations of systems of three fixed spherical particles; and 3. Use many-particle Monte Carlo simulations to investigate the conditions under which clustering or roping occurs.
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Cleaver, David. "Low Reynolds number flow control through small-amplitude high-frequency motion." Thesis, University of Bath, 2011. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.547620.
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There is currently growing interest in the field of Micro Air Vehicles (MAVs). A MAV is characterized by its low Reynolds numbers flight regime which makes lift and thrust creation a significant challenge. One possible solution inspired by nature is flapping flight, but instead of the large-amplitude low-frequency motion suited to the muscular actuators of nature, small-amplitude high-frequency motion may be more suitable for electrical actuators. In this thesis the effect of small-amplitude high-frequency motion is experimentally investigated focusing on three aspects: general performance improvement, deflected jets, and the effect of geometryResults presented herein demonstrate that using small-amplitude high-frequency plunging motion on a NACA 0012 airfoil at a post-stall angle of attack of 15° can lead to significant thrust production accompanying a 305% increase in lift coefficient. At low Strouhal numbers vortices form at the leading-edge during the downward motion and then convect into the wake. This ‘mode 1’ flow field is associated with high lift but low thrust. The maximum lift enhancement was due to resonance with the natural shedding frequency, its harmonics and subharmonics. At higher Strouhal numbers the vortex remains over the leading-edge area for a larger portion of the cycle and therefore loses its coherency through impingement with the upward moving airfoil. This ‘mode 2’ flowfield is associated with low lift and high thrust. At angles of attack below 12.5° very large force bifurcations are observed. These are associated with the formation of upwards or downwards deflected jets with the direction determined by initial conditions. The upwards deflected jet is associated with the counter-clockwise Trailing Edge Vortex (TEV) loitering over the airfoil and thereby pairing with the clockwise TEV to form a dipole that convects upwards. It therefore draws fluid from the upper surface enhancing the upper surface vortex leading to high lift. The downwards deflected jet is associated with the inverse. Deflected jets were not observed at larger angles of attack as the asymmetry in the strength of the TEVs was too great; nor at smaller amplitudes as the TEV strength was insufficient. To understand the effect of geometry comparable experiments were performed for a flat plate geometry. At zero degrees angle of attack deflected jets would form, as for the NACA 0012 airfoil, however their direction would switch sinusoidally with a period on the order of 100 cycles. The lift coefficient therefore also switched. At 15° angle of attack for Strouhal numbers up to unity the performance of the flat plate was comparable to the NACA 0012 airfoil. Above unity, the upper surface and lower surface leading-edge vortices form a dipole which convects away from the upper surface resulting in increased time-averaged separation and reduced lift.
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Cole, Sarah Elizabeth S. B. Massachusetts Institute of Technology. "Design of two-tailed swimmer to swim at low-Reynolds number." Thesis, Massachusetts Institute of Technology, 2009. http://hdl.handle.net/1721.1/54530.
Full textAbstract:
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 30).
In the realm of systems with Reynolds numbers less than 1, swimming is a difficult task. Viscous forces from the fluid dominate inertial forces. In order to propel itself, a mechanism must be designed to overcome the viscous forces from the fluid and satisfy the non-reciprocal, cyclic motion requirements of the Scallop Theorem. Furthermore, a swimmer must employ one of the three mechanisms stated by Purcell to be capable of swimming at low Reynolds number, a three link swimmer, a corkscrew, or a flexible tail. Three devices utilizing the flexible-tail paradigm of swimming were tested using silicon oil to simulate a Reynolds number of approximately 0.6. Design parameters were uncovered which determine the successfulness of the swimmer and can be used for creating future successful flexible-tail swimmers.
by Sarah Elizabeth Cole.
S.B.
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Hammett, Victoria N. (Victoria Nicole). "Optimizing the stroke of Purcell's rotator, a low Reynolds number swimmer." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/74440.
Full textAbstract:
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 24).
Purcell's rotator is a theoretical low Reynolds number swimmer that can act as a model of more complex natural microorganisms, such as E.coli. Because of the low Reynolds number environment, the swimmer has approximately no inertia and it's motion is dominated by viscous forces. The version of Purcell's rotator examined in this paper is two dimensional and has three rigid links which rotate about the center of the body. It is able to propel itself by moving these links in a repetitive, nonreciprocal stroke motion. Using a mathematical model of the swimmer, two strokes were found, one which optimizes its rotation of the swimmer and one which optimizes its translation.
by Victoria N. Hammett.
S.B.
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Strong, Elizabeth Ford. "Hydrodynamic loading of a porous plate at low Reynolds number conditions." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/113751.
Full textAbstract:
Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 59-63).
In this thesis, we discuss our experimental work conducted to investigate the flow through and around porous disks driven through a viscous fluid at low Reynolds number conditions. Specifically, we present the results from a series of experiments in which we investigated the hydrodynamic drag experienced by thin (thickness to diameter ratio is t/d < 5%), circular disks of constant porosity (void fraction, [phi] = 69±2%). We characterize the dependence of the hydrodynamic loading on the size and shape of the perforations in the disk using a parameter called drag ratio, which compares the magnitudes of drag that porous and impermeable disks experienced. These experiments were conducted using a displacement controlled experimental apparatus, which, to the best of our knowledge, is the first of its kind. We benchmarked this experimental apparatus with a second experiment, and we found excellent agreement between experimental results and the analytical prediction. We find that the drag ratio depends on the effective void radius, but not on the thickness of the disks. We rationalize our results by comparing them to an existing analytical solution by way of a scaling analysis.
by Elizabeth Ford Strong.
S.M.
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Roy, Valentin. "Numerical analysis of airfoils with gurney flaps at low reynolds number." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=119634.
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A two-dimensional numerical investigation was performed to determine the effect of a Gurney flap on symmetric and cambered airfoils at very low Reynolds number. A Gurney flap is a device consisting of a small tab attached at the trailing edge and perpendicular to the chord line of the airfoil. Originally installed on a race car wing, a Gurney flap was proven to have a positive influence on the lift to drag ratio, and leading therefore to better efficiency and performance. First studies of Gurney flap showed that the optimal length of such a device is in the order of 1-4% of the chord length. The increase in lift comes primarily from the increased effective camber of the airfoil without disturbing the upper surface flow. The flow field around the airfoil is numerically studied in this thesis using an efficient numerical analysis based on a pseudo-time integration method using artificial compressibility. Gurney flap of sizes ranging from 1 to 4% of the airfoil chord were studied. The numerical solutions show the pressure distribution along the airfoil and flap as well as the lift and drag coefficients. The streamline contours illustrating the flow separation regions have also been generated. The influence of the flap length and the angle of attack on the aerodynamic coefficients was thoroughly studied.Une etude numérique bi-dimensionelle a été menée afin de determiner l'influence de la présence d'un aileron de type Gurney sur des profils d'aile symétriques et cambrés pour de très faibles nombres de Reynolds. Un rabat Gurney consiste en une petite pièce droite fixée au bord de fuite de l'aile et perpendiculaire à la corde de celle-ci. A l'origine installé sur l'aileron d'une voiture de grand tourisme, il a été observe que le rabat Gurney pouvait augmenter le coefficient de portance sans toutefois déteriorer le rapport portance/trainée, resultant ainsi en de meilleures performances. Les premières etudes faisant intervenir le rabat Gurney ont montrées que la longueur optimale de ce dernier est de l'ordre de 1 a 4% de la corde. L'augmentation de la portance vient premièrement du fait de l'augmentation effective de la cambrure de l'aile, sans toutefois venir perturber l'écoulement extrado. Le champ aérodynamique autour de l'aile a été calculé numériquement par le biais d'un schema numérique base sur une méthode d'intégration en pseudo-temps incluant le concept de compressibilité artificielle. Des volets dont la taille varie de 1à 4% de la corde de l'aile ont été etudiés. Des visualisations incluant les lignes de courant ainsi que les zones de recirculation ont également été génerées Cette etude détaille l'influence des parameters nombre de Reynolds, configuration de l'aile, angle d'attaque, taille du rabat sur l'écoulement.
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Ihi, Rafael Nascimento. "Low and high reynolds number study of fluid-structure interaction problems." Instituto Tecnológico de Aeronáutica, 2014. http://www.bd.bibl.ita.br/tde_busca/arquivo.php?codArquivo=2945.
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The present work is concerned with studying fluid-structure interaction problems using a high-fidelity representation for the fluid. In particular, the research aims to analyze the aeroelastic behavior of rigid airfoils and cylinders with elastic constraints, with emphasis in the effects of the inclusion of viscous terms in the aerodynamic formulation. The aerodynamic operator is constructed from the results of flow simulations using a computational fluid dynamics (CFD) tool which solves the 2-D Reynolds-averaged Navier-Stokes (RANS) equations with appropriate turbulence closures. Both low and high Reynolds number flow conditions are addressed in the present investigation. An in-house developed CFD solver is used for the simulations. Studies of low Reynolds number flows are directed towards addressing the physical phenomena present in the wake of cylinders, as well as their effects on the bodies present in the flow. The typical applications of interest in such cases are vortex-induced vibration problems which can arise in many practical scenarios, ranging from satellite launch vehicles at the launch platform to underwater risers in the petroleum industry. The study of such low Reynolds number flows has also been used as a building block in the process of developing the computational tools for addressing the fluid-structure interaction problems of interest here, since the computational requirements in such cases are much less stringent. Studies performed at high Reynolds number flows are directed towards typical aeroelastic stability analyses of lifting surfaces at transonic conditions. The aeroelastic system of interest is represented by a rigid NACA 0012 airfoil-based typical section with both plunge and pitch elastic degrees of freedom. Root locus stability analyses of the aeroelastic system are performed in order to predict the flutter onset point for a given flight condition. Results obtained in the present work indicate that the simulation capability implemented is adequate for handling the fluid-structure interaction problems of interest. However, as expected, computational requirements become very severe for the high Reynolds number flows and several numerical techniques have to be brought to bear in order to allow treatment of such aeroelastic problems in a sufficiently efficient manner.
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Johansen, Todd A. "Optimization of a Low Reynolds Number 2-D Inflatable Airfoil Section." DigitalCommons@USU, 2011. https://digitalcommons.usu.edu/etd/864.
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A stand-alone genetic algorithm (GA) and an surrogate-based optimization (SBO) combined with a GA were compared for accuracy and performance. Comparisons took place using the Ackley Function and Rastrigin's Function, two functions with multiple local maxima and minima that could cause problems for more traditional optimization methods, such as a gradient-based method. The GA and SBO with GA were applied to the functions through a fortran interface and it was found that the SBO could use the same number of function evaluations as the GA and achieve at least 5 orders of magnitude greater accuracy through the use of surrogate evaluations.
The two optimization methods were used in conjunction with computational fluid dy- namics (CFD) analysis to optimize the shape of a bumpy airfoil section. Results of opti- mization showed that the use of an SBO can save up to 553 hours of CPU time on 196 cores when compared to the GA through the use of surrogate evaluations. Results also show the SBO can achieve greater accuracy than the GA in a shorter amount of time, and the SBO can reduce the negative effects of noise in the simulation data while the GA cannot.
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Hammer, Patrick Richard. "A Discrete Vortex Method Application to Low Reynolds Number Aerodynamic Flows." University of Dayton / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1311792450.
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Ewing, Mark E. "Fundamental studies of enhanced heat exchanger surfaces at low Reynolds number." The Ohio State University, 1998. http://rave.ohiolink.edu/etdc/view?acc_num=osu1302011882.
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Webb, Charles. "Separation and Vorticity Transport in Massively-Unsteady Low Reynolds Number Flows." Wright State University / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=wright1244864717.
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Marks, Christopher R. "Surface Stress Sensors for Closed Loop Low Reynolds Number Separation Control." Wright State University / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=wright1309998636.
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Chang, Huakang. "The steady Navier-Stokes problem for low Reynolds' number viscous jets." Thesis, University of British Columbia, 1991. http://hdl.handle.net/2429/30968.
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The classical existence theorem for the steady Navier-Stokes equations, based on a bound for the solution's Dirichlet integral, provides little qualitative information about the solution.
In particular, if a domain is unbounded, it is not evident that the solution will be unique even when the data are small. Inspired by the works of Odqvist for the interior problem and of Finn for the problem of flow past an obstacle, we give a potential theoretic construction of a solution of the steady Navier-Stokes equations in several domains with noncompact boundaries. We begin by studying a scalar quasilinear elliptic problem in a half space, which serves as a model problem for the development of some of the methods which are later applied to the Navier-Stokes equations. Then, we consider Navier-Stokes flow in a half space, modeling such phenomena as a jet emanating from a wall, with prescribed
boundary values. The solution which is obtained decays like |x|⁻² at infinity and has a finite Dirichlet integral. Finally, we solve the problem of flow through an aperture in a wall between two half spaces, with a prescribed net flux through the aperture, or with a prescribed pressure drop between the two half spaces. A steady solution is constructed which decays like |x|⁻² at infinity. For small data, uniqueness is proven within the class of functions which decay like |x|⁻¹ at infinity and have finite Dirichlet integrals.Science, Faculty of
Mathematics, Department of
Graduate
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Pantelidis, Konstantinos. "Reynolds number effects on the aerodynamics of compact axial compressors." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/284940.
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An axial compressor for a domestic appliance can be designed to be smaller than an equivalent centrifugal compressor. However, the performance of such a compact axial compression system is limited by increased viscous losses and reduced flow turning at low Reynolds numbers ($Re$). In domestic appliance compressors, $Re$ is typically in the range $10^4$ - $10^5$. Although the aerodynamics of isolated aerofoils operating at these $Re$ have been studied extensively, the flow fields within low $Re$ axial compressors have not been investigated in detail. This dissertation aims to develop an improved understanding of loss variation at low $Re$ and to explore how the losses can be reduced through design changes. Experiments on a 5 times scaled-up single stage axial compressor have been conducted across a range of $Re$ of $10^4$ - $10^5$. The flow field has been characterised using detailed area traverses with a miniaturised five-hole probe at the rotor inlet, rotor exit and stator exit and a miniature hot-wire at the rotor exit. The probe was specifically designed and calibrated for the scale of the experiments and methods to improve the accuracy of the measurements have been applied including a probe geometry correction. The traverse experiments were performed at the design operating condition ($\phi=0.55$ and $Re= 6\times10^4$) and at a condition close to stall for a datum stage design, a stage with an improved stator design and two stators with compound lean. It was found that losses in the rotor were greater than the stator losses across the whole range of $Re$. As expected, the loss decreased with increasing $Re$ for both the stator and rotor. The losses were also increased by three-dimensional flow, with typical loss coefficients at the hub and tip of the blade rows in the range of $20-30\%$. A major contributor to the rotor loss was an unexpected hub separation that increased in size as $Re$ was reduced. At higher $Re$, the major loss sources were the rotor tip leakage, the stator wake and the stator hub separation. The results indicate that an improved stator design that accounts for the actual, measured, rotor exit flow field at low $Re$ could reduce the $Re$ at which blade row losses start to rise dramatically as well as reduce the loss across all $Re$. The improved stator design was better matched to the radial distribution of rotor exit flow angle, which led to a decrease in stator loss across all $Re$. For all stator designs, however, the measured stage stall margin was identical at all $Re$. This, along with the increase in velocity deficit in the rotor tip region at off-design indicates that stall occurred in the rotor and was neither $Re$ nor stator design dependent. The introduction of compound lean to the the stator design had the expected result of decreasing the endwall corner separation loss and increasing midspan losses. The experiments have shown that there was a loss increase in both the midspan and casing region much greater than the corresponding decrease in the stator hub. Also the mass flow redistribution in the experiments was larger that the redistribution predicted by the CFD. Three-dimensional RANS computations at low $Re$ of the same designs as experimentally studied were also conducted in order to investigate the predictive accuracy of industry standard CFD. The simulation results predicted the overall loss distribution but overestimated the end-wall losses and failed to capture the drop in stage performance at low $Re$. The differences with the experiments were caused by the inherent limitations of a fully turbulent solver that cannot reproduce transitional flow-features. Similarly to the experiments, there was no stall margin dependency on $Re$ in the simulations. This thesis has shown that with axial compressors designed specifically for low $Re$, the $Re$ at which the losses start increasing exponentially can be shifted from $10\times10^4$ to $ 4\times10^4$. The loss increase is predominantly caused by the rotor hub corner separation.
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Trivilos, Epameinondas. "Performance and flow regimes in plane 2-D diffusers with exit channels at low Reynolds numbers." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 2003. http://library.nps.navy.mil/uhtbin/hyperion-image/03sep%5FTrivilos.pdf.
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Thesis (Mechanical Engineer and M.S. in Mechanical Engineering)--Naval Postgraduate School, September 2003.Thesis advisor(s): Knox T. Millsaps. Includes bibliographical references (p. 79-80). Also available online.
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Pitt, Ford Charles William. "Unsteady aerodynamic forces on accelerating wings at low Reynolds numbers." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608219.
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Lee, Jaejin. "Numerical Study of Three Dimensional Low Magnetic Reynolds Number Hypersonic Magnetohydrodynamic Flows." Scholarly Repository, 2011. http://scholarlyrepository.miami.edu/oa_dissertations/698.
Full textAbstract:
Hypersonic vehicles generate shocks that can heat the air sufficiently to partially ionize the air and create an electrically conducting plasma that can be studied using the equations of single fluid magnetohydrodynamics (MHD). Introducing strong applied magnetic and electric fields into the flow could have beneficial effects such as reducing heat damage, providing a sort of MHD parachute, and generating electric power or thrust in the vehicle. The Low Diffusion E-CUSP (LDE) scheme with a fifth order WENO scheme has recently been developed by Zha et al. [1, 2]. The purpose of this work is to incorporate the low magnetic Reynolds number MHD model and the thermodynamics of high temperature air to the above CFD algorithm so that it can be used to simulate hypersonic flows with MHD effects. In this work we compare results treating air as chemically frozen, neglecting all high temperature real gas effects with results obtained treating the air as a real gas in thermodynamic equilibrium, whose thermodynamic properties are changed by the high temperature. The hypersonic flows at high altitudes considered in this study have low Reynolds numbers. The Reynolds numbers range from about 2000 to 5000 for Mach 6 flows and reach up to 1200000 for Mach 15 flows. Thus, the flows are treated as laminar for the former cases and as turbulent for the latter using the Baldwin-Lomax turbulence model.
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Gunaydinoglu, Erkan. "Low Reynolds Number Aerodynamics Of Flapping Airfoils In Hover And Forward Flight." Master's thesis, METU, 2010. http://etd.lib.metu.edu.tr/upload/12612397/index.pdf.
Full textAbstract:
The scope of the thesis is to numerically investigate the aerodynamics of flapping airfoils in hover and forward flight. The flowfields around flapping airfoils are computed by solving the governing equations on moving and/or deforming grids. The effects of Reynolds number, reduced frequency and airfoil geometry on unsteady aerodynamics of flapping airfoils undergoing pure plunge and combined pitch-plunge motions in forward flight are investigated. It is observed that dynamic stall of the airfoil is the main mechanism of lift augmentation for both motions at all Reynolds numbers ranging from 10000 to 60000. However, the strength and duration of the leading edge vortex vary with airfoil geometry and reduced frequency. It is also observed that more favorable force characteristics are achieved at higher reduced frequencies and low plunging amplitudes while keeping the Strouhal number constant. The computed flowfields are compared with the wide range of experimental studies and high fidelity simulations thus it is concluded that the present approach is applicable for investigating the flapping wing aerodynamics in forward flight. The effects of vertical translation amplitude and Reynolds number on flapping airfoils in hover are also studied. As the vertical translation amplitude increases, the vortices become stronger and the formation of leading edge vortex is pushed towards the midstroke of the motion. The instantaneous aerodynamic forces for a given figure-of-eight motion do not alter significantly for Reynolds numbers ranging from 500 to 5500.
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Nowak, Lisa M. "Computational investigations of a NACA 0012 airfoil in low Reynolds number flows." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1992. http://edocs.nps.edu/npspubs/scholarly/theses/1992/Sep/92Sep_Nowak.pdf.
Full textAbstract:
Thesis (Degree in Aeronautical and Astronautical Engineer)--Naval Postgraduate School, Sept. 1992.Thesis advisor(s): M.F. Platzer and M. Chandrasekhara. "September 1992." Includes bibliographical references. Also available online.