Kenneth S Breuer

• PhD
• Brown University

This study investigates the coupled deformation and flow behavior through thin, hyper-elastic, porous membranes subjected to pressure loading. Using bulge test experiments, optical deformation measurements, and flow rate characterization, we analyze the structural and fluid dynamic responses of membranes with varying material stiffness and porosity...

We report on experimental and numerical studies aimed at developing an improved paradigm for modeling avian flights at moderate Reynolds numbers. A series of experiments were performed to characterize the behaviors of aerodynamic forces and moment associated with a quasi-steady rectangular wing over a range of incidence angles, α. We demonstrate th...

The strength and trajectory of a leading edge vortex (LEV) formed by a pitching-heaving hydrofoil (chord c) is studied. The LEV is identified using the Q-criterion method, which is calculated from the 2D velocity field obtained from particle image velocimetry measurements. The relative angle of attack at midstroke, αT/4, proves to be an effective m...

Bat wing skin is exceptionally compliant and cambers significantly during flight. Plagiopatagiales proprii, arrays of small muscles embedded in the armwing membrane, are activated during flight and hypothesised to modulate membrane tension. We examined the function of these muscles using Jamaican fruit bats, Artibeus jamaicensis. When these muscles...

The strength and trajectory of a leading edge vortex (LEV) formed by a pitching-heaving hydrofoil (chord $c$) is studied. The LEV is identified using the $Q$-criterion method, which is calculated from the 2D velocity field obtained from PIV measurements. The relative angle of attack at mid-stroke, ${\alpha_{T/4}} $, proves to be an effective method...

Oscillating foils in synchronized pitch/heave motions can be used to harvest hydrokinetic energy. By understanding the wake structure and its correlation with the foil kinematics, predictive models for how foils can operate in array configurations can be developed. To establish a relationship between foil kinematics and wake characteristics, a wide...

Oscillating foils in synchronized pitch/heave motions can be used to harvest hydrokinetic energy. By understanding the wake structure and its correlation with the foil kinematics, predictive models for how foils can operate in array configurations can be developed. To establish a relationship between foil kinematics and wake characteristics, a wide...

Flapping flight using passive pitch regulation is a commonly used mode of thrust and lift generation in insects and has been widely emulated in flying vehicles because it allows for simple implementation of the complex kinematics associated with flapping wing systems. Although robotic flight employing passive pitching to regulate angle of attack ha...

For many animals, movement through complex natural environments necessitates the evolution of mechanisms that enable recovery from unexpected perturbations. Knowledge of how flying animals contend with disruptive forces is limited, however, and is nearly nonexistent for bats, the only mammals capable of powered flight. We investigated perturbation...

The aeroelastic response and energy harvesting performance of an elastically mounted hydrofoil subject to a prescribed pitching motion are experimentally studied using a cyber- physical force-feedback control system in a uniform flow. By taking advantage of this cyber-physical system, we systematically sweep through the parameter space of the elast...

Wall confinement effects on the energy harvesting performance by a flapping hydrofoil (aspect ratio = 4.5) have been investigated in a circulating water flume at a Reynolds number of 50,000. Measurements of hydrodynamic forces are taken for three different confinement configurations (unconfined, one-wall and two-wall confinement) and a series of co...

We analyze the effects of morphology and wing kinematics on the performance of hovering flight. We present a simplified dynamical model with body translational and rotational degrees of freedom that incorporates the flapping, long-axis wing rotation and folding of the wing. To validate our simulation, we compare our results with direct measurements...

The energy harvesting performance of an elastically-mounted hydrofoil (chord, c, span, s) subject to a prescribed pitching motion is studied using a cyber-physical force-feedback control system. We vary the mass, m, the frequency of the pitching motion, w, the parameters of the elastic support (stiffness, k and damping, b) and the Reynolds number,...

Animals respond to changes in power requirements during locomotion by modulating the intensity of recruitment of their propulsive musculature, but many questions concerning how muscle recruitment varies with speed across modes of locomotion remain unanswered. We measured average muscle recruitment intensity (aEMG) for pectoralis major and biceps br...

Aspect ratio (AR) is one parameter used to predict the flight performance of a bat species based on wing shape. Bats with high AR wings are thought to have superior lift-to-drag ratios and are therefore predicted to be able to fly faster or to sustain longer flights. By contrast, bats with lower AR wings are usually thought to exhibit higher manoeu...

We present a self-consistent theory to predict the behavior of compliant membrane wings subject to aerodynamic loads. The theory incorporates the Young–Laplace equation to treat nonlinear deformation of the membrane at low angles of attack and uses a potential flow model to estimate the aerodynamic load associated with the thin wing. The model is a...

Measurements of flow structure and power extraction by tandem pitching and heaving hydrofoils are conducted in a flume. The leading and trailing hydrofoils are synchronized and aligned parallel to the oncoming flow. Force measurements and time-resolved PIV are used to characterize the system. The system efficiency of tandem foils with the same kine...

We compare kinematics and wake structure over a range of flight speeds (4.0–8.2 m s ⁻¹ ) for two bats that pursue insect prey aerially, Tadarida brasiliensis and Myotis velifer . Body mass and wingspan are similar in these species, but M. velifer has broader wings and lower wing loading. By using high-speed videography and particle image velocimetr...

Bat wings, like other mammalian forelimbs, contain many joints within the digits. These joints collectively affect dynamic three-dimensional (3D) wing shape, thereby affecting the amount of aerodynamic force a wing can generate. Bats are a speciose group, and show substantial variation in the number of wing joints. Additionally, some bat species ha...

Measurements of energy harvesting using a heaving and pitching hydrofoil with an aspect ratio 4.5 were taken in three different situations: unconfined, in close proximity to one wall and in close proximity to two walls. Measured lift and torque forces were used with the hydrofoil positions to calculate the efficiency of energy extracted from the fl...

The remarkable maneuverability of flying animals results from precise movements of their highly specialized wings. Bats have evolved an impressive capacity to control their flight, in large part due to their ability to modulate wing shape, area, and angle of attack through many independently controlled joints. Bat wings, however, also contain many...

Two different particle tracking velocimetry techniques are used to measure the fluid velocities close to the substrate in the vicinity of both receding and advancing contact lines. The slip velocity is found to be as much as 60% of the substrate speed near the contact line and persists as far as 10 μm from the liquid-gas interface. The estimated sl...

Membrane wings with variable compliance have a great potential to improve the maneuverability and performance of micro air vehicles. Moreover, changes in membrane wing compliance might be used by flying animals, such as bats, to control aerodynamic performance. In this work, the mechanical properties and aerodynamic performance of a low-aspect-rati...

Significance Using a digital tracking microscope that provides both cell position and orientation, we have correlated the detailed motion of the cell body of a fast-swimming bacterium, Caulobacter crescentus , with its swimming motility. Contrary to the prevailing view that the rotating flagellum is the only means to propel the cell, we show that w...

Aerodynamic theory has long been used to predict the power required for animal flight, but widely used models contain many simplifications. It has been difficult to ascertain how closely biological reality matches model predictions, largely because of the technical challenges of accurately measuring the power expended when an animal flies. We desig...

In bats, the wing membrane is anchored not only to the body and forelimb, but also to the hindlimb. This attachment configuration gives bats the potential to modulate wing shape by moving the hindlimb, such as by joint movement at the hip or knee. Such movements could modulate lift, drag, or the pitching moment. In this study we address: 1) how the...

Unlike flapping birds and insects, bats possess membrane wings that are more similar to many gliding mammals. The vast majority of the wing is composed of a thin compliant skin membrane stretched between the limbs, hand, and body. Membrane wings are of particular interest because they may offer many advantages to micro air vehicles. One critical fe...

Bats display a wide variety of behaviors that require different amounts of aerodynamic force. To control and modulate aerodynamic force, bats change wing kinematics, which, in turn, may change the power required for wing motion. There are many kinematic mechanisms that bats, and other flapping animals, can use to increase aerodynamic force, e.g. in...

Aerodynamic theory has long been used to predict the power required for animal flight, but widely used models contain many simplifications. It has been difficult to ascertain how closely biological reality matches model predictions , largely because of the technical challenges of accurately measuring the power expended when an animal flies. We desi...

This paper presents the experimental approach and the design of a bat inspired four degree of freedom flapping wind tunnel model. The flapping trajectories were determined from highspeed image captures of live Glossophaga soricina. The scaled wind tunnel model has half wingspan of 34 cm and flaps at 4 Hz. Wind tunnel testing was conducted to measur...

We study the microscale propulsion of a rotating helical filament confined by a cylindrical tube, using a boundary-element method for Stokes flow that accounts for helical symmetry. We determine the effect of confinement on swimming speed and power consumption. Except for a small range of tube radii at the tightest confinements, the swimming speed...

We apply the boundary-element method to Stokes flows with helical symmetry, such as the flow driven by an immersed rotating helical flagellum. We show that the two-dimensional boundary integral method can be reduced to one dimension using the helical symmetry. The computational cost is thus much reduced while spatial resolution is maintained. We re...

Gliding is an efficient form of travel found in every major group of terrestrial vertebrates. Gliding is often modelled in equilibrium, where aerodynamic forces exactly balance body weight resulting in constant velocity. Although the equilibrium model is relevant for long-distance gliding, such as soaring by birds, it may not be realistic for short...

The morphology, kinematics and stiffness properties of lifting surfaces play a key role in the aerodynamic performance of vertebrate flight. These surfaces, as a result of their flexible nature, may move both actively, owing to muscle contraction, and passively, in reaction to fluid forces. However, the nature and implications of this fluid-structu...

There are many challenges to measuring power input and force output from a flapping vertebrate. Animals can vary a multitude of kinematic parameters simultaneously, and methods for measuring power and force are either not possible in a flying vertebrate or are very time and equipment intensive. To circumvent these challenges, we constructed a robot...

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We measure the swimming speed of a cylindrical version of Taylor's swimming sheet in viscoelastic fluids, and find that depending on the rheology, the speed can either increase or decrease relative to the speed in a Newtonian viscous fluid. The swimming stroke of the sheet is a prescribed propagating wave that travels along the sheet in the azimuth...

Low Reynolds number aerodynamic experiments with flapping animals (such as bats and small birds) are of particular interest due to their application to micro air vehicles which operate in a similar parameter space. Previous PIV wake measurements described the structures left by bats and birds and provided insight into the time history of their aero...

We investigated the deposition and accumulation of droplets on both smooth substrates and substrates textured with square pillars, which were tens of micrometers in size. After being coated with a hydrophobic monolayer, substrates were placed in an air flow with a sedimenting suspension of micrometer-sized water droplets (i.e., fog). We imaged the...

All bats experience daily and seasonal fluctuation in body mass. An increase in mass requires changes in flight kinematics to produce the extra lift necessary to compensate for increased weight. How bats modify their kinematics to increase lift, however, is not well understood. In this study, we investigated the effect of a 20% increase in mass on...

Summary of ANCOVA analyses of kinematic variables in response to loading and speed. (PDF)

This paper presents the design of a bat-like micro aerial vehicle with actuated morphing wings. NiTi shape memory alloys (SMAs) acting as artificial biceps and triceps muscles are used for mimicking the morphing wing mechanism of the bat flight apparatus. Our objective is twofold. Firstly, we have implemented a control architecture that allows an a...

Flying vertebrates change the shapes of their wings during the upstroke, thereby decreasing wing surface area and bringing the wings closer to the body than during downstroke. These, and other wing deformations, might reduce the inertial cost of the upstroke compared with what it would be if the wings remained fully extended. However, wing deformat...

Bats are unique among mammals for their ability to fly. A substantial body of research has focused on understanding how they do so, and in 1990, Norberg's landmark volume provided an up-to-date understanding of diverse aspects of bat flight (Norberg, 1990). Building on work accomplished before 1990, our understanding of bat flight has changed signi...

To date, wake measurements using particle image velocimetry (PIV) of bats in flight have studied only three bat species, all fruit and nectar feeders. In this study, we present the first wake structure analysis for an insectivorous bat. Tadarida brasiliensis, the Brazilian free-tailed bat, is an aerial hunter that annually migrates long distances a...

We precisely measure the force-free swimming speed of a rotating helix in viscous and viscoelastic fluids. The fluids are highly viscous to replicate the low Reynolds number environment of microorganisms. The helix, a macroscopic scale model for the bacterial flagellar filament, is rigid and rotated at a constant rate while simultaneously translate...

Maximum range in steady-state glides is achieved when the lift coefficient is chosen to maximize the lift-to-drag ratio. Whether or not this same steady-state result applies to animal gliders is examined. Because animal gliders spend relatively little time in the air and glide relatively short distances, it is expected that the transient behavior a...

We precisely measure the force-free swimming speed of a rotating helix in viscous and viscoelastic fluids. The fluids are highly viscous to replicate the low Reynolds number environment of microorganisms. The helix, a macroscopic scale model for the bacterial flagellar filament, is rigid and rotated at a constant rate while simultaneously translate...

In this paper we present a novel method to reconstruct the 3D posture of flying bats from sparse multiple view video. Specifically, we incorporate biomechanical and geometric knowledge about bats into an articulated model. We then estimate the bats time-varying pose by tracking a set of known markers using a Square Root Unscented Kalman filtering m...

Flying animals generate forces to move through the air with the coordinated movement of their wings. Bats have evolved a particularly impressive capacity in flight control. With more than 24 wing joints bats are able to manipulate wing area, angle of attack, and camber to control their flight through altering aerodynamic forces on their wings. Here...

We discuss a mechanical experimental model of a flexible sheet swimming with a prescribed wave pattern - a Taylor swimmer - through a fluid. Our study is motivated by a need for a fundamental understanding of microorganism locomotion through non-Newtonian fluids. In order to simplify the problem, we suspend a tall flexible cylindrical sheet concent...

A hydrophobic surface patterned with micro-pillars can exhibit ``superhydrophobic'' behavior, wherein drops only touch the tops of pillars (the so-called Cassie-Baxter state). This state, however, requires drops much larger than the pillar size/spacing. In contrast, drops smaller than the microtexture can adsorb both on top of and in- between these...

This work isolates the heaving motion of flapping flight in order to numerically investigate the fluid-structure interaction at Reynolds numbers relevant to birds and bats. Although there has been much focus on insect flight, larger vertebrates fly at a higher Reynolds number, which leads to different dynamics in terms of flow separation, reattachm...

We present data from a mechanical model that we have used to explore a physical mechanism that may have aided transition from gliding to flapping flight over fifty million years ago. The model is composed of a cantilevered flat plate with a hinged trailing flap and is tested in a low-speed wind tunnel. For slow wind speeds the model is stationary,...

Directly probing the liquid-vapor interface shape near moving contact lines is critical to understanding the microscopic physics of dynamic contact line behavior. Using a novel high resolution optical technique, we report on the meniscus shape in the immediate vicinity of moving contact lines. The liquid is fluorescently-dyed and illuminated with a...

Membrane wings are characteristic of flying animals such as bats, as well as low Reynolds number Micro Air Vehicles. These wings exhibit interesting features such as self-cambering, soft stall, and good performance at large angles of incidence. The interaction between the membrane and the vortical structures over the wing play an important role in...

Particle Image Velocimetry (PIV) has become a well-established tool to study flows associated with flying animals. The wake shed by flying bats as it is seen in the Trefftz plane is by now well described for several bat species. However, to complete the understanding of the three-dimensional wake structures, additional views are necessary. To meet...

Live bacteria often live in polymer suspensions, and are inevitably subjected to the effects of fluid viscoelasticity. To study the viscoelastic effect on bacterial motility, we have constructed a scaled-up model system and use particle image velocimetry (PIV) to measure the flow field generated by a rigid helical filament that rotates and translat...

More than 50 million years ago bats evolved from a gliding‐fluttering flight to power flapping. 1 The conditions or reasons that led from gliding to power flight are still unknown. In this work we explore ‘passive’ flight (no actuation of the wing) and the transition between fixed wing gliding and passive flapping using a simple two degree of freed...

An efficient system for high-resolution measurements of a bat wing’s membrane during flight is presented, proving the feasibility of dynamic strain measurements on bats wing membranes during flapping. Data were collected from wind tunnel wind-off flights of a Jamaican fruit bat, Artibeus jamaicensis, a nocturnal and frugivorous specie trained by Br...

The center of mass (COM) of a flying animal accelerates through space because of aerodynamic and gravitational forces. For vertebrates, changes in the position of a landmark on the body have been widely used to estimate net aerodynamic forces. The flapping of relatively massive wings, however, might induce inertial forces that cause markers on the...

Bacteria achieve motility by eluding the constraints of kinematic reversibility, for instance, by rotating a helical flagellum. We study experimentally the motility of the flagellum with a scaled-up model system, a motorized helical coil that rotates along its axial direction. The rotating helix is tethered on a linear stage that advances at a pred...

The metabolic cost of flight increases with mass, so animals that fly tend to exhibit morphological traits that reduce body weight. However, all flying animals must sometimes fly while carrying loads. Load carrying is especially relevant for bats, which experience nightly and seasonal fluctuations in body mass of 40% or more. In this study, we exam...

In this study we compared the wing kinematics of 27 bats representing six pteropodid species ranging more than 40 times in body mass (M(b)=0.0278-1.152 kg), to determine whether wing posture and overall wing kinematics scaled as predicted according to theory. The smallest species flew in a wind tunnel and the other five species in a flight corridor...

Employing high-speed particle tracking, we experimentally investigate the slip boundary condition in the vicinity of a moving contact line. A liquid bridge was established between a stationary hydrophobic glass slide and a rod. By translating the rod at a controlled speed, we establish a well-controlled moving contact line. The liquid was seeded, e...

The flexible nature of lifting and propulsive surfaces is a common characteristic of aquatic and aerial locomotion in animals. These surfaces may not only move actively, but also passively or with a combination of both. What is the nature of this passive movement? What is the role of this passive motion on force generation, efficiency and muscle co...

Low Reynolds number experiments with flapping animals (such as bats and small birds) are of current interest in understanding biological flight mechanics, and due to their application to Micro Air Vehicles (MAVs) which operate in a similar parameter space. Previous PIV wake measurements have described the structures left by bats and birds, and prov...

Recent research has suggested significant modification in the structure of turbulent flow of water over a superhydrophobic surface. The changes, which may include large reductions in skin friction, are due to the modification of the no-slip boundary condition at the liquid-solid interface. We present experimental and computational results from an o...

The complex behavior of drop deposition on a hydrophobic surface is considered by looking at a model problem in which the evolution of a constant-volume liquid bridge is studied as the bridge is stretched. The bridge is pinned with a fixed diameter at the upper contact point, but the contact line at the lower attachment point is free to move on a s...

We investigated the detailed kinematics and wake structure of lesser dog-faced fruit bats (Cynopterus brachyotis) flying in a wind tunnel. High speed recordings of the kinematics were conducted to obtain three-dimensional reconstructions of wing movements. Simultaneously, the flow structure in the spanwise plane perpendicular to the flow stream was...

The Brazilian free-tailed bat, Tadarida brasiliensis, roosts in very large colonies, con-sisting of hundreds of thousands of individuals. Each night, bats emerge from their day roosts in dense columns in a highly coordinated manner. We recorded short segments of an emergence using three spatially-calibrated and temporally-synchronized thermal infra...

We present a visual exploration tool that facilitates biologists navigating through complex bat wing geometry by combining a novel modeling method and an interactive visualization approach. Our work contributes to the following: a new method to quantify the dynamic kinematics during flight, a new curve fitting method that measures camber, and a new...

Flow over a compliant membrane is a complex problem where the interaction between fluid and membrane determines the nature of the aerodynamic characteristics of the membrane wing. This investigation is concerned with the deformation and oscillatory motion of a membrane under aerodynamic loading. The approach is computational, but the analytical sol...

Hydrodynamic synchronization is important in many biological systems such as beating cilia and cell motility. Here we study a model problem, in which flexible paddles rotate in a viscous liquid. We examine a three-paddle system and show that, depending on the arrangement, paddles can exhibit either a periodic phase difference variation or converge...

The remarkable maneuverability of flying animals results from precise movements of their highly specialized wings. Among these flyers, bats have evolved a particularly impressive capacity to control their flight. This adeptness is, in part, determined by bats' ability to modulate their wing shape through many independently controlled joints. Howeve...

Total internal reflection velocimetry (TIRV) is applied to measure the dynamics of 17nm diameter, colloidal quantum dot (QD) tracer particles within 200nm of a microchannel wall at shear rates in excess of 20,000 s−1. QDs are quickly developing into viable tracer particles for measuring microscale fluid dynamics. However, the low emission intensiti...

Motivated by the observed coordination of nearby beating cilia, we use a scale model experiment to show that hydrodynamic interactions can cause synchronization between rotating paddles driven at constant torque in a very viscous fluid. Synchronization is only observed when the shafts supporting the paddles have some flexibility. The phase differen...

We present results from a model system designed to study the interactions between vortex shedding and structural compliance, as might be exhibited in systems as diverse as flying animals with compliant wings or traffic signs subject to hurricane-force winds. A sharp-edged plate is mounted at high angle of attack such that vortex shedding from the l...

The structure and strength of the vortex wake behind a airplane or animal flying with a fixed or flapping wing contains valuable information about the aerodynamic load history. However, the amount of vorticity measured in the trailing vortex is not always in agreement with the known lift generated, and the behavior of these vortices at relatively l...

We have used experimental and theoretical methods to study the evolution and pinch-off of a liquid bridge fixed at the upper attachment point, but with a free contact line at the lower attachment point. High speed video shows that the contact line motion consists of two stages: a slow retraction at the beginning and a rapid retraction immediately p...

It has long been known that a continuum hydrodynamic description using a no-slip boundary condition breaks down near a moving contact line. Theoretical models including microscopic effects, such as velocity slip and a diffuse interface, have been proposed to relieve the contact line singularity. Although experimental testing of the theoretical mode...

We present detail regarding the flow field generated by the high-speed motion of a long, thin elastic fiber immersed in a viscous fluid. The fiber, made of glass, or carbon, is approximately 1 mm in length, has a diameter of 7 microns, and is immersed in water, seeded with sub-micron tracer particles. The fiber is oscillated back and forth at 32 kH...

Flight kinematics and morphology differ greatly between the approx. 1200 bat species and the goal of our project is to understand how these differences affect the flight mechanisms, the generation of aerodynamic forces, and the resultant wake structures. Multiple individuals of three diverse species of bat were flown in the wind tunnel. The three s...

Recent research has suggested that large reductions in both laminar and turbulent skin friction might be realized in water flows over over superhydrophobic surfaces due to the modification of the no-slip boundary condition at the liquid-solid interface by a thin layer of trapped air. In our work, superhydrophobic surfaces have been fabricated by fi...