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© 2012 Diego Gustavo Campos To my parents


I would like to thank Dr. Lawrence Ukeiley for his guidance and support on my research. Thank you to my committee members, Dr. Peter Ifju and Dr. Rick Lind, for reviewing my work and for their valuable input. I would also like to thank Adam Hart for the time spent teaching me how to use all the equipment and for his advice, Dr. Erik Sällström for his insight and assistance in developing many of the data processing routines used in this study and Amory Timpe for his help throughout this process, and for his help outside of research. Thank you to all my friends for their support. I would like to acknowledge the support of AFOSR through a MURI program managed by Dr. Smith and the Florida Center for Advanced Aero Propulsion.










Scaling Parameters

Reynolds number

Strouhal number

Reduced frequency

Fluid-Structure Interaction Scaling Parameters

Effective stiffness

Cauchy number

Elastoinertial number

Frequency ratio

Aeroelastic studies (Benefits of flexibility)

Chordwise flexible studies

Spanwise flexible studies

Combined spanwise and chordwise flexible studies

Current Study


Aerodynamic Characterization Facility

Plunging Device

Motion Analysis

PIV Synchronization

Wing Models

Experimental Parameters

Stereo Particle Image Velocimetry

SF PIV Measurement Setup

SFPP PIV Measurement Setup

Force Transducer

Laser Doppler Vibrometer



Force Estimation

Pressure Estimation

Momentum Balance

Stereo Particle Image Velocimetry

Scheimpflug Condition



Modal Analysis

Fully Supported Root Studies

Flow Field Analysis

SF3 model

SF1 model

Wing Deformation/Comparison Studies

Force Calculations

Spanwise Flexible Wings with Passive Pitch

Flow Field and Deformation Analysis

SFPP3 Model

Frequency ratio of 0.4

Frequency ratio of 0.9

SFPP2 Model

Frequency ratio of 0.4

Frequency ratio of 0.9

SFPP1 Model

Frequency ratio of 0.4

Frequency ratio of 0.9

Momentum Balance




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2-1 Dimensionless kinematic scaling parameters

2-2 Kinematic and geometric properties

2-3 Material properties

2-4 Dimensionless fluid-structure scaling parameters.

2-5 ATI Nano 17 sensing range

2-6 ATI Nano resolution for a 16-bit DAQ

4-1 Modal Analysis on the wing models

4-2 Frequency ratios for the studies in wind and water tunnel.

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2-1 Plunging device with wing attached.

2-2 Sinusoidal motion analysis.

2-3 Zimmerman planform wings.

2-4 PIV planes and coordinate system.

2-5 SF experiment setup.

2-6 SFPP experiment setup.

2-7 Shaker and wing setup.

3-1 Schematic of the Scheimpflug condition.

3-2 Lavision calibration target.

4-1 First Mode (Bending) for wings tested in the SF experiment.

4-2 Second Mode (Twist-Bending) for 2 wings tested in SFPP study.

4-3 Vorticity and Q-criterion at beginning of the downstroke for the SF3 wing at the 50% location.

4-4 Vorticity and Q criterion at phase 75° for SF3 model at 50% spanwise location.

4-5 Vorticity and Q criterion at beginning of downstroke for SF3 wing at 75% spanwise plane.

4-6 Vorticity and Q criterion at 75° phase for SF3 wing at 75% spanwise plane...... 71 4-7 Isosurfaces of Q. slices are at 50% and 75% spanwise planes.

4-8 Normalized velocity at phase 75° for SF3 wing at 75% spanwise plane............. 72 4-9 Vorticity and Q criterion at beginning of the downstroke for SF1 wing................ 72 4-10 Vorticity and Q-criterion near midstroke for SF1 wing.

4-11 Displacements as a funtion of phase for SF1 and SF3 wings.

4-12 Vorticity plots for phases where the LEV leaves TE for SF3 wing

4-13 Phase averaged force calculations using Momentum Balance approach........... 74 4-14 Vorticity fields (ω*) for SFPP3 model

4-15 Vorticity fields (ω*) for SFPP2 model

4-16 Vorticity fields (w*) for SFPP1 model.

4-17 Spanwise deformation for the SFPP3 wing.

4-18 Spanwise deformation for SFPP2 wing.

4-19 Spanwise deformation for SFPP1 wing.

4-20 Force in the streamwise direction for SFPP3 wing using momentum balance method.

4-21 Force in the streamwise direction for SFPP3 wing using momentum balance method.

4-22 Force in the streamwise direction for SFPP1 wing using momentum balance method.

–  –  –

Chair: Lawrence S. Ukeiley Major: Aerospace Engineering In recent years there has been an interest in studying and understanding natural flyers to incorporate some of their features into small engineered flying systems. Natural flyers display desirable flight characteristics, such as increased maneuverability, that could be used in the design of micro air vehicles. The present study is aimed at understanding the effect of flexibility on the aerodynamic performance and flow around plunging flexible wings. The value of wing stiffness is varied using a predetermined scaling parameter, defined as the ratio of elastic to aerodynamic forces. The first part of the study consisted of matching conditions from previous water tunnel studies to investigate the requirement of dynamic similarity using the wing stiffness parameter.

This study investigates the unsteady flow phenomena generated from plunging wings with varying flexibility. The structures are then analyzed to understand the mechanisms for force production. The measurements showed that large deflections at the tip produced strong leading edge vortices. However, when the tip-root lag is greater than 70°, the effects are adverse resulting in no leading edge vortex development. In order to understand wing flexibility further, experiments were performed using a laser Doppler Vibrometer to understand the modal properties for each wing with varying flexibility parameter.

The second set of experiments consists of studying wings that do not have twist constrained. The ratio of plunging to natural frequency is varied between the value for maximum propulsive efficiency and for maximum propulsive force. This set will allow for comparison with the previous studies, in an attempt to understand the effect of the twist on the flow, and performance. It will also serve to understand the flow phenomena for the cases of maximum efficiency and maximum force. This will provide a framework for the study of wing flexibility using force, flow and structural measurements.

–  –  –

Natural flyers display many desirable flight characteristics that could be used in the design of micro air vehicles (MAVs)[1]–[6]. Insects and birds are able to handle wind gusts, avoid objects with great maneuverability, as well as hover. The kinematics observed in biological flyers is complex, often involving highly deformable wing shapes and coordinated wing-tail movement[7]–[11]. The motions involve flexing, twisting, bending and rotating the wings throughout the flapping cycle, leading to a complex fluidstructure interaction that is not fully understood[12], [13]. This creates highly coupled nonlinearities in structural dynamics and fluid dynamics making it a rich field of study [14].

MAVs capable of mimicking natural flyers would prove helpful in areas such as remote sensing and information gathering for both civilian and homeland security applications. Recently there have been many vehicle concepts that were developed in order to address these mission requirements. The Aerovironment Nano Hummingbird, a small hovering ornithopter with a wingspan of 16.5 cm, has demonstrated its ability to achieve controlled flight (hover and forward) strictly with the use of its flapping wings, a feature only achievable by biological flyers previously[15]. It hovers and sustains flight for several minutes, and transmits color video to a remote station, while closely resembling a hummingbird. Other examples of successful flapping wing MAVs are the Delfly I,II and Micro[16] from the Delft University of Technology, the Microrobotic Fly from the University of Harvard[17], the Mentor from SRI international and UTIAS[18], and the commercially available iBird[19]. Even though there are a number of working vehicles, the aerodynamics, structural, and control implications of the many modes seen in biological flight are not understood enough for efficient design[14].

One of the main features common to many natural flyers is their deformable wing structure. Wing flexibility has been recognized as an important aspect for insect and bird flight aerodynamics[20]–[22]. Studies conducted by Wootton [23] on butterflies concluded that flexible wing surfaces adapt their shape in response to external fluid forces, thus changing aerodynamic force production during flight. Combes & Daniel [24]–[26] have performed a series of experiments to understand the variation of wing flexural stiffness across all insects and the influence of wing venation and force contributions to deformation. They found that flexural stiffness varies significantly amongst insects, spanning about four orders of magnitude. Likewise, venation patterns across insects are diverse. In the case of flapping insect wings they concluded that wing flexibility was governed primarily by the inertia of wing rather than the aerodynamic forces from the aeroelastic interaction, thus they believed that wings could be treated as purely inertial, flexible structures.

These studies show the intricacy in trying to model bio-inspired wings, and the many characteristics of biological flight. Rather than mimicking the characteristics of a specific flyer, this study will focus on understanding certain characteristics and effects of wing deformation. This study is part of a multidisciplinary effort aimed at developing a better understanding of the effects of flexibility on the flow behavior, and on the force production. This study concentrates on experimental investigations of the flow, deformation and forces produced by wings. The wings have isotropic bending characteristics and are studied under forward flight conditions. A plunging motion is used, in order to simplify the deformation characteristics of the wing. This will allow for a better understanding of how the spanwise and chordwise bending of the wing affects the flow and how the fluidic structures formed relate to the generation of aerodynamic forces.

The following sections of Chapter 1 contain a review of the research related to the study of wing flexibility. It is then followed by Chapter 2 describing the experimental setup and Chapter 3 with analytical methods used to investigate the data. Chapter 4 presents the two sets of experiments conducted; one on spanwise flexible wings fully supported at the root, and the second on wings partially supported. While Chapter 5 presents a summary of the research, the conclusions, and future work.

–  –  –

As outlined in the previous section, in recent years there has been an interest in studying and understanding natural flyers to incorporate some of their characteristics into engineered flying vehicles. In order to achieve this goal, a better understanding of the unsteady fluid phenomena generated by the wing kinematics of insects and birds to achieve the aerodynamic forces necessary for flight is desired. The mechanisms that govern the generation of aerodynamic forces are associated with the formation and shedding of vortices into the flow. Therefore, an understanding of the fluid-structure interactions between the vortex dynamics and the structural properties of the wing is of great importance.

Scaling Parameters Non-dimensional scaling parameters that relate wing material characteristics and the kinematics to the free stream conditions allow for a better understanding of the effects of wing stiffness. By using scaling the number of parameters that describe the system can be reduced, and an assessment of which combination of parameters are important for certain condition can be analyzed[13]. There have been several studies that have performed a non-dimensional analysis to identify the characteristic properties of flapping systems. Depending on the models used the parameters vary. The parameters involved are divided into two types: parameters related to the fluid dynamics and the wing kinematics, and the parameters related to the fluid-structure interaction.

The following sections will present a review of the relevant parameters used in the study of wing flexibility, and the findings of the parametric investigations performed with these parameters.

Reynolds number Reynolds number (Re) represents the ratio of inertial forces to viscous forces. In kinematics. The reference velocity is either the mean wing tip velocity ( ) or the forward flapping flight the Re is selected depending on the flight conditions and the wing forward flight velocity (∞ ). The Re for hovering flight and forward flight based on wing tip velocity is shown in Equation 1-1.

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