Visual Image Correlation Research Articles

Fluid–structural dynamic characterization of an electroactive membrane wing

The interactions between low-Reynolds-number fluid flow and an electroactive membrane wing is characterized to illustrate changes in harmonic and transient behavior from electric field excitation of the membrane wing. The wing is constructed of a dielectric elastomer material that changes its tension as a function of the applied field. The field excitation leads to changes in the shape of the wing under aerodynamic loads and subsequently, increased lift and delay of stall. Prior work in time-averaged lift and drag characterization is extended to better understand dynamic characteristics. Benchtop membrane structural dynamics are compared with visual image correlation, hotwire measurements, and particle image velocimetry within a low-Reynolds-number wind tunnel. An applied electric field leads to a 13.7% reduction in structural resonance of the membrane under ambient conditions while wind tunnel measurements illustrate a 5.1% reduction in resonance under the same applied field. Despite these differences, the structural and fluid dynamic harmonics are closely correlated for low-Reynolds-number flow at 10 m/s.

Modal Analysis of Hummingbird Inspired MAV Flapping Wings

Natural flyers are the best source of inspiration for making successful MAVs. Hummingbirds are known for their excellent flight characteristics such as long duration hovering, backward flying, high agility, etc. Giant hummingbird is chosen as the bio-inspiration for designing the wing. Wings are required to be light, strong, and fatigue resistant to be used for MAV applications. Carbon nanotubes (CNTs)/Polypropylene (PP) composite is chosen as the wing membrane material whereas carbon fiber (CF)/epoxy (E) composite is chosen for wing frame. Two types of wings are fabricated, one is CNTs/PP wing and another is CF/E frame with CNTs/PP membrane wing. Kinematics, structural dynamics, and aerodynamics are the main component of flapping flight studies. Modal analysis of fabricated wings is done using 3D visual image correlation (VIC-3D) and laser displacement sensor setup. In the end, the results of both type wings are compared with experimental results and a good correlation has been seen. The validation of results is done using Ansys.

Proper Orthogonal Decomposition of Flexible Clap and Fling Elastic Motions via High-Speed Deformation Measurements

Many complex unsteady mechanisms are thought to facilitate the high efficiency and agility commonly observed in small biological flyers. One of these, the flexible clap and fling maneuver, has not been extensively studied; an experimental characterization is the focus of this work. The clap–fling mechanism is approximated with a single flexible membrane flapping wing, replacing the symmetry plane between two wings with a splitter plate simulating the pair wing. This produces a complex vibro-impact aeroelastic problem, the deformation resulting from which is measured with a high-speed visual image correlation system. A low-dimensional representation of the ensuing large data set is obtained with proper orthogonal decomposition. The POD modes, and the relative importance of each, can help elucidate crucial mechanisms and relationships within the flapping system, and are computed for various membrane wing structures and flapping frequencies, with or without the presence of the splitter plate.

Aerodynamic control of micro air vehicle wings using electroactive membranes

Dielectric elastomer materials are ideal candidates for developing high-agility micro air vehicles due to their electric field–induced deformation. Consequently, the aero-structural response and control authority of the dielectric elastomer material, VHB 4910, are characterized on an elliptical membrane wing. An experimental membrane wing platform was constructed by stretching VHB 4910 over a rigid elliptical wing-frame. The low Reynolds number (chord Reynolds number < 106) and aerodynamics of the elliptical wing were characterized when different electrostatic fields were applied to the membrane. We observe an overall increase in lift with maximum gains of 20% at an applied voltage of 4.5 kV and demonstrate the ability to delay stall. The time-averaged aerodynamic surface pressure is also investigated by comparing sting balance data and membrane deformation measured using visual image correlation. The experimental results are compared to a nonlinear finite element membrane model to further understand the effects of aerodynamic load and electric fields on membrane displacements. Model predictions of surface pressure provide insight into how the electrostrictive constitutive relations influence the fluid–structure interactions of the membrane. This is validated by comparing lift predictions from the model with time-averaged wind tunnel lift measurements near stall.

Energy Absorption Behavior of a Hyperelastic Membrane for Micro Air Vehicle Wings: Experimental and Finite Element Approaches

The rubber latex membrane is used for micro air vehicles construction and plays an important role in their wings performance. This paper presents finite element (FE) models for investigating the deformation and energy absorption behavior of the latex membrane at static loading, validated by the experimental results. The membrane at different pre-tension levels are attached to a circular steel ring and statically loaded using steel spheres of different sizes placed at the center of the membrane. The deformation of the membrane is measured by the visual image correlation (VIC), a non-contact measurement system and strain energy is calculated based on the Mooney–Rivlin hyperelastic material model. It is found that the deflection and strain energy of the membrane estimated by experiments and FE models are correlated well although discrepancy is expected among experimental and FE results within reasonable limits due to the variation of the thickness of the membrane. The deformation and energy absorption of the membrane increase with the weight of the sphere. But the deformation decreases and the energy absorption increases with increase of the pre-tension level of the membrane due to a specific weight of the sphere.

A Bendable Load Stiffened Wing for Small UAVs

A bendable load stiffened wing, developed at the University of Florida, has the ability to load stiffen in the positive flight load direction while remaining compliant in the opposite direction, enabling UAV storage inside smaller packing volumes. The wing employs an under-cambered airfoil with a swept planform providing dissimilar stiffness in the flight load and the folding direction. A comparative experimental study is performed using two wing geometries; straight camber and swept camber. The load stiffening ability is tested by performing three point bend tests while monitoring the wing root airfoil shape change using a visual image correlation technique. For the wing utilizing a swept camber design, increase in the root airfoil camber with increased loading resulted in a load stiffening structure. Swept camber wing showed a higher load carrying capacity (7 g's load factor) over a straight camber wing design (2 g's load factor), still maintaining the compliant nature in the folding direction. Long term storage induced creep deformations are small in both of the wing geometries. By increasing the wing stiffness, sweepback helps in reducing spanwise residual creep strain. Wind tunnel tests at Re = 7×104 of both the straight camber and the swept camber wing show similar L/D ratios. The sweepback helps in improving the static stability of the wing. Thus the bendable load stiffened wing has a clear advantage of offering stiffness improvement and reducing storage induced creep residual strains while maintaining the aerodynamic efficiency and improving the static stability of the wing.

The Validity Range of Low Fidelity Structural Membrane Models

Three classes of structural models can be considered for the characterization of thin extensible pressurized membranes. A low-fidelity, linear model assumes that all of the membrane’s resistance to a transverse pressure is provided from geometric stress-stiffening, and that movement along the membrane is purely out-of-plane. A medium-fidelity model can include geometric nonlinearity (finite strains, non-conservative pressure loads); while the highest level of membrane modeling assumes material nonlinearity: specifically, hyperelasticity. Common modeling applications such as performance prediction, failure prevention, and structural optimization all require repeated function evaluations. As computational cost is proportional to model fidelity, what is the validity range of the two lower fidelity models discussed above? The presence of a large pre-tension within the membrane is known to lessen the role of nonlinear elastic effects: the range of linear transverse deformation increases with pre-tension. Conversely, very low pre-tensions require the use of a nonlinear model, as the linear model becomes unbounded. This work studies the Hencky–Campbell problem for model validation purposes: hydrostatic inflation of a thin flat circular membrane, clamped along its boundary, with an arbitrary initial tension. A full-field, non-contact visual image correlation system is used to estimate the material properties of the rubber membrane, measure the state of pre-tension in the circular sheet, and document the displacement and strain fields as a function of applied pressure. The resulting data set is then compared directly with numerical simulations, in order to estimate the location of the surface of data points wherein a particular low fidelity model (either linear or geometrically nonlinear) loses its predictive capability.

Wind-Tunnel Testing and Modeling of a Micro Air Vehicle with Flexible Wings

The field of micro air vehicles is relatively immature; consequently, high-fidelity simulations do not yet exist for a generic aircraft. The fidelity of flight dynamic simulations is closely correlated to the reliability of models representing the vehicle's aerodynamic and propulsion characteristics in the entire flight envelope, including the nonlinear region. This paper discusses wind-tunnel experiments performed to investigate the aerodynamic and mechanical characteristics of micro air vehicles with flexible wings in different conditions of propeller type, motor power, and elevator deflections. Visual image correlation was used to measure the deformation of the flexible wings to quantify general features such as variations in aerodynamic and geometric twist angle. Aerodynamic and propulsion results were used to formulate empirical models of the relevant coefficients in the form of multiple linear regressions and to estimate the effectors' functional dependencies and interactions. High-order nonlinear interactions were confirmed between the coefficients of lift, drag, and pitching moment with the independent variables. The rates of the dependencies with elevator deflections and angle of attack were found, to some extent, to be motor voltage and dynamic pressure dependent, evincing a strong coupling with the propeller speed.

Static Aeroelastic Model Validation of Membrane Micro Air Vehicle Wings

A low-aspect-ratio, low-Reynolds-number membrane wing has been identified as a viable platform for micro air vehicle applications. Desirable flying qualities include high lift and larger stability margins. Several challenges are associated with the numerical modeling of such a wing, including highly three-dimensional flows, separation bubbles, and nonlinear membrane behavior. A thorough model validation and system identification effort is therefore required. A novel experimental setup integrates a wind tunnel with a visual image correlation system for simultaneous measurement of wing displacements, strains, and aerodynamic loads. These three metrics are used for a direct comparison of numerical and experimental data for both pre- and poststall angles of attack. Suitable correspondence is demonstrated for moderate angles of attack; methods for increasing the model fidelity can be made for angles with poor predictive capability. Computed flow structures reveal further information concerning the aeroelastic behavior of membrane wings.

Aerodynamic Coefficients and Deformation Measurements on Flexible Micro Air Vehicle Wings

This paper documents the elastic deformations and corresponding aerodynamic coefficients of flexible wings used for micro air vehicles (MAVs). These low-aspect ratio wings, developed and fabricated at the University of Florida, incorporate an elastic latex membrane skin covering a thin carbon fiber skeleton. The wings were tested in a unique low-speed wind tunnel facility integrating a visual image correlation (VIC) system with a six-component strain gauge sting balance. Model characteristics are presented, along with the appropriate specimen preparation techniques and wind tunnel instrumentation. The static response characteristics, including full-field displacements and plane strain measurements, for three distinct MAV wing designs are presented. The full-field deformation results show how passive wing flexibility preferably affects aerodynamic performance when compared to a rigid model of similar geometry.

Static Finite Element Validation of a Flexible Micro Air Vehicle

The flexible-wing approach has proven to be a successful method for designing micro air vehicles. The wing’s passive deformation under wind loads can allow for gust rejection, delayed stall, or improved longitudinal stability. As such, an accurate structural model of the flexible wing can provide greater understanding of the aforementioned phenomena. This paper seeks to formulate a static finite element wing model, with a particular emphasis on accuracy. The wing is broken into three different types of elements: beams, plates, and membranes. Individual element types are characterized and validated by constructing simple structures from the appropriate material, and then comparing experimental and numerical deformation fields. Experimental results are found through a visual image correlation system. The elements are then combined to form the complete wing model, which is also validated through experiments. The resulting finite element model is found to be very accurate, able to predict the complicated structural response of a composite wing. Due to observations made during standard wind tunnel testing, the structural response of a typical membrane MAV wing in steady level pre-stall flight is thought to be quasi-static. As such, the finite element model formulated in this work will be indispensable towards future numerical static aeroelastic optimization research efforts aimed at improving the efficiency, agility, and sensitivity of practical micro air vehicles.