Sergio Preidikman

Influence of flexibility on the aerodynamic performance of a hovering wing

SUMMARY In the present study, a computational investigation was carried out to understand the influence of flexibility on the aerodynamic performance of a hovering wing. A flexible, two-dimensional, two-link model moving within a viscous fluid was considered. The Navier–Stokes equations governing the fluid dynamics were solved together with the equations governing the structural dynamics by using a strongly coupled fluid–structure interaction scheme. Harmonic kinematics was used to prescribe the motions of one of the links, thus effectively reducing the wing to a single degree-of-freedom oscillator. The wings flexibility was characterized by the ratio of the flapping frequency to the natural frequency of the structure. Apart from the rigid case, different values of this frequency ratio (only in the range of 1/2 to 1/6) were considered at the Reynolds numbers of 75, 250 and 1000. It was found that flexibility can enhance aerodynamic performance and that the best performance is realized when the wing is excited by a non-linear resonance at 1/3 of the natural frequency. Specifically, at Reynolds numbers of 75, 250 and 1000, the aerodynamic performance that is characterized by the ratio of lift coefficient to drag coefficient is respectively increased by 28%, 23% and 21% when compared with the corresponding ratios of a rigid wing driven with the same kinematics. For all Reynolds numbers, the lift generated per unit driving power is also enhanced in a similar manner. The wake capture mechanism is enhanced, due to a stronger flow around the wing at stroke reversal, resulting from a stronger end of stroke vortex at the trailing edge. The present study provides some clues about how flexibility affects the aerodynamic performance in low Reynolds number flapping flight. In addition, it points to the importance of considering non-linear resonances for enhancing aerodynamic performance.

Nonlinear free and forced oscillations of piezoelectric microresonators

Free and forced oscillations of piezoelectric, microelectromechanical resonators fabricated as clamped?clamped composite structures are studied in this effort. Piezoelectric actuation is used to excite these structures on the input side and piezoelectric sensing is carried out on the output side. A refined integro-partial differential model is developed for a clamped?clamped composite beam structure and used for studying the nonlinear transverse vibrations of these resonators. This model accounts for the longitudinal extension due to transverse vibrations, distributed actuation and axially varying properties across the length of the structure. Free oscillations about a post-buckled position are studied, and for weak damping and weak forcing, the method of multiple scales is used to obtain an approximate solution for the response to a harmonic forcing. Analytical predictions are also compared with experimental observations. The model development and the analysis can serve as a basis for analysing the responses of other composite microresonators.

Novel strategy for suppressing the flutter oscillations of aircraft wings

A new strategy, based on the nonlinear phenomenon of saturation, is proposed for controlling the flutter of a wing. The concept is illustrated by means of an example with a rather flexible, high-aspect wing of the type found on such vehicles as high-altitude long-endurance aircraft and sailplanes. The wing is modeled structurally as an Euler-Bernoulli beam with coupled bending and twisting motions. A general unsteady nonlinear vortex-lattice technique is used to model the flow around the wing and provide the aerodynamic loads. The structure, the flowing air, and the controller are considered the elements of a single dynamic system, and all of the coupled equations of motion are simultaneously and interactively integrated numerically in the time domain. The results indicate that the aerodynamic nonlinearities alone can be responsible for limit-cycle oscillations and that the saturation controller can effectively suppress the flutter oscillations of the wing when the controller frequency is actively tuned.

Modified Unsteady Vortex-Lattice Method to Study Flapping Wings in Hover Flight

A numerical-simulation tool is developed that is well suited for modeling the unsteady nonlinear aerodynamics of flying insects and small birds as well as biologically inspired flapping-wing micro air vehicles. The present numerical model is an extension of the widely used three-dimensional general unsteady vortex-lattice model and provides an attractive compromise between computational cost and fidelity. Moreover, it is ideally suited to be combined with computational structural dynamics to provide aeroelastic analyses. The present numerical results for a twisting, flapping wing with neither leading-edge nor wing-tip separations are in close agreement with the results obtained in previous studies with the Euler equations and a vortex-lattice method. The present results for unsteady lift, mean lift, and frequency content of the force are in good agreement with experimental data for the robofly apparatus. The actual wing motion of a hovering Drosophila is used to compute the flowfield and predict the lift ...

A semi-analytical tool based on geometric nonlinearities for microresonator design

In this paper, a computational mechanics model specifically tailored for composite microresonators with piezoelectric actuation and piezoelectric sensing is developed and used as a design tool for these microresonators. The developed model accounts for the structural properties and the electromechanical coupling effect through finite-element analysis. It is assumed that the deflection is large and that the geometric nonlinearity must be included. The dynamic admittance model is derived by combining the linear piezoelectric constitutive equations with the modal transfer function of the multi-layered microresonator structure. The resonator receptance matrix is constructed through modal summation by considering a limited number of dominant modes. The electromechanical coupling determination at the input and output ports makes use of converse and direct piezoelectric effects. In the development of the finite-element models, the boundary conditions, the shapes of electrodes and distributed parameters such as varying elastic modulus across the length of the structure have been taken into account. The developed semi-analytical tool can be used to carry out parametric studies with respect to the following: (i) the resonator beam thickness and length, (ii) the influence of constant axial forces on the transverse vibrations of clamped–clamped microresonators, (iii) the geometry of the drive and sense electrodes and (iv) imperfect boundary conditions due to mask imperfections and fabrication procedure. The semi-analytical development has been validated by comparing model predictions with prior results available in the literature for clamped–clamped resonators and experimental measurements. A detailed discussion of modeling considerations is also presented.

Development of a Kinematical Model to Study the Aerodynamics of Flapping-Wings

The kinematics that characterizes the “natural flight” of insects is quite complex. It involves simultaneous rotations, oscillations and significant changes in the angle of attack. All this permits the wings to follow an extremely complex trajectory producing different flight mechanisms that are efficient at low to moderate Reynolds numbers. Some of these mechanisms, such as the delayed stall, the additional circulation generated by the rotation of the wing, and the wake capture amongst others, offer unique advantages with respect to the well-known fixed-wing aerial vehicles. Such advantages are better lift and thrust generation without the need to increase weight. This paper presents a general kinematical model that permits studying the movements of the wings of a scale robot of a house fly, the ‘RoboFly’, built at UC Berkeley, USA. Additionally, this general kinematical model allows studying the kinematics of the wings of a flying insect considering both the body orientation and the stroke plane orientation of the creature in the 3D space. This work provides a nexus between the descriptive language used by biologists and the predictive language used by engineers. This connection between scientific disciplines allows one to study and characterize the principal kinematic parameters that intervene in a stroke cycle, as well as to determine how these variables modify the trajectories of the material points on the wings.

Time-domain simulation for evaluating smart wing concepts for reducing gust loads

A numerical simulation for evaluating methods of predicting and controlling the response of an elastic wing in an airstream is discussed. The technique employed interactively and simultaneously solves for the response in the time domain by considering the air, wing, and controller as elements of a single dynamical system. The method is very modular, allowing independent modifications to the aerodynamic, structural, or control subsystems and it is not restricted to periodic motions or simple geometries. To illustrate the technique, a High Altitude, Long Endurance aircraft wing is used. The wing is modeled structurally as a linear Euler-Bernoulli beam that includes dynamic coupling between the bending and torsional oscillations. It is discretized via finite elements. The general, nonlinear, unsteady vortex lattice method, which is capable of simulating arbitrary subsonic maneuvers of the wing and accounts for the history of the motion, is employed to model the aerodynamics and feedback control via a distributed actuator is used for flutter and gust-load alleviation. The aerodynamic and structural grids do not have to be coincident. A controller that responds to induced loads and bending moments on the wing via a distributed actuator (e.g., piezoelectrics) by simultaneously decreasing the angle of attack is proposed.

Computational Dynamics of Flapping Wings in Hover Flight: A Co-Simulation Strategy

A co-simulation strategy for modeling the unsteady dynamics of flying insects and small birds as well as biologically inspired flapping-wing micro-air-vehicles is developed in this work. In particu...

A Numerical Model to Study the Nonlinear and Unsteady Aerodynamics of Bioinspired Morphing-Wing Concepts

In the present paper, a numerical model to study the nonlinear and unsteady aerodynamics of morphing-wing concepts inspired by bird flight is developed. The model includes: i) a wing topology inspired by gull wings; ii) a kinematical model to describe the process of wing adaptation based on one mechanism observed in the flight of gulls (folding-wing approach); and iii) a version of the unsteady vortex-lattice methods (UVLM) that allows taking nonlinear and unsteady aerodynamic phenomena into account. The model was specially developed to study the aerodynamic behavior during wing adaptation. A simulation for a twisting-flapping wing was performed in order to validate the numerical model. The present results are in close agreement with those obtained in previous studies based on the Euler equations, but required much less execution time. The numerical simulations of a bioinspired morphing wing showed the strong dependence between the prescribed kinematics and the aerodynamic characteristics, which evidences the importance of studying the process of wing adaptation. UVLM is shown to be ideal for preliminary analysis of bioinspired morphing wings.

An Exact Solution for the Natural Frequencies of Flexible Beams Undergoing Overall Motions

We investigate the free vibrations of a flexible beam undergoing an overall two-dimensional motion. The beam is modeled using the Euler-Bernoulli beam theory. An exact solution for the natural frequencies and corresponding mode shapes of the beam is obtained. The model can be extended to beams undergoing three-dimensional motions.