1,299 research outputs found

    A two-step hybrid approach for modeling the nonlinear dynamic response of piezoelectric energy harvesters

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    An effective hybrid computational framework is described here in order to assess the nonlinear dynamic response of piezoelectric energy harvesting devices. The proposed strategy basically consists of two steps. First, fully coupled multiphysics finite element (FE) analyses are performed to evaluate the nonlinear static response of the device. An enhanced reduced-order model is then derived, where the global dynamic response is formulated in the state-space using lumped coefficients enriched with the information derived from the FE simulations. The electromechanical response of piezoelectric beams under forced vibrations is studied by means of the proposed approach, which is also validated by comparing numerical predictions with some experimental results. Such numerical and experimental investigations have been carried out with the main aim of studying the influence of material and geometrical parameters on the global nonlinear response. The advantage of the presented approach is that the overall computational and experimental efforts are significantly reduced while preserving a satisfactory accuracy in the assessment of the global behavior

    Optimization of force-limiting seismic devices connecting structural subsystems

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    This paper is focused on the optimum design of an original force-limiting floor anchorage system for the seismic protection of reinforced concrete (RC) dual wall-frame buildings. This protection strategy is based on the interposition of elasto-plastic links between two structural subsystems, namely the lateral force resisting system (LFRS) and the gravity load resisting system (GLRS). The most efficient configuration accounting for the optimal position and mechanical characteristics of the nonlinear devices is obtained numerically by means of a modified constrained differential evolution algorithm. A 12-storey prototype RC dual wall-frame building is considered to demonstrate the effectiveness of the seismic protection strategy

    Energy harvesting from earthquake for vibration-powered wireless sensors

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    Wireless sensor networks can facilitate the acquisition of useful data for the assessment and retrofitting of existing structures and infrastructures. In this perspective, recent studies have presented numerical and experimental results about self-powered wireless nodes for structural monitoring applications in the event of earthquake, wherein the energy is scavenged from seismic accelerations. A general computational approach for the analysis and design of energy harvesters under seismic loading, however, has not yet been presented. Therefore, this paper proposes a rational method that relies on the random vibrations theory for the electromechanical analysis of piezoelectric energy harvesters under seismic ground motion. In doing so, the ground acceleration is simulated by means of the Clough-Penzien filter. The considered piezoelectric harvester is a cantilever bimorph modeled as Euler-Bernoulli beam with concentrated mass at the free-end, and its global behavior is approximated by the dynamic response of the fundamental vibration mode only (which is tuned with the dominant frequency of the site soil). Once the Lyapunov equation of the coupled electromechanical problem has been formulated, mean and standard deviation of the generated electric energy are calculated. Numerical results for a cantilever bimorph which piezoelectric layers made of electrospun PVDF nanofibers are discussed in order to understand issues and perspectives about the use of wireless sensor nodes powered by earthquakes. A smart monitoring strategy for the experimental assessment of structures in areas struck by seismic events is finally illustrated

    Topology optimization of multi-story buildings under fully non-stationary stochastic seismic ground motion

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    Topology optimization has been mainly addressed for structures under static loads using a deterministic setting. Nonetheless, many structural systems are subjected to uncertain dynamic loads, and thus efficient approaches are required to evaluate the optimal topology in such kind of applications. Within this framework, the present paper deals with the topology optimization of multi-story buildings subjected to seismic ground motion. Because of the inherent randomness of the earthquakes, the uncertain system response is determined through a random vibration-based approach in which the seismic ground motion is described as filtered white Gaussian noise with time-varying amplitude and frequency content (i.e., fully non-stationary seismic ground motion). The paper is especially concerned with the assessment of the dynamic response sensitivity for the gradient-based numerical solution of the optimization problem. To this end, an approximated construction of the gradient is proposed in which explicit, exact derivatives with respect to the design variables are computed analytically through direct differentiation for a sub-assembly of elements (up to a single element) resulting from the discretization of the optimizable domain. The proposed strategy is first validated for the simpler case of stationary base excitation by comparing the results with those obtained using an exact approach based on the adjoint method, and its correctness is ultimately verified for the more general case of non-stationary seismic ground motion. Overall, this validation demonstrates that the proposed approach leads to accurate results at low computational effort. Further numerical investigations are finally presented to highlight to what extent the features of the non-stationary seismic ground motion influence the optimal topology

    Towards a Wind Tunnel Testing Environment for Rotorcraft Operations Close to Obstacles

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    The correct identification of the aerodynamic loads due to interaction between rotorcraft and obstacles requires to run computationally intensive numerical models characterized by a high level of uncertainty. Wind tunnel data can be used as a source of information to improve those models. The present paper investigates the aerodynamic interaction of a helicopter and ship airwake exploiting wind tunnel data. A series of wind tunnel experiment, using a scaled helicopter model and Simple Frigate Shape 1, has been performed to measure forces and moments acting on the rotor, while the helicopter is approaching the flight deck. In addition, the velocity components along the longitudinal symmetry plane of the rotor have been visualized using PIV technique. With the rotor positioned at the starting point of the landing trajectory, the load measurements are used to modify the distribution of the inflow over the rotor in multibody simulation environment, in order to generate same loads, including thrust, torque and in-plane moments. Then, an identification algorithm is developed to capture the effect of ship airwake on the rotor loads during the maneuvers, modeling it as an external gust to the rotor inflow. The gust velocity is obtained through an optimization algorithm with the objective of generating same load coefficients as the experiment. The simulation results show that the same load coefficients as the experiment can be generated by implementing a linear gust over the rotor with a magnitude that changes as the rotor moves through the wake of ship. The experiment showed that this test setup could be used for identification of aerodynamic interaction to be used for maneuver analysis

    Numerical simulation of aileron buzz using an adaptive-grid compressible flow solver for dynamic meshes

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    The paper presents numerical results from a novel scheme for the solution of the flow equations in two dimensional domains by an Arbitrary Lagrangian Eulerian formulation able to cope with deforming and adaptive two dimensional grids without recurring to any explicit interpolation scheme. The method is applied to the investigation of a classical transonic aeroelastic instability phenomenon: the aileron buzz. By resorting to deforming and adaptive grids, the method allows to highlight the dependency of the aeroelastic stability boundaries on the mesh spacing

    Voluntary Pilot Action Through Biodynamics for Helicopter Flight Dynamics Simulation

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    This work presents the integration of detailed models of a pilot controlling a helicopter along the heave axis through the collective control inceptor. The action on the control inceptor is produced through a biomechanical model of the pilot’s limbs, by commanding the activation of the related muscle bundles. Such activation, in turn, is determined by defining the muscle elongations required to move the control inceptor in order to obtain the control of the vehicle according to a high-level model of the voluntary action of the pilot acting as a regulator for the vehicle. The biomechanical model of the pilot’s limbs and the aeromechanical model of the helicopter are implemented in a general-purpose multibody simulation. The helicopter model, the biomechanical model of the pilot’s limbs, the cognitive model of the pilot, and their integration are discussed. The integrated model is applied to the simulation of simple, yet representative, mission task elements
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