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1. Improving Output Power of a Torsional-Flutter Harvester in Stochastic Thunderstorms by Duffing—Van Der Pol Restoring Torque

   https://doi.org/10.1115/1.4065532  

   Caracoglia, Luca (December 2024, ASCE-ASME Journal of Risk and Uncertainty in Engineering Systems, Part B: Mechanical Engineering)

   Wind energy harvesters are usually designed to operate in the low wind speed range. They rely on smaller swept areas, as a complement to larger horizontal-axis wind turbines. A torsional-flutter-based apparatus is investigated herein to extract wind energy. A nonlinear hybrid restoring toque mechanism, installed at equally spaced supports, is used to produce energy through limit-cycle vibration. Energy conversion and storage from the wind flow are enabled by eddy currents. The apparatus is used during thunderstorm outflows to explore its efficiency in nonideal wind conditions. The thunderstorm flow model accounts for both nonstationary turbulence and slowly varying mean wind speed, replicating thunderstorm's intensification and decay stages. This paper evolves from a recent study to examine stochastic stability. More specifically, the output power is derived as a random process that is found numerically. Various thunderstorm features and variable apparatus configurations are evaluated. Numerical investigations confirm the detrimental effect of nonideal, thunderstorms on harvester performance with, on average, an adverse increment of operational speed (about +30%). Besides nonlinear damping, the “benign” flutter-prone effect is controlled by the square value of the flapping angle. Since flapping amplitudes are moderate at sustained flutter, activation of the apparatus is delayed and exacerbated by the nonstationary outflow and aeroelastic load features. Finally, efficiency is carefully investigated by quantification of output power and “quality factor.” 

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2. Exploiting Stochastic Resonance Principles to Influence the Efficiency of a Torsional-Flutter Energy Harvester in Turbulent Winds

   https://doi.org/10.1115/1.4067394  

   Caracoglia, Luca (April 2025, ASME Letters in Dynamic Systems and Control)

   Abstract Wind energy harvesters are emerging as a viable alternative to standard, large horizontal-axis wind turbines. This study continues a recent investigation on the operational features of a torsional-flutter-based apparatus, proposed by the author to extract wind energy. The apparatus is composed of a non-deformable, flapping blade-airfoil. A nonlinear torsional spring mechanism, either simulated by a Duffing model or a hybrid Duffing–van der Pol model, installed at equally spaced supports, enables limit-cycle, post-critical vibration. To enhance the output power, stochastic resonance principles are invoked through a novel, negative stiffness mechanism that is coupled to the eddy current device for energy conversion. The output power is explored by numerically solving the stochastic differential equation of the model, accounting for incoming flow turbulence. Three main harvester types with variable configuration are examined; the chord length of the blade-airfoil, used for energy harvesting, varies between 0.5 and 1 m; the spanwise-length-to-chord aspect ratio is four. The flapping frequency varies between 0.10 and 0.25 Hz. The study demonstrates that exploitation of negative stiffness mechanism can improve the performance of the harvester. 

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3. Stochastic flutter analysis of a torsional-vibration-based energy harvester affected by random aeroelastic loads and wind turbulence

   https://doi.org/10.1088/1742-6596/2647/11/112009  

   Piciucco, Davide; Caracoglia, Luca (June 2024, Journal of Physics: Conference Series)

   This paper investigates the energy production of a “meso-scale”, wind-based energy harvester that exploits the torsional aeroelastic instability of a rigid blade-airfoil, elastically supported at equidistant supports. Torsional flutter is a single mode aeroelastic instability phenomenon, in which a diverging dynamic angular rotation of a body occurs. The apparatus relies on a simple mechanism that uses flow-induced pitch motion to extract and convert airflow kinetic energy to electrical energy. The system is composed by a rigid blade-airfoil, connected to a support structure through a non-linear restoring force (torsional spring-like) mechanism that enables the rotation about a reference pivot axis. The proposed technology is designed to be efficient in the range of low and medium wind speeds (10-13 m/s), in which horizontal-axis wind turbines and other harvesters are not efficient. Deterministic pre-flutter, incipient flutter and post-critical vibrations of the apparatus have been already explored in a previous study. This work aims to further investigate the aeroelastic behavior of the “flapping foil” by examining the effect of turbulence, random experimental error and modeling simplifications of the aeroelastic forces. The analysis is conducted at incipient flutter in the frequency domain using classical unsteady force models. Monte Carlo methods are employed to solve for the probability of incipient flutter speed. Several configurations are considered to improve the efficiency of the energy harvester. 

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4. Exploring Stochastic Dynamics and Stability of an Aeroelastic Harvester Contaminated by Wind Turbulence and Uncertain Aeroelastic Loads

   Caracoglia, Luca (September 2022, 8th European African Conference on Wind Engineering, Bucharest, Romania, September 20-23, 2022. Conspress, 2022, ISBN 978-973-100-532-4)

   Calotescu, Ileana; Chitez, Adriana; Coşoiu, Costin; Vlăduţ, Alexandru Cezar (Ed.)

   The paper expands a recently developed model that examines the stochastic stability of a torsional-flutter-based harvester. The new model accounts for both uncertainty in the aeroelastic loads and wind turbulence in the incoming flow. Since the blade-airfoil is three-dimensional, three-dimensional flow effect are simulated through η3D, i.e., a reduction parameter of the static lift slope, dependent on the aspect ratio of the apparatus. The first uncertainty source is a byproduct of the modelling simplifications of the aeroelastic loads, which are described by indicial function approach and ideally applicable to two-dimensional flow. The second source is the flow turbulence that operates by modifying the Parametric stochastic perturbations are applied to the parameter describing the memory-effect of the load, simulating “imperfections” in the load measurement and approximate description through η3D. Stochastic flutter stability is examined by mean squares. Post-critical states are also discussed. 

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5. Discrete Vortex Modeling of a Flapping Foil with Activated Leading Edge Motion

   Prier, M.W.; Liburdy, J.A. (July 2019, ASME-JSME-KSME 2019 Joint Fluids Conference)

   Energy harvesting performance for a flapping foil device is evaluated to determine how activated leading edge motion affects the aerodynamic forces and the cycle power generated. Results are obtained for a thin flat foil that pitches about the midchord and operates in the reduced frequency range of k = f c/U of 0.06 - 0.10 and Reynolds numbers of 20,000 and 30,000 with a pitching amplitude of 70 and heaving amplitude of h0 = 0.5c. Time resolved data are presented based on direct force measurements and are used to determine overall cycle efficiency and coefficient of power. These results are compared against a panelbased discrete vortex model to predict power production. The model incorporates a leading edge suction parameter predictor for vortex shedding and empirical adjustments to circulatory forces. It is found that the leading edge motions that reduce the effective angle of attack early in a flapping stroke generate larger forces later in the stroke. Consequently, the energy harvesting efficiencies and power coefficients are increased since the generated aerodynamic loads are better synchronized with the foil motion. The efficiency gains are reduced with increasing reduced frequencies. 

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