An official website of the United States government
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.
more »
« less
COMPUTATIONAL STUDY OF CANTILEVERED PLATE IN HIGH SPEED AXIAL FLOW
A cantilevered plate at an angle of attack in axial high speed flow represents a prototypical control surface. Using a computationally efficient modal structural model cou- pled with a novel enhanced piston theory aerodynamic method, a nonlinear aeroelastic model is developed. Results presented include pre-flutter stable steady state solutions, flutter points, and post-flutter limit cycle oscillations. Because of the computationally efficient methodology, many simulations can be run to define sensitivities to mathematical modeling, model parame- ters, and numerical methods.
more »
« less
- Award ID(s):
- 1907500
- PAR ID:
- 10347596
- Date Published:
- Journal Name:
- International Forum on Aeroelasticity and Structural Dynamics
- Issue:
- IFASD 2022
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Flag flutter frequently features a marked difference between the onset speed of flutter and the speed below which flutter stops. The hysteresis tends to be especially large in experiments as opposed to simulations. This phenomenon has been ascribed to inherent imperfections of flatness in experimental samples, which are thought to inhibit the onset of flutter but have a lesser effect once a flag is already fluttering. In this work, we present an experimental confirmation for this explanation through motion tracking. We also visualize the wake to assess the potential contribution of discrete vortex shedding to hysteresis. We then mould our understanding of the mechanism of bistability and additional observations on flag flutter into a novel, observation-based, semiempirical model for flag flutter in the form of a single ordinary differential equation. Despite its simplicity, the model successfully reproduces key features of the physical system such as bistability, sudden transitions between non-fluttering and fluttering states, amplitude growth and frequency growth.more » « less
-
Many studies over the 1960’s reported failure in predicting accurate flutter boundaries using the classical theory of unsteady aerodynamics even at zero angle of attack and/or lift conditions. Since the flutter phenomenon lies in the intersection between unsteady aerodynamics and structural dynamics, and because the structural dynamics of slender beams can be fairly predicted, it was inferred that the problem stems from the classical theory of unsteady aerodynamics. As a result, a research flurry occurred over the 1970’s and 1980’s investigating such a theory, with particular emphasis on the applicability of the Kutta condition to unsteady flows. There was almost a consensus that the Kutta condition must to be relaxed at high frequencies and low Reynolds numbers, which was also concluded from several recent studies of the unsteady aerodynamics of bio-inspired flight. Realizing that vorticity generation and lift development are essentially viscous processes, we develop a viscous extension of the classical theory of unsteady aerodynamics, equivalently an unsteady extension of the boundary layer theory. We rely on a special boundary layer theory that pays close attention to the details in the vicinity of the trailing edge: the triple deck theory. We use such a theory to relax the Kutta condition and determine a viscous correction to the inviscid unsteady lift. Using the developed viscous unsteady model, we develop a Reynolds-number-dependent lift frequency response (i.e., a viscous extension of Theodorsen’s). It is found that viscosity induces a significant phase lag to the lift development beyond Theodorsen’s inviscid solution, particularly at high frequencies and low Reynolds numbers. Since flutter, similar to any typical hopf bifurcation, is mainly dictated by the phase difference between the applied loads and the motion, it is expected that the viscosity-induced lag will affect the flutter boundary. To assess such an effect, we couple the developed unsteady viscous aerodynamic theory with a structural dynamic model of a typical section to perform aeroelastic simulation and analysis. We compare the flutter boundary determined using the developed viscous unsteady model to that of Theodorsen’s.more » « less
-
The current trend in offshore wind energy is to design and install systems with larger swept areas that yield unprecedented efficiency. Long and slender blades are needed to achieve this objective. As a result of aerodynamic and structural tailoring, slender blades are particularly susceptible to various dynamic instability phenomena during standard operations. One of these phenomena is the bending-torsion flutter that may lead either to structural failure or system breakdown. The research author has been examining blade flutter under the influence of stochastic perturbations, which include both flow turbulence and aeroelastic load variability. A reduced-order Markov model has been used to describe the effects of the various random perturbations. Mean-square stability has been recently explored; results suggest that perturbations may negatively impact the flutter angular speed and increase the risk of failure. In this study the model is employed to investigate moment stability beyond mean squares, observing that dynamic instability involves nonlinear propagation of the perturbations and may exhibit amplitude dependency. Third-order instability is investigated and compared against previous numerical results. The NREL 5MW reference wind turbine blade is used as a benchmark example.more » « less
-
With the computational resources becoming available, data-driven methods have emerged as powerful means for equation discovery and model construction. Sparse regression methods such as SINDy (Sparse Identification for Nonlinear Dynamical Systems) can be used for developing reduced-order models of nonlinear systems. In this study, the authors examine how SINDy can be used for developing low-dimensional models for airfoil systems, which experience unsteady aerodynamic loads and flutter instabilities. For a system of multiple closely spaced airfoil oscillators, analytical models are not readily available to determine flutter instabilities, and one has to take recourse to experimental and numerical means. In this work, as a starting point, data collected through simulations of unsteady aerodynamics of a single airfoil oscillator system are considered and a reduced-order model is constructed based on this data.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- The DOI is not currently available.
