Experimental study of inertia-based passive flexibility of a heaving and pitching airfoil operating in the energy harvesting regime

Abstract

The effects of passive, inertia-induced surface flexibility at the leading and trailing edges of an oscillating airfoil energy harvester are investigated experimentally at reduced frequencies of k = fc/U∞ = 0.10, 0.14, and 0.18. Wind tunnel experiments are conducted using phase-resolved, two-component particle image velocimetry to understand the underlying flow physics, as well as to obtain force and pitching moment estimates using the vortex-impulse theory. Results are obtained for leading and trailing edge flexibility separately. It is shown that both forms of flexibility may alter the leading edge vortex inception and detachment time scales, as well as the growth rate of the circulation. In addition, surface flexibility may also trigger the generation of secondary vortical structures and suppress the formation of trailing edge vortices. The total energy harvesting efficiency is decomposed into contributions of heaving and pitching motions. Relative to the rigid airfoil, the flexible leading and trailing edge segments are shown to increase the energy harvesting efficiency by approximately 17% and 25%, respectively. However, both the flexible leading and trailing edge airfoils operate most efficiently at k = 0.18, whereas the peak efficiency of the rigid airfoil occurs at k = 0.14. It is shown that the flexible leading and trailing edge airfoils enhance the heaving contribution to the total efficiency at k = 0.18 and the negative contribution of the pitching motion at high reduced frequencies can be alleviated by using a flexible trailing edge.

Force and torque reactions on a pitching flexible aerofoil

“…The flow-structure interaction (FSI) problem is solved numerically using the coupled fluid and solid equations of motion discretized in both the fluid and plate domains separated by a moving boundary. In addition to (2.5a,b) for the fluid, more general differential structural equations inside the plate are used instead of the thickness-averaged nonlinear equation of motion (2.2), as described in Sanmiguel-Rojas & Fernandez-Feria (2021), but now for the pitching motion (2.1) imposed at the leading edge instead of a heaving motion. For both fluid and solid, the finite-volume-based solver Ansys Fluent is used, with the well known k-ω SST turbulence model for the fluid, and the intrinsic FSI algorithm to simulate the fluid-structure coupling.…”

“…It must also be emphasized that the aerofoil's inertia effects on the force and moment measurements are approximately a thousand times more important in air than in water, greatly hindering direct measurements of the fluid force and moment on non-stationary aerofoils in a wind tunnel. In fact, inertial and aerodynamic forces are of the same order of magnitude in oscillating foils in air for reduced frequencies of interest in propulsion and energy harvesting applications (Siala, Kamrani Fard & Liburdy 2020). For a rigid aerofoil, the inertial contributions can be computed by just measuring synchronously with the force and torque the pitch angle , but this is not enough for deformable aerofoils.…”

Reconstructing the pressure field around a swimming fish using a physics-informed neural network

“…Multi-directional integration schemes utilize the scalar property of pressure, i.e., its local value is independent of the path taken, to improve the accuracy of the pressure estimation. Using this approach, Dabiri et al (2014) developed an unsteady pressure reconstruction algorithm, known as Queen 2.0, to study animal locomotion (Dabiri et al, 2020; Dagenais et al, 2020; Siala et al, 2020; Costello et al, 2021; Gemmell et al, 2021; Kasoju et al, 2021; Thandiackal et al, 2021; Guo et al, 2022).…”

Flapping wing propulsion: Comparison between discrete vortex method and other models

“…The interaction between the leading edge suction and the shear layer developing on the airfoil during dynamic stall is modeled 14 . Wind tunnel experiments are performed on the effects of passive, inertiainduced surface deformation, at the leading and trailing edges, of an oscillating airfoil energy harvester 15 .…”