Effect Of Chordwise Flexibility Research Articles

Effect of chordwise flexibility on propulsive performance of high inertia oscillating-foils

We performed a parametric experimental study to quantify the effects of structural properties and kinematic parameters on the propulsive performance of a low-aspect ratio oscillating-foil at a chord-based Reynolds number of 80,000. Multiple foils of the same shape but varied construction allowed explicit comparison of the effect of stiffness and inertia. Forces exerted on the foil were directly measured using a load cell and decomposed into thrust and efficiency values. Quantitative patterns of phase-averaged flow velocity and out-of-plane vorticity in the near-wake of the foil were obtained using particle image velocimetry (PIV). Flexibility was shown to improve the thrust generation and efficiency of the oscillating-foils in comparison to a rigid foil baseline, particularly in heave-only kinematics. The angle of the trailing edge of the foils with respect to the direction of the incident flow was observed to have the highest influence on thrust production, and the thrust results were insensitive to whether this was achieved by passive deformation of the foil or through a prescribed pitching motion.

Effect of chordwise flexibility on pitching foil propulsion in a uniform current

The role of the chordwise flexural stiffness in a rectangular foil undergoing pure pitching has been investigated experimentally. Digital Particle Image Velocimetry (DPIV) and load measurement with a 6-axes balance have been done to study the flow patterns and hydrodynamic forces around the flapping flexible foils. We have found a transition of thrust generation in the semi − flexible foil with θ0=72∘ for all the Re studied between the range of 0.25≤St≤0.6 where it is known that maximize the thrust by a flapping foil. As a result, the optimum flexural stiffness has been determined with a semi − flexible foil up to 69% efficiency for Re = O(104).

Effects of chordwise flexibility on 2D flapping foils used as an energy extraction device

The effects of chordwise pressure- and inertia-driven deformations on the power extraction potential of a kinematically constrained oscillating thin plate that undergoes a heaving and pitching motion are investigated. 2D fully coupled fluid-structure interaction simulations are performed at a Reynolds number of 1100 with thin flexible plates having a thickness over chord ratio of 1%. Three flexibility distributions are tested: a uniformly flexible plate as well as plates with only the rear part or the front part being flexible. The results are compared with those of rigid plates operating at the same conditions. Flexible plates show very promising results as they can extract up to more than twice the power of the corresponding rigid ones. The investigation of semi-flexible foils has highlighted the fact that considering a completely flexible foil with constant chordwise mechanical properties is likely not an optimal configuration. Indeed, different mechanisms are beneficial whether the front part or the rear part of the foil is flexible: pressure-driven deformations usually increase the performances of front flexible foils while inertia-driven deformations are beneficial to rear flexible foils. In cases involving weak fluid-structure interactions (e.g.: heavy plate in a light fluid), the resonance of the first deformation mode of the plate's tail or head may appear. It is observed that this phenomenon can either deteriorate or improve the performances of the flapping plate. In this context, the dimensionless flexibility appears to be a key factor as it is involved in the resonance phenomenon that turns out to greatly influence the flow field.

Aerodynamic performance of two-dimensional, chordwise flexible flapping wings at fruit fly scale in hover flight

Fruit flies have flexible wings that deform during flight. To explore the fluid-structure interaction of flexible flapping wings at fruit fly scale, we use a well-validated Navier–Stokes equation solver, fully-coupled with a structural dynamics solver. Effects of chordwise flexibility on a two dimensional hovering wing is studied. Resulting wing rotation is purely passive, due to the dynamic balance between aerodynamic loading, elastic restoring force, and inertial force of the wing. Hover flight is considered at a Reynolds number of Re = 100, equivalent to that of fruit flies. The thickness and density of the wing also corresponds to a fruit fly wing. The wing stiffness and motion amplitude are varied to assess their influences on the resulting aerodynamic performance and structural response. Highest lift coefficient of 3.3 was obtained at the lowest-amplitude, highest-frequency motion (reduced frequency of 3.0) at the lowest stiffness (frequency ratio of 0.7) wing within the range of the current study, although the corresponding power required was also the highest. Optimal efficiency was achieved for a lower reduced frequency of 0.3 and frequency ratio 0.35. Compared to the water tunnel scale with water as the surrounding fluid instead of air, the resulting vortex dynamics and aerodynamic performance remained similar for the optimal efficiency motion, while the structural response varied significantly. Despite these differences, the time-averaged lift scaled with the dimensionless shape deformation parameter γ. Moreover, the wing kinematics that resulted in the optimal efficiency motion was closely aligned to the fruit fly measurements, suggesting that fruit fly flight aims to conserve energy, rather than to generate large forces.

Numerical Modelling of Flexible Heaving Foils

We consider the effects of chordwise flexibility on the aerodynamic performance of flapping wings using numerical simulation. The two-dimensional Navier-Stokes equations are solved using a Fourier pseudo-spectral method with no-slip boundary conditions imposed by the volume penalization method. The flexible wing is modelled with a non-linear beam equation. Our numerical simulations of heaving plates show that the maximum thrust is achieved at a stroke frequency lower than resonant, which is in agreement with experiments. The oscillatory part of the force only increases in amplitude when the frequency increases. We also consider aerodynamic interactions between two heaving foils.

The Effect of Chordwise Flexibility on the Lift of a Rapidly Accelerated Airfoil

SummaryThe effects of finite chordwise flexibility on the development of lift on a rapidly started airfoil are studied. This is a generalization of the so-called Wagner problem of unsteady aerodynamics. It is found that the lift varies with time in a non-monotonous fashion, including more than one extremum. The lift at large times is found to be lower, the more flexible the foil. Changes in the wake vortex sheet are also discussed.