Fluid–Structure Interaction Dynamics of a Flexible Foil in Low Reynolds Number Flows
01 Jan 2020-pp 449-459
TL;DR: In this paper, the aerodynamic characteristics of a chord-wise flexible filament-like structure subjected to a fluctuating inflow in terms of the wake of a rigid cylinder situated upstream in the low Reynolds number regime were investigated with a strongly coupled partitioned fluid-structure interaction (FSI) solver based on finite volume approach.
Abstract: The present paper numerically investigates the aerodynamic characteristics of a chord-wise flexible filament-like structure subjected to a fluctuating inflow in terms of the wake of a rigid cylinder situated upstream in the low Reynolds number regime (RED = 500, where D is the diameter of the cylinder). The numerical simulations are performed with a strongly coupled partitioned fluid–structure interaction (FSI) solver based on finite volume approach. An incompressible Navier–Stokes solver is used to capture the unsteady viscous flow features and the flexible structural model is considered to be nonlinear and elastic. The foil is fixed at its leading edge, and the structural properties (mass ratio (µ), flexural rigidity (EI), etc.) are chosen appropriately to have a comparable fluid and structural inertia with dominant FSI effects to significantly augment the aerodynamic loads. It can be seen that the chord-wise flexibility of the wing manifests a passive pitching achieving greater propulsive efficiency. The deflection envelope reflects the fundamental bending modes of the structure. The interactions between the wake of the rigid cylinder and the flexible structure are investigated with the help of vorticity contours as well as Lagrangian coherent structures to have a clear understanding of the vortex-induced FSI dynamics. Moreover, the aerodynamic forces generated by the flexible foil are compared with that of a rigid stationary foil of same length and subjected to similar upstream flow fluctuations. It is observed that the chord-wise flexibility enhances the lift and thrust generation remarkably compared to the rigid foil.
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TL;DR: In this article, the authors use modeling and simulations guided by initial experiments to study thin foils which are oscillated at the leading edge and are free to move unidirectionally under the resulting fluid forces.
Abstract: We use modeling and simulations guided by initial experiments to study thin foils which are oscillated at the leading edge and are free to move unidirectionally under the resulting fluid forces. We find resonant-like peaks in the swimming speed as a function of foil length and rigidity. We find good agreement between the inviscid model and the experiment in the foil motions (particularly the wavelengths of their shapes), the dependences of their swimming speeds on foil length and rigidity, and the corresponding flows. The model predicts that the foil speed is proportional to foil length to the −1/3 power and foil rigidity to the 2/15 power. These scalings give a good collapse of the experimental data.
13 citations
References
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01 Jan 1996
TL;DR: This text develops and applies the techniques used to solve problems in fluid mechanics on computers and describes in detail those most often used in practice, including advanced techniques in computational fluid dynamics.
Abstract: Preface. Basic Concepts of Fluid Flow.- Introduction to Numerical Methods.- Finite Difference Methods.- Finite Volume Methods.- Solution of Linear Equation Systems.- Methods for Unsteady Problems.- Solution of the Navier-Stokes Equations.- Complex Geometries.- Turbulent Flows.- Compressible Flow.- Efficiency and Accuracy Improvement. Special Topics.- Appendeces.
7,054 citations
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TL;DR: In this article, a review of the recent progress in flapping wing aerodynamics and aeroelasticity is presented, where it is realized that a variation of the Reynolds number (wing sizing, flapping frequency, etc.) leads to a change in the leading edge vortex (LEV) and spanwise flow structures, which impacts the aerodynamic force generation.
Abstract: Micro air vehicles (MAVs) have the potential to revolutionize our sensing and information gathering capabilities in areas such as environmental monitoring and homeland security. Flapping wings with suitable wing kinematics, wing shapes, and flexible structures can enhance lift as well as thrust by exploiting large-scale vortical flow structures under various conditions. However, the scaling invariance of both fluid dynamics and structural dynamics as the size changes is fundamentally difficult. The focus of this review is to assess the recent progress in flapping wing aerodynamics and aeroelasticity. It is realized that a variation of the Reynolds number (wing sizing, flapping frequency, etc.) leads to a change in the leading edge vortex (LEV) and spanwise flow structures, which impacts the aerodynamic force generation. While in classical stationary wing theory, the tip vortices (TiVs) are seen as wasted energy, in flapping flight, they can interact with the LEV to enhance lift without increasing the power requirements. Surrogate modeling techniques can assess the aerodynamic outcomes between two- and three-dimensional wing. The combined effect of the TiVs, the LEV, and jet can improve the aerodynamics of a flapping wing. Regarding aeroelasticity, chordwise flexibility in the forward flight can substantially adjust the projected area normal to the flight trajectory via shape deformation, hence redistributing thrust and lift. Spanwise flexibility in the forward flight creates shape deformation from the wing root to the wing tip resulting in varied phase shift and effective angle of attack distribution along the wing span. Numerous open issues in flapping wing aerodynamics are highlighted.
785 citations
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TL;DR: The dynamics of swimming fish and flapping flags involves a complicated interaction of their deformable shapes with the surrounding fluid flow, and it is found that, for a single filament, there are two distinct, stable dynamical states.
Abstract: The dynamics of swimming fish and flapping flags involves a complicated interaction of their deformable shapes with the surrounding fluid flow. Even in the passive case of a flag, the flag exerts forces on the fluid through its own inertia and elastic responses, and is likewise acted on by hydrodynamic pressure and drag. But such couplings are not well understood. Here we study these interactions experimentally, using an analogous system of flexible filaments in flowing soap films. We find that, for a single filament (or 'flag') held at its upstream end and otherwise unconstrained, there are two distinct, stable dynamical states. The first is a stretched-straight state: the filament is immobile and aligned in the flow direction. The existence of this state seems to refute the common belief that a flag is always unstable and will flap. The second is a flapping state: the filament executes a sinuous motion in a manner akin to the flapping of a flag in the wind. We study further the hydrodynamically coupled interaction between two such filaments, and demonstrate the existence of four different dynamical states.
571 citations
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01 Jan 2006
TL;DR: In this article, the authors describe new benchmark settings for the rigorous evaluation of different methods for fluid-structure interaction problems, which consist of laminar incompressible channel flow around an elastic object which results in self-induced oscillations of the structure.
Abstract: We describe new benchmark settings for the rigorous evaluation of different methods for fluid-structure interaction problems. The configurations consist of laminar incompressible channel flow around an elastic object which results in self-induced oscillations of the structure. Moreover, characteristic flow quantities and corresponding plots are provided for a quantitative comparison.
439 citations
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TL;DR: In this article, a review of recent developments in the understanding and prediction of flapping-wing aerodynamics is presented, with a special emphasis on the dependence of thrust, lift, and propulsive efficiency on flapping mode, amplitude, frequency, and wing shape.
Abstract: It is the objective of this paper to review recent developments in the understanding and prediction of flapping-wing aerodynamics. To this end, several flapping-wing configurations are considered. First, the problem of single flapping wings is treated with special emphasis on the dependence of thrust, lift, and propulsive efficiency on flapping mode, amplitude, frequency, and wing shape. Second, the problem of hovering flight is studied for single flapping wings. Third, the aerodynamic phenomena and benefits produced by the flapping-wing interactions on tandem wings or biplane configurations are discussed. Such interactions occur on dragonflies or on a recently developed micro air vehicle. The currently available two- and three-dimensional inviscid and viscous flapping-wing flow solutions are presented. It is shown that the results are strongly dependent on flapping frequency, amplitude, and Reynolds number. These findings are substantiated by comparison with the available experimental data.
427 citations
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