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Journal ArticleDOI

A high-fidelity numerical study on the propulsive performance of pitching flexible plates

03 May 2021-Physics of Fluids (AIP Publishing LLC AIP Publishing)-Vol. 33, Iss: 5, pp 051901
TL;DR: In this article, the authors employed a body-conforming fluid-structure interaction solver for a high-fidelity numerical study of three-dimensional pitching flexible plates with varying flexibility and trailing edge shapes.
Abstract: In this paper, we numerically investigate the propulsive performance of three-dimensional pitching flexible plates with varying flexibility and trailing edge shapes. We employ our recently developed body-conforming fluid-structure interaction solver for our high-fidelity numerical study. To eliminate the effect of other geometric parameters, only the trailing edge angle is varied from 45 ° (concave plate), 90 ° (rectangular plate) to 135 ° (convex plate) while maintaining the constant area of the flexible plate. For a wide range of flexibility, three distinctive flapping motion regimes are classified based on the variation of the flapping dynamics: (i) low bending stiffness K B low, (ii) moderate bending stiffness K B moderate near resonance, and (iii) high bending stiffness K B high. We examine the impact of the frequency ratio f * defined as the ratio of the natural frequency of the flexible plate to the actuated pitching frequency. Through our numerical simulations, we find that the global maximum mean thrust occurs near f * ≈ 1 corresponding to the resonance condition. However, the optimal propulsive efficiency is achieved around f * = 1.54 instead of the resonance condition. While the convex plate with low and high bending stiffness values shows the best performance, the rectangular plate with moderate K B moderate is the most efficient propulsion configuration. To examine the flow features and the correlated structural motions, we employ the sparsity-promoting dynamic mode decomposition. We find that the passive deformation induced by the flexibility effect can help in redistributing the pressure gradient, thus, improving the efficiency and the thrust production. A momentum-based thrust evaluation approach is adopted to link the temporal and spatial evolution of the vortical structures with the time-dependent thrust. When the vortices detach from the trailing edge, the instantaneous thrust shows the largest values due to the strong momentum change and convection process. Moderate flexibility and convex shape help to transfer momentum to the fluid, thereby improving the thrust generation and promoting the transition from drag to thrust. The increase in the trailing edge angle can broaden the range of flexibility that produces positive mean thrust. The role of added mass effect on the thrust generation is quantified for different pitching plates and the bending stiffness. These findings are of great significance to the optimal design of propulsion systems with flexible wings.
Citations
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Journal Article
TL;DR: In this paper, the effects of added mass and fluid damping on a flapping membrane are quantified using a simple damped oscillator model, and an analytic model based on thin airfoil theory coupled with a membrane equation is developed to characterize the steady and unsteady aeroelastic behavior of compliant membrane wings under different conditions.
Abstract: We present a theoretical framework to characterize the steady and unsteady aeroelastic behaviour of compliant membrane wings under different conditions. We develop an analytic model based on thin airfoil theory coupled with a membrane equation. Adopting a numerical solution to the model equations, we study the effects of wing compliance, inertia and flapping kinematics on aerodynamic performance. The effects of added mass and fluid damping on a flapping membrane are quantified using a simple damped oscillator model. As the flapping frequency is increased, membranes go through a transition from thrust to drag around the resonant frequency, and this transition is earlier for more compliant membranes. The wake also undergoes a transition from a reverse von Karman wake to a traditional von Karman wake. The wake transition frequency is predicted to be higher than the thrust–drag transition frequency for highly compliant wings.

27 citations

Journal ArticleDOI
TL;DR: In this article , the sparsity-promoting dynamic mode decomposition was used to study the flow physics in the wake of a propeller, with particular emphasis placed on identifying the underlying temporal and spatial scales that play important roles in the onset of propeller wake instabilities.
Abstract: Propeller wakes under different loading conditions obtained by the improved delayed detached eddy simulation method were studied based on the flow decomposition technique. The sparsity-promoting dynamic mode decomposition was used to study the flow physics in the wake of a propeller, with particular emphasis placed on identifying the underlying temporal and spatial scales that play important roles in the onset of propeller wake instabilities. The morphology of flow structures of different modes selected by the sparsity-promoting algorithm at different frequencies characterizes the instability process of the wake system. It shows that the circumferential diffusion of tip vortex structures promotes the approaching of adjacent tip vortices, enhancing the interaction of the vortex pairs, which plays an important role in the instability triggering mechanism of the propeller wake, especially the mutual inductance between neighboring tip vortices. The present study further extends knowledge of propeller wake instability inception mechanisms under different loading conditions.

11 citations

Journal ArticleDOI
TL;DR: This survey of recent studies on highly simplified bio-inspired models that aim to reveal the flow physics associated with FSI plays a key role in the dynamics of such structures immersed in fluids.
Abstract: Flexible slender structures are ubiquitous in biological systems and engineering applications. Fluid-structure interaction (FSI) plays a key role in the dynamics of such structures immersed in fluids. Here, we survey recent studies on highly simplified bio-inspired models (either mathematical or mechanical) that aim to revealthe flow physics associated with FSI. Various models from different sources of biological inspiration are included, namely flexible flapping foil inspired by fish and insects, deformable membrane inspired by jellyfish and cephalopods, beating filaments inspired by flagella and cilia of microorganisms, and flexible wall-mounted filaments inspired by terrestrial and aquatic plants. Suggestions on directions for future research are also provided.

10 citations

Journal ArticleDOI
TL;DR: In this article , the authors propose a decoupling of bi-directional fluid-structure interactions into simpler mono-irectional components to overcome nonlinearity by the Koopman theory and transform bi-FSI into a linear superposition of the fluid-to-structures, structure-tofluid, and interactive subcases.
Abstract: We propose a novel thinking of decoupling bi-directional fluid-structure interactions (bi-FSI) into simpler mono-directional components for analytical insights. The decoupling aims to overcome nonlinearity by the Koopman theory and transform bi-FSI into a linear superposition of the fluid-to-structure, structure-to-fluid, and interactive subcases. This first of a serial effort presents the wind tunnel experimental and computational fluid dynamics numerical actualizations of the fluid-to-structure and structure-to-fluid subcases via static and forced vibration models, which are indispensable requisites to the forthcoming Koopman analysis. The results have been analysed with respect to flow field phenomenology, and the role of forced vibration, hence cross structure motion alone, has been isolated and elucidated. Compared to the static case, crosswind motion weakens leading-edge separation, promotes shear layer curvature and the impingement of the asymmetric wall jets, and hastens reattachment. Consequently, it causes premature shedding of the roll substructure and delays the formation of the rib substructure, effectively altering the Kármán shedding frequency. It also reduces three-dimensional suppression of the Kármán shedding near the fix- and free-end boundary conditions, overarchingly devolumizing wake coherent structures and weakening the Kármán street's intensity. Results also suggest that increasing the wind speed from the characteristic speed of the vortex-induced vibration (VIV) to that of galloping intensifies vortical activities but causes no fundamental change in flow field phenomenology. Therefore, the underlying causes of VIV and galloping are not attributed to the flow field nor structure motion alone but to the interactive mechanisms unique to bi-FSI.

9 citations

References
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Journal ArticleDOI
TL;DR: In this article, the phase angle between transverse oscillation and angular motion is the critical parameter affecting the interaction of leading-edge and trailing-edge vorticity, as well as the efficiency of propulsion.
Abstract: Thrust-producing harmonically oscillating foils are studied through force and power measurements, as well as visualization data, to classify the principal characteristics of the flow around and in the wake of the foil. Visualization data are obtained using digital particle image velocimetry at Reynolds number 1100, and force and power data are measured at Reynolds number 40 000. The experimental results are compared with theoretical predictions of linear and nonlinear inviscid theory and it is found that agreement between theory and experiment is good over a certain parametric range, when the wake consists of an array of alternating vortices and either very weak or no leading-edge vortices form. High propulsive efficiency, as high as 87%, is measured experimentally under conditions of optimal wake formation. Visualization results elucidate the basic mechanisms involved and show that conditions of high efficiency are associated with the formation on alternating sides of the foil of a moderately strong leading-edge vortex per half-cycle, which is convected downstream and interacts with trailing-edge vorticity, resulting eventually in the formation of a reverse Karman street. The phase angle between transverse oscillation and angular motion is the critical parameter affecting the interaction of leading-edge and trailing-edge vorticity, as well as the efficiency of propulsion.

1,209 citations

Journal ArticleDOI
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.

877 citations

Journal ArticleDOI
TL;DR: The vortex wake shed by the tail differs between eel-like fishes and fishes with a discrete narrowing of the body in front of the tail, and three-dimensional effects may play a major role in determining wake structure in most fishes.
Abstract: What mechanisms of flow control do animals use to enhance hydrodynamic performance? Animals are capable of manipulating flow around the body and appendages both passively and actively. Passive mechanisms rely on structural and morphological components of the body (i.e., humpback whale tubercles, riblets). Active flow control mechanisms use appendage or body musculature to directly generate wake flow structures or stiffen fins against external hydrodynamic loads. Fish can actively control fin curvature, displacement, and area. The vortex wake shed by the tail differs between eel-like fishes and fishes with a discrete narrowing of the body in front of the tail, and three-dimensional effects may play a major role in determining wake structure in most fishes.

684 citations

Journal ArticleDOI
TL;DR: In this paper, a sparsity-promoting variant of the standard dynamic mode decomposition (DMD) algorithm is developed, where sparsity is induced by regularizing the least-squares deviation between the matrix of snapshots and the linear combination of DMD modes with an additional term that penalizes the l 1-norm of the vector of the DMD amplitudes.
Abstract: Dynamic mode decomposition (DMD) represents an effective means for capturing the essential features of numerically or experimentally generated flow fields. In order to achieve a desirable tradeoff between the quality of approximation and the number of modes that are used to approximate the given fields, we develop a sparsity-promoting variant of the standard DMD algorithm. Sparsity is induced by regularizing the least-squares deviation between the matrix of snapshots and the linear combination of DMD modes with an additional term that penalizes the l1-norm of the vector of DMD amplitudes. The globally optimal solution of the resulting regularized convex optimization problem is computed using the alternating direction method of multipliers, an algorithm well-suited for large problems. Several examples of flow fields resulting from numerical simulations and physical experiments are used to illustrate the effectiveness of the developed method.

678 citations