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Open accessJournal ArticleDOI: 10.1063/5.0049217

On the propulsive performance of 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. To eliminate the effect of other geometric parameters, only the trailing edge angle is varied from 45° (concave), 90° (rectangular) to 135° (convex) while maintaining the constant area of the flexible plate. 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 bending stiffness is the most efficient propulsion configuration. Through dynamic mode decomposition, we find that the passive deformation can help in redistributing the pressure gradient thus improving the efficiency and thrust production. A momentum-based thrust evaluation approach is adopted to link the instantaneous 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 transfer momentum to the fluid, thereby improving thrust generation and promoting the transition from drag to thrust. The increase of the trailing edge angle can broaden the range of flexibility that produces positive mean thrust.

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Topics: Thrust (59%), Trailing edge (56.99%), Propulsive efficiency (52%) ... show more

5 results found

Journal ArticleDOI: 10.1063/5.0055686
Lan Yao1, Csaba Hefler1, Wei Shyy1, Huihe Qiu1Institutions (1)
21 Jul 2021-Physics of Fluids
Abstract: This paper addresses hydrodynamic performance of fins regarding their trailing edge convexity–concavity and flexibility distribution. The effects of trailing edge convexity–concavity on propulsive performance and vortex dynamics were investigated experimentally utilizing time-resolved particle image velocimetry and force sensors. It was found that the convex trailing edge shape always outperforms the concave shape. Wake contracting by the bent shape of the trailing edge vortex of a convex trapezoidal form resulted in higher thrust and efficiency. The results also showed that the rounded edges of fish fins did not provide additional hydrodynamic advantages. Furthermore, we found that a gradually flexible fin delivered better propulsive performance over a uniformly flexible fin. The hydrodynamic performance of the flexible fins depended on the strength and relative positions of the trailing edge vortexes shed by each fin, which were affected by the flexible deformations of the fins. In the lower Reynolds number operation (approaching, but below the first resonant mode), the fins with larger camber produced a stronger momentum footprint especially considering the far wake elements, while in the higher Reynolds number range due to resonant deformation the extent of trailing edge excursion became dominant in affecting the propulsive performance. The results showed that gradually flexible fins can improve the performance of future watercraft.

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Topics: Fin (64%), Trailing edge (60%)

3 Citations

Journal ArticleDOI: 10.1063/5.0064219
21 Sep 2021-Physics of Fluids
Abstract: The locomotion of a flapping flexible plate with different shapes and non-uniform chordwise stiffness distribution in a stationary fluid is studied numerically. The normalized effective bending stiffness K∗ for three-dimensional plates with arbitrary stiffness distribution and shape parameters is proposed, and the overall bending stiffness of non-uniform plates with different shapes is reasonably characterized. It is found that the propulsion performance in terms of cruising speed and efficiency of the self-propelled flapping plate mainly depends on the effective bending stiffness. Plates with moderate flexibility K∗ show better propulsion performance. Meanwhile, both a large area moment of the plate and a flexible anterior are favorable to significantly improve their propulsive performance. The evolution of vortical structures and the pressure distribution on the upper and lower surfaces of the plate are analyzed, and the inherent mechanism is revealed. These findings are of great significance to the optimal design of propulsion systems with different fins or wings.

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Topics: Bending stiffness (61%), Flapping (54%), Stiffness (53%)

Open accessJournal ArticleDOI: 10.1016/J.JFLUIDSTRUCTS.2021.103428
Abstract: In this study, we present a global Fourier mode decomposition framework for unsteady fluid–structure interaction. We apply the framework to isolate and extract the aeroelastic modes arising from a coupled three-dimensional fluid–membrane system. We investigate the frequency synchronization between the vortex shedding and the structural vibration via mode decomposition analysis. We explore the role of flexibility in the aeroelastic mode selection and perform a systematic comparison of flow features among a rigid flat wing, a rigid cambered wing and a flexible membrane. The camber effect can enlarge the pressure suction area on the membrane surface and suppress the turbulent intensity, compared to the rigid flat wing counterpart. With the aid of our mode decomposition technique, we find that the dominant structural mode exhibits a chordwise second and spanwise first mode at different angles of attack. The structural natural frequency corresponding to this mode is estimated using an approximate analytical formula. By examining the dominant frequency of the coupled system, we show that the dominant membrane vibration mode is selected via the frequency lock-in between the dominant vortex shedding frequency and the structural natural frequency. From the fluid modes and the mode energy spectra at α = 2 0 ∘ and 25°, the aeroelastic modes corresponding to the non-integer frequency components lower than the dominant frequency are observed, which are associated with the bluff body vortex shedding instability. The non-periodic aeroelastic behaviors observed at higher angles of attack are related to the interaction between aeroelastic modes caused by the frequency lock-in and the bluff-body-like vortex shedding. Using the mode decomposition analysis, we suggest a feedback cycle for flexible membrane wings undergoing synchronized self-sustained vibration. This feedback cycle reveals that the dominant aeroelastic modes are selected through the mode and frequency synchronization during fluid–membrane interaction to exhibit similar modal shapes in the membrane vibration and the pressure pulsation.

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Topics: Vortex shedding (56.99%), Natural frequency (53%), Vibration (53%)

Open accessJournal ArticleDOI: 10.1063/5.0065562
05 Nov 2021-Physics of Fluids
Abstract: Fins or fairings are typically streamlined structures employed to reduce the vortex-induced unsteady forces acting on a bluff body by preventing shear layer roll-up in the near-wake region. In this work, fins would refer to thin plate-like structures attached tangentially to the bluff body's top and bottom surfaces. Of particular interest here are flexible fins that can undergo static deformation or coupled fluid-elastic vibrations due to the non-linear interactions with the shear layer from a circular cylinder and the roll-up of shear layers at the trailing edge of the fin. We present a numerical analysis to realize the effect of fin flexibility on the performance with regard to vortex-induced forces by varying non-dimensional flexural rigidity, KB∈ [0.01, 10], of the fins. Two-dimensional simulations are carried out for a fixed non-dimensional fin mass ratio, m*=0.1, and Reynolds number, Re = 100. In this study, we consider two fins attached tangentially to the top and bottom surfaces of a fixed circular cylinder. We show that as the flexibility of the fins increases progressively, the stability of the fins is lost and the fins undergo a coupled flapping motion. As a function of KB, three distinct dynamic response regimes of the flexible fins are identified: (i) fixed-point stability for KB>1, (ii) periodic outward flapping 0.025≤KB≤0.1, and (iii) periodic flapping about the initial position with large amplitudes KB<0.025. Flexibility and inclination angle of fins are observed to be effective in minimizing the vortex-induced forces.

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Topics: Fin (70%), Flapping (51%), Trailing edge (51%)


64 results found

Open accessJournal ArticleDOI: 10.1017/S0022112097008392
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.

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Topics: Vortex (56.99%), Vorticity (55%), Particle image velocimetry (55%) ... show more

1,099 Citations

Journal ArticleDOI: 10.1016/J.PAEROSCI.2010.01.001
Wei Shyy1, Hikaru Aono1, Satish Kumar Chimakurthi1, Pat Trizila1  +3 moreInstitutions (2)
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.

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Topics: Wing root (70%), Wing twist (69%), Angle of attack (62%) ... show more

785 Citations

Open accessJournal ArticleDOI: 10.2514/3.10246
Manoochehr Koochesfahani1Institutions (1)
24 Mar 1987-AIAA Journal
Abstract: The vortical flow patterns in the wake of a NACA 0012 airfoil pitching at small amplitudes are studied in a low speed water channel. it is shown that a great deal of control can be exercised on the structure of the wake by the control of the frequency, amplitude and also the shape of the oscillation waveform. An important observation in this study has been the existence of an axial flow along the cores of the wake vortices. Estimates of the magnitude of the axial flow suggest a linear dependence on the oscillation frequency and amplitude.

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Topics: NACA airfoil (59%), Wake (55%), Airfoil (53%) ... show more

619 Citations

Journal ArticleDOI: 10.1146/ANNUREV.FLUID.38.050304.092201
Frank E. Fish1, George V. Lauder2Institutions (2)
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.

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601 Citations

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