Lift-drag and flow structures associated with the "clap and fling" motion
TL;DR: In this paper, the influence of kinematic parameters such as the percentage overlap between translational and rotational phase ξ, the separation between two wings δ and Reynolds numbers Re on the evolvement of lift and drag has been investigated.
Abstract: The present study focuses on the analysis of the fluid dynamics associated with the flapping motion of finite-thickness wings. A two-dimensional numerical model for one and two-winged “clap and fling” stroke has been developed to probe the aerodynamics of insect flight. The influence of kinematic parameters such as the percentage overlap between translational and rotational phase ξ, the separation between two wings δ and Reynolds numbers Re on the evolvement of lift and drag has been investigated. In addition, the roles of the leading and trailing edge vortices on lift and drag in clap and fling type kinematics are highlighted. Based on a surrogate analysis, the overlap ratio ξ is identified as the most influential parameter in enhancing lift. On the other hand, with increase in separation δ, the reduction in drag is far more dominant than the decrease in lift. With an increase in Re (which ranges between 8 and 128), the mean drag coefficient decreases monotonously, whereas the mean lift coefficient decreases to a minimum and increases thereafter. This behavior of lift generation at higher Re was characterized by the “wing-wake interaction” mechanism which was absent at low Re.
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TL;DR: In this article, the formation and shedding of vortices in two vortex-dominated flows around an actuated flat plate are studied to develop a better method of identifying and tracking coherent structures in unsteady flows.
Abstract: The formation and shedding of vortices in two vortex-dominated flows around an actuated flat plate are studied to develop a better method of identifying and tracking coherent structures in unsteady flows. The work automatically processes data from the 2D simulation of a flat plate undergoing a $$45^{\circ }$$
pitch-up maneuver, and from experimental particle image velocimetry data in the wake of a continuously pitching trapezoidal panel. The Eulerian $$\varGamma _1$$
, $$\varGamma _2$$
, and Q functions, as well as the Lagrangian finite-time Lyapunov exponent are applied to identify both the centers and boundaries of the vortices. The multiple vortices forming and shedding from the plates are visualized well by these techniques. Tracking of identifiable features, such as the Lagrangian saddle points, is shown to have potential to identify the timing and location of vortex formation, shedding, and destruction more precisely than by only studying the vortex cores as identified by the Eulerian techniques.
50 citations
Cites background from "Lift-drag and flow structures assoc..."
...Arora et al. (2014) studied the roles of LEV and TEV with respect to lift and drag evolution with a “clap and fling” stroke....
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TL;DR: The wing kinematics for miniature-insect species of different sizes are measured and the aerodynamic forces are computed to produce the required vertical force for flapping as larger insects.
Abstract: Miniature insects fly at very low Reynolds number (Re); low Re means large viscous effect. If flapping as larger insects, sufficient vertical force cannot be produced. We measure the wing kinematics for miniature-insect species of different sizes and compute the aerodynamic forces. The planar upstroke commonly used by larger insects changes to a U-shaped upstroke, which becomes deeper as size or Re decreases. For relatively large miniature insects, the U-shaped upstroke produces a larger vertical force than a planar upstroke by having a larger wing velocity and, for very small ones, the deep U-shaped upstroke produces a large transient drag directed upwards, providing the required vertical force.
37 citations
TL;DR: In this article, the effect of air viscosity on the flow around an insect wing increases as insect size decreases, and the authors observed very small insects using drag to support their weight and explained how a net vertical force is generated when the drag principle is applied.
Abstract: The effect of air viscosity on the flow around an insect wing increases as insect size decreases. For the smallest insects (wing length of that by the real wing kinematics; i.e. they must use the special wing movements to overcome the problem of large viscous effects encountered by the commonly used flapping kinematics. We have observed for the first time very small insects using drag to support their weight and we explain how a net vertical force is generated when the drag principle is applied.
37 citations
TL;DR: The results suggest that the bristled wings can provide unique aerodynamic benefits via increasing lift to drag ratio during clap and fling for Re between 5 and 15.
Abstract: In contrast to larger flight-capable insects such as hawk moths and fruit flies, miniature flying insects such as thrips show the obligatory use of wing–wing interaction via “clap and fling” during the end of upstroke and start of downstroke. Although fling can augment lift generated during flapping flight at chord-based Reynolds number (Re) of 10 or lower, large drag forces are necessary to clap and fling the wings. In this context, bristles observed in the wings of most tiny insects have been shown to lower drag force generated in clap and fling. However, the fluid dynamic mechanism underlying drag reduction by bristled wings and the impact of bristles on lift generated via clap and fling remain unclear. We used a dynamically scaled robotic model to examine the forces and flow structures generated during clap and fling of: three bristled wing pairs with varying inter-bristle spacing, and a geometrically equivalent solid wing pair. In contrast to the solid wing pair, reverse flow through the gaps between the bristles was observed throughout clap and fling, resulting in: (a) drag reduction; and (b) weaker and diffuse leading edge vortices that lowered lift. Shear layers were formed around the bristles when interacting bristled wing pairs underwent clap and fling motion. These shear layers lowered leakiness of flow through the bristles and minimized loss of lift in bristled wings. Compared to the solid wing, peak drag coefficients were reduced by 50–90% in bristled wings. In contrast, peak lift coefficients of bristled wings were only reduced by 35–60% from those of the solid wing. Our results suggest that the bristled wings can provide unique aerodynamic benefits via increasing lift to drag ratio during clap and fling for Re between 5 and 15.
34 citations
Cites background or methods from "Lift-drag and flow structures assoc..."
...[30] found via numerical simulations that for symmetrical clap and fling motion of a solid wing pair, increasing the overlap between the translational and rotational phases resulted in significant increase in lift and drag forces....
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...[30] varied the amount of overlap between rotation and translation....
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...UST was used as the velocity scale in calculating Re, identical to previous studies of low Re clap and fling aerodynamics [13,14,28,30]....
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...UST was used as the velocity scale in calculating both lift and drag coefficients, identical to previous studies of low Re clap and fling aerodynamics [28,30]....
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...UST was used as the velocity scale for non-dimensionalization of the instantaneous wingtip velocity identical to previous studies of low Re clap and fling aerodynamics [13,14,28,30]....
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TL;DR: In this article, a nonlinear aeroelastic model suitable for flexible insect-like flapping wings in hover is presented by coupling a structural dynamic model based on MARC, with a potential-flow-based approximate aerodynamic model that consists of leading-edge vortices and a wake model.
Abstract: A nonlinear aeroelastic model suitable for flexible insectlike flapping wings in hover is presented. The aeroelastic model is obtained by coupling a nonlinear structural dynamic model based on MARC, with a potential-flow-based approximate aerodynamic model that consists of leading-edge vortices and a wake model. The aeroelastic response is obtained using an updated Lagrangian method. The paper describes validation studies conducted on the structural dynamic model, aerodynamic comparisons, and aeroelastic studies conducted on isotropic and anisotropic Zimmerman wings. The results demonstrate the suitability of MARC for modeling anisotropic wings undergoing insectlike wing kinematics. For the aeroelastic cases considered, the approximate model shows acceptable agreement with computational-fluid-dynamics-based and experimental results. The approximate model captured several important trends correctly.
33 citations
References
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TL;DR: An overview of the lattice Boltzmann method, a parallel and efficient algorithm for simulating single-phase and multiphase fluid flows and for incorporating additional physical complexities, is presented.
Abstract: We present an overview of the lattice Boltzmann method (LBM), a parallel and efficient algorithm for simulating single-phase and multiphase fluid flows and for incorporating additional physical complexities. The LBM is especially useful for modeling complicated boundary conditions and multiphase interfaces. Recent extensions of this method are described, including simulations of fluid turbulence, suspension flows, and reaction diffusion systems.
6,565 citations
TL;DR: In this paper, the authors show that the enhanced aerodynamic performance of insects results from an interaction of three distinct yet interactive mechanisms: delayed stall, rotational circulation, and wake capture.
Abstract: The enhanced aerodynamic performance of insects results from an interaction of three distinct yet interactive mechanisms: delayed stall, rotational circulation, and wake capture. Delayed stall functions during the translational portions of the stroke, when the wings sweep through the air with a large angle of attack. In contrast, rotational circulation and wake capture generate aerodynamic forces during stroke reversals, when the wings rapidly rotate and change direction. In addition to contributing to the lift required to keep an insect aloft, these two rotational mechanisms provide a potent means by which the animal can modulate the direction and magnitude of flight forces during steering maneuvers. A comprehensive theory incorporating both translational and rotational mechanisms may explain the diverse patterns of wing motion displayed by different species of insects.
2,246 citations
TL;DR: The multi-objective optimal design of a liquid rocket injector is presented to highlight the state of the art and to help guide future efforts.
Abstract: A major challenge to the successful full-scale development of modem aerospace systems is to address competing objectives such as improved performance, reduced costs, and enhanced safety. Accurate, high-fidelity models are typically time consuming and computationally expensive. Furthermore, informed decisions should be made with an understanding of the impact (global sensitivity) of the design variables on the different objectives. In this context, the so-called surrogate-based approach for analysis and optimization can play a very valuable role. The surrogates are constructed using data drawn from high-fidelity models, and provide fast approximations of the objectives and constraints at new design points, thereby making sensitivity and optimization studies feasible. This paper provides a comprehensive discussion of the fundamental issues that arise in surrogate-based analysis and optimization (SBAO), highlighting concepts, methods, techniques, as well as practical implications. The issues addressed include the selection of the loss function and regularization criteria for constructing the surrogates, design of experiments, surrogate selection and construction, sensitivity analysis, convergence, and optimization. The multi-objective optimal design of a liquid rocket injector is presented to highlight the state of the art and to help guide future efforts.
2,152 citations
08 Mar 2001
TL;DR: A comprehensive theory incorporating both translational and rotational mechanisms may explain the diverse patterns of wing motion displayed by different species of insects.
Abstract: The enhanced aerodynamic performance of insects results from an interaction of three distinct yet interactive mechanisms: delayed stall, rotational circulation, and wake capture. Delayed stall functions during the translational portions of the stroke, when the wings sweep through the air with a large angle of attack. In contrast, rotational circulation and wake capture generate aerodynamic forces during stroke reversals, when the wings rapidly rotate and change direction. In addition to contributing to the lift required to keep an insect aloft, these two rotational mechanisms provide a potent means by which the animal can modulate the direction and magnitude of flight forces during steering maneuvers. A comprehensive theory incorporating both translational and rotational mechanisms may explain the diverse patterns of wing motion displayed by different species of insects.
2,133 citations
TL;DR: In this article, a general technique for simulating solid-fluid suspensions is described, which combines Newtonian dynamics of the solid particles with a discretized Boltzmann equation for the fluid phase; the many-body hydrodynamic interactions are fully accounted for, both in the creeping flow regime and at higher Reynolds numbers.
Abstract: A new and very general technique for simulating solid–fluid suspensions is described; its most important feature is that the computational cost scales linearly with the number of particles. The method combines Newtonian dynamics of the solid particles with a discretized Boltzmann equation for the fluid phase; the many-body hydrodynamic interactions are fully accounted for, both in the creeping-flow regime and at higher Reynolds numbers. Brownian motion of the solid particles arises spontaneously from stochastic fluctuations in the fluid stress tensor, rather than from random forces or displacements applied directly to the particles. In this paper, the theoretical foundations of the technique are laid out, illustrated by simple analytical and numerical examples; in a companion paper (Part 2), extensive numerical tests of the method, for stationary flows, time-dependent flows, and finite-Reynolds-number flows, are reported.
2,073 citations