Vortex flow structures and interactions for the optimum thrust efficiency of a heaving airfoil at different mean angles of attack

Martín-Alcántara, Antonio; Fernández‐Feria, R.; Sanmiguel‐Rojas, E.

doi:10.1063/1.4926622

Cited by 52 publications

(34 citation statements)

References 58 publications

(74 reference statements)

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“…Since the problem is so complex, many studies have explored the simplified configuration of a heaving and/or pitching 2D airfoil [5,6,7,8,9,10,11]. Such studies have shown and quantified the role that the vortical structures formed during the oscillating cycle play in the process of force generation.…”

Section: Introductionmentioning

confidence: 99%

From flapping to heaving: A numerical study of wings in forward flight

Gonzalo

Arranz

Moriche

et al. 2018

Journal of Fluids and Structures

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Direct Numerical Simulations of the flow around a pair of flapping wings are presented. The wings are flying in forward flight at a Reynolds number Re = 500, flapping at a reduced frequency k = 1. Several values of the radius of flapping motion are considered, resulting in a database that shows a smooth transition from the wing rotating with respect to its inboard wingtip (flapping), to a vertical oscillation of the wing (heaving). In this transition from flapping to heaving, the spanwise-averaged effective angle of attack of the wing increases while the effect of the Coriolis and centripetal accelerations becomes weaker. The present database is analyzed in terms of the value and surface distribution of the aerodynamic forces, and in terms of 2D and 3D flow visualizations. While the former allows a decomposition of the force in pressure (i.e., the component of the force normal to the surface of the wing) and skin friction (i.e., tangential to the surface of the wing), the latter allows the identification of specific flow structures with the corresponding forces on the wing. It is found that the aerodynamic forces in the vertical direction (lift) tend to increase for wings moving with larger radius of flapping motion, becoming maximum for the heaving configuration. This is mostly due to the increase of the spanwise-averaged effective angle of attack of the wing with the radius of the flapping motion. Also, the local changes in the effective angle of attack have a strong effect on the structure of the leading edge vortex, resulting in changes in the distribution of suction along the span near the leading edge of the wing. The effect of the apparent accelerations is mostly felt on the spanwise position where the separation of the LEV occurs. On the other hand, the differences in the force in the streamwise direction (thrust/drag) between the configurations with different radius of flapping motion seems to be linked to the position of the stagnation point dividing the suction and pressure side boundary layers, which seems to be controlled by the local effective angle of attack. Finally, the results of the DNS are used to evaluate the performance of an unsteady panel method, and to explain its deficiencies.

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Section: Introductionmentioning

confidence: 99%

From flapping to heaving: A numerical study of wings in forward flight

Gonzalo

Arranz

Moriche

et al. 2018

Journal of Fluids and Structures

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“…In order to quantify the differences we define the errors in the mean for a reference position of the airfoil, and then rotated (using 20) and translated to the position of the airfoil at each time instant. Finally, it should be noted that the auxiliary potential functions φ x and φ z only have analytical solutions for simple geometries, like ellipses [Martín-Alcántara et al, 2015]. For other geometries, like the NACA 0012 considered in the present work, they need to be computed numerically.…”

Section: A Grid Refinement Studymentioning

confidence: 99%

“…where the analytical expressions for the auxiliary potentials of a flat plate are obtained from [Martín-Alcántara et al, 2015]. Note that while the first term in equation (10) is the same appearing in equation (11), the former has some extra terms depending explicitly onθ andθ.…”

Section: Force Decomposition and Modelling Of Case B090mentioning

confidence: 99%

On the aerodynamic forces on heaving and pitching airfoils at low Reynolds number

2017

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The influence that the kinematics of pitching and heaving 2D airfoils have on the aerodynamic forces is investigated using Direct Numerical Simulations and a force decomposition algorithm. Large amplitude motions are considered (of the order of one chord), with moderate Reynolds numbers and reduced frequencies of order O(1), varying the mean pitch angle and the phase shift between the pitching and heaving motions. Our results show that the surface vorticity contribution (viscous effects) to the aerodynamic force is negligible compared to the contributions from the body motion (fluid inertia) and the vorticity within the flow (circulation). For the range of parameters considered here, the latter tends to be instantaneously oriented in the direction normal to the chord of the airfoil. Based on the results discussed in the paper, a reduced order model for the instantaneous aerodynamic force is proposed, taking advantage of the force decomposition and the chord-normal orientation of the contribution from vorticity within the flow to the total aerodynamic force. The predictions of the proposed model are compared to those of a similar model from the literature, showing a noticeable improvement on the prediction of the mean thrust, and a smaller improvement on the prediction of mean lift and the instantaneous force coefficients.

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“…32 and by Martín-Alcántara et al. 14 have shown that only the vortical structures close to the airfoil contribute to the aerodynamic force.…”

Section: Resultsmentioning

confidence: 97%

“…10 Although the main physical mechanisms of force generation in this configuration are fairly well understood, it is difficult to systematize the knowledge, since the flow is governed by several nondimensional parameters: the Reynolds number ( Re ), the Strouhal number ( St ), the reduced frequency ( k ), the airfoil shape, etc. This has led to a large number of numerical 10–14 and experimental 15–21 studies. Only few studies have combined numerical simulations and experiments.…”

Section: Introductionmentioning

confidence: 99%

Assessing aerodynamic force estimation with experiments and simulations of flapping-airfoil flows on the verge of three-dimensionality

Moriche

Raiola

Discetti

et al. 2019

Proceedings of the Institution of Mechanical Engineers, Part G:

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This paper reports a combined experimental and numerical study of the flow over a rigid airfoil in flapping motion. The setup consists of a heaving and pitching airfoil at a moderate Reynolds number ([Formula: see text]), at a Strouhal number St = 0.1. The aim is to assess the accuracy of two-dimensional direct numerical simulations in predicting aerodynamic forces in a flow configuration, which is nominally two-dimensional but is at the verge of three-dimensionality. The assessment is carried out with experiments, including flow field and aerodynamic force measurements with particle image velocimetry and a load cell. The comparative study shows a good qualitative agreement between the experiments and the simulations at comparable Reynolds numbers both in terms of forces and flow fields, but with some quantitative differences. The quantitative discrepancies between experiments and simulation are analyzed and reduced to inherent differences between experimental and computational setups. It is observed that the significant differences are apparent almost exclusively in the wake evolution. Nonetheless, this is shown to have a minor effect on the aerodynamic force estimation. Overall, the trends observed when varying the mean pitch angle and the pitching amplitude are the same in both experiments and simulations. This suggests that two-dimensional/three-dimensional effects do not alter significantly the relationship between the unsteady flow mechanisms (i.e. leading edge vortex) and the aerodynamic forces in the parametric range considered here.

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Vortex flow structures and interactions for the optimum thrust efficiency of a heaving airfoil at different mean angles of attack

Cited by 52 publications

References 58 publications

From flapping to heaving: A numerical study of wings in forward flight

From flapping to heaving: A numerical study of wings in forward flight

On the aerodynamic forces on heaving and pitching airfoils at low Reynolds number

Assessing aerodynamic force estimation with experiments and simulations of flapping-airfoil flows on the verge of three-dimensionality

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