Recent progress in flapping wing aerodynamics and aeroelasticity

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.

Flapping Wing Aerodynamics: Progress and Challenges

The topic of this article is a review of the approach to extract pressure fields from flow velocity field data, typically obtained with particle image velocimetry (PIV), by combining the experimental data with the governing equations. Although the basic working principles on which this procedure relies have been known for quite some time, the recent expansion of PIV capabilities has greatly increased its practical potential, up to the extent that nowadays a time-resolved volumetric pressure determination has become feasible. This has led to a novel diagnostic methodology for determining the instantaneous flow field pressure in a non-intrusive way, which is rapidly finding acceptance in an increasing variety of application areas. The current review describes the operating principles, illustrating how the flow-governing equations, in particular the equation of momentum, are employed to compute the pressure from the material acceleration of the flow. Accuracy aspects are discussed in relation to the most dominating experimental influences, notably the accuracy of the velocity source data, the temporal and spatial resolution and the method invoked to estimate the material derivative. In view of its nature of an emerging technique, recently published dedicated validation studies will be given specific attention. Different application areas are addressed, including turbulent flows, aeroacoustics, unsteady wing aerodynamics and other aeronautical applications.

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PIV-based pressure measurement

Here, we present a detailed analysis of the deforming wing kinematics of free-flying hoverflies ( Eristalis tenax , Linnaeus) during hovering flight. We used four high-speed digital video cameras to reconstruct the motion of approximately 22 points on each wing using photogrammetric techniques. While the root-flapping motion of the wing is similar in both the downstroke and upstroke, and is well modelled as a simple harmonic motion, other wing kinematic parameters show substantial variation between the downstroke and upstroke. Whereas the magnitude of the angle of incidence varies considerably within and between different hoverflies, the twist distribution along the wing is highly stereotyped. The angle of incidence and camber both show a recoil effect as they change abruptly at stroke reversal. Pronation occurs consistently after stroke reversal, which is perhaps surprising, because this has been found to reduce lift production in modelling studies. We find that the alula, a hinged flap near the base of the wing, operates in two discrete states: either in plane with the wing, or flipped approximately normal to it. We hypothesize that the alula may be acting as a flow-control device.

Deformable wing kinematics in free-flying hoverflies

Inert gas influence on the laminar burning velocity of methane-air mixtures.

The unsteady aerodynamic forces acting on a square cross-sectional cylinder are investigated by means of time-resolved particle image velocimetry (TR-PIV) at Reynolds number 4,900. The objective of the investigation is to prove the feasibility of non-intrusive force measurements around two-dimensional bodies. The PIV measurements performed at a rate of 1 kHz enable a time resolved (TR) description of the vortex shedding phenomenon occurring at 10 Hz and to follow the time evolution of vortex dominated wake. The instantaneous aerodynamic force coefficients are obtained from the integration of the force equations within a control volume enclosing the object. The required instantaneous pressure distribution is inferred making use of two physical models: Bernoulli relation is adopted in the potential slowly-evolving flow region; in the turbulent wake, the Navier–Stokes equations are invoked to determine the pressure gradient spatial distribution, which integrated in space yields the pressure distribution. The spatial acceleration field is directly obtained from the temporal difference of the time-filtered velocity field. For a choice of the control volume approximately one model height away from the surface the contributions to the aerodynamic forces coming from the different terms of the force equation are individually examined. The convective term dominates the unsteady lift forces whereas the pressure term prevails for the drag. The temporal evolution of C
 L
 returns a clear periodic pattern in phase with the vortex shedding at a frequency of 10.1 Hz (Strouhal number St = 0.128) with oscillation amplitude of 0.9, whereas C
 D
 barely shows periodicity. The measurement uncertainties associated to the evaluation of all the terms in the force equation and especially in relation to TR-PIV measurements are discussed.

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Unsteady aerodynamic forces estimation on a square cylinder by TR-PIV

Even for the stationary airfoils, due to the boundary and shear layer interactions of upper and lower surface of the airfoils, alternating vortex patterns form and the flow becomes time dependent. In the current study, the unsteady behavior of the flow around a symmetric airfoil is considered as incidence angle increases. The flow patterns are presented for wide range of angles of attack values. The vortex pattern generated is analyzed numerically for different angles of attack at Re=1000 around NACA 0012 airfoil. At this Reynolds number, the flow is laminar and boundary layers are quite thick. Flow separation and unsteady vortex shedding is observed even at low angles of attack. For NACA 0012 airfoil, the unsteady vortex pattern is observed at about 8° angle of attack for Re=1000. Spectral analysis is performed for angles of attack ranging from 0° to 90°. It is presented that amplitude spectrum of lift coefficient (C1) start to shows a peak at 8° for NACA 0012 and the aerodynamic forces presents oscillat...

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On the Unsteady Behavior of the Flow around NACA 0012 Airfoil with Steady External Conditions at Re=1000

An experimental study on CH 4 –CO 2 –air flames at various pressures is conducted by using both laminar and turbulent Bunsen flame configurations. The aim of this research is to contribute to the characterization of fuel lean methane/carbon dioxide/air premixed laminar and turbulent flames at different pressures, by studying laminar and turbulent flame propagation velocities, the flame surface density and the instantaneous flame front wrinkling parameters. PREMIX computations and experimental results indicate a decrease of the laminar flame propagation velocities with increasing CO 2 dilution rate. Instantaneous flame images are obtained by Mie scattering tomography. The image analysis shows that although the height of the turbulent flame increases with the CO 2 addition rate, the flame structure is quite similar. This implies that the flame wrinkling parameters and flame surface density are indifferent to the CO 2 addition. However, the pressure increase has a drastic effect on both parameters. This is also confirmed by a fractal analysis of instantaneous images. It is also observed that the combustion intensity S T / S L increases both with pressure and the CO 2 rate. Finally, the mean fuel consumption rate decreases with the CO 2 addition rate but increases with the pressure.

CO2 addition and pressure effects on laminar and turbulent lean premixed CH4 air flames

The aim of the present work is to understand the aerodynamic phenomena and the vortex topology of an unsteady flapping motion by means of numerical and experimental methods. Instead of the use of real insect/bird wing geometries and kinematics which are highly complex and difficult to imitate by an exact modeling, a simplified model is used in order to understand the unsteady aerodynamics and vortex formation mechanisms during the different phases of the flapping motion. The flow is assumed to be laminar with a Reynolds number of 1,000. Direct numerical simulations, laser sheet visualizations and particle image velocimetry (PIV) measurements are performed for the phenomenological analysis of the flow. The vortex dynamics and their identification are put in evidence with PIV measurements by considering velocity magnitude, streamlines, second invariant of velocity gradient (Q-criteria), vorticity contours and Eurlerian accelerations.

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Dilek Funda Kurtulus

Papers

Unsteady aerodynamic forces estimation on a square cylinder by TR-PIV

On the Unsteady Behavior of the Flow around NACA 0012 Airfoil with Steady External Conditions at Re=1000

CO2 addition and pressure effects on laminar and turbulent lean premixed CH4 air flames

Aerodynamic characteristics of flapping motion in hover

Synthesis of a torsional spring mechanism with mechanically adjustable stiffness using wrapping cams