• Journal of computational fluids engineering
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• v.8 no.2
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• pp.24-32
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• 2003

In this work, we propose a new idea of flapping airfoil design for optimal aerodynamic performance from detailed computational investigations of flow physics. Generally, flapping motion which is combined with pitching and plunging motion of airfoil, leads to complex flow features such as leading edge separation and vortex street. As it is well known, the mechanism of thrust generation of flapping airfoil is based on inverse Karman-vortex street. This vortex street induces jet-like flow field at the rear region of trailing edge and then generates thrust. The leading edge separation vortex can also play an important role with its aerodynamic performances. The flapping airfoil introduces an alternative propulsive way instead of the current inefficient propulsive system such as a propeller in the low Reynolds number flow. Thrust coefficient and propulsive efficiency are the two major parameters in the design of flapping airfoil as propulsive system. Through numerous computations, we found the specific physical flow phenomenon which governed the aerodynamic characteristics in flapping airfoil. Based on this physical insight, we could come up with a new kind of airfoil of tadpole-shaped and more enhanced aerodynamic performance.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.787-790
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• 2002

In this work, numerical experiments ave conducted to find out the optimal shape of flapping-airfoil using thickness variation airfoils. In the previous study of flapping-airfoil, we had found that the thrust efficiency of thicker airfoil is better than thinner one, but the latter has higher thrust coefficient. Therefore, we have combined thin(NACA0009) and thick(NACA0015)airfoil to overcome these demerits of each airfoil. Using this combined airfoil, we can achieve acceptable aerodynamic performances from thrust efficiency and coefficient points of view. In order to computational study, we have used parallel-implemented incompressible Wavier-Stokes solver. Computational results show how to design leading and trailing edge shapes.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.3
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• pp.6-13
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• 2006

Aerodynamic characteristics for two-dimensional cyclic motion profile of flapping airfoil in low Reynolds number flows are investigated. Plunging motion and lead-lag motion in the two dimensional space with different plunging and lead-lag amplitudes are combined to cyclic motion profile and the flow around the airfoil is simulated. Present result shows that the improved aerodynamic efficiencies for a given flapping airfoil by adding periodic lead-lag motion of airfoil rather than the pure plunging case. The thrust coefficient and lift coefficient are compared with each cycle during the flapping period and aerodynamic characteristics are obtained on upstroke motion and downstroke motion.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.4
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• pp.44-52
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• 2002

Aerodynamic characteristics of a flapping airfoil in low Reynolds number flows are numerically studied using the unsteady, incompressible Navier-Stokes flow solver with a two-equation turbulence model. For more efficient computation of unsteady flows over flapping airfoil, the flow solver is parallel-implemented by MPI programming method Unsteady computations are performed for low Reynolds number flows over a NACA four-digit series airfoils. Effects of pitching, plunging, and flapping motion with different reduced frequency, amplitude, thickness and camber on aerodynamic characteristics are investigated. Present computational results yield a better agreement in thrust at various reduced frequency with experimental data.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.21 no.1
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• pp.45-50
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• 2013

In this study, lumped-vortex element method and thin airfoil theory were used to analyze aerodynamic characteristics of airfoils with relative motion that had camber lines of NACA $44{\times}{\times}$ airfoil in 2-dimensional unsteady incompressible potential flow. Velocity disturbance due to airfoil was calculated by lumped-vortex element model and force distribution on airfoil by unsteady Bernoulli's equation. Variables in relative motion were considered the period p, the amplitude of flapping $A_f$ and pitching $A_p$, and the phase difference between flapping and pitching ${\phi}_p$ and the angle of attack ${\alpha}$. Due to movement of an airfoil, dag was induced in 2-dimensional unsteady incompressible potential flow. The numerical results show that the aerodynamic characteristics of the airfoil with flapping and pitching at the same time are illustrated. Especially the mean lift coefficient became smaller, but drag coefficient became larger.

• 한국전산유체공학회:학술대회논문집
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• 2006.05a
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• pp.263-267
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• 2006

The path of a flapping airfoil during upstroke and down-stroke is optimized for maximum thrust and propulsive efficiency. The periodic flapping motion in combined pitch and plunge is described using Non-Uniform B-Splines(NURBS). A gradient based algorithm is employed for optimization of the NURBS parameters. Unsteady, low speed laminar flows are computed using a Navier-Stokes solver in a parallel computing environment based on domain decomposition. It is shown that the thrust generation is significantly improved in comparison to the sinusoidal flapping motion. For a high thrust generation, the airfoil stays at a high effective angle of attack for short durations.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.1
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• pp.32-41
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• 2006

In this study, the thrust generation by simultaneous flapping airfoils in tandem configuration is parametrically studied with respect to flapping frequency, amplitude and relative location. Navier-Stokes solver with overset grid topology is employed to calculate the unsteady flowfields. The computation results indicate that when the two airfoils stroke in-phase - flapping phase lag is zero - the maximum propulsive efficiency and thrust can be obtained for most frequency and amplitude range. At a flapping amplitude of 0.2 chord and a reduced frequency of 0.75, the propulsive efficiency of aft airfoil is enhanced by about 37 % compared with that of forward airfoil. However, if flapping frequency exceeds some critical value, the strength of the leading edge vortex of aft airfoil is fortified by the trailing edge vortex of the forward airfoil, resulting in poor propulsive efficiency. It is also found that out-of-phase flapping has relatively low propulsive efficiency and thrust since vortical wake of the forward airfoil interacts with the leading edge vortex of aft airfoil in the unfavorable fashion. The total thrust and propulsive efficiency are shown to decrease with the horizontal miss distance of the aft airfoil. On the contrary, the vertical miss distance has little effect on the overall aerodynamic performance.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.12
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• pp.1146-1151
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• 2008

In this paper, the effects of torsional flexibility on a flapping airfoil are investigated. The aerodynamic forces of a torsional flexible flapping airfoil is computed using 2-D unsteady vortex panel method. A typical-section aeroelastic model is used for the aeroelsatic calculation of the flapping airfoil. Torsional flexibility and excitation frequency are considered as main effective parameters. Under heavy airfoil condition , the thrust peak is observed at the points where the frequency ratio is about 0.75. Based on this peak criterion, there exists two different motions. One is an inertia driven deformation motion and the other is an oscillation driven deformation motion. Also, in the thrust peak condition, the phase angle is kept 85 degrees, independent of the torsional flexibility and the excitation frequency.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.3
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• pp.1-5
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• 2006

Birds and insect in nature morph their mean camberline shapes to obtain both lift and thrust simultaneously. Previous unsteady thin airfoil theories were derived mainly for a rigid flapping airfoil. An extended unsteady thin airfoil theory for a deformable airfoil is required to analyze the unsteady two-dimensional aerodynamic characteristics of a biomorphing wing. Theodorsen's approach is extended to calculate the unsteady aerodynamic characteristics of a biomorphing airfoil. The mean camberline of the airfoil is represented as a polynomial. The unsteady aerodynamic characteristics of the morphing airfoil are represented as noncirculatory and circulatory terms. Present theory can be applied to the unsteady aerodynamic analysis of a flapping biomorphing airfoil and the aeroelastic analysis of a morphing wing.

• International Journal of Aeronautical and Space Sciences
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• v.7 no.2
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• pp.86-103
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• 2006

Using a combination of force transducer measurement to quantify net lift force, high frame rate camera to quantify and subtract inertial contributions, and Digital Particle Image Velocimetry (DPIV) to calculate aerodynamic contributions in the spanwise plane, the contribution of spanwise flow to the generation of lift force in wings undergoing a pure flapping motion in hover is shown as a function of flapping angle throughout the flapping cycle. These experiments were repeated at various flapping frequencies and for various wing planform sizes for flat plate and span wise cambered wings. Despite the previous identification of the importance of span wise fluid structures in the generation of lift force in flapping wings throughout the existing body of literature, the direct contribution of spanwise flow to lift force generated has not previously been quantified. Therefore, in the same manner as commonly applied to investigate the chordwise lift distribution across an airfoil in flapping wings, spanwise flow due to bulk flow and rotational fluid dynamic mechanisms will be investigated to validate the existence of a direct component of the lift force originating from the flapping motion in the spanwise plane instead.