Kijung Kwon

Wind tunnel tests for a flapping wing model with a changeable camber using macro-fiber composite actuators

In the present study, a biomimetic flexible flapping wing was developed on a real ornithopter scale by using macro-fiber composite (MFC) actuators. With the actuators, the maximum camber of the wing can be linearly changed from −2.6% to +4.4% of the maximum chord length. Aerodynamic tests were carried out in a low-speed wind tunnel to investigate the aerodynamic characteristics, particularly the camber effect, the chordwise flexibility effect and the unsteady effect. Although the chordwise wing flexibility reduces the effective angle of attack, the maximum lift coefficient can be increased by the MFC actuators up to 24.4% in a static condition. Note also that the mean values of the perpendicular force coefficient rise to a value of considerably more than 3 in an unsteady aerodynamic flow region. Additionally, particle image velocimetry (PIV) tests were performed in static and dynamic test conditions to validate the flexibility and unsteady effects. The static PIV results confirm that the effective angle of attack is reduced by the coupling of the chordwise flexibility and the aerodynamic force, resulting in a delay in the stall phenomena. In contrast to the quasi-steady flow condition of a relatively high advance ratio, the unsteady aerodynamic effect due to a leading edge vortex can be found along the wing span in a low advance ratio region. The overall results show that the chordwise wing flexibility can produce a positive effect on flapping aerodynamic characteristics in quasi-steady and unsteady flow regions; thus, wing flexibility should be considered in the design of efficient flapping wings.

Experimental investigation on the aerodynamic characteristics of a bio-mimetic flapping wing with macro-fiber composites

This study describes the development of a bio-mimetic flapping wing and the aerodynamic characteristics of a flexible flapping wing. First, the flapping wing is designed to produce flapping, twisting, and camber motions by using a bio-mimetic design approach. A structural model for a macro-fiber composite (MFC) actuator is established, and structural analysis of a smart flapping wing with the actuator is performed to determine the wing configuration for maximum camber motion. The analysis model is verified with the experimental data of the smart flapping wing. Second, aerodynamic tests are performed for the smart flapping wing in a subsonic wind tunnel, and the aerodynamic forces are measured for various test conditions. Additionally, the effects of camber and chordwise wing flexibility on unsteady and quasi-steady aerodynamic characteristics are discussed. The experimental results demonstrate that the effect of the camber generated by the MFC produces sufficient aerodynamic benefit. It is further found that chordwise wing flexibility is an important parameter in terms of affecting aerodynamic performance, and that lift produced in a quasi-steady flow condition is mostly affected by the forward speed and effective angle of attack.

Acoustic streaming pattern induced by longitudinal ultrasonic vibration in an open channel using particle imaging velocimetry

Acoustic streaming, which is induced in an open channel by a cylindrical exciter vibrating at 30kHz, is experimentally investigated using particle imaging velocimetry. Induced streaming pattern and velocity field for the channel gap of 18mm are presented. The symmetric distinctive steady rotational flow with local eddies is observed. The maximum streaming velocity measured stands at 0.16cm∕s with a vibration amplitude of 50μm at an excitation frequency of 30kHz.

Particle Image Velocimetry Measurement of Laminar Boundary Layer in a Streamwise Corner

Measurement of velocity profiles along a laminar boundary layer in a streamwise corner was carried out by using particle image velocimetry in contrast to conventional hot-wire anemometry. To make the measurement successful, the laser light source for particle image velocimetry was placed at a far-downstream station, and a light sheet was emitted in parallel to the corner intersection line. The platewas tilted at very small angles relative to the freestream to realize the laminar flow starting from the corner leading edge. Discussions on the characteristic features of the velocity profile in the bisector plane are given based on the measurement data. It was found that the measured velocity profiles correspond to the Blasius branch solution of the theoretical corner layer equations.

Induced rolling moment for NACA4412 plain and flapped wing

Purpose – The paper aims to compute rolling moments on a follower aircraft wing due to vortices generated by a plain and flapped NACA4412 wing using experimental particle image velocimetery (PIV) data.Design/methodology/approach – This paper describes the detailed variation of the induced rolling moment on a follower aircraft wing derived from a PIV velocity field measurement. A rectangular wing of a subsonic wall interference model is used as a vortex generator in two different configurations: plain wing of NACA4412 cross‐section profile; and flapped wing with trailing edge flap of NACA0012 cross‐section profile extended at 20°.Findings – Results obtained showed that the maximum induced rolling moment coefficient depends on the strength of the vortex produced by the generating aircraft wing and increases linearly with the increment of the angle of attack.Originality/value – This paper provides an insight on the effects of different angles of attacks for plain and flapped wings on the induced rolling mome...

Experimental Investigation of Plain- and Flapped-Wing Tip Vortices

Particle image velocimetry was used in a low-speed wind tunnel to investigate and characterize wing tip vortex structures. A rectangular wing of a subsonic wall interference model was used as a vortex generator in two different configurations: 1) plain wing and 2) flapped wing with the trailing-edge flap extended at 20 degrees. Vortex flow quantities and their dependence on angle of attack at Reynolds numbers of 32:8 � 103 and 43:8 � 103 were evaluated. Assessment of measured data reveals that the peak values of tangential velocities, vortex strength, and vorticities are directly proportional to the angle of attack. The vortex core radius value grows slowly as the angle of attack is increased. Both plain and flapped configurations showed similar trends. The peak tangential velocities and circulation almost doubled when the flapped configuration was used instead of the plain wing.

Influence of Differential Spoiler Settings on the Wake Vortex Characterization and Alleviation

Experimental investigations using particle image velocimetry technique have been carried out for the evaluation of the differential spoiler setting capabilities in modifying the spanwise wing load and further reduce the wake vortex hazard. The aircraft half model (at high lift configuration) was investigated for two differential spoiler settings. Results reveal a noticeable inboard shift of spanwise wing loading. Implementation of a differential spoiler setting results in a substantial redistribution of the flap tip vortex circulation with an increase in the diameter of the merged vortex by a factor of up to 2.72 relative to the undisturbed flap tip vortex. Inspection of the cross-stream distribution of axial vorticity shows a reduction by a factor of up to 2.33 in the peak vorticity value. A 44% decrease of the maximum crossflow velocity relative to the undisturbed flap tip vortex crossflow velocity was recorded for the case of deployed spoilers. Finally assessment of the differential spoiler setting capabilities as a wake vortex attenuation device reveals that, while position of the maximum induced rolling moments in the flap tip area is little influenced by the differential spoiler setting, the maximum induced rolling moment coefficient was reduced to nearly one third of the undisturbed flap tip vortex value.

Wake Characteristics of Vane-Type Vortex Generators in a Flat Plate Laminar Boundary Layer

Experimental and numerical investigations were conducted to identify the wake characteristics downstream of two vanetype vortex generators over laminar flat plate boundary layer. Experimental study was carried out by using the stereoscopic particle image velocimetry. To describe the flow field around the vortex generator in detail, numerical study was performed. We considered two different planform shapes of vortex generator: triangular and rectangular shape. The height of the generator was chosen to be about the boundary layer thickness at the position of its installation. Two different lengths of the generator were chosen: two and five times the height. Wake measurements were carried out at three angles of attack for each configuration. Wake characteristics for each case such as overall vortical structure, vorticity distribution, and location of vortex center with downstream distance were obtained from the PIV data. Wake characteristics, as expected, were found to vary strongly with the geometry and angle of attack so that no general tendency could be deduced. Causes of this irregular tendency were explained by using the results of the numerical simulation.

Numerical Study of the Aerodynamic Characteristics of an Airfoil with Thickness Uncertainty for a Wind Tunnel Testing

Numerical investigation is performed to understand the effects of thickness uncertainty of a supporting airfoil due to manufacturing processes on the aerodynamic characteristics of an airfoil used for measuring data in a wind tunnel testing. This is done by comparing the coefficients of lift, drag and moment of the airfoils. In this work, the airfoil model consists of three parts, one located in the center for measuring and two outer parts used for supporting. The study is carried out with a NACA64-418 airfoil and the turbulence model of Transition SST. It is found that the effect of thickness uncertainty of the airfoils used for supporting is not significant to the performance of the test airfoil at various angles of attack and Reynolds numbers.

Evolution of NACA23012 Wake Vortices Structure Using PIV

The formation and development of a wing-tip vortex in a near and extended near field were studied experimentally. A swept-back tapered wing with a NACA23012 cross-section was used as a vortex generator. Particle image velocimetry (PIV) as a whole field velocity measurement technique was used in a low-speed wind tunnel to measure and characterize the wing-tip vortex. Wake structures at successive downstream planes crosswise to the axis of the wake vortices were evaluated in internal and external core radius, maximum tangential velocities, vorticity, and circulation distributions. The effect of angle of attack on vortex parameters was examined at one downstream location. Internal core radius and circulation distributions were nearly constant along the downstream direction. A direct dependence of the circulation and tangential velocity distribution on the angle of attack was evident. The centers of the wing-tip vortices scatter in a circle of radius nearly equal to 1% of the mean wing chord. Meandering ampli...