Jo-Won Chang

Vortex Flow Visualization of a Yawed Delta Wing with Leading Edge Extension

The development and interaction of vortices over a yawed delta wing with leading-edge extension (LEX) was investigated through off-surface flow visualization using micro water droplets and a laser beam sheet. Angles of attack of 12, 16, 20, and 24 deg were tested at sideslip angles of 0, -5, and -10 deg. The flow Reynolds number based on the main-wing chord was 1.82 × 10 5 . The wing vortex and the LEX vortex coiled around each other while maintaining comparable strength and identity at a zero sideslip. The increase of angle of attack intensified the coiling and shifted the cores of the wing and LEX vortices inboard and upward. By sideslip, the coiling, the merging and, the diffusion of the wing and LEX vortices increased on the windward side, whereas they became delayed significantly on the leeward side. Also the migration behavior of vortices on the windward and leeward sides of the wing changed considerably

Boundary Layer and Near-Wake Measurements of NACA 0012 Airfoil at Low Reynolds Numbers

A study on the boundary layers and near-wake behavior of a NACA 0012 airfoil at low Reynolds numbers was conducted in order to gain knowledge about flow properties applicable to turbine blades and MAVs. Hot-wire anemometers were used to measure the boundary layers and near-wakes at angles-of-attack of α =0°, 3°, and 6°; and Reynolds Numbers of Rec=2.3×10, 3.3×10, and 4.8×10. The boundary layers were measured with small intervals along the direction normal to the airfoil surface by a 3-D traverse system. The near-wakes were measured at four downstream stations of x/C=0.1, 0.3, 0.5, and 1.0. The results show that the laminar separation of the boundary layers occurred at α =3°. The reattachment of the separated boundary layers was clearly observed for the case of Re=4.8×10 at α =6°, showing the formation of a long separation bubble. The properties of the long separation bubble were then observed and found to be different compared to the properties of short separation bubbles studied previously. The shear flow instability wave was investigated with respect to the formation of coherent vortical structures in the separated shear layer. It was found that the Reynolds number critically influenced the shear layer instability wave rather than the angle-of-attack. The near-wakes in the present paper were analyzed in detail from the boundary layer characteristics that directly affect nearwakes. In particular, the near-wake properties for Rec=4.8×10 and α =6° represented a clearly different evolutions compared to the other Reynolds number cases owing to reattachment.

Reynolds number dependency of an insect-based flapping wing

Aerodynamic characteristics depending on Reynolds number (Re) ranges were studied to investigate the suitable design parameters of an insect-based micro air vehicle (MAV). The tests centered on the wing rotation timing and Re ranges, and were conducted to understand the lift augmentations and unsteady effects. A dynamically scaled-up flapping wing controlled by a pair of servos was installed underwater with a micro force/torque sensor. A high-speed camera and a laser sheet were also put in front of the water tank for the time-resolved digital particle image velocimetry (DPIV). The lift augmentations clearly appeared at low Re and were well reflected on the insects flight range. In the case of the high Re, however, the peak standing for the wing–wake interaction was delayed, and the pitching-up rotation was not able to lead to another lift enhancement, i.e., rotational lift. In such Re, the mean CL and the L/D of the advanced rotation were substantially decreased from those of the other rotations. The DPIV results at high Re well described turbulent characteristics such as the irregular, unstable, and high-intensity vortex structures with a short temporal delay. In the advanced rotation, the LEV in the rotational phase could not maintain the attachment. Thus, the rotational lift was not able to work. On the contrary, the temporal response delay benefitted the wing in the delayed rotation. Therefore, the wing in the delayed rotation had both a similar level of the mean CL and a higher marked L/D than those of the advanced rotation. Such results indicate that the high Re could interrupt lift augmentation mechanisms, and these augmentations would not be suitable for a heavier MAV. In conclusion, using adequate wing kinematics to acquire estimations of the weight and range of the Re is highly recommended at the aerodynamic design step.

Role of Trailing-Edge Vortices on the Hawkmothlike Flapping Wing

A time-course force measurement and time-resolved particle image velocimetry study were conducted to investigate the unsteady characteristics of an insect wing. In most cases, the tendencies of the aerodynamic forces in the stroke phase were extremely similar to the stroke velocity profiles, which indicated the appositeness of the steady aerodynamic model. The time-course forces showed that the wing–wake interaction appeared in temporally and spatially restricted sections right after the stroke reversal. The time-resolved particle image velocimetry taken near the stroke reversal demonstrated the vortex-dominated flowfields including the leading-edge vortex and the trailing-edge vortices. This was in contrast to the middle of the stroke, which only had a stable leading-edge vortex. The results showed that the unsteadiness was highly associated with the trailing-edge vortex structures. In particular, the wing–wake interaction were substantially influenced by the behavior of the number 2 trailing-edge vortex...

Flow Visualization and Force Measurement of an Insect- based Flapping Wing

An experimental study on the flow around the flapping wing of an insect hovering in flight is carried out in order to obtain basic design parameters for an insect-based MAV. A pair of 4-bar linkages operated with small phase differences were designed to imitate the insect flapping motion and assembled on the insect-based model with a pitch actuator. The wing planform was based on the wing of a fruit fly, Drosophila melanogaster, which has been reported in numerous studies. The aerodynamic forces were measured by using tandem straingages and DAQ-system, and the flow structure was observed through the dye flow visualization to investigate the effect of the Reynolds number, which was considered for the range of the MAV flight, and the timing of the wing rotation. Mean lift coefficients increase with the Reynolds numbers; however, additional lift was generated as the unsteady effect, called ‘wake-capturing’, faded as it increased. Generated downwash became stronger as the Reynolds number increased, and the wake does not affect the wing surface directly. Such results indicate that the effect of the wake appeared at the high Reynolds number, and the additional lift produced by the wake is difficult to apply to insect-based MAVs.

Near-Wake Characteristics of an Oscillating NACA 4412 Airfoil

An experimental study is carried out to investigate the near-wake characteristics of an airfoil oscillating in pitch. An NACA 4412 airfoil is sinusoidally pitched about the quarter chord point between an angle of attack of -6 and +6 deg. A hot-wire anemometer is used to measure the phase-averaged velocity and turbulence intensity in the near-wake region of an oscillating NACA 4412 airfoil. The freestream velocities of the present work are 3.4, 12.4, 26.2 m/s, the corresponding Reynolds numbers are 5.3×10, 1.9×10, 4.1×10, and the reduced frequency is 0.1. Streamwise velocity and turbulence intensity profiles are presented to show the Reynolds number effects in the near-wake region behind an airfoil oscillating in pitch. All the cases in these measurements show that the velocity defects are very large at the lowest Reynolds number of Re = 5.3×10, and are small at the other Reynolds numbers of Re = 1.9 × 10 5 and 4.1×10 in the near-wake region. __________________________________________ † Assistant Professor, ‡ Associate Professor Dept. of Aeronautical Science and Flight Operation * Associate Professor Department of Aerospace Engineering Copyright  2002 The American Institute of Aeronautics, Inc. All rights reserved. A significant difference in turbulence intensity between 5.3×10 and 1.9×10 is observed. This study shows that a critical value for the Reynolds number in the nearwake of an oscillating airfoil of the present work exists in the range between 5.3×10 and 1.9×10.

Delta-Wing Vortex Visualization Using Micro-Sized Water Droplets Generated by an Ultrasonic Humidifier

An off-surface visualization method using micro water droplets and a laser beam sheet was developed. The average size of the water droplets generated by the home-style ultrasonic humidifier was about 5–10 μ. This method was pollution-free, eliminated the problem of toxicity, and provided a sufficient density of tracing particles for good visibility. The method was successfully applied to a visualization of the complex vortex flow of a double-delta wing with strake.

Reduced frequency effects on the near-wake of an oscillating elliptic airfoil

An experimental study was carried out to investigate the reduced frequency effect on the near-wake of an elliptic airfoil oscillating in pitch. The airfoil was sinusoidally pitched around the center of the chord between -5° and +25° angles of attack at an airspeed of 3.4 m/s. The chord Reynolds number and reduced frequencies were 3.3 X104, and 0.1, 0.7, respectively. Phase-averaged axial velocity and turbulent intensity profiles are presented to show the reduced frequency effects on the near-wake behind the airfoil oscillating in pitch. Axial velocity defects in the near-wake region have a tendency to increase in response to a reduced frequency during pitch up motion, whereas it tends to decrease during pitch down motion at a positive angle of attack. Turbulent intensity at positive angles of attack during the pitch up motion decreased in response to a reduced frequency, whereas turbulent intensity during the pitch down motion varies considerably with downstream stations. Although the true instantaneous angle of attack compensated for a phase-lag is large, the wake thickness of an oscillating airfoil is not always large because of laminar or turbulent separation.

Flow Visualization and Aerodynamic-Force Measurement of a Dragonfly-Type Model

An unsteady flow visualization and force measurement were carried out in order to investigate the effects of the reduced frequency of a dragonfly-type model. The flow visualization of the wing wake region was conducted by using a smoke-wire technique. An electronic device was mounted below the test section in order to find the exact position angle of the wing for the visualization. A load-cell was employed in measuring aerodynamic forces generated by a plunging motion of the experimental model. To find the period of the flapping motion in real time, trigger signals were also collected by passing laser beam signals through the gear hole. Experimental conditions were as follows: the incidence angles of the foreand hind-wing were 0° and 10°, respectively, and the reduced frequencies were 0.150 and 0.225. The freestream velocities of the flow visualization and force measurement were 1.0 and 1.6m/sec, respectively, which correspond to Reynolds numbers of 3.4 × 103 and 2.9 × 103. The variations of the flow patterns and phase-averaged lift and the thrust coefficients during one cycle of the wing motion were presented. Results showed that the reduced frequency was closely related to the flow pattern that determined flight efficiency, and the maximum lift coefficient and lift coefficient per unit of time increased with reduced frequency.

Numerical Flow Visualization of First Cycle and Cyclic Motion of a Rigid Fling-Clapping Wing

A flow visualization of the two-dimensional rigid fling-clap motions of the flat-plate wing is performed to get the knowledge of fling-clapping mechanism that might be employed by insects during flight. In this numerical visualization, the time-dependent Navier-Stokes equations are solved for two types of wing motion; ‘fling followed by clap and pause motion’ and ‘cyclic fling-clapping motion’. The result is observed regarding the main flow features such as the sequential development of the two families of separation vortex pairs and their movement. For the ‘fling followed by clap and pause motion’, a strong separation vortex pair of counterrotation develops in the opening between the wings in the fling phase and they then move out from the opening in the following clap phase. For ‘the cyclic fling-clapping motion’, the separation vortex pair developed in the outside space in the clap phase move into the opening in the following fling phase. The separation vortex pair in the opening developed in the fling phase of the cyclic motion is observed to be stronger than those of the ‘fling followed by clap and pause motion’. Regarding the strong fling separation vortex and the weak clap separation vortex above it in the opening, the flow pattern of the fling phase of the cyclic fling and clap motion is different to that of the fling phase of the first cycle. The flow pattern of the third cycle of the cyclic fling-clapping motion is observed to be almost same as that of the second cycle. Therefore, a periodicity of the flow pattern is established after the second cycle.