Jong-Seob Han

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...

Hovering and forward flight of the hawkmoth Manduca sexta: trim search and 6-DOF dynamic stability characterization.

We show that the forward flight speed affects the stability characteristics of the longitudinal and lateral dynamics of a flying hawkmoth; dynamic modal structures of both the planes of motion are altered due to variations in the stability derivatives. The forward flight speed u e is changed from 0.00 to 1.00 m s(-1) with an increment of 0.25 m s(-1). (The equivalent advance ratio is 0.00 to 0.38; the advance ratio is the ratio of the forward flight speed to the average wing tip speed.) As the flight speed increases, for the longitudinal dynamics, an unstable oscillatory mode becomes more unstable. Also, we show that the up/down (w(b)) dynamics become more significant at a faster flight speed due to the prominent increase in the stability derivative Z(u) (up/down force due to the forward/backward velocity). For the lateral dynamics, the decrease in the stability derivative L(v) (roll moment due to side slip velocity) at a faster flight speed affects a slightly damped stable oscillatory mode, causing it to become more stable; however, the t(half) (the time taken to reach half the amplitude) of this slightly damped stable oscillatory mode remains relatively long (∼12T at u(e) = 1 m s(-1); T is wingbeat period) compared to the other modes of motion, meaning that this mode represents the most vulnerable dynamics among the lateral dynamics at all flight speeds. To obtain the stability derivatives, trim conditions for linearization are numerically searched to find the exact trim trajectory and wing kinematics using an algorithm that uses the gradient information of a control effectiveness matrix and fully coupled six-degrees of freedom nonlinear multibody equations of motion. With this algorithm, trim conditions that consider the coupling between the dynamics and aerodynamics can be obtained. The body and wing morphology, and the wing kinematics used in this study are based on actual measurement data from the relevant literature. The aerodynamic model of the flapping wings of a hawkmoth is based on the blade element theory, and the necessary aerodynamic coefficients, including the lift, drag and wing pitching moment, are experimentally obtained from the results of previous work by the authors.

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.

Photoluminescence studies of poly(2-fluoro-1,4-phenylene vinylene) : Comparison with 1,4-bis(2-fluorostyryl)-2-fluorobenzene, a model oligomer

We report photoluminescence property of poly(2-fluoro-1,4-phenylene vinylene) (PFPV) comparing with that of poly(1,4-phenylene vinylene) (PPV). They were investigated by using steady-state and time-resolved photoluminescence spectroscopy. PFPV showed higher PL quantum efficiency and longer PL lifetime than PPV, which was explained in terms of a trapping of excitons into shallow traps formed by donor-acceptor states between PPV and fluorine substituent. A model oligomer, 1,4-bis(2-fluorestyryl)-2-fluorobenzene, was synthesized through Wittig reaction to understand the PL property of PFPV. Comparison of the results for the oligomer and the polymer provides a useful insight into the properties of the polymer, PFPV.

Extended Unsteady Vortex-Lattice Method for Insect Flapping Wings

An extended unsteady vortex-lattice method is developed to study the aerodynamics of insect flapping wings while hovering and during forward flight. Leading-edge suction analogy and vortex-core growth models are used as an extension, which is incorporated into a conventional unsteady vortex-lattice method in an effort to overcome the challenges that arise when simulating insect aerodynamics such as wing–wake interaction and leading-edge effects. A convergence analysis was carried out to derive an optimal aerodynamic mesh and a time-step size for flapping-wing models. A parallel computing technique was used to reduce computational time. The aerodynamics of hawkmoth (Manduca sexta) wing models was simulated, and the results were validated against previous numerical and experimental data.

Effect of body aerodynamics on the dynamic flight stability of the hawkmoth Manduca sexta.

This study explores the effects of the body aerodynamics on the dynamic flight stability of an insect at various different forward flight speeds. The insect model, whose morphological parameters are based on measurement data from the hawkmoth Manduca sexta, is treated as an open-loop six-degree-of-freedom dynamic system. The aerodynamic forces and moments acting on the insect are computed by an aerodynamic model that combines the unsteady panel method and the extended unsteady vortex-lattice method. The aerodynamic model is then coupled to a multi-body dynamic code to solve the system of motion equations. First, the trimmed flight conditions of insect models with and without consideration of the body aerodynamics are obtained using a trim search algorithm. Subsequently, the effects of the body aerodynamics on the dynamic flight stability are analysed through modal structures, i.e., eigenvalues and eigenvectors in this case, which are based on linearized equations of motion. The solutions from the nonlinear and linearized equations of motion due to gust disturbances are obtained, and the effects of the body aerodynamics are also investigated through these solutions. The results showed the important effect of the body aerodynamics at high-speed forward flight (in this paper at 4.0 and 5.0 m s-1) and the movement trends of eigenvalues when the body aerodynamics is included.

Experimental Study on the Unsteady Aerodynamics of a Robotic Hawkmoth Manduca sexta model

Time-course force measurement and time-resolved PIV studies were conducted in order to investigate unsteady aerodynamic characteristics. A dynamically scaled-up robotic wing model was operated underwater within the Reynolds number range of 7.4×10 – similar to a hovering hawkmoth. In the stroke section, the tendencies of each CL were substantially obeyed to the translational velocity profiles. This indicated the appositeness of the quasisteady estimation in the stroke phase. Also, the CL traces in the rotational phase showed that the wing-wake interaction was a localized phenomenon that appeared in a temporally and spatially narrow section right after the stroke reversal. It was found that the wing-wake interaction was impacted by the rotational profiles, i.e. the level of the rotational velocity rather than the change of the translational profiles. The PIV results demonstrated that the vortical structures near the stroke reversal, which were clearly described the LEV of the previous stroke, the TEVs due to the wing rotation, and the LEV of the next stroke. Timeline vorticity distributions showed that the TEV1 was generated by the impulsive start of the wing rotation. Such structures pointed out that the characteristics of the wing-wake interaction were not associated with the LEV of the next stroke, but substantially related with the TEV2.

The effect of the abdomen deformation on the longitudinal stability of flying insects

In this paper, we derive longitudinal nonlinear equations of motion of a hovering insect with deformable abdomen to investigate the effect of the abdominal motion to the longitudinal dynamics. The blade-element theory, which is based on experimentally obtained aerodynamic coefficients, is used for the periodic force and moment excitation to the system. Here, we focus on the role of the deformable abdomen to investigate whether or not the flexible body is a decisive factor to the longitudinal flight dynamic stability. Three cases: 1) rigid connection between the thorax and abdomen, 2) flexible connection, and 3) active connection with a feedback control, are compared to check the role of the abdomen deformation on the longitudinal flight dynamic stability, by examining eigenvalues of the linearized system model of each case. The results show that an active control of the abdominal angle can stabilize the longitudinal flight dynamics of the insect modeled in this study.

Buckled bistable beam actuation with twisted strings

In this study, the use of the twisted string concept with a pin, serving as a moment arm, is proposed to produce the snapthrough of a pre-compressed beam so that the whole system can be used as an effective on/off actuator. The twisted string mechanism is to produce a horizontal pulling force to the pin, which triggers the snap-through of the beam. The actuation moment required to trigger the bistable beam in this study is 24.3 Nmm, corresponding to a horizontal force of 0.81 N. The twisted string actuator is able to produce a pulling force of 1 N, which is further pulled through a distance of 5-mm. Static performance of the integrated system based on the effects of the length of the string on the required input motor voltage, torque, and the overall system response time is experimentally investigated. The snap-through sequence during the static experiment is also captured with a high-speed camera. The input voltage to the motor increases as the length of the string is increased. The length of the string also affects the overall system response, motor speed and torque. The whole snap-through of the beam happens within 100 msec after the trigger signal is sent.