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The three-dimensional deformable wing kinematics was described as an input condition in the commercial CFD solver (Fluent 16

The three-dimensional deformable wing kinematics was described as an input condition in the commercial CFD solver (Fluent 16

3.3putational method

2 package) to estimate the force generation and flow structure around the wings during the flapping motion. The wing motion was defined by using a user-defined function at a flapping frequency of 20 Hz to obtain a Reynolds number of approximately 15 000. Young & Lai used a dynamic mesh feature for a turbulent model, which was found to have no difference with the laminar model in terms of the force generation of a flapping wing at Reynolds numbers ranging from 100 to 50 000. Therefore, in this study, an incompressible laminar model was chosen to simulate the airflow around the wing. Similar CFD with laminar model was explored in previous studies [49,56,57].

Only one wing was simulated with a symmetric condition, as the flapping mechanism was designed to flap the wings symmetrically in a symmetric plane as shown in figure 4. The computational domain included a half cylinder with a diameter (D) and a length (L) of 12R (840 mm), as show in figure 4a. The wing was placed behind the inlet at a distance of 6R (420 mm), and the wing surface was considered as a membrane without any roughness. In the hovering condition, there was no inflow velocity at the inlet. Six flapping cycles were simulated at a time step of 1/1000 of a cycle. The motion of a wing was a combination of flapping around the flapping axis (z-axis) and rotation around the feather axis (?-axis), which was attached to the leading edge of the wing, as shown in figure 4b. The flapping angle, denoted by ?, was defined as the angle between the x-axis and the feather axis. The rotation angle, denoted by ?r, was determined by the angle between the ?-axis and the wing chord. The distance from the flapping axis to the symmetric plane was 8 mm (d/2 = 8 mm), which is equal to half the distance between the flapping axes of the two wings. In order to investigate the effect of the clap and fling, the forces generated in this case were compared with those in the other case where the distance between the flapping axis and the symmetric plane extended to 20 mm (d/2 = 20 mm), which was sufficiently far to eliminate the clap-and-fling effect. Henceforth, the other case is referred to as the non-clap-and-fling case, in this study.

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3.4. Experimental method

The forces generated by the flapping wings were measured by using a 6-axis load cell (Nano 17, Stainless steel, ATI Industrial Automation, USA, force resolution of 2.94 mN) as shown in figure 5. The flapping-wing system was excited by an external power supply (E3646A, Agilent, Malaysia) at the same flapping frequency of 20 Hz as that applied to the CFD model. The flapping-wing system was operated for approximately 100 flapping cycles in each test. The measured forces acquired from more than 10 experiments were averaged.

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A flapping-wing system was fabricated with an extended distance between two flapping axes, as shown in figure 6, to investigate the manner in which the forces changed without the effect of the clap and fling at the stroke reversals. All other design parameters in this model were theoretically the same as those in the flapping-wing system with the clap-and-fling effect. The only difference was the distance between two flapping axes of the two wings, which was extended to 40 mm or 1.6c. A study by Sun & Yu indicated that this distance was sufficiently far to minimize the interaction effect of the wings at each stroke reversal. The flapping-wing system was also installed in the load cell for force measurement at a frequency of 20 Hz https://hookupdate.net/shagle-review/, and compared in terms of force generation with the flapping-wing system with the clap-and-fling effect. The time history of the force generation during the flapping motion was obtained by filtering the raw data using a low-pass filter with a cut-off frequency that was five times higher than the flapping frequency to eliminate the high-frequency effect from noises and structure vibrations.

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