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Experiments in Fluids

, 60:164 | Cite as

Influence of aspect ratio on wing–wake interaction for flapping wing in hover

  • Reynolds Addo-Akoto
  • Jong-Seob Han
  • Jae-Hung HanEmail author
Research Article
  • 191 Downloads

Abstract

The interaction between an insect wing and its wake during hovering flight is inevitable. The effect of this wing–wake interaction (WWI) during stroke reversal is an intricate phenomenon that is difficult to forecast. Previous studies have mostly concentrated on the effect of aspect ratio (AR) on the leading-edge vortex (LEV) at the middle of stroke motion. However, the effect of AR on LEV during stroke reversal, which is where the intricate WWI phenomenon occurs, has not been fully exploited. This study aimed at revealing how AR affects WWI during stroke reversal at Re of ~ 104, and the role LEV plays. The experimental time-course measurement of force showed that the WWI at stroke reversal was strengthened by decreasing AR irrespective of pitching duration. A time-varying digital particle image velocimetry (DPIV) measurement revealed a jet-like flow induced by a pair of counter-rotating trailing edge vortex (TEV) as the primitive source of the interaction. This study found a WWI that appeared at the wing root contrary to the conventional type in literature, which appeared at the wing tip. The size of the LEV at the wing root played a key role by facilitating the effectiveness of the wing rotation. AR = 2 wing benefited much from this form of WWI by generating strong coherent shear layer at the end of stroke, and relatively weak LEV during stroke reversal. Thus, this type of WWI might be mostly beneficial for low AR wings (AR < 3).

Graphic abstract

List of symbols

Latin symbols

J

Advanced ratio

AR

Aspect ratio

\(C_{D}\)

Drag coefficient

EI

Flexural rigidity

\(R\)

Distance from the pivot to the wingtip

\(C_{L}\)

Lift coefficient

\(\hat{r}_{2}\)

Non-dimensional radius of gyration

P

Petiolation

\(R\)

Reference length to the wing tip

\(R_{2}\)

Reference length to the location of the radius of gyration

\(U_{\text{ref}}\)

Reference velocity

Re

Reynolds number

\(Ro\)

Rossby number

\(J_{1}\)

Tip velocity ratio

\(S\)

Wing area

\(c\)

Wing chord length

\(\Delta R\)

Wing root offset distance

\(b\)

Wing span length

E

Young’s modulus

Greek symbols

\(\varPi_{1}\)

Effective stiffness

\(\Delta \tau\)

Non-dimension duration

\(\Delta \tau_{\alpha }\)

Non-dimensional duration of the wing rotation

\(\Delta \tau_{\phi }\)

On-dimensional duration for acceleration and deceleration

\(\tau\)

Non-dimension time

\(\tau_{\text{TR}}\)

Non-dimensional timing rotation

\(\hat{\omega }\)

Normalized vorticity

\(\alpha_{0}\)

Pitch amplitude

\(\phi_{0}\)

Stroke amplitude

Notes

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1A2B4005676).

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  • Reynolds Addo-Akoto
    • 1
  • Jong-Seob Han
    • 1
  • Jae-Hung Han
    • 1
    Email author
  1. 1.Department of Aerospace EngineeringKorea Advanced Institute of Science and Technology (KAIST)DaejeonRepublic of Korea

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