An experimental study of the aerodynamics of micro corrugated wings at low Reynolds number

https://doi.org/10.1016/j.expthermflusci.2020.110286Get rights and content

Highlights

  • PIV was used to study the flow over reconstructed micro-corrugated Odonata wings.

  • We employed a non-destructive photogrammetry method to fabricate the wing.

  • The accuracy of corrugation patterns with fabricated wing was evaluated by Micro-CT.

  • Photogrammetry is advantageous to reconstruct the corrugated wing of MAVs.

  • The obtained aerodynamic results of corrugated wing benefit next generation of MAVs.

Abstract

In this paper, an experimental study was performed to examine the flow characteristics of fabricated micro corrugated wings by fully mimicking the real 3D Odonata wing. For this purpose, a true scale hind wing from the Orthetrum caledonicum species with a semi span length of 60 mm was reconstructed by non-destructive close-range photogrammetry and fabricated with an advanced 3D printer. The accuracy of the proposed reconstruction technique was evaluated and compared with a 3D model of the same wing created using a Micro-Computed Tomography (CT) scanning technique to show that the close-range photogrammetry method was able to predict the pattern of micro corrugation of the wings with satisfactory fidelity. To do that, the corrugation patterns of both reconstructed wings were compared at different sections of the wings. Then, high-resolution Particle Image Velocimetry was used to investigate the flow field of the wing during gliding flight at three low Reynolds numbers Re = 5 × 103, Re = 8 × 103 and Re = 12 × 103, and angle of attack 10°. The results include free stream velocity, vorticity distribution, boundary layer, and flow visualization. The velocity contour and vorticity boundary layer of both wings were compared experimentally. The flow behavior around the corrugated patterns reconstructed from both methods were compared with satisfactory agreement. The results support that the corrugations of the wing act as turbulators to generate unsteady vorticity to transition the boundary layer from laminar to turbulent quickly, leading to delayed stall and improved aerodynamic performance. Moreover, this study shows the application of the presented photogrammetry method for corrugated wing reconstruction, which is fast, low-cost, non-destructive, with high replication accuracy for the next generation of micro air vehicles.

Introduction

Developing small aircraft, particularly micro air vehicles (MAVs), presents challenging aspects such as low Reynolds number (Re ≈ 102 to 104) flight [1], small dimensions, and low weight [2]. There has been increasing interest in insect flight to better understand the aerodynamic characteristics of MAVs. The wings of insects are corrugated [3] instead of being smooth or conventionally cambered. Among insects, dragonflies arguably have the best flight performance and maneuverability due to their four-wing planform and control [4], combined with their sophisticated wing design comprising veins, corrugations and membranes [5].

More than 3600 dragonflies species exist in the world [6] with wing various corrugations and shapes that might be functionally different in terms of flight performance and aerodynamic characteristics, of which only a few have been analyzed in either flapping [7], [8], [9], [10] or gliding flight [11], [12]. Kesel’s experimental studies were carried out in [13] from the fore wing (FW) of Aeshna Cyana. Other studies considered simplified corrugated profiles; extruded 2D profiles of sections from 3D wings with diminished corrugation complexity [12], [14].

Various studies were performed to investigate the properties of corrugated dragonfly wings [15], [16] compared to a flat plate and a cambered plate at low Reynolds numbers, showing that the corrugated cross-section indeed provided desirable aerodynamic performance. They concluded that flow separation vorticities were trapped in the valleys, leading to delayed stall and flow separation, with increasing angle of attack (AoA) [11], [17], also causing an increase in the lift to drag ratio [18].

This study for the first time presents flow analysis of fabricated micro-corrugated wings copied from Orthetrum caledonicum in gliding flight. For comparative purposes, gliding flight lends itself to more definitive measurements than the infinite variations possible in flapping flight kinematics. The dragonfly has a high aspect ratio wing with high lift to drag [19], that appears to have evolved for both modes of flight. There remains an open question as to the importance of different scales of surface structure on resulting air flow features and ultimately performance of the wing. Past studies have used a highly simplified corrugated wing chord copied from a cut dragonfly wing [12], [20], extruded into a finite 2D wing. This study used a full 3D wing copied from a dragonfly using two different technologies with different resolutions, then replicated using a 3D printer. Wings were modeled using a low cost, nondestructive photogrammetry technique and the more costly, higher resolution, but destructive Micro-CT technique. Very limited data exists on insect wing flow visualization, and none has considered the effect of scales of reproduction of the wing surface structure. The study was performed using Particle Image Velocimetry (PIV) on a fabricated Orthetrum caledonicum hind wing (HW) at low Reynold number (Re) at 10° AoA at Re = 5 × 103, Re = 8 × 103, Re = 12 × 103, and at different corrugation sections. This species is a common medium sized Australian dragonfly known as the “blue skimmer” and their common resting position is holding their wings swept forward. The three-dimensional shapes of the fabricated wing were generated from scans of actual wings, thus varying across chord and span as a true 3D wing, precisely to scale.

Section snippets

3D reconstruction and fabrication

In this study, the micro-corrugation HW of Orthetrum caledonicum, with the semi span of 60 mm, were fabricated and analyzed experimentally in the wind tunnel using the PIV method. The wings have complex variations in 3D structure in the span-wise and chord-wise directions. The HW has a larger wing area and mean chord compared to the FW [21]. An analysis of the diversity of species in terms of shape and planform, has shown that the HW has more variation and a deep cord between species compared

Results and discussion

Previous experimental investigations [16], [28] were conducted on large extruded corrugated profiles of wings with a chord dimension of 100 mm, whereas, this study performed experimental analysis on a full scale copy of a dragonfly wing (semi span length of ~60 mm and chord lengths of ~24 mm and 27 mm). The vorticity behavior over the biomimetic wing compared with a flat wing is shown in Fig. 7. The results from the biomimetic wing in Fig. 7 (left), show flow separation vortices trapped in the

Conclusion

This study analyzed flow over full 3D models of insect wings sampled directly from nature (the 3D model is available in the Dryad [32]), and replicated digitally in engineering materials. We have shown that the wing had extremely complex surface structure, demonstrated by sampling at two different resolutions with the photogrammetry and Micro-CT techniques, although primary flow features were visible in both reconstructed models. The flow patterns were measured at three different low Reynolds

CRediT authorship contribution statement

N. Chitsaz: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization, Project administration. K. Siddiqui: Methodology, Resources, Software, Supervision, Writing - review & editing. R. Marian: Supervision, Writing - review & editing. J. Chahl: Supervision, Funding acquisition, Methodology, Resources, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The first author is the recipient of the William T. Southcott Scholarship from the University of South Australia. We would like to thank Timothy McIntyre and Frank O’Riley for their assistance in setting up the camera and wing manufacturing process. Authors acknowledge that technical help received by Kadeem Dennis and Justin Lantaigne at the Western University for the Particle Image Velocity Measurements. This work was performed in part at the South Australian node of the Australian National

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