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Beneficial Aerodynamic Effect of Wing Scales on the Climbing Flight of Butterflies

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Beneficial Aerodynamic Effect of Wing Scales on the Climbing Flight of Butterflies Digital Commons @ George Fox University Faculty Publications - Biomedical, Mechanical, Department of Biomedical, Mechanical, and and Civil Engineering Civil Engineering 2017 Beneficial Aerodynamic Effect of Wing Scales on the Climbing Flight of Butterflies Nathan Slegers George Fox University, [email protected] Michael Heilman Jacob Cranford Amy Lang University of Alabama - Tuscaloosa John Yoder University of Alabama - Tuscaloosa See next page for additional authors Follow this and additional works at: https://digitalcommons.georgefox.edu/mece_fac Part of the Engineering Commons Recommended Citation Slegers, Nathan; Heilman, Michael; Cranford, Jacob; Lang, Amy; Yoder, John; and Habegger, Maria Laura, "Beneficial Aerodynamic Effect of Wing Scales on the Climbing Flight of Butterflies" (2017). Faculty Publications - Biomedical, Mechanical, and Civil Engineering. 97. https://digitalcommons.georgefox.edu/mece_fac/97 This Article is brought to you for free and open access by the Department of Biomedical, Mechanical, and Civil Engineering at Digital Commons @ George Fox University. It has been accepted for inclusion in Faculty Publications - Biomedical, Mechanical, and Civil Engineering by an authorized administrator of Digital Commons @ George Fox University. For more information, please contact [email protected]. Authors Nathan Slegers, Michael Heilman, Jacob Cranford, Amy Lang, John Yoder, and Maria Laura Habegger This article is available at Digital Commons @ George Fox University: https://digitalcommons.georgefox.edu/ mece_fac/97 IOP Bioinspiration & Biomimetics Bioinspir. Biomim. 12 2017 Beneficial aerodynamic effect of wing scales on the climbing © 2017 IOP Publishing Ltd flight of butterflies BBIICI Nathan Slegers1, Michael Heilman1, Jacob Cranford2, Amy Lang3, John Yoder4 and Maria Laura Habegger5 1 Department of Mechanical Engineering, George Fox University, Newberg, OR 97132, USA 2 Gloyer-Taylor Laboratories LLC, 112 Mitchell Blvd. Tullahoma, TN 37388, USA 016013 3 Department of Aerospace Engineering and Mechanics, University of Alabama, Tuscaloosa, AL 35487, USA 4 Department of Biological Sciences, University of Alabama, Tuscaloosa, AL 35487, USA 5 Department of Integrative Biology, University of South Florida, Tampa, FL 33620, USA N Slegers et al E-mail: [email protected] Keywords: Butterfly, insects, surface patterning, unsteady aerodynamics, flapping, aerodynamic efficiency Abstract It is hypothesized that butterfly wing scale geometry and surface patterning may function to improve aerodynamic efficiency. In order to investigate this hypothesis, a method to measure butterfly BB flapping kinematics optically over long uninhibited flapping sequences was developed. Statistical results for the climbing flight flapping kinematics of 11 butterflies, based on a total of 236 individual 10.1088/1748-3190/aa551d flights, both with and without their wing scales, are presented. Results show, that for each of the 11 butterflies, the mean climbing efficiency decreased after scales were removed. Data was reduced to a 1 single set of differences of climbing efficiency using are paired t-test. Results show a mean decrease in climbing efficiency of 32.2% occurred with a 95% confidence interval of 45.6%–18.8%. Similar analysis showed that the flapping amplitude decreased by 7% while the flapping frequency did not 1748-3190 show a significant difference. Results provide strong evidence that butterfly wing scale geometry and surface patterning improve butterfly climbing efficiency. The authors hypothesize that the wing 1 scale’s effect in measured climbing efficiency may be due to an improved aerodynamic efficiency of the butterfly and could similarly be used on flapping wing micro air vehicles to potentially achieve similar gains in efficiency. 12 1. Introduction that butterflies need their scales to fly, no reasonable aer- odynamic explanation as to how the scales benefit the The area of bio-inspired engineering has only just butterfly has been proven and accepted. Within a given begun to make vital discoveries. Insects in particular, species, scale size only varies proximodistally. Recent while being the subject of study for decades [1, 2], can work has shown that the scales on the hindwing of the still yield many untold breakthroughs. The rapidly Blue Pansy (Junonia orithya), an aggressive flyer, gener- developing field of micro air vehicles (MAVs) has ally decrease in size from the wing base towards the edge recently created a surge in the study of insect flight for [8]. This reduction in scale size can be as much as 40%, biomimicry. Recent progress in low Reynolds numbers and the reason for this occurrence was attributed to a 4 (10–10 ) flapping flight and unsteady aerodynamics has maturation wave derived from morphogenetic factors shown that increases in generated lift can be achieved emanating from the postbasal hinge region. However, [2, 3]. Insect flight kinematics have also been extensively no real biological function for the size decrease was pro- investigated, providing tools to develop dynamic posed. Rather, the principle function of the morphoge- models for flapping wing MAVs and validation for netic wave was hypothesized to regulate scale coloration advancing aerodynamic theories [4, 5]. while consequently affecting scale size [8]. The fact that Butterflies and their scales have been extensively higher velocities would be induced towards the tip in studied for their optical properties [6] as well as for their flapping flight suggests that the scale geometry and sur- super-hydrophobic nature [7]. However, the aerody- face patterning may function to improve aerodynamic namic benefit of the surface patterning resulting from efficiency of the butterfly. the scale microgeometry shown in figure 1 has yet to be Nachtigal [9] was one of the first to consider the 30 fully documented. While there exists a misconception beneficial aerodynamic effect of butterfly scales and January 2017 Figure 1. (A) Monarch butterfly dorsal scales imaged using SEM. (B) Sagitall cut through wing showing scales. (C) Dorsal view of orange Monarch scales. (D) Monarch dorsal scales showing how they form in rows perpendicular to the veins on the wing. attempted to determine the lift and drag on dead speci- by Bushnell [15] as a possible means of achieving mens under gliding conditions in a wind tunnel experi- drag reduction (a flow phenomenon he referred to as ment. His results indicated increased lift of 15% with the ‘micro-air bearing’ effect). However, Savill [16] the presence of the scales. Later research by Chebbi and later observed that a turbulent boundary layer form- Tavoularis [10], and Savoie and Gangno [11], began to ing above the surface caused fluid to enter and leave look at low Reynolds number experiments and simula- the cavities resulting in an overall increase in surface tions to study the vortex formation within a triangular drag. Experimental studies on axisymmetric bodies cavity modeled after the shingle-like pattern observed with 2D transverse grooves have resulted in a measure- on butterfly scales. This research documented vortex able reduction in form drag [17] where they attributed formation at various Reynolds numbers but failed to separation control to the formation of the embedded adequately resolve any aerodynamic function of the vortices obviating the no-slip condition. scales [11]. This work further investigates how microgeometry Surface patterning such as dimples and riblets have from surface patterning of butterfly scales may contrib- similarities to butterfly scales and are well-known. The ute to flight performance. The flight of a butterfly is a use of small dimples, such as on a golf ball, are known very complex flow, for the wings are used to generate to delay flow separation and thereby decrease the pres- thrust and lift while also generating drag. If the animal sure drag. However, dimples do more than just trip accelerates or increases its overall potential energy, as the boundary layer to the turbulent state. It has been in the upward flights in this study, then the impeding shown by Choi et al [12], that the formation of embed- forces of drag and weight are less than the thrust and lift. ded cavity vortices, or small localized regions of separa- The sources of drag can come from three components: tion within the surface that allow the outer boundary skin friction drag, pressure drag and induced drag. layer flow to skip over the dimples, is a crucial aspect Induced drag is a drag due to lift and is seen as the energy to this mechanism of separation delay and results in a of the vortices shed into the wake. If scale microgeom- wider Re range of decreased drag than generic rough- etry contributes to a very low Re ‘roller bearing effect’ ness elements. Riblets (ridges) or streamwise grooves, whereby fluid is trapped between the scales and forms first inspired by the grooves found on shark denticles embedded vortices, the outer flow would pass over the [13], have been demonstrated as a passive mechanism scales potentially decreasing the component of fric- to decrease turbulent skin friction drag. Bechert et al tion drag resulting in an increased flight efficiency [14] performed exhaustive experiments with optimiz- (figure 1(a)). Conversely, flow passing over the scales ing riblet spacing and cavity geometry resulting in a in the direction of the rows could increase skin friction riblet tape for application to aircraft capable of a local drag, but there exist regions during flight where this 8% reduction
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