In the course of the development of animal species, flying has been “invented” several times. The first winged insects began to populate the forest of the Carboniferous period about 350 million years ago - the wing surfaces of these insects were smooth. Butterflies with scaled wings began to fly into the sky of the Jurassic period. Through natural selection, the butterflies have been experimenting with scale coverage and scale microstructure for 210 million years. Among all present day insects, butterflies and moths with scale coverage are the record holders of two titles: long distance travel (butterfly Danaus plexippus L. (Figure 1) and flight speed (the flight speed of the moth Agrotis ipsilo was 113 km/h) [Sappington et al., 1992].

Figure 1: Butterfly Monarch (Danaus plexippus L.), weighing about an average of half a gram, covers the migration distance of 3200 km in only three weeks.

Butterflies and moths both belong to the insect order Lepidoptera. These insects are usually called Lepidopterans. The world “Lepidoptera” is derived from the Greek word meaning “scale wing”. The surface of the wings of present day butterflies is covered with millions of tiny movable appendages – scales (30-200 μm in size). The scales are arranged in highly ordered rows in the same fashion as skate tiles on a roof. When we handle butterflies, the “dust” that comes off is composed of these very small scales.

Investigation of the structures, forms and functions of scales was begum in medieval times. Theodore de Mayerne, physician to Charles I (England, late 17^th century) described colors and patterns on the wings of butterflies. The development of the microscope and of scientific knowledge had a substantial impact on the research of cuticular appendages of insect wings. It was shown that the scale coverage was multifunctional. The tiny appendages create the wonderful colors on butterfly wings, influence regulation of body temperature, and increase the lift force of fixed-wings.

Figure 2: A vertical cross-section of Danaus plexippus butterfly wing scale.
UL – upper lamina; LL – lower lamina, T – trabecula.

The microstructure of a butterfly scale is a true miracle of nature. Each butterfly scale is a long and flattened extension of cuticle and generally resembles a gathered sack consisting of lower and upper laminae (Figure 2). These laminae are separated by an air cavity. The lower lamina is a flat plate from which trabeculae rise to join the upper lamina. The upper lamina is a complex structure consisting of ridges with an inverted V-profile and grooves which discrete openings. The inverted V-profile of the ridges form the micro channels, which are disposed between the air permeable upper laminae and airproof lower laminae.

I continued the investigation of Lepidopterans in laboratory and in nature, and discovered three major effects of the butterfly scale coverage. The first effect was to increase the aerodynamic forces of wings in flapping flight, and to extend the movement capability (maneuverability). Furthermore, the air cavity of a butterfly scale increases the lift force [Kovalev, 2008]. The second effect was to minimize the vibration. The final effect was to decrease the noise produced by the flying insect [Kovalev, 2005]. These properties allow Lepidopterans to overcome the predator attacks in the sky.

Biomimicry is a progressive orientation in engineer work. Its main objective is to create new kinds of highly effective machines that function like living organisms. For this biomimicry prepares the ground by systematically investigating the multiplicity of biological structures, form and processes and ways that are functionally interrelated. For example, Otto Lilienthal (Germany, late 19^th century) together with his brother Gustav investigated the flight of birds and the airflow past a wing. He came to appreciate the importance of the arched profile.

A wind turbine, usually called a windmill, is a machine that converts the energy of the wind into more useful forms (mechanical energy, or electricity) using rotating blades. A history of first practical wind turbine development is usually begun with mention of windmills in eastern Persia (9^th century) [Ahmad Y. H., Donald R. H., 1986]. The initial development of the wind turbine faced two major problems [George A.R., Chou S.-T., 1983]. The first problem was to develop a light and strong design for turbine blades while maintaining good aerodynamic efficiency. The second problem was to minimize the vibration of wind turbine blades.

Figure 3: A vertical cross-section of the ‘butterfly skin’.
LW - lower wall; R - ridges; UW - upper wall.

The matchless multifunctional microstructure of wing appendages, which have functioned flawlessly for 210 million years, suggested developing the design of highly effective skin which eliminated the problem of wind turbine blades. I devised a special design of the wing skin, called ‘butterfly skin’ (metallic version of the butterfly scale), and experimentally investigated the influence of the skin on the lift force and vibration performance of a wind turbine blade [Kovalev, 2010]. Two different wind turbine blades were used. The skin of the first wind turbine blade was imitated from the Danaus plexippus butterfly wing scale (Figure 2). ‘Butterfly skin’ was composed of two layers (Figure 3). The upper metal wall UW and the lower metal wall LW were separated by an air cavity. Both sides of the upper wall were covered with a large number of span wise grooves. The ridges R (spacing 1 mm) with an inverted V-profile were formed between grooves. The grooves of the external surface were provided with lines of perforations. The inverted V-profile of the ridges formed the channels, which were disposed between the upper metal wall and lower wall. The lower metal wall was similar to a thin sheet. The skin thickness was 1 mm.

The second turbine blade was geometrically similar to the first wing (the blade airfoil was NACA 230). It was the principal concern of this study to qualitatively determine the effect of ‘butterfly skin’ on aerodynamic forces of a wing. Therefore, the skin of the second turbine blade was mono-layered, smooth and airproof. The design of the skin is traditional for modern wind turbines. The aerodynamic properties of the first turbine blade were compared with that of the second turbine blade.

Results of the studies indicated that the ‘butterfly skin’ of a wing increased the lift force by a factor of 1.15, and reduced both the aerodynamic friction, and the frequency of an oscillating blade. Moreover, aerodynamic influence on the rotor blade with the ‘butterfly skin’ in unsteady conditions was more lasting than on the rotor blade with the smooth skin. This result is in good agreement with the experimental data of the lift given by Nachtigall for a moth wing with scales [Nachtigall, 1965], and with the data of the drag reduction given by Becher for a shark skin [Becher et al., 1985].

Figure 4: The influence of the flows on the surface configurations of ‘butterfly skin’.
1 – mean flow; 2 – aspiration of air; 3 – secondary flow; 4 – discharge of air; HP - high pressure; LP - low pressure.

The modification of the aerodynamic effects and of the vibration performances on the turbine blade was due to an increase in the volume of the air which influences the ‘butterfly skin’ and is set in motion together with the wing. On the one hand the turbine blade with the ‘butterfly skin’ interacts with mean flow1, enter flow 2, pass flow 4, and second flow 3 (Figure 4). On the other hand the smooth and airproof skin interacts only with mean flow.

It is evident that the higher performance of wind turbine with the ‘butterfly skin’ will extract more energy from the wind, and the power of the wind turbine with the ‘butterfly skin’ will become more economic.

References

Ahmad Y. al-Hassan, Donald R. Hill, 1986. Islamic Technology: An illustrated history, p. 54, Cambridge University Press.
Bechert, D.W., Hoppe G., and Reif W. - E., 1985, "On the drag reduction of the shark skin", American Institute of Aeronautics and Astronautics, Paper No 85 - 0546, March 12 – 14, 18 p.
George A.R., Chou S.-T., 1983, Comparison of broadband noise mechanism, analyses, and experiments on helicopters, propellers, and wind turbines, NASA Center, AIAA, Aero acoustics Conference, 8th, Atlanta, GA, Apr. 11-13, 38 p.
Kovalev I., 2005, "Butterflies and helicopters," Bulletin of the Entomological Society ofCanada, Vol. 37 (3), September, pp. 140 -142.
Kovalev I.S. 2008, “The Functional Role of the Hollow Region of the Butterfly Pyrameis atalanta (L.) Scale”, Journal of Bionic Engineering, 5(3), pp. 224-230.
Kovalev I.S. 2010, From Butterfly to Wind Turbine. Wind engineering, vol. 34, No 4, 351-360 p.
Nachtigall W. 1965, Die aerodinamische Funrtion der Schmetterlingsschuppen // Naturwissenschaften. Bd 52. H.9. S. 216 - 217.
Sappington T.W., Showers W.B., 1992, Reproductive maturity, mating status, and long-duration flight behaviour of Agrotis ipsilon (Lepidoptera: Noctuidae) and the conceptual misuse of the oogenesis-flight syndrome by entomologists: Environmental Entomology, 21, pp. 677-688.

Image Credits:

other diagrams: Igor Kovalev

Igor Kovalev is with the Kinneret College on the Sea of Galilee.

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'Butterfly Skin' for Wind Turbines (Igor Kovalev)

References