AIAA 2017-0296 AIAA SciTech Forum 9 - 13 January 2017, Grapevine, Texas 25th AIAA/AHS Adaptive Structures Conference
Effects of birds’ wing color on their flight performance for biomimetics purposes
M. Hassanalian1, H. Abdelmoula1, S. Ben Ayed2, and A. Abdelkefi3
The thermal effects of the birds color on their flight performances are investigated. In most of the large migrating birds, the top of their wings is black. Considering this natural phenomenon in the migrating birds, such as albatross, a thermal analysis of the boundary layer of their wings is performed during the year depending on the solar insolation. It is shown that the temperature difference between the bright and dark colored top wing surface is around 10 degrees Celsius. The dark color on the top of the wing increases the temperature of the boundary layer over the wing which consequently reduces the drag forces over the wing. This reduction in the drag force over the wing can be considered as one of the effective factors for long endurance of these migrating birds. The main purpose of this investigation is to propose a novel efficient design of the drones by applying the inspired colors which can help drones increase their endurance.
I. Introduction The flying of birds is similar to aircraft so that when their wings move through the air, because of the pressure difference between the top and bottom of the wings, a distributed force called lift is generated. This lift force acts perpendicular to the wing surface and keeps the birds or airplane in the air. Generally, birds have different flight modes which can be divided into two modes, namely, unpowered flight (soaring and gliding fight) and powered fight (hovering and flapping flight)1. In gliding flight modes, usually birds extend their wings and do not flap which causes the production of a lift force by the action of air flow on the wings. In other words, by converting their gravitational potential energy into kinetic energy, birds generate speed and consequently lift force2. Flying animals, such as insects, bats, and birds usually generate both lift and thrust forces by flapping their wings. But, in gliding mode, they just keep their wings stretched out which produces only lift, not thrust. The thrust force can be generated by the gravity force. This type of flapping mode and also soaring flight can be seen in birds, such as vultures, albatrosses, pelicans, and storks with a high lift-to-drag ratio2,3. These mentioned birds can perform these types of flight modes because of their larger wings. The next flight mode is a special kind of glide which is called soaring. In this flight mode, birds fly into a Downloaded by UNIVERSITY OF COLORADO on January 10, 2017 | http://arc.aiaa.org DOI: 10.2514/6.2017-0296 rising air current to maintain or gain height and instead of using gravity, they use energy in the atmosphere. Pennycuick3 has defined soaring flight mode where energy is extracted from atmospheric motions. There are different types of soaring forms including slope, thermal, and dynamic soaring4. In thermal soaring, the birds just use convection currents, called thermals, to stay in the air without any additional power source. Thermals are some localized parts of the atmosphere which are created by solar radiation heating the ground and moving
1PhD student, Department of Mechanical and Aerospace Engineering, New Mexico State University, Las Cruces, NM 88003, USA. 2College Professor, Department of Mechanical and Aerospace Engineering, New Mexico State University, Las Cruces, NM 88003, USA. 3Assistant Professor, Department of Mechanical and Aerospace Engineering, New Mexico State University, Las Cruces, NM 88003, USA. 1
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Copyright © 2017 by M. Hassanalian, H. Abdelmoula, S. Ben Ayed, A. Abdelkefi. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. upwards with a speed in the range of 1–10m/s4. The mentioned birds can gain height by circling in thermals with their spread wings. Various researches, such as Pennycuick5, Welch6, and Leshemand Yom-Tov7 have investigated various birds, such as storks, eagles, hawks, and vultures which can fly by applying soaring in thermal updrafts. Birds have several types of feathers with various sizes and shapes depending on their functions. These feathers, which are divided to primary and secondary types, enable birds to fly and help them regulate their body temperature. They are also useful for identifying and attracting mates8. Birds have different feathers with a variety of colors ranging from dull neutrals to bright and sometimes iridescent hues. Investigations on birds show that there are two distinct sources from which feathers get their color. Red and yellow colors are created by a pigment in the feather and the brilliant iridescent blue and green colors are caused by light refraction. The created color by pigments is independent of the structure of the birds’ feathers. Pigment colorization in birds comes from carotenoids, melanins, and porphyrines9. Melanins occur as tiny granules of color in feathers of birds. Depending on their location and concentration, this material can produce different colors ranging from the darkest black to reddish browns. Melanin provides more than just coloration. Feathers that contain this material are more resistant to wear and stronger than birds that do not have melanin. Feathers without any pigmentation are the weakest of all10. Usually, most of the white birds have black feathers on their wings or wingtips. The melanin causing the tips to appear black also provides extra strength. The color of some types of birds is the result of a combination of these two methods. Nowadays, researchers try to interpret the color of each bird from gynandromorphy view or for protection from predators. For example, colorful plumage uses its colorful feathers to attract the opposite sex and some birds change their color for camouflage11. One aspect of the birds’ colors which has been neglected in the litterature is the effect of their color on their flight features and modes. Birds have different types of flight modes, such as flapping, soaring, gliding, hovering, and bounding1. The flight modes of birds vary depending on their type and class of flight. In this study, the effects of the color on albatross which has the ability to do soaring, high endurance, and high altitude flight are investigated. In other words, the thermal effects of the color of the wings are studied. The heat transfer equations are applied and their effects on the boundary layer of the flow over the wing are investigated. The main purpose of this investigation is to propose a novel efficient design of micro air vehicles with high endurance, which is nowadays a challenge in the design process of the fixed and flapping wings12-15.
II. The color of birds with soaring ability As mentioned above, various birds including storks, eagles, hawks, and vultures can fly without flapping by applying thermal soaring. Compared to other birds, albatrosses have long and slender wings with high aspect ratio. The high-aspect-ratio wings of soaring seabirds minimize their drag during the flight16. Albatrosses also have very high wing loading and they are close to the structural limits of wing length and wing loading. Other seabirds usually apply their high aspect-ratio wings and wing loading to benefit from the slope lift17. In this study, the effects of color of wings is investigated in these types of birds. Even though all of the mentioned birds can perform the soaring flight by taking advantage from the thermal features of atmosphere, the effects of their wings’ color can also be considered in their flight performance. In this work, we consider the thermal analysis of the albatross wing.The same process can be expanded to other birds. Albatrosses whose wings can reach up to 3.5m across and have one of the longest wingspans among the birds, are capable of flying
Downloaded by UNIVERSITY OF COLORADO on January 10, 2017 | http://arc.aiaa.org DOI: 10.2514/6.2017-0296 10,000 miles in a single journey18. The studies show that these types of birds are taking advantages from the dynamic soaring. In Figure 1, schematic views of this bird, its wing shape, and color are shown.
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Figure 1. Views of the top and bottom parts of the albatross wings.
As shown in Figure 1, usually the top part of the albatross wing is black and the bottom part is white. It is obvious that objects absorb heat at different rates. Some of them are excellent absorbers while others are very poor absorbers. Generally, the heat absorption of dark colored objects is better than lighter colors objects. For example, dark colors (black) heat up more than light colors (white). On the other hand, some portions of the heat energy is bounced or reflected in the opposite direction, as shown in Figure 2. The incident radiation hits the wing. A portion is absorbed, another is reflected, and the rest is transmitted. Since the wings are considered as opaque surfaces, the transmitted portion is neglected.
Figure 2. View of different portions of irradiation on albatross wings.
III. Thermal analysis of the wings The top part of the bird wing is the most important to model since it is the part exposed to solar insolation.
Downloaded by UNIVERSITY OF COLORADO on January 10, 2017 | http://arc.aiaa.org DOI: 10.2514/6.2017-0296 For the sake of simplicity, the wing is supposed to be insulated from the bottom, which allows us to use an adiabatic boundary condition. The most significant heat transfer modes in this model are radiation and convection. In fact, as shown in Figure 2, the absorbed portion of the incoming solar insolation rises the temperature of the wing. This temperature is found using an energy balance on the wing. The absorbed heat is then delivered to the environment by both radiation and convection. Radiation is expressed through the Stefan- Boltzman law and convection is expressed though the Newton’s law of cooling. The amount of the absorbed heat by the surface depends on the surface absorptivity which is a surface property that depends on both the material and the color. In fact, dark colored surfaces have a higher absorptivity. The objective of the wing thermal modeling is to compare the flight performance of the bright to that of the dark colored wings. For the sake of example, we assume the absorptivity of the bright wing to be 0.3 and that of the dark wing to be 0.8. Since the wing is not assumed to be a black body, we associate an emissivity to the material. Following19, the emissivity value of the feathures is considered to be equal to 0.8. The solar radiation intensity can be calculated according to the longitude, latitude, and altitude and the month (or season) of the year. The migration 3
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trajectory of albatross and the time of their migration are first investigated. Generally, migration of birds is affected not only by food supply, but also by wind and oceans currents. These factors influence the routes of their migration. Many birds migrate from northern areas in summer to southern wintering grounds. Some types of birds migrate horizontally to take advantages of the milder coastal climates in winter. There are some types of birds which migrate in terms of altitude; moving higher up a mountain in summer, and wintering on the lowlands. They may shift altitude to find the best wind. Winds at higher altitudes are usually blowing strongly and larger birds, such as albatross rely on these types of winds and also on thermalsto gain altitude by simply soaring. One of the points that can be investigated in this work is the influence of solar radiation intensity on the season or time of birds’ migration. In Figure 3, a schematic view of the albatross flight trajectory is shown19.
Figure 3. A schematic view of the albatross flight trajectory20.
It should be noted that albatrosses usually fly very close to the sea level, which varies between 10m to 20m21,22. Therefore, it can be assumed that the solar insolation at sea level is applicable for our calculations. The analysis is performed for different seasons and places, such as the northern hemisphere in winter and summer, and the tropical regions as shown in Figure 4. The sunlight intensity is considered hourly.
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Figure 4. View of sunlight intensity comparison between tropical and northern hemisphere countries23.
Concerning the convection between the wing and the environment, we suppose that the wing is at a uniform temperature Ts and the free stream is at a uniform temperature T∞. This free stream temperature at the sea level 4
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in winter and summer is equal to 288.2K (15CO) and 298.2K (25CO), respectively. The convective heat transfer coefficient h varies from 10 to 100W/m2K. It is approximated by24: h10 . 45 v 10 v (1) where v is the relative speed of the object through the air in meter per second. The average velocity of migrating birds in the air is almost equal to 68mph (≈30m/s). Therefore, the convective coefficient of the air above the albatrosses’ wings is almost equal to 35.5W/m2K. The thermal balance of the wing surface for the different wing colors yields a difference of approximately 10oC. In order to understand the effect of this temperature difference on the flight performance, we evaluate the drag force for the bright and the dark wings. For that, we need to understand the convection boundary layer physics. As a first apporoximation, the curvature of the albatross wing is ignored and the boundary layer on a flat plate is assumed. In Figure 5, schematic views of the albatross airfoil and the simulated boundary layer are presented.
Figure 5. Schematic views of (a) albatross airfoil and (b) simulated boundary layer over it.
In Figure 5, δ denotes the boundary layer thickness and u is the flow velocity. As shown in Figure 5(b), at the wing surface, the velocity of the flow is zero, and in the next layers the particles are slowed down by the boundary layer effect until the freestream velocity is recovered. Identically, the thermal boundary layer is shown in Figure 6. T∞ is the temperature of the free flowing fluid and Ts is the temperature of the wing surface which is higher than the free stream temperature. Therefore, heat will transfer from the wing to the air flow25.
Figure 6. Schematic view of thermal boundary layer over the wing.
The continuity, momentum, and energy equations are defined as follows, respectively:
Downloaded by UNIVERSITY OF COLORADO on January 10, 2017 | http://arc.aiaa.org DOI: 10.2514/6.2017-0296 uv (2) 0 xy u v 2 u (3) u v v x y y 2 TTT 2 (4) uv x y y 2
For the boundary conditions, we have: (5) Atx0 u,yu 0 T,yT 0 5
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(6) Aty0 ux, 0 0 vx, 0 0 Tx,T 0 s (7) Asy ux, u Tx,T Since the flow regime is laminar, we consider the Blasius solution of the boundary layer equations under the defined boundary conditions. The effects of the thermal boundary layer on the drag forces is calculated for different seasons. Sutherland's formula26 can be used to derive the dynamic viscosity of an ideal gas as a function of the temperature as shown in the following equation: 3 (8) 2 TC0 T 0 TCT 0 where μ is the dynamic viscosity at input temperature T, μ0 is a reference viscosity at a reference temperature T0 -5 (μ= 1.827×10 pa.s, T0=291.15K), T and T0 are the input and reference temperature (K), respectively, and C is Sutherland's constant for the gaseous material, which is equal to 120K for air. The effects of temperature on the density in the boundary layer is expressed as27: (9) ()TT where ρ is the air density at the input temperature T, ρ∞ is the reference air density at the reference temperature 3 3 T∞ which is equal to 1.225kg/m , and β is a constant that is equal to 0.004kg/m K for air. Finally, the drag force (D) over the wing can be expressed as follows27: 3 (10) D 0.664 b U L
where b and Lμ are the wingspan and the mean aerodynamic chord. For albatross, they are considered equal to 3.5m and 0.22m, respectively.
IV. Wing surface color effects on the drag force and performance of the albatross Considering the sunlight intensity shown in Figure 4, the results for the drag forces on the dark and the bright colored wings are obtained as a function of the hour throughout the day. Considering the effects of the temperature on the viscosity and the density of the air in equations (8) and (9), the drag force can be calculated from equation (10) in different seasons. It follows from the plotted graphs in Figures 8(a), 8(b), and 8(c) that a dark colored wing generates less drag than a white colored one for winter, summer and tropical regions. Even when we compare the two wings in all the weather cases and everywhere, as shown in Figure 8(d), it is clear that the dark colored wing still generates less drag than the bright colored wing. This means that for the albatross, its black top wings are helping it to improve his flight performance by reducing the drag force.
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(a) (b)
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(c) (d) Figure 8. Views of drag forces versus days hours for the bright and the dark colored wings during (a) winter (b) tropical regions, (c) summer, (d) all of the seasons and regions.
V. Conclusions The effects of wings’ colors of migrating birds with soaring and gliding capabilities on their flight performance have been investigated and discussed. Taking the Albatross’ wing as an example, the temperature of the top surface of the wing was calculated through an energy balance. Then, the Blasius solution for laminar boundary layer was used to calculate the generated drag forces of the wing. The results showed that, through using the thermal balance of the wing surface for the different wing colors, the temperature difference between using black and bright surface colors is approximately equal to 10oC.It was also found that the black top wing color of the albatross improves its flight performance by reducing its drag, regardless of the weather or the place. The observed reduction in the drag force over the wing can be one of the main reasons behind the high endurance and efficiency of these migrating birds. The performed analysis in this study showed the impacts of the surface wing colors on the effectiveness of migrating birds. This analysis is so beneficial and can be utilized to mimic new generation of biologically inspired drones with high endurance due to the accurate selection of the wing surface colors.
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