Bionic - The animal kingdom as a model for industry

16/07/2019 Engineering

Aerodynamics and how it is measured

Aerodynamics does more than just affect streamlined racing cars, low-resistance aircraft wings or particularly energy-efficient trains. The term comes from the Greek and is made up of the two words for air and power. As a sub-discipline of physics, aerodynamics describes flow processes in gases and is therefore of great importance for a multitude of daily processes.

In order to be able to quantitatively evaluate the aerodynamics of moving bodies, the so-called drag coefficient was introduced. The coefficient, also known as the cw-value, is a dimensionless measure of the flow resistance of a moving body. It is calculated as the quotient of resistance and the product of dynamic pressure and reference surface. What sounds complicated can be explained quite simply: The lower the drag coefficient of a body, the better its aerodynamic properties. While a long rectangular plate, for example, has a very unfavourable cw-value of 2.0, the streamlined drop with a cw-value of 0.02 is considered to have a perfect shape. For comparison: modern cars usually have a resistance value between 0.25 and 0.40. As early as 1921, Edmund Rumpler had the idea of imitating the body of a car with a falling drop. The Rumpler drop car achieved a cw value of 0.28, which is still considered excellent by today's standards.
In industry, the cw-value is usually determined in special flow chambers. Whether normal cars for road traffic, racing cars for Formula 1 or wings of airplanes: engineers always have an interest in making the flow resistance as low as possible. The cost- and time-intensive measures to optimize the cw-value are worthwhile: cars consume less fuel and produce less CO², racing cars can increase their maximum speed and aircraft glide through the air more quietly. Particularly in view of ambitious energy efficiency targets, it quickly transpires that aerodynamics can make an important contribution to greater environmental protection.

Practical applications of aerodynamics

In industry, aerodynamics is particularly often associated with automotive engineering. This is hardly surprising, since there is rarely an industry where the future requirements for energy efficiency are as strict. The goal announced by politicians of reducing the specific emission value of vehicle fleets to 95 grams per kilometre by 2020 demands everything from engineers in terms of aerodynamics, lightweight construction, engine development and tyre characteristics. Despite the trend towards greater comfort, larger engines and a wider track gauge, the automotive industry has succeeded in recent years in continuously reducing the cw-value. However, experts assume that further progress in the area of series production vehicles will only be achieved in very small steps in the future.
 
Many vehicle characteristics are tested in motorsport and find their way into daily traffic - its wings and shapes help to reduce downforce and improve cornering control.
In aircraft construction, the primary objective is not to achieve maximum speeds, but to reduce fuel consumption and noise emissions and to optimize flight characteristics. Scientists and engineers therefore use modern computer simulations and huge wind tunnels to research perfect aerodynamics. The fact that aerodynamic applications are by no means limited to the automotive and aerospace industries is shown by examples from mechanical and plant engineering, such as air compressors or aerodynamic bearings.


Which lessons do we learn from the animal world?

The past has shown that the animal world offers a large pool of techniques that inspire people and stimulate innovation. In the field of aerodynamics, we learn above all from animals that live in the air or water. A particularly astonishing example can be seen when we take a look at the underwater world: with a cw-value of 0.06, the boxfish - although apparently resembling a box with little wind resistance - has less resistance than a Porsche. Mercedes-Benz recognised the potential of the boxfish and built the bionic car with an advanced flow coefficient of 0.19 based on its model. According to the manufacturer, the car presented in Washington in 2005 consumes 4.3 litres per 100 km in the EU driving cycle and reaches a top speed of 190 km/h.
The study initially focused on a completely different animal: the penguin. However, since its body structure is not suitable as a model for automotive engineering, it now serves aviation as a model for efficient flow behavior. The "penguin aircraft" has not yet been technically implemented due to the high effort involved, but it impressively demonstrates the possibilities of optimized aerodynamics. Other animals were more successful here.


The fascinating properties of owl wings

The owl and its extraordinarily quiet, efficient flight behaviour arouses the curiosity of science. After all, the ever-increasing volume of air traffic has the primary goal of drastically reducing aircraft noise. In order to be able to handle all flights according to schedule in the future, there will be no alternative but night flights and therefor take-offs and landings. This will only be possible, however, if the aircraft can be operated with a correspondingly low noise level. While modern engines already do their job with minimum noise levels, air turbulence on the wings is still a source of noise that should not be underestimated. A concern that could be resolved with the characteristics of the owls. Their wing beat is almost inaudible. Owl wings have at least three characteristics that contribute to a silent flight. For example, the wings have a comb of extremely stiff feathers on the front, flexible fringes on the opposite side of the wing and a very soft, almost downlike surface on the upper side of the wing.
Even though various research groups are intensively engaged in the characteristics of the silent night hunters - groundbreaking successes for a timely implementation cannot be shown at present. However, theoretical research has shown that the fine, extremely flexible structure at the ends of the owl wings has a major influence on noise reduction during flight. However, it is still unclear how exactly the tips of the feathers influence the noise level. It is assumed that these tips ensure that the air currents of the upper and lower sides of the wings meet more gently, thus reducing the noise level. The velvety upper side of the owl wings is the currently least researched part of the owl wing. Here, researchers suspect that the structure eliminates noise in a previously unknown way, quite unlike that of known sound-reducing materials. The stiff feather comb at the front of the wing is probably combined with the velvety soft surface of the wing to form micro-turbulence, which improves the adhesion of the airflow at the top of the wing.

Challenges and significance

According to common doctrine, it will be more than 20 years before the knowledge gained from science can be implemented in the aircraft industry. The reason is the so-called Reynolds number. It measures the ratio of inertial to tenacity forces of certain objects in fluid mechanics. Only if the ratio were the same for owls and flying machines it would be possible to easily transfer the insight gained from bionics. Nevertheless, the Reynolds number of the owl wing is much smaller than that of the aircraft wing. A manufacturer of fans is already using the owl's wing as a model for particularly quiet products. Here, the comb on the front wings has been copied and transferred to the rotor blades of the devices, thus reducing the noise level during operation.
A transfer of the owl wing principle is also conceivable on a larger scale in the near future. Wind turbines or their rotor blades could already be redesigned in a few years' time according to nature's example and thus be operated even more quietly. Once the principle of the noise-reducing wing surface of owls is fully understood, a transfer to the surface of cars or trucks is also imaginable. One thing is certain - the research departments of all major manufacturers of aircraft, cars, trucks or even ships show great interest in the broad field of bionics, its results and their transfer to technology.

The stork - model to optimize the turbulences on the wing

There is a simple physical cause for the formation of the vortices. The air flows at different speeds over the top and bottom of the wings, creating lift. The shape of the wings, especially their curvature, means that the particles have to travel a longer distance on the upper side than on the lower side. This creates negative pressure on the upper side and positive pressure on the underside. The resulting force is sufficient to get several hundred tons into the air if there is sufficient acceleration. High and low pressure areas meet at the tips of the wings. Since different pressure ratios always want to balance each other, balancing currents are created at the wing tips. These two counter-rotating swirls do not generate lift, but do require energy, this is called induced air resistance.
Whirls on the wings are a weak point, especially for energy efficiency. Wake vortices that occur after lift must also be reduced so that following aircraft are not endangered. Waiting times resulting from this are unthinkable, especially at highly frequented airports such as London Heathrow (EGGL) or Frankfurt/Main (EDDF). One solution is the stretching of the wings, as this can reduce end vortices and thus the induced air resistance. However, the aspect ratio has a disadvantage: the stability of the wings as well as the maneuverability of the aircraft suffer from the strongly stretched wings. The space required would also present further challenges for many aircraft and airports.

Winglets und Sharklets

With the help of winglets the wing is extended and "bent" upwards. The winglets of a Boeing 737-800 are about 2.40 m high, which leads to a fuel saving of about 5 %. They also increase stability around the vertical axis (yaw axis) as well as maneuverability around the longitudinal axis (roll axis).
The first winglets were quite unusual, today there are a lot of variations. At Boeing the winglets are mostly angular and straight or diagonal upwards. Newer versions are blended winglets, which can be seen on almost all aircraft flying in this country. These form a flowing transition from the wing to the winglet. TUIfly was the first airline in Germany that used Split Scimitar Winglets. Of course, Airbus planes are also flying with winglets. However, the winglets newly developed for the A320 family since 2009 are called sharklets. The reason: the wing spreaders remind in their shape of shark fins. Small winglets are also known as wingtip fences and can be oriented upwards or downwards. This form of winglets is often seen on Airbus aircraft.

The shark as a model for aviation

The shark travels up to 70 km/h at the top, the water flows past it and even though it saves energy when swimming. For a long time it was a mystery how these speeds can be achieved with low energy consumption. With the help of a scanning electron microscope it was possible to gain an insight into the biological structure. The shark's skin has a serrated surface and is one of the prime examples of bionics. The applications for aviation are manifold.


Sharkskin effect

The skin of a "Lamniform" feels rough, almost like sandpaper. The reason for this lies in the microscopic tooth-like platelets that cover the entire body of the animal. The teeth are all oriented towards the caudal fin, thus forming grooves and ribs, water particles can now flow orderly along the body of the shark. The natural structure of the shark's skin reduces frictional resistance and decimates the resulting turbulence. The amount of energy used is low for the shark.

Application to aviation

In the 1990s, scientists at the TU Berlin tackled the question of whether the structure could be transferred to an aircraft fuselage and whether it could compensate for temperature fluctuations from -55 to +70 degrees Celsius, high UV radiation and high speeds. The result of this research is a foil that can significantly reduce the kerosene consumption of passenger planes. The applied foil reduced frictional resistance by up to 8 percent, saving 2.4 tons of fuel on a long-haul flight. The film is now used in all kinds of applications. It was tested for the first time on an Airbus A340 on the scheduled flight of Cathay Pacific Airways. In this test, 30% of the aircraft, especially the fuselage and the leading edges of the wings, were covered with the film.
A corresponding film was also tested by Lufthansa Technik. Within the framework of the "Multifunctional Coating" research project, patches measuring 10 cm x 10 cm were tested on the fuselage and wings. In addition to many advantages, however, there is also an additional material requirement and the application to three-dimensional surfaces was not always easy. This is where the Fraunhofer IFAM developed a coating. In 2010, this and the associated scientific achievement were awarded the Joseph von Fraunhofer Prize. With the help of nanoparticles and a matrix, the paint simulates the structure of a shark's skin and is UV-resistant, withstands extreme temperature fluctuations and mechanical stress. A special coating machine uses a negative mould to apply the riblet structure to the surface and hardens the coating with UV radiation. This means that the coating can also be applied in a regular maintenance cycle. If the coating is applied on every aircraft, a total volume of 4.48 million tons of fuel could be saved. Testing of the coating has already been completed, but the seven to ten-year approval process makes its current use more difficult.
In addition to active support in aircraft construction, the paint is also of interest to the shipping and wind turbine industries. In shipbuilding, the novel "sharkskin paint" has a further function. The paint prevents the colonization of "Balanidae" (barnacles), which attack the hull of the ship and thus increase frictional resistance.

Bionics in industry - a look into the future

The fact that aerodynamic optimization measures do not always go hand in hand with an aesthetic design is shown by the example of the duck's beak shape of high-speed trains, whose bionically optimized design results in extremely low air resistance at high speeds.

But nature does not always represent the optimum in terms of efficiency and technology. Rather, it is a matter of being inspired by nature's ideas and applying them to the problems of mankind. ARTS is fascinated by the technical achievements that the animal world has in store for humans. Day after day, our experts are inspired by bionics and work to make the high-tech industry more efficient, more sustainable and better. As a technology-oriented company, we actively shape the future of industry and give our customers a head start with our expertise and experience. We are always looking for motivated experts who share the same goals and are as enthusiastic about bionics as we are.

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