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ABSTRACT
For centuries now, wind tunnels have been a key element in scientific research in a number of fields. Experimenting with racecars, airplanes, weather patterns, birds, and various other areas has been made much easier because of its development. In the racing field, for example, the information gathered from this testing can mean the difference between winning and losing a race. Weather simulations can also provide valuable information regarding building stability and safety. This has become very important when designing buildings today. Valuable information concerning bird flight has also been collected based on wind tunnel testing. Experts have experimented with a variety of birds and gathered extremely useful information regarding rate of mass lost during flight, respiratory information, and flight kinematics. Wind tunnels have a variety of important uses in the world today.
A wind tunnel simulates the conditions of an aircraft in flight by causing a high-speed stream of air to flow past a model of the aircraft (or part of an aircraft) being tested. The model is mounted on wires so that lift and drag forces on it can be measured by measuring the tensions in the wire. The paths of the air stream around the model can also be studied by attaching tufts of wool (which align themselves with the wind direction) to various parts of the model, by injecting thin streams of smoke into the tunnel to render the airflow visible, or by using certain optical devices. Pressures on the model surface are measured through small flush openings in its surface. Forces exerted on the model may be determined from measurement of the airflow upstream and downstream of the model. In wind tunnels operating well below the speed of sound, the air stream is created by large motor-driven vanes. At velocities near or above the speed of sound, the air stream is created either by releasing highly compressed air from a tank at the upwind end of the tunnel or by allowing air to rush through the tunnel into a previously evacuated vacuum tank at its downwind end. Sometimes these methods are combined, especially for the production of hypersonic velocities, i.e., velocities at least five times as great as the speed of sound.
The effect of wind on other vehicles, e.g., automobiles, and on stationary objects such as buildings and bridges may also be studied in wind tunnels. In many instances, wind tunnels have been rendered obsolete by computer modeling.
Fields of Operation
Some operations ordinarily performed in the wind tunnel are the following:
Drag/Lift measurements on aircraft, helicopters, missiles, racing cars.
Drag/Lift/Moment characteristics of airfoils and wings.
Static stability of aircraft and missiles
Dynamic stability derivatives of aircraft
Surface Pressure distributions on nearly all systems.
Flow visualizations (with smoke, oil, talcum).
Propeller performances (torque, thrust, power, efficiency, etc.).
Performances of air-breathing engines.
Wind effects on buildings, towers, bridges, automobiles.
Heat transfer properties of engines and aircraft.
HISTORY
Experimentation with aerodynamics began in the nineteenth century. Originally, aeronauts were unaware of aerodynamic forces, such as drag and lift, on various objects. There was a need for a machine that was capable of measuring these forces. Before wind tunnels were invented, the whirling arm was the main machine used in finding aerodynamic patterns. When whirling arms were taken a step further, the result was the creation of a wind tunnel. Wind tunnels provided controlled and systematic airflow in an enclosed space (Baals and Corliss, 2000).
Benjamin Robins, an English mathematician, began experimenting with whirling arms in the 1700s. The whirling arm consisted of a free falling weight which pulled an extended arm in a circular motion. During this time period, this was one of the only reliable methods of moving air over a test subject to observe aerodynamic forces. Based on his experiments, Robins discovered that different shapes with the same area do not necessarily have the same wind resistance and drag. Adding to these discoveries, Sir George Cayley continued experimentation with whirling arms. He is credited for creating the first whirling arm apparatus. He found that in order to create lift, a steam powered engine would provide the forward motion of the object, and lift could then be achieved by wings. Despite the advances made by the whirling arm in measuring aerodynamic forces, there was still room for improvement. A whirling arm created a great amount of air, which caused turbulence. In addition, it was difficult to attach instruments and measure the small forces while the arm was rapidly moving. The needed improvement was a wind tunnel. A wind tunnel provided an enclosed area with controlled airflow. Many previous experiments with whirling arms were done outside, creating an environment in which experiments were prone to gusts of wind, creating an inaccurate measurement of the drag and lift on an object. Air in a wind tunnel, however, can be created by a machine or a fan.
In a wind tunnel, an object’s lift and drag are found based on its relativity to a central axis inside the tunnel. The first wind tunnel was created by Frank H. Wenham in 1871, who was a member of the Aeronautical Society of Great Britain.
Due to his disappointment with previous experiments with whirling arms, Wenham raised the funds for the building of a wind tunnel. After the creation of the wind tunnel, uses of the whirling arm for testing purposes became rare. Wenham’s early experiments involved placing shapes in the tunnel and evaluating the movement of the air rushing by the object. The lift and the drag of an object were found by the movement of the air. Researchers continued to experiment with wind tunnels, comparing the lift and drag of different objects. Eventually, researchers found the present day “aspect ratio.” The aspect ratio states that the longer and more narrow the wings of an object are, the greater lift an object can attain.
After the creation of wind tunnels, experimenters wanted to be able to find the lift and drag on airplanes. However, to build a wind tunnel big enough for an airplane would be highly expensive. A scientist named Osborne Reynolds discovered that scale models of airplanes had the same flow of patterns, ease of heat transfer, and turbulence as a full model. Scale models’ drag and lift are called Reynolds number. However, later scientists began to realize that scale models were not as accurate and did not match full-scale models’ results. In order to make the small scale models more accurate, models were placed under high pressures to create a more realistic condition.
Another scientist, Hiram Maxim, built a wind tunnel which tested power manned flight. It was twelve feet long and three feet wide powered by a steam engine blowing air at 50 mph. He found that curved airfoils, or curved wings, will create greater lift and less drag as opposed to flattened airfoils. He also found that the total drag of an object is greater than the sum of the drag of each individual component. Another aeronautical experimenter, Samuel Langley, experimented with whirling arms. Unlike Maxim, Langley was known for his failures.
However, he did have success with gliders, which he believed were the precursor to power manned planes.
Wilbur and Orville Wright are credited for the first flying machine in 1899. However, their success came as a result of much experimentation. After their first glider failed at Kitty Hawk, they used natural winds to compare the effectiveness of various wing shapes. The Orville brothers had then created a “wind tunnel without walls” and realized that past aerodynamic measures were incorrect.
With this discovery, the Wright brothers continued their experimentation. They built makeshift wind tunnels at first to test flight and find drag and lift. Their instruments were crude and primitive. As a result of their new discoveries and proof that older theories were incorrect, they were required to write a handbook. This also meant that they needed to build a wind tunnel. In 1901, the Wright brothers built their first wind tunnel. It consisted of a tube directing air flow, a fan, and balance in the path of the air flow. When the air was at full force, the balance would move one way or the other, showing the relative lifting forces. From these experiments, the Wright brothers obtained vital data for future flight attempts.
The Wright brothers’ wind tunnel consisted of two balances, one for drag and one for lift. After the Wright brothers built the wind tunnel and tested their aircraft, they tested their model at Kitty Hawk. Although a few mistakes were made in the assembly of the wind tunnel, alterations and corrections were later made. Through the use of wind tunnels, the Wright brothers became the first in flight.
WHAT ARE WIND TUNNELS ?
• Wind tunnels are research tools developed to assist with studying the effects of air moving over or around solid objects. The air is blown or sucked through a specialized duct. A viewing port and instrumentation are included in the duct, to which geometric shapes or models can be mounted in order to be studied. The airflow and geometry observed within wind tunnels is generally compared to theoretical results in order to test their accuracy. This study requires taking into account the Reynolds number, which is the ratio of inertial forces to viscous forces, and the Mach number, which is the the ratio of speed of an object or flow relative to the speed of sound in the medium through which it travels.
• Threads are sometimes attached to the object surface in wind tunnels in order to learn more about flow direction and the speed of the air flow in a specific air flow situation. In this situation, dye is injected into the air stream created in wind tunnels, and the particles that result are photographed so they can be studied in varying time frames. Probes can also be inserted at certain points within the air flow in wind tunnels in order to measure air pressure.
PARAMETERS AFFECTING AN AIRPLANE
When an airplane flies through the air, it is affected by four different forces:
1. Gravity pulls the plane down toward the ground.
2. Lift pulls the plane up. Lift is created by the movement of air around the plane’s wings. The air which goes above the wings must travel farther than the air which passes below them, and thus travels faster. This produces an area of lowered air pressure above the wings, which helps the plane rise.
3. Thrust, usually produced by an engine, is the power that moves the plane forward.
4. Drag, caused by friction as the plane moves through the air, pushes the plane backwards.
Understanding how these four forces affect an airplane is very important. Because small changes, such as the angle of the plane’s wings or how much extra weight it is carrying, can make big differences in how well the plane flies. Airplane designers use “wind tunnels” to see how their planes will fly and how design changes will affect their performance against these four forces.
BASIC PARTS AND WORKING OF WIND TUNNEL
Although wind tunnels test extremely complex concepts, most are made up of five basic components. The diagram below, illustrate each part and describe its operation (“The Parts of a Wind Tunnel,”).
As air enters the settling chamber, the first section of the tunnel, the airflow is straightened in order to reduce turbulence. Uneven wind flows and turbulence can cause unpredictable forces inside the test section, reducing the tunnel’s ability to simulate actual flying conditions. Most chambers contain a honeycomb flow straightener, which prevents swirling currents in the tunnel, and wire mesh smoothing screens. Together, these devices generate a smooth air flow.
Next, the air enters the contraction cone. The purpose of the contraction cone is to take a large volume of low-velocity air and reduce it to a small volume of high-velocity air. As the air moves from the wider area of the cone into the narrower area, the speed of the air increases.
Once through the contraction cone, the air enters the test section and is brought to the desired velocity. Various objects, such as models of wings, planes, or racecars, are placed inside. Sensors then measure the effects of forces, such as lift and drag, on the test item. Lift and drag are two of the most important forces affecting an object in the test section. Lift is the force on the object opposite the force of gravity. In other words, lift is what holds a plane in the air. Drag is the force on the object in the direction of the airflow. Plane engines, for example, must overcome drag to move through the air. Lift and drag measurements provide relationships between the test articles and their environment.
Based on these relationships, accurate predictions of real-life performances can be made.
The next section of the wind tunnel is the diffuser. The shape of the diffuser causes the air to slow down prior to exhausting or recirculating*. As the area of wind flow widens, the air speed decreases.
The final section of the wind tunnel is called the drive section. This section provides the force that drives the air through the tunnel.
Normally, this force is provided by large fans that can reach sizes of up to 40 feet wide.
• *Wind tunnels can be categorized into two groups: open- or closed-loop systems.
• In an open-loop system, intake and exhaust ends of the tunnels are not connected. However, this system is not very economical, and thus, most wind tunnels today are closed loop.
• In contrast to an open-loop system, closed loop systems recycle air using special vanes to turn the airflow around the corners of the tunnel while minimizing power loss and turbulence.
The comparison of the two systems below (open on left, closed on right) shows the obvious advantages to a closed-loop system and why it is an overall economically more efficient use of the tunnel