Different Ways Air Moves

The following terms and definitions are used by aerodynamicists to define the way a fluid moves in or around an object. In order to get a good picture of what is happening about a wing, for example, the aerodynamicist must know the velocity of the plane, the altitude of the plane, the size and shape of the wing, and the properties of the air. He or she will use the terms and concepts discussed in this list to define fluid flow.

Speed of Sound

If a person is standing very far from an explosion, he or she will not hear it right away. It takes time for the sound waves to travel. This is because sound travels in invisible waves of changing pressure through a fluid (usually air, but sometimes liquid). A person standing closer to the explosion will hear it sooner. At sea level, on a typical day (not too hot, not too cold), the speed of sound (how fast the sound waves travel) is about 760 miles per hour (mph).

The speed of sound depends on the pressure and density of the fluid in question. Since the pressure and the density can change with temperature or altitude, the aerodynamicist must compute the speed of sound at the altitude, pressure, and density where the plane is flying. This means the speed of sound could be more or less than 760 mph under different conditions.

Mach Number

The numbers Mach 1, Mach 2, Mach 3...etc. are used to show the pilot's speed in comparison to the speed of sound. Mach 2 is two times the speed of sound, for example. Remember, the speed of sound can change according to conditions in the atmosphere. An airplane at a low altitude flying at Mach 0.8 will have the same airflow behavior over the wing as the same airplane flying at a high altitude at Mach 0.8. The speed of sound decreases as the altitude increases, so in order for the airplane at the higher altitude to be flying at Mach 0.8, its velocity will be slower than that of the plane flying at the lower altitude! The behavior of airflow over the wing, however, will be the same on both planes.

The Mach number is named for Ernst Mach (1838 -1916), who conducted the first meaningful experiments in supersonic flight at the University of Prague, Germany.

Air flow, over a wing, changes around Mach 1.0. Different mathematical procedures are used to compute flow behavior. Air flow under Mach 1.0 is called subsonic flow. Air flow over Mach 1.0 is called supersonic flow. If the Mach number is greater than 5.0, that regime (pattern) is called hypersonic flow. However, an airplane traveling between Mach 0.75 and Mach 1.20 will have surface areas that are experiencing both subsonic and supersonic airflow; aerodynamicists have named it the transonic regime. Airflow calculations must be done carefully in this area.

It is interesting to see what happens to air flow regimes (patterns) as an airplane approaches Mach 1.0. At subsonic speeds the waves of changing pressure about the plane travel out in all directions at the speed of sound for that altitude.

As the plane flies faster and approaches the transonic regime (still below Mach 1.0), the waves in front of the plane don't travel that much faster than the plane itself.

At the sonic barrier, Mach = 1.0, the front of the sound waves and the plane are traveling at the same speed.

As the plane flies faster than the speed of sound (Mach number greater than 1.0), the waves compress into a cone-shaped envelope around the plane. the flow conditions of the air ahead of the plane remain unchanged until the plane flies past. Only the region inside the cone is affected by the plane. This conical compression is called a shock wave, and it will be discussed in greater detail in a later section.

Friction

Anything that moves against another object causes friction or resistance to motion between the two objects. If a person tries to push a box across the floor, he or she must push hard to overcome the resistance. If the person applies a push, or force that is stronger (larger) than the frictional force, the box will move. If the push isn't strong enough the box won't move.

The friction between two moving objects can be affected by the surfaces of the objects. For example, it is easier to push a heavy box across a smooth wood floor, or a sheet of ice, than it is to push it across thick, bumpy carpet. That means the frictional force between the box and the smooth floor or ice sheet is less than the frictional force between the box and the thick carpet, so it takes less of a push to get it moving.

When a fluid like air flows across a surface such as a wing, there is friction resisting the motion. How much friction is dependent on two factors, the viscosity of the fluid and the smoothness of the surface. A very viscous fluid like honey (a fluid with high viscosity) will resist flowing, even down a smooth surface. The friction force is very strong at the surface. Af fluid like water with much lower viscosity will travel much faster down a smooth surface; the frictional force between the water and the surface is much smaller. However, if water flows across a very rough surface, like carpet, it will travel down more slowly than on the smooth surface. Because the surface is rougher, the friction force is stronger, the velocity is slower.

Boundary Layer

Because of this friction force, when a fluid flows over a surface, an interesting pattern develops. The fluid actually stops; there is no velocity or movement at the surface. A new layer develops on top of the stopped flow. There is less friction on this new surface so there is some movement of the flow. New layers develop, each with less friction, until some distance away from the original surface, there is no effect of the slowed flow, and the remaining layers of the fluid travel at the original velocity. The distance from the original surface to the layer of the flow traveling at the original velocity is called the boundary layer thickness.

In general, the boundary layer gets thicker as the flow moves along the surface. How fast and how big the boundary layer grows is a function of the smoothness of the surface, the shape of the surface, and how fast the flow is travelling.

Laminar Boundary Layer

For lower velocities, fluid flowing over a smooth surface that is relatively short and flat will only devleop a very thin boundary layer. The flow inside the boundary layer will be smooth and orderly, meaning that the layers will basically stay in layers, without mixing. This condition is called laminar boundary layer.

Unfortunately, nature tends towards disorder, so it is rare to be able to maintain a laminar boundary layer for very long.

Turbulent Boundary Layer

As a fluid moves over a long, relatively flat surface, the boundary layer will get thicker, and the layers will start to mix and swirl around each other.. This swirling, rolling layer is called a turbulent boundary layer. The mixing and swirling is called trubulence; if the swirling is regular and repeatable, it is called a vortex or an eddy.

Since most of the boundary layers over an airplane will be turbulent, aerodynamicist will try to design the surfaces to minimize the amount of turbulence or disorder.

Transition

The region in the boundary layer where the orderly laminar layers start to mix together, but before they really start swirling, is called the transition region. Most of the time it is a fairly small region. The aerodynamicist will design the surface to keep the turbulent region small.

Flow Separation

Sometimes a boundary layer will be forced to move away from the surface. When this happens the flow inside the boundary layer gets so mixed up it starts to circulate and flow back towards the front of the surface! The outside, original fluid will move over a large bubble created by the circulating layer. This is called flow separation. The front of the bubble, where the outside fluid turns sharply away from the surface, is called the point of separation; the back of the bubble, where the outside fluid turns back to follow the surface again, is called the point of reattachment. If the region of flow separation extends past the surface, this region is called a wake.

Pilots and engineers usually don't like it when the flow separates on a wing. This is a condition known as stall. When a wing stalls, the lift (a force that helps a plane to fly; see later section) decreases sharply. The plane loses altitude, and if the stall is not corrected, the plane will crash. To land a plane however, a pilot will wait until the plane is close to the ground, then initate a slight, controlled stall to gently drop the plane to the runway.

Buoyancy

Buoyancy is a force that is directed upward, or opposite of weight (which is considered a downward force). There is always buoyancy in a fluid. The fluid may be moving or stationary. The Greek scientist Archimedes (287 - 212 B.C.) deduced that the buoyancy force was equal to the weight of the fluid displaced by the body.

If an object, dropped in water, weighs less than the water displaced (pushed away) then it will float; if it weighs more then it will sink.

The density of liquids is much higher than for gases, like air. Therefore the buoyancy force of a liquid is much higher than in a gas. Naval architects and ship designers must use the buoyancy forces in their calculations. The buoyancy forces for airplanes are so small that they are usually ignored (not used). Hot air balloons and blimps do use the buoyancy force to get afloat, but they displace such an extremely large volume of air that the computed buoyancy force exceeds their weight so that they can fly.

Streamlines and Flow Patterns

Aerodynamicists and other engineers like to know where the flow is going. A streamline traces out the path of an element or piece of fluid as it travels in space and time around or through an object. Streamlines are computed mathematically from the velocities in the flow region. Streamlines are usually plotted as smooth lines, and they sometimes have arrows on them to show the direction of the flow. They can be used to show how the air travels around an airfoil (the cross-section or slice of a wing), with some of the air flowing over the top of the airfoil, and the rest flowing below the airfoil. In a previous section, for example, streamlines were used to show how flow separation appears on a wing.

Shocks

As discussed in the Mach number section, when a plane flies faster than the speed of sound, a shock wave is created. This is the conical-shaped enveleope formed around the plane as it flies at supersonic speed. When a shock wave is formed, fluid properties such as pressure, density, temperature, and velocity change drastically and instantaneously through a shock wave.

Theoretically, once a shock wave is formed it will travel on to infinity. In nature, however, atmospheric winds cause the shock to weaken and disperse. When an aircraft flying at supersonic speeds is at a high altitude, the shock wave is diffused (scattered) long before it reaches the earth's surface. If a plane, flying at supersonic speed, flies too close to the ground, however, the shock will hit the earth's surface. It will be heard and felt by observers on the ground (it's called a sonic boom). If the shock is strong enough, it will cause buildings to shake and windows to break!

The space shuttle has a shock wave around it as it returns to earth through the atmosphere. There is a section of southwestern Georgia that is along the flight path of the returning shuttle when it lands at Cape Canaveral. when the shuttle travels along this path, it is still slightly supersonic, and it is close enough that the people on the ground hear the sonic boom as it travels over head. The shuttle can't be seen, but it can be heard! Before the shuttle flies low enough for the shock wave to cause any damage, however, it has dropped its speed below Mach 1.0 and the shock is gone.

In the early days of flight, the aerodynamics of transonic and supersonic flight were not well understood. As pilots went faster and approached the sonic region (called the sound barrier, back then) their airplanes would begin to shake and even fall apart! Some people were sure that there was an invisible barrier and that humans were not intended to go faster than the speed of sound.

In the late 1940's, designers started to understand high speed aerodynamics and began to design aircraft to fly in the supersonic regime. On October 14, 1947, Captain Charles Yeager, flying the experimental aircraft Bell XS-1, flew the first successful supersonic flight. Today many pilots regularly fly faster than the speed of sound.

Perfect Gas Law

The perfect gas law establishes the relationship between the pressure, density, and temperature of a gas at any instant in time or space. Air is treated as a perfect gas, even though it is a mixture of gases; it is mostly nitrogen. Engineers regularly use the perfect gas law to compute air flow properties.

Bernoulli's Theorem

Daniel Bernoulli (1700 -1782) was the first to develop a mathematical formula and theory that showed the relationship between fluid velocity and pressure: when the velocity in the flow increases, the pressure decreases, and when the velocity decreases, the pressure increases. This was an important discovery. As more people began to experiment with flying they were able to use Bernoulli's theorem to design airfoils. The theorum shows how lift is created when an airstream goes over a wing. This was the vital information needed to make flight possible.

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Last modified: Wed Jul 28 11:45:12 PDT 1999

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