Frank Boyd, age 14, of Birmingham, Ala., for his question:
WHAT EXACTLY IS STREAMLINING?
Streamlining is the shaping of a body so that it meets the smallest amount of resistance as it moves through a fluid, such as liquid or gas. The best streamlined shape for a body depends on whether it is to travel slower or faster than sound through the fluid.
Subsonic travel is slower than sound. In streamlining for subsonic travel,a body should be somewhat blunt and rounded in front and then tapered to a point at the tail. Submarines and subsonic airplanes have this shape. In nature,fish also have this type of streamlining.
For supersonic travel, or travel that is faster than sound,a body sould have a sharply pointed front. Supersonic airplanes androckets have this shape.
The resisting force acting on a body as it travels through a fluid is called drag. The amount of drag depends on how smoothly the fluid flows around the body.
The path that any bit of fluid follows around the body is called a streamline. If a body is streamlined, the streamlines separate smoothly at the front, pass smoothly around the body and meet again at the tail.
But if the body is not streamlined, the fluid may swirl and twist violently as it passes around the body.These motions arecalled eddy currents. The fluid may separate from the surface of the body and cause a partial vacuum behind it.The amount of drag increases because of the lack of pressure behind the body to balance pressure in front.
The effects of streamlining can be measured in a wind tunnel. In the tunnel, air is blown past a body so the drag can be measured. Streamlines can be made visible by adding smoke to the air at several points.
In addition to a body's shape, three other factors affect the drag the density of the fluid, the amount of the body's area that meets the fluid and the speed of the body through the fluid.
The drag doubles if either the fluids density or the area of the body meeting the fluid is doubled. Doubling the speed of the body multiplies the drag by four.
Wind tunnels are built in many shapes and for different purposes. Some are big enough to test full sized experimental airplanes while others can test only small models.
Tunnels in which the speed of the wind is less than the speed of sound are called subsonic tunnels. Those with speeds faster than sound are supersonic.
Tunnels that use air speeds five or more times faster than sound are called hypersonic. In some tunnels, temperatures can be made very low, in order to represent conditions at high altitudes. These tunnels are used to investigate icing on aircraft.
In others, temperature is made very high to represent conditions met by a guided missile in its flight through the earths atmosphere. Large electric fans make the wind in most subsonic tunnels. Air compressors like those used in paint sprayers, make the wind in supersonic tunnels.