UNIT 3 Characteristics of Atmospheric Turbulence

One of the obvious characteristics of wind is its variability. We are aware of random temporal variations of the wind through fluttering of flags and leaves, variations in the force of wind on us, and other common experiences. Spatial variations are obvious when one looks at a field of "waving grain" or at "cat's paws" on a lake. We are also aware that the range of variability is large. We see very small-scale fluctuations in "heat waves" on hot summer days and feel or hear the effects of very large-scale fluctuations as wind gusts which blow dust or shake the house. All of these characteristics of wind with which we are intimately acquainted are characteristics of turbulent flow. Except for a thin layer of air close to surfaces, the atmosphere is essentially always turbulent, or, in other words, characterized by random fluctuations in wind speed and direction caused by a swirling or eddy motion of the air. These swirls or eddies are generated in two ways. As wind moves over natural surfaces, the friction with the surface generates turbulence. This is called mechanical turbulence. Turbulence is also generated when air is heated at a surface and moves upward due to buoyancy. This is called thermal or convective turbulence. The size of the eddies produced by these two processes is different. The fluctuations from mechanical turbulence tend to be smaller and more rapid than thermal fluctuations. A striking demonstration of these types of turbulence can be seen by watching the plume from a smokestack on a hot day. The plume is called a looping plume because, in addition to the small scale mechanical turbulence that tears the plume apart and spreads it with distance, the thermal updrafts and downdrafts cause the entire plume to be transported upward or downward.

Large eddies, which are produced either mechanically or thermally, are unstable and decay into smaller and smaller eddies until they are so small that viscous damping by molecular interactions within the eddies finally turns their energy into heat. The size of the smallest eddies produced by mechanical and convective motion (rather than breakdown of larger eddies) is called the outer scale of turbulence. The eddy size at which significant molecular interaction (viscous dissipation) begins is called the inner scale of turbulence. The outer scale is generally taken to be a few meters and the inner scale a few millimeters.

Define the following words using a dictionary

Random, temporal, to flutter, eddy, updraft, a plume, buoyancy

Complete the sentences

1. As was previously mentioned,…

2. Spatial variations are obvious…

3. All of these characteristics of wind with which we are intimately acquainted…

4. Except for a thin layer of air…

5. As wind moves over natural surfaces,…

6. Turbulence is also generated when…

7. A striking demonstration of these types of turbulence can be seen when…

8. The plume is called a looping plume because…