7: Flow in Rotating Environments
- Page ID
- 4189
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- 7.1: Playing on a Rotating Table
- The fictitious side force that seems to act on moving bodies in a rotating environment is called the Coriolis force, after the nineteenth-century French mathematician who first analyzed the effect. And the apparent acceleration of the sphere (it is a radial acceleration, not a tangential acceleration, in that only the direction changes, not the speed) is called the Coriolis acceleration. The entire effect is called the Coriolis effect.
- 7.2: The Coriolis Effect on the Earth's Surface
- Fluid flows you observe on the Earth’s surface experience a Coriolis acceleration because the Earth is rotating, and both you and the flowing fluid are rotating with it. The effects you discovered on your turntable show up in those flows as well. The only places this should seem really obvious to you are at the North Pole and the South Pole—where the Earth’s surface is perpendicular to the axis of rotation. But the Coriolis acceleration affects fluid motions everywhere else on the Earth.
- 7.3: The Rossby Number and Inertia Currents
- How can we gain a general idea about whether the motion of a fluid or a solid object on or near the Earth’s surface would manifest non-negligibly the Coriolis effect? The answer lies in a dimensionless parameter called the Rossby number. Any such motion, whether it is a flow of a fluid or the flight of a bullet or an artillery shell or a rocket, has some characteristic speed U and moves over some characteristic distance L .
- 7.4: The Ekman Spiral
- This section deals briefly with some of the intricacies of the Coriolis effect on wind-driven currents. A wind blowing over a water surface exerts a force on the surface, and that force tends to drag or push the water in the direction of the wind. Surface currents of this kind are called pure drift currents. This is in addition to the more readily observable effect of generation of surface waves. The wind-driven current is in the direction of the wind, and that its effect decreases downward from
- 7.5: Geostrophic Motion
- All of the large-scale motions of the oceans and atmosphere, of the kind you would see on a weather map of North America or a chart of North Atlantic currents, owe their existence to horizontal pressure gradients: changes in pressure from place to place when viewed at the same altitude (in the atmosphere) or the same depth (in the oceans).
- 7.6: Ekman Layers
- There has to be something more to the phenomenon of geostrophic motion than what I have shown above—because the air somehow has to get down the pressure gradient, or else the pressure gradients would keep on increasing. Somehow there has to be movement of the air across the isobars, as in the more direct flow that would be set up in the absence of rotation. The way out of this dilemma is to include the effect of friction forces in the lowermost part of the atmosphere.