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Glossary
Discharge or Flux
'Discharge', 'Flux' or the 'rate of fluid flow' is the volume of fluid which passes through a given area per unit time. Given an area A, and a fluid flowing perpendicularly through it with uniform speed u, then the flux Q is:
Q = u · A
If the velocity of the fluid incides on the area with an angle ¸ (away from the perpendicular), then the flux is
Q = A(cosθ)u
The average discharge of the river Rhine in the Netherlands is approximately 2200 cubic meter per second.
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Doppler
The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. For waves, such as sound waves, that propagate in a wave medium, the velocity of the observer and the source are reckoned relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from both motion of the source and motion of the observer. Each of these effects is analyzed separately. The effect was first analyzed by Christian Andreas Doppler in 1842. In 1845 the Dutch meteorologist Christoforus Buys Ballot tested his analysis for sound waves by standing next to a rail line and listening to a car full of musicians as they approached him and after they passed him. He confirmed that sound's pitch was higher as the sound source approached him, and lower as the sound source receded from him, to the degree that he had predicted. Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848 (in France, the effect is sometimes called "effet Doppler-Fizeau"). It is important to realize that the frequency of the sounds that the source emits does not actually change. To understand what happens, consider the following analogy. Someone throws one ball every second in your direction. Assume that balls travel with constant velocity. If the thrower is stationary, you will receive one ball every second. However, if he is moving towards you, you will receive more than that because there will be less spacing between the balls. The converse is true if the person is moving away from you. So it is actually the wavelength which is affected; as a consequence, the perceived frequency is also affected. If the moving source is emitting waves with an actual frequency f0, then an observer stationary relative to the medium detects waves with a frequency f given by: f =f0(V / V - Vs) (where V is the speed of the waves in the medium and Vs is the speed of the source with respect to the medium (positive if moving towards the observer, negative if moving away)
Source (edited): Wikipedia |
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