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Atmospheric Winds.-A regular wind is one which has a constant velocity (speed and direction of course), and such an ideal wind is a convenient standard for the comparison of atmospheric winds. The artificial wind of a research wind channel may be regarded as the nearest approximation to a regular wind. The fluctuations of the speed of the wind in the wind channel at the National Physical Laboratory are easily maintained within one-half per cent. of the mean wind speed.
The range of speed variation of a “ steady” atmospheric wind is roughly proportional to the mean speed of the wind, and positive and negative speed fluctuations of 25 per cent. of the mean speed, with occasional speed excursions of 35 per cent., are to be expected, so that a steady wind of a mean speed of 20 miles per hour will probably have a speed range of 13 miles per hour to 27 miles per hour. A steady wind also has fluctuations of direction, although no definite connection appears to exist between the speed and the direction fluctuations. Low surface winds are usually of a “gusty” nature, chiefly due to the eddy motion created by obstacles.
A horizontal eddy—that is, a revolving roller of air with a horizontal axis—may be encountered at the horizontal edge of a high vertical cliff, if the general wind impinges directly upon the face of the cliff, whilst a well-defined vertical edge of such a cliff will favour the formation of a vertical eddy. If the fluctuations of the wind velocity are much greater than the range of the normal gust excursions of a steady wind, the wind is said to be “ squally.” The most dangerous winds encountered in England are termed Line Squalls, and they are usually accompanied by a sudden fall of temperature, a rise of pressure, a veer of the general wind direction, and a squall of hail, rain, or snow. Line squalls, so called because they advance with the characteristic line front of a tidal wave, traverse the surface of the country at a great speed. These squalls, which would have a disastrous action upon air-craft, are probably formed by the spreading of the upper cold air layers before a rising warm air current.
On account of surface irregularities, ground winds are of an entirely different character from the winds of the upper atmospheric regions. The speed and steadiness of a surface wind increase with the height above the earth's surface, until the wind velocity reaches a more or less limiting value. There is evidence to show that the gustiness of surface winds is due to the effect of the ground, and it is convenient to assume that gustiness disappears, and the limiting velocity is reached, in the air strata which are removed from ground influences. The upper layers probably have a regular periodic motion.
A surface wind blowing in from the sea has its velocity continuously diminished as it passes inland, on account of the retarding influences of surface friction and obstacles.
The experience of balloonists and pilots has proved the existence of vertical atmospheric currents. The formation of clouds, as is well known, depends upon the rising of warm moist air currents into the colder upper regions, although the absence of clouds does not necessarily imply the absence of ascending air currents. Occasionally, the velocity of these ascending currents is sufficiently great to support large hailstones. A cumulus cloud is generally the cap of an ascending air current, and a cumulus sky may be regarded as a dangersignal indicating the prevalence of ascending and descending air currents. Rising currents, chiefly formed by the local heating of the earth's surface by the sun's rays, are always accompanied by descending air currents. The low-pressure centre of a cyclonic disturbance is also partial evidence of the existence of an ascending current, and, conversely, the highpressure core of an anticyclone indicates the presence of a descending air current.
There are no well-pronounced atmospheric signs to indicate
presence of descending air currents, although the bright fine weather which occasionally follows a rainy period is probably due to descending currents. Cyclonic and anticyclonic motions have no resemblance to those perfect columns of rotating air, the product of the imagination. Moreover, most eddies are of short duration, as the low pressure of the core can only be maintained in most exceptional circumstances. Horizontal eddies, which are certainly dependent upon peculiarities of the earth's surface, are of less frequent occurrence than vertical eddies.
Wind Velocity.- Pitot and Static Pressure Tube. The velocity of the wind may be determined very accurately by a Pitot and Static Pressure tube. A sketch of such a tube is given in fig. 1. The inner tube or Pitot tube A faces the wind, and is surrounded by a concentric tube B.
The annular space C between the two concentric tubes is connected to the outside air by a series of small holes D.
To measure the velocity of the wind, the tube A is connected to one end of a sensitive tilting water gauge and the annular space B to the other end of the gauge, and the pressure difference is balanced by a head of water. Adopting consistent units, let
v=velocity of the wind,
h=head of water. Now, it has been proved experimentally that the pressure reading of the annular space equals p, and the pressure reading
1 of the Pitot tube
2 Hence, when the pressures have been balanced by the head of water, we have
be the pres
Pitot and Suction Tube.—Fig. 2 is a sketch of a Pitot
and Suction tube commonly employed upon aeroplane. If
Pi sure at the mouth E of the suction tube F, then, adopting the same notation as formerly,
It has been experimentally
established that the difference of the pressure readings of the two tubes F and B is
1 proportional to 2 puz, and hence,
(p-pa)=(K – 1)(2002). We see, then, that the difference between the static pressure and the pressure in the cone at the point E is proportional to v2.
A Comparison of Forces acting upon Similar Bodies. Let two similar bodies A and B be similarly situated in two different fluids. For the body A let L represent a linear dimension, F represent a force, and V represent the velocity of the body relatively to the fluid. Also let pa=density, and va=coefficient of kinematic viscosity of the fluid surrounding body A.
The coefficient of kinematic viscosity is equal to the ordinary coefficient of viscosity divided by the density of the fluid.
Then for the body B let l represent a similar dimension, and f the force corresponding to the force F of the body A, also v the velocity of the body relatively to the fluid. Assume Po to be the density, and vg the coefficient of kinematic viscosity of the second fluid.
VL vl Now, if
F_Kp L2V2 then
f Kpl?v2 if we ignore the compressibility of the fluids. This statement may be deduced from a mathematical discussion of dimensional equations.
Moreover, the flow around the body A will be exactly similar to the flow around the body B, and the photographs of the flows, when taken upon plates of the same size, will be exactly the same.
Application of the Preceding Discussion to the Case of Air.For the same fluid PA=Pb=p, say, and
VA=vb=v, say; and if we make vl=VL, and also ignore compressibility, then
1202 When the wind speed is above 125 miles an hour, compressibility effects, although very small, may have to be considered. The accurate determination of air forces acting upon full-sized machines may be deduced from the results of experiments upon models, if the product of the linear dimension and the velocity for the machine is the same as that of the model.
Pressure upon Square Plates.—The pressure upon a square plate may be written at P=KV2, where P is in lbs. per square foot, and V is in feet per second.