風の構造,風速及びその測定に就きて
1937 年 15 巻 7 号 p. 265-277
詳細
1937 年 15 巻 7 号 p. 265-277
The wind does not flow as a laminar motion, but as a very complex turbulent motion. One of the characters of such turbulent motion is the socalled “ turbulence ”
Hence the present author tried to analyse the turbulence from the records of the daily observation of wind, such as those due to Dines' pressure tube anemometer and wind vane and obtained the following results.
(1) The velocity distribution of tubulence in steady state is given by
(2) Gustiness g, which is defined as the ratio of the velocity of turbulence to the mean wind velocity V, is given approximately by g=a+b/V
where a and b are appropriate constants.
(3) The greatest wind velocity in a given time interval, say an hour, is about twice of the mean velocity.
(4) g is greater in day than in night.
Generally the word “wind velocity” is not used in true sense of hydrodynamics, but it will mean some kind of average of the velocity of the air motion. In daily observation of wind velocity are used three anemometers that is, pilot balloon, Dines' anemometer and Robinson's anemometer. The motion of these anemometers are discussed here and it is found that they give different sorts of wind velocity so long as turbulence exists. The velocities which are measured by these anememeters are given respectively by
(1) Pilot balloon: Vp=uo
(2) Dine's anemometer:
(3) Robinson's anemometer: hence we have
Vp<Va<VrThe first means the velocity of the mass motion of the fluid, the second is the mean of the wind speed and not the mean of the velocity and the last is difficult to be explained physically, but it will be something like a mean energy of the wind. Thus, the velocity given by the Robinson's anemometer gives the greatest and that given by the pilot balloon the smallest, and hence, the Robinson constant shonld be taken smaller value than normal (about 0.8 times) to make the calculated value of velocity agree with the velocity obtained by pilot balloon. Then the wind velocity obtained by the Robinson anemometer will also become to have a rather natural meaning.
