In the previous paper, the author investigated on windbreak nets and relatively compared with vertical and horizontal profiles of wind speed formed by various single-windbreak nets in a wind tunnel (Maki, 1982). In this paper, an experiment was carried out in the wind tunnel using four representative windbreak nets and the relations among the net density, decreasing rate of wind speed, minimum and maximum wind speeds and so on were obtained.
The results were mainly as follows:
(1) In the case of successive windbreak nets at every 13
H times (distance expressed as a multiple of the net height,
H=15.0cm), the effect of decreasing wind speed was accumulated up to the third net at 30 to 40% of density (
D) and to the second net at about 50%, and the accumulation of decreasing wind speed was not recognized at about 75%. The relative efficiency of windbreak goes down in successive net number.
(2) The effect of decreasing wind speed at about 10m/s (200rpm of fan rotation number) is higher than those at about 4m/s (80rpm) and 25m/s (500rpm).
(3) The spreading windbreak nets in a region of paddy rice cultivation in Hokkaido are vinylone cheese cloth (CC-110) and polyethylene russell (PE-9G) ones, however, the effectiveness of the former is higher than that of the latter. As the relative minimum wind speed is about 40% around 2
H for CC-110 and the effective area below 50% is by about 10
H in the open field, it is considered that CC-100 is more reasonable than CC-110 for the purpose of spread of effective area.
(4) According to the results of experiment in the wind tunnel and of observation in the open field, the effective distance of settled successive nets seems to be 35 to 45
H for CC-110 and 40 to 45
H for CC-100. It is necessary to select the variety of net on the basis of limiting wind speed to crop protection. The net interval is 35 to 45
H in the cases of 40 to 80% of
D, i.e., for 2m-high windbreak net of CC-100 or CC-110, about 80m at the paddy rice field.
(5) The minimum wind speed (
uri) decreases with the increase of
D. The
uri value deteriorates with the increment of successive net number at the lower
D for CC-200 and PE-9G, on the contrary at the higher
D for PE-18G,
vice versa. At the middle
D for CC-110,
uri decreases up to the 2nd net, but recovers slightly at the 3rd net. The distance of appearance point of
uri in the leeward side diminished with the enlargement of
D.
(6) The maximum wind speed (
ura), and the distance (
x axis) and height (
z axis) of the appearance point of
ura increase linearly in proportion to
D. The wind speed and the distance of
x direction decrease in the order of the 1st, 2nd and 3rd successive nets, on the other hand, the height of
z direction enlarges.
(7) The wind speed ratio (
u1/
u-1) decreases,
u2/
u1 decreases below about 50% of
D and increases over that, and
u3/
u2 increases, with the increment of
D. The variation of wind speed ratios at 200rpm is more moderate than that at 80rpm. Although the order of
u1/
u-1 is the same as that of
D at the 1st net,
u3/
u2 is the opposite at the third. Notations are same as in Fig. 8.
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