and an identical structure is mounted on the bottom half of the central pole. The three
blades of the top and bottom windmills are fixed at an equal circumferential angle of
120
, but a circumferential phase gap of 60
is given between the top and bottom
windmills. If both windmills are fixed in the same phase, it would be difficult to
initiate movement of the windmill; if the wind begins to blow in the direction facing
one blade, the remaining two blades are located at both sides in symmetry, and the
windmill cannot start to move since the moment of the two symmetrical blades is
balanced. Therefore, a phase gap is introduced between the top and bottom windmills. The results in Figs. 12.3 and 12.4 were obtained at the same site as that shown
in the photograph in Fig. 12.5.
Figure 12.6 illustrates the variation in relative wind velocity toward the blades
and the lifting force acting on the blades during an entire revolution of the windmill,
where a constant wind of a given velocity is blowing from the left-hand to the righthand side. The shape of the blade is based on a popular design for airplanes,
NACA4412, and the angle of attack is set as À2
. Wind velocity is represented by
a horizontal line (light blue arrow) of identical length at every point of the revolution.
During one revolution of the windmill, the blade has a certain velocity in the
tangential direction (dark blue arrow), even if the air does not move (i.e., the wind
does not blow). In such circumstances, the resultant velocity of the air toward the
blade has a different direction (green arrow), depending on its point within one
revolution of the windmill. The lifting force on the blade acts perpendicular to the
resultant velocity, as indicated by red arrows in Fig. 12.6. The acting direction of the
lifting force thus analyzed does not pass through the center of the revolution; a
Fig. 12.6 Variation in relative wind velocity and lifting force acting on blades during one
revolution of the windmill
12 Building a Global Low-Carbon Society Based on Hybrid Use of Natural Clean Energy 223
blades of the top and bottom windmills are fixed at an equal circumferential angle of
120
, but a circumferential phase gap of 60
is given between the top and bottom
windmills. If both windmills are fixed in the same phase, it would be difficult to
initiate movement of the windmill; if the wind begins to blow in the direction facing
one blade, the remaining two blades are located at both sides in symmetry, and the
windmill cannot start to move since the moment of the two symmetrical blades is
balanced. Therefore, a phase gap is introduced between the top and bottom windmills. The results in Figs. 12.3 and 12.4 were obtained at the same site as that shown
in the photograph in Fig. 12.5.
Figure 12.6 illustrates the variation in relative wind velocity toward the blades
and the lifting force acting on the blades during an entire revolution of the windmill,
where a constant wind of a given velocity is blowing from the left-hand to the righthand side. The shape of the blade is based on a popular design for airplanes,
NACA4412, and the angle of attack is set as À2
. Wind velocity is represented by
a horizontal line (light blue arrow) of identical length at every point of the revolution.
During one revolution of the windmill, the blade has a certain velocity in the
tangential direction (dark blue arrow), even if the air does not move (i.e., the wind
does not blow). In such circumstances, the resultant velocity of the air toward the
blade has a different direction (green arrow), depending on its point within one
revolution of the windmill. The lifting force on the blade acts perpendicular to the
resultant velocity, as indicated by red arrows in Fig. 12.6. The acting direction of the
lifting force thus analyzed does not pass through the center of the revolution; a
Fig. 12.6 Variation in relative wind velocity and lifting force acting on blades during one
revolution of the windmill
12 Building a Global Low-Carbon Society Based on Hybrid Use of Natural Clean Energy 223