Pressure distributions
Figure 27 to figure 31 shows the wind pressure distributions on the
blade surface obtained through a CFD analysis, and it is in the form of
contours. Here in the pressure contours, the maximum pressure obtained
is closer to the tip and less at the section near the hub. For each
velocity and pitch angles, pressure contours on both top surface
(pressure) and bottom surface (suction) of the blade are produced. As
can be seen from the figure, the negative pressure is observed at the
edge of the bottom surface (suction side), and the positive pressure
observed at the edge of top surface (pressure side).however as the pitch
angle of the blade changes, the position of blade shows some deviation
with the airflow which further cause stagnation point to change its
position and move towards the suction side and results with negative
pressure on the opposite side of the bladed due to reducing air
velocity. Now, in this section, the Distribution of pressure is
justified following the three-stall region. One is a pre-stall region
that occurs before 9 m/s; second is the dynamic stall region that comes
into play when wind speed is 13 m/s to 17 m/s and third is deep stall
region which occurs at velocity more than 20 m/s. As can be seen from
the graph that in the pre stall region (5m/s, 7 m/s), the pressure
distribution shows some significant change as the pitch angle of the
blade changes. When the velocity is 5 m/s, the trend of maximum pressure
decreases exponentially from 753.7 Pascal at -100pitch angle to 400 Pascal at 400 pitch angle. However,
the trend of minimum pressure shows some up and down variation, such as
at -100 the highest negative pressure is -1000 Pascal
followed by -811.7 Pascal for 100 pitch angle which
further shows some variation by changing the pressure to -938.4 Pascal
for 200degree pitch angle and subsequently ended to
-951.9 Pascal and -865.8 Pascal for the pitch angle
300 and 400. Now for 7 m/s, the
trend of maximum pressure shows the same variation as obtained from 5
m/s but for minimum pressure, the pattern is such that it first
decreases from -1417 Pascal at -100 pitch angle to
-811.7 Pascal at 100 pitch angle and then increases as
it progresses the pitch angle from 200 to
400 in step of 100. The position of
maximum pressure occurs at the center position near to the tip. The
variation of color tone is symmetric on both sides of the maximum
pressure location. Here in the contour diagram, the red color tone
indicates maximum pressure and dark blue reflects minimum pressure.
Whereas, yellow and green color tones are displaying the average
pressure range which can be seen at the maximum place over the surface,
may it be top surface or bottom surface. In the deep-stall region, for
the pitch angles 100 and -100, the
pressure distribution of the blade is nearly similar when the wind
velocity is 20 m/s and 25 m/s. however, the distribution of maximum
pressure and minimum pressure over the top surface of the blade shows
some discrepancy when the wind velocity reaches 25.1 m/s. the
distributions are such that the bottom surface is no more dominated with
blue color tone(highest negative pressure) as seen in the pre-stall
region and also when the velocity is 20 m/s and 25 m/s for a deep stall
region. The top surface in the other hand also shows some deviation in
the smoothness of the red color tone (highest positive pressure) and is
overshadowed by green color one near the root. Now, when pitch angle of
200, 300, 400 was
taken and deep stall velocities are considered for the analysis, the
pressure distributions show high deviation from the pre-stall one. With
an increase in pitch angle, all deep stall velocities possess variation
in pressure distribution.
Moreover, in the deep-stall region, the effects of the leading-edge
suction become insignificant, and strong suction occurs at the isolated
small area of the leading edge near the hub, which has little influence
on the aerodynamic force. Here, a greater differential of the pressure
distribution on the blade surface is mainly due to the three-dimensional
rotation effect and flow separation on the blade. As can be seen from
the graph above, the position that is closer to the root of blade
exhibit larger angle of attack, which results in stalling of airfoil and
flow separation, which further leads to Separation of the vortex at
trailing edge and thus create a pressure distribution on the surface of
the blade and induced a violent pressure fluctuation will be induced.
Because of this, pressure distribution shown in figure 27 to figure 31
exerts irregular in color band, especially near the trailing edge. A
greater differential is shown at the leading edge than the trailing
edge, especially near the tip of the blade. There is a low-pressure area
at the tip of the blade on the pressure surface of the blade. It is
mainly caused by three-
Dimensional rotation effect at the tip of the blade. In this area, part
of the air flows from the pressure surface to suction surface through
the tip of the blade