3. TUNNEL TEST - RESULTS AND
DISCUSSIONS
3.1 Discussion
on the equipment and the wind tunnel
Calibrating wind speed from anemometer to wind
tunnel pitot tube
The wind speed measured from anemometer and pitot tube are shown in Table
1. The wind speed readings from anemometer were taken from the testing
area just before the tunnel contraction part while the wind speed readings
from pitot tube were from the small area just after contraction as shown
in Figure 1. The calculation for the wind velocity from pitot tube water
height difference is shown in Calculation 1. The result in Table 1 shows
that the anemometer overestimated the wind speed if the pitot tube is
considered accurate. The pitot to anemometer ratio increased slightly
with wind speed but average ratio of 0.812 were used through this report
to correct all the wind speed values.
| Tunnel |
Velocity from |
|
Pitot tube |
Velocity |
Pitot to |
| rpm |
anemometer (m/s) |
|
water height |
calculated |
anemometer |
| |
at front |
at test area |
difference (mm) |
from pitot (m/s) |
ratio |
| 1000 |
2.75 |
3.15 |
36.75 |
2.54 |
0.806 |
| 1100 |
3.10 |
3.50 |
46.00 |
2.84 |
0.811 |
| 1200 |
3.45 |
3.75 |
53.75 |
3.07 |
0.819 |
Table 1: Comparison of wind speed readings from
pitot tube and anemometer
Figure 1: Rough sketch of wind stream in the tunnel
Calculation 1. Wind speed from wind tunnel’s
pitot tube
Wind distribution in tunnel with opened side door
The wind sucked into tunnel from the side opening was continuous and very
constant. The fluctuation range was less than 0.2m/s (±2% from
wind speed of 4.5m/s) as observed from readings taken before and after
testing (Refer Table 2). The actual fluctuation might even be lower as
the orientation of the anemometer can easily alter the readings. However,
the wind followed a curved path, as sketched in Figure 1, so the rotor
have to be positioned as backward as possible to avoid the direction problem.
Also, the wind was not uniform horizontally over the cross section. Table
2 shows that the measured wind speeds decreased from centre to the wall
opposite to the opening side. On the opening side, the wind speed became
slower and very turbulent as the plastic side wall obstructed the wind
flow as shown in Figure 1. The horizontal variation of wind speed across
the rotor swept area did not exceed 1.0m/s while the vertical variation
remained quite constant. The clearance between the rotor and the wall
opposite to opening was ½ times the rotor radius. The wind managed
to cover nearly 1.5 times the width of rotor.
| Tunnel |
Wind speed in front of rotor from |
|
|
Uncorrected |
Corrected |
| rpm |
anemometer (m/s) |
|
|
average |
average |
| |
Left column |
Centre |
Right column |
speed (m/s) |
speed (m/s) |
| 1000 (before) |
4.6-4.7 |
4.2-4.3 |
4.0-4.1 |
|
|
| (after test) |
4.7-4.8 |
4.2-4.4 |
3.9-4.1 |
4.33 |
3.52 |
| 1100 (before) |
4.9-5.0 |
4.6-4.7 |
4.4-4.5 |
|
|
| (after test) |
5.0-5.1 |
4.7-4.7 |
4.3-4.4 |
4.69 |
3.81 |
| 1200 (before) |
5.4-5.6 |
4.9-5.1 |
4.6-4.7 |
|
|
| (after test) |
5.4-5.6 |
5.0-5.1 |
4.6-4.7 |
5.06 |
4.11 |
Table 2: Wind speeds before and after testing
with correction of 0.812
|
