SOLUTION Experimental Question 1 page 1 of 11 WIND POWER AND ITS METROLOGIES (20 points) A. Theoretical Background (1.0 points) A.1 (0.4 pts) 2 0 1 2 w v dm P dt = 0 0 3 1 2 w P Av ρ = n = 3 A.2 (0.4 pts) ( ) 2 3 3 2 3 0 0 1 (1 )1 2 2 (1 ) 4 R v P A Av ρ λ λ λ λ λ ρ + − + − = − = 2 0 1 2 3 0 R dP d λ λ λ → − − = = 1 3 λ = A.3 (0.2 pts) Betz efficiency: 16 2 ~ 59.26% 7 P W R C P P λ = = B. The Wind Tunnel (3.2 points) B.1 (0.8 pts) We move the motor generator blade manually, and the voltage at the opto-sensor signal will increase every time the sensor hits the reflective sticker in the blade. This signal provides frequency signal to the meter. V(V) B.2 (2.4 pts) 0 0 0 0 2 2 A A M M M M n n W M P P P P Av Av ρ ρ η η = = → = Wind tunnel diameter: D T = 13.5 cm. 2 2 0 0.0143m T A R π = = . 0 ln ln ln 2 A M M A n v y a b P x ρ η = + → = + From the plot and linear regression below we obtain the power factor: 3.0 n = , in good agreement with the theory thus showing that the wind
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SOLUTION
Experimental Question 1
page 1 of 11
WIND POWER AND ITS METROLOGIES (20 points)
A. Theoretical Background (1.0 points) A.1 (0.4 pts)
20
12w v dmP
dt=
0 031
2wP A vρ=
n = 3 A.2 (0.4 pts) ( )2
3 32 3001 (1 ) 1
2 2(1 )
4RvP A Avρλ λ λ λ λρ + −+ −= − =
20 1 2 3 0RdPd
λ λλ
→ − −= =
13
λ =
A.3 (0.2 pts)
Betz efficiency: 162~ 59.26%
7PW
RC PP λ
= =
B. The Wind Tunnel (3.2 points)
B.1 (0.8 pts)
We move the motor generator blade manually, and the voltage at the opto-sensor signal will increase every time the sensor hits the reflective sticker in the blade. This signal provides frequency signal to the meter. V(V)
B.2 (2.4 pts)
0 0 0 0
2 2A A
MM M M
n nW
MP PP P
A v A vρ ρηη
= = → =
Wind tunnel diameter: DT = 13.5 cm. 2 2
0 0.0143mTA Rπ= = .
0ln ln ln2A
MM
A n v y a bP xρη
= + → = +
From the plot and linear regression below we obtain the power factor: 3.0n = , in good agreement with the theory thus showing that the wind
SOLUTION
Experimental Question 1
page 2 of 11
power PW ~ v3.
0
22.8%A
M aeAρη = =
Results for full score / grading scheme (sampling from several setups):
(3 2)%Mη = ± , to anticipate wide variability in motor quality. Connection diagram:
Note that for best results the voltmeter has to be placed right across the motor to avoid extra voltage drop across the amperemeter.
C. Ping Pong Ball Anemometer (3.5 points) C.1 (0.7 pts)
Force diagram at static equilibrium:
tan2
mD D B
B B
F C A vW m g
ρθ Α= =
t n2 aB
D A
mvC
gA
m θρ
=
C.2 (2.8 pts) 2
ln tan ln lnD A B
BmC A m v y a bx
gρθ = + → = +
The deflection can be calculated from: tan /x hθ = Δ , where Δx is the displacement and h is the height from the ruler. The cross section of the ball is: 2 / 4B BA dπ= where 0.0395mBd = . In the tunnel the wind velocity is given as (Eq. 4) in the tunnel 1 Mv c f= where 1 0.0873mc = . In this example we have:
2.2m b= =
SOLUTION
Experimental Question 1
page 4 of 11
2 0.40aBD
A B
gC eAmρ
==
Results for full score / grading scheme (sampling from several setups):
NOTE: These values are in very good agreement with the theoretical and established value of n = 2, i.e. the drag force is proportional to the square of the velocity. The drag coefficient CD is in close to the ideal known value: CD = 0.47 for smooth ball with particle Reynold number Re ~ 103 – 105 as shown below. In this experiment the maximum Reynold number is:
SOLUTION
Experimental Question 1
page 5 of 11
6
1.2 2.5 0.038Re 624018.3 10
BB
v DρµΑ
−Α
× ×= = =×
(1)
Discrepancy in our CD values could be due to finite boundary of our wind tunnel.
Figure 1. Drag coefficient as a function of the particle Reynold number
D. Hotwire Anemometer (HWA) (6.7 points)
[D.1] Constant Temperature (3.2 points) D.1.1 (0.4 pts)
In the Wheatstone bridge we have:
1W
W INP INPW B
RV V c VR R
= =+
22
0 021
22
0 2
( ) ( )( )
( ) ( )( ) (
( )
)
c cW Ww w INP W W
W
c cW BINP W W
W
RA T T V A T T a bvR c
R RV A T T a bv c a
a
b
V b
R
v
v
− → = − +
+= − + = +
= +
Where 2
2 0( ) ( )W B
W WW
R Rc A T TR+= − is a constant.
2
2
c aBAc b
==
D.1.2 (0.3 pts)
2
0
1INPVyV
⎛ ⎞= −⎜ ⎟⎝ ⎠
with 0V A= is the input potential when there is no
We use DMM in ohmmeter mode, we first measure the lead resistance of the ohmmeter cables by shorting them to get the lead resistance. RDMM,0 ~ 0.2 Ω Then we measure the resistance of the motor directly with DMM several times, looking for its minimum values and subtract it with the lead resistance RDMM,0. We get :
(0.8 0.2)MR = ± Ω
E.2 (1.2 pts)
To eliminate the contact or lead resistance we can perform: (1) Two point resistance measurement at several lengths. (2) Four point resistance measurement with current source (using
CCA box) ampmeter and voltmeter as shown below.
This yields: 0(0.21 .02) / cmRλ ± Ω=
SOLUTION
Experimental Question 1
page 9 of 11
(a) Two-wire method (b) Four-wire method
Method #1: Using two-wire measurement
Method #2: Using four-wire method at l = 250 mm.
V (mV) I (mA) 198.5 37.5 169.2 31.9 131.2 24.7 103.5 19.4
78.3 14.6 69.4 13
y = 5.2614x + 1.2869R² = 1
0
50
100
150
200
250
0 5 10 15 20 25 30 35 40
V (m
V)
I (mA)
Nichrom length = 25 cm
E.3 (2.4 pts)
We vary the nichrom wire length and calculate the power output, we obtain peak at: 1.0LR = Ω
SOLUTION
Experimental Question 1
page 10 of 11
Results for full score / grading scheme (sampling from several setups): (1.0 0.4)LR = ± Ω
Our turbine has range of efficiency <2.5% and tends to increase with higher TSR as the turbine spin faster. Note that at some point this efficiency will drop, unfortunately this is beyond the capability of our wind generator fan.