- #1
Nurah
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Homework Statement
A long cylindrical antenna of 9 mm (0.009 m) diameter is mounted vertically on a car moving at 100 km/h (27.778 m/s). Antenna is 920 mm long (0.92 m). Calculate the power required to move antena through the air with standard kinematic viscositiy ν = 1.46 ⋅ 10-5 m2/s.
The given result is P = 104 W.
2. Homework Equations
F = 0.5 ⋅ ρ ⋅ v2 ⋅ CD ⋅ A
where:
F - drag force
ρ = 1.2 kg/m3 - density of the air
v = 27.778 m/s - speed
CD - drag coefficient
A - relevant area
P = F ⋅ v
where:
P - reguired power
3. The Attempt at a Solution
First, I tried to find drag coefficient. To do that, I must find Reynolds number:
Re = (v ⋅ d) / ν
Re = (27.778 ⋅ 0.009) / (1.46 ⋅ 10-5)
Re = 17123.4
According to the diagram, for infinitely long cylinders, CD ≈1
Also, relevant area is:
A = diameter ⋅ height
A = 0.009 ⋅ 0.92
A = 8.28 ⋅ 10-3 m2
Now, the drag force is:
F = 0.5 ⋅ 1.2 ⋅ 27.7782 ⋅ 1 ⋅ 8.28 ⋅ 10-3
F = 3.83 N
Required power is equal to:
P = F ⋅ v
P = 4.37 ⋅ 27.778
P = 106.4 W
Is this the correct way to calculate this? Am I getting 2 watts more just because of "not so accurate" reading from the diagram above?
Thanks.