PowerFromActuatorDisk:

```--------------------------------------------------------------------------
Compute power and power coefficient using actuator disk theory.
This provides a limit on how much power a wind turbine can produce.
"a" is defined from the relationship v3 = v0*(1 - 2*a) where v0 is the
wind speed in front of the machine and v3 is the wind speed at the end
of the wake. Note that v3 = 0 is NOT the optimum value. The optimum
value is with a = 1/3 so cP = 16/27. This can never be achieved in
practice due to aerodynamic losses.

The rotor swept area is pi*r^2 for a HAWT and hd for a VAWT.

Type PowerFromActuatorDisk for a demo.

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Form:
[p, cP] = PowerFromActuatorDisk( rho, A, v0, a )
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------
Inputs
------
rho                 (1,1)  Atmospheric density (kg/m^3)
A                   (1,1)  Rotor swept area (m^2)
v0                  (1,:)  Wind velocity (m/s)
a                   (1,1)  See above

-------
Outputs
-------
p                   (1,:)  Power (W)
cP                  (1,:)  Power coefficient

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Reference: Wortman, A. J., "Introduction to Wind Turbine Engineering,"
Butterworth Publishers, 1983, pp. 3-4.
--------------------------------------------------------------------------
```

Children:

```Common: Graphics/Plot2D
```