Multiple streamtubes momentum model for Darrieus rotor
– continued 2nd page
Airfoil velocity and force diagram (important sign convention)
The value of induction factor has to be solved numerically by some root
finding algorithm. First, the starting value of induction factor is guessed.
With the induction factor known, the value of the normalized velocity,
angle of attack, lift, drag, normal, and tangent coefficient can be calculated.
The first thrust coefficient derived from the aerodynamics forces is calculated
next. Then, the second thrust coefficients derived from the wind momentum
loss rate is calculated. If the guessed value of induction factor is correct,
both thrust coefficient values will be the same. If not, then guess another
value of induction factor and repeats the previous calculation. The induction
factor normally falls in the range between 0 to 0.5 so the starting value
should be set in here for less iteration. Once the induction factors for
all the streamtubes are found, the torque and power coefficient can be
calculated as below.
References and related papers
J.H. Strickland (1975) "The Darrieus Turbine: A Performance Prediction
Model Using Multiple Streamtubes" SAND75-0431
G.F. Homicz (1991) "Numerical Simulation of VAWT Stochastic Aerodynamic
Loads Produced by Atmospheric Turbulence: VAWT-SAL Code" SAND91-1124
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