Supporting arms and clamps drag losses
The loss caused by the aerodynamics drag force on the supporting arms
was estimated with the following drastic assumptions. First, the spanwise
(directed along the flat plates longitudinal axis) velocity component
was neglected (refer figure below), so the wind was assumed to hit the
flat plates at normal direction only. The spanwise wind component decreases
with increasing tip speed ratio, so the accuracy of this assumption increases
with tsr. Without this assumption, numerical integration will be required
because the drag coefficient data is not constant but depends on the angle
of wind relative velocity. The drag coefficients data is only available
for condition where the wind is directed normal to the plates, so other
assumptions might be needed. Second, the induced velocity was neglected.
This resulted in higher than actual velocity, so a higher drag force was
estimated.
Wind distribution along the support arms with spanwise velocity neglected
The drag loss by the clamps (bolt joints between airfoils and flat plates)
was estimated using the above same method. A simple geometry and ‘equivalent’
size were selected to replace the complex geometry of the actual clamps
for obtaining drag data. Result can be very much affected by this subjective
selection.
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