Aerodynamics and Design for Ultra-Low Reynolds Number Flight

Chapter 4
of the pitch angle at the rotor with the vertical ring spacing held constant. Other models
are possible, such as varying the separation at fixed strength, but this model is straight
forward and allows the wake length to be rigidly defined as an input.
This procedure is repeated until the wake structure reaches equilibrium, typically five to
ten cycles. The ring model extends downstream for five rotor radii. The variations in
the computed induced velocities appear to be negligible for larger wake lengths while
any increase in length also increases the computational cost due to additional rings. The
equilibrium wake form is axisymmetric and symmetric from end to end, with a ‘bell-
mouth’ at the stream-tube exit equal to the rotor disk area. Additional wake length
moves the exit ‘bell-mouth’ further downstream but does not significantly affect the
solution and adds to the computational expense. The initial and converged ring
configuration for a candidate rotor are displayed in Figures 4.5 and 4.6. The rotor plane
is at x=0 and the streamtube exit is below the lower extent of the figures.
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