Phase II Final Report - NASA's Institute for Advanced Concepts

Chapter 3.0 Vehicle Design
3.3 Wing Aerodynamics
Figure 3-71: Pressure Contours, α = 45.79 o , Reynolds Number = 5,100, Time = 3.6848 s
3.3.1.4.1 Low Reynolds Number, High-α Results
A new cambered, thin wing modeled after the North American cicada wing was used in this
series of simulations. Chord Reynolds number was varied in the range 510-5,100. A high angle
of attack value was used to characterize the formation and evolution of the LEV. These timeaccurate
simulations were done using a very fine mesh and small integration time step. Both the
mesh size and the time step were varied and the corresponding simulation results compared to
ensure the results can be treated as independent of mesh size and time step.
Figures 3-72 and 3-73 show, respectively, the lift and drag coefficient variations for U = 1.4 m/s
(Reynolds number = 510, Case A) and α = 34.8 o . The corresponding plots for U = 14 m/s (Reynolds
number = 5,100, Case D) are shown in Figures 3-74 and 3-75. These four plots reveal several
interesting characteristics not emphasized in previous work by other investigators of low
Reynolds number, high angle of attack flow. Both cases show that there are distinct frequencies
associated with each, and both the lift and drag variations have fairly large amplitudes. The
salient features are summarized in Table 3-7. These results indicate that the flow is dominated
by the formation and shedding of the LEV. The LEV forms at the leading edge, stays attached to
the top surface and grows as it convects downstream. During this phase, the airfoil has a high C L
value, and then it drops as the vortex detaches from the surface, leading to the low C L phase of
the cycle. The c d variation has a phase difference of approximately 180 o from the C L variation.
From the summary results given in Table 3-7, several useful design guidelines can be drawn. For
example, for Case D (Reynolds number = 5,100) the dominant frequency is f ~ 7.23 Hz. To
investigate the relationship to the well known Karman vortex shedding from bluff bodies, the
Strouhal number, defined as
91