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Three-Dimensional Wing Aerodynamics
Low Reynolds number flyers use low AR wings, typically no larger than 5. For the MAVs developed by Ifju et al. [17] the AR is close to 1. Consequently, it is important to investigate the 3D flow structures around a low AR wing at low Reynolds numbers.
Lian and Shyy [154] and Viieru et al. [155] reported flow structures around a low AR rigid wing. The geometry follows the design of Ifju et al. [17] as discussed earlier. The wing has a span of 15 cm, a camber of 6 percent, a root chord of 13.3 cm, and a wing area of 160 cm2.
To confirm the capabilities of the Navier-Stokes solver, the computational results are first compared with wind-tunnel data measured for a MAV rigid wing with a
12.5 cm span, which has a smaller planform area than those used by Lian and Shyy [154] and Viieru et al. [155]. However, the overall shape and AR are similar. The experiment is conducted in a horizontal, open-circuit low-speed wind tunnel. It has a square entrance of a bell-mouth-inlet type, and it has several screens that provide low turbulence levels, less than 0.1 percent, in the test section. The test section is
91.4 cm x 91.4 cm and has a length of 2 m. The model under test is attached to a six – component strain-gauge sting balance used to measure the aerodynamic forces and moments. The AoA is controlled by computer and can be set in any sequence, steady or variable, in time. The force balance is calibrated from 1 gram to 500 grams, from precisely defined loading points. For more detailed information on the experimental measurements and uncertainty, we refer to Albertani et al. [156].
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![Подпись: 0 Figure 2.35. Phase and shape factors during one gust cycle on an SD7003 airfoil at the nonsteady Reynolds number, Rens = 1.98 x 104: (a) phase; (b) shape factor [117].](/img/3131/image172_4.gif)
The 12.5 cm wing configuration is tested at two different Reynolds numbers (7.1 x 104 and 9.1 x 104) based on the root chord length. The experimental data are obtained by averaging the values from multiple tests for each AoA and Reynolds number. In Figure 2.37a the lift versus drag curves are plotted for the two Reynolds numbers just mentioned. The figure demonstrates agreement between the computational and experimental data. As shown in Figure 2.37b, within the considered Reynolds number range, the lift-to-drag ratio does not vary much. Furthermore, both experimental and computation data show that the best lift-to-drag ratio is reached for an AoA between 4° and 9°.
