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1892 LOPERA ET AL. 0.6 0.5 0.4 0.3 CL 0.2 0.1 0.0 -0.1 0 4 8 12 16 20 24 28 α Fig. 8 angles on the coefficient of lift vs angle of attack for baseline (control off) configurations. CL 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -12 -8 -4 0 4 8 12 16 20 24 28 α No ramp 20-deg ramp 30-deg ramp 40-deg ramp Off On: 25% duty cycle Comparison of the effect of windward-surface backward ramp Coefficient of lift vs angle of attack; plasma actuators pulsed Fig. 10 with a 25% duty cycle, F 1, fmod 395 Hz; 20-deg windward- surface separation ramp. Off On: 12.5% duty cycle Off On: 100% duty cycle (t d) CL 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -12 -8 -4 0 4 8 12 16 20 24 28 α 0.5 0.4 0.3 0.2 0.1 CL Coefficient of lift vs angle of attack; plasma actuators pulsed 0.0 -0.1 -0.2 -0.3 -0.4 -12 -8 -4 0 4 8 12 16 α Coefficient of lift vs angle of attack; plasma actuators pulsed Fig. 9 with a 12.5% duty cycle, F 1, and fmod 395 Hz; 20-deg windward- surface separation ramp. Fig. 11 with a 100% duty cycle (steady); 20-deg windward-surface separation ramp. baseline (actuator off) configuration. In our study, we conducted experiments to compare the effect of different windward-surface separation-ramp angles, including a baseline (no ramp) case, to assess the aerodynamic performance of the air vehicle under different windward configurations. Results in Fig. 8 show that, overall, the use of a windward-surface separation ramp does not have a detrimental effect on the baseline lift coefficient. Results show that the separation ramp leads to a slight increase in the lift coefficient at lower angles of attack.Forhigheranglesofattack,>16 deg,resultsshowthatthe 40-deg ramp results in a considerable increase in the lift coefficient when compared with the baseline design. Drag measurements showed small difference in the drag coefficient at low angles of attack (which is the flight envelope for the UAV design examined) for the baseline (no ramp) case and the modified UAV designs with different windward-surface separation-ramp angles. 2. Effects of Ramp Angle and Duty Cycle The first separation-ramp angle tested was a 20-deg windward- surface ramp near the trailing edge. Figure 9 shows the lift coefficient for a baseline case (actuator off) and a controlled case (actuator on), with the plasma actuators operating in an unsteady mode at a 12.5% duty cycle. Results show that the plasma actuators produced a consistent shift in the CL– curve for all angles of attack from 2 to 22 deg. The effect of the windward-surface plasma actuator is a decrease in the CL , ranging from 6 to 25% from 2 to 20 deg. Figure 10 shows results for a case when the plasma actuators were pulsed at a 25% duty cycle. Similar reductions in the lift coefficient were observed for 2 to 20 deg. In this case, the most significant changes were observed from 2 to 18 deg, with a decrease in the lift coefficient ranging from 5 to 30%. Figure 11 shows results from a test conducted with a steady plasma actuator. Results show that a steady plasma actuator has a negligible effect on the lift coefficient. These results demonstrate that a pulsed (unsteady) actuator has a stronger effect on the flowfield than a steady actuator. It has been conjectured that an unsteady actuator produces a series of vortices convecting downstream. When the pulse frequency produces a sufficient number of stable vortices over the surface continuously, flow reattachment occurs. This finding led us to conduct only unsteady plasma actuation experiments for the subsequent investigations (30- and 40-deg ramps). Subsequent studies showed that the most significant control, measured by a decrease in CL, was achieved by operating the unsteady plasma actuators at a duty cycle of 12.5%. Additional tests were conducted to verify this effect of the unsteady plasma actuators at a 12.5% duty cycle by ensemble averaging three data sets. Lift coefficients for the three separate tests were ensemble- averaged for the baseline and controlled cases and are presented in Fig. 12. Noticeable reductions in the lift coefficient are observed for 0 to 20 deg. Results show that the plasma actuator decreases the lift coefficient by 4.8 to 25% for 4 to 20 deg. These results validate the use of a plasma actuator mounted at the onset of aPDF Image | Aerodynamic Control Using Windward-Surface Plasma Actuators on a Separation Ramp
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