.4. Control efficiency analysis
Firstly, the flow control effect of the computational model at different angles of attack is analyzed. The cruising parameters of the aircraft are Ma=0.60and Re=21.28 ×106. The jet actuators (R11–R14) are selected and the single actuator jet parameters are set at cμ=0.00408%, F+=1.0583, and the jet deflection angle is set at θjet=90◦, and normal jet control is adopted. Fig.4shows a comparison of the model pitching moment characteristics before and after flow control. It can be seen that under small angles of attack, the flow is attached and the flow control technology has no
Fig.2.Schematicdiagramofactuatorlayoutinthemodel.impact on the aerodynamic characteristics of the model. As the an-gle of attack is increased, the onset of flow separation is observed. More specifically, the applied jet at the leading edge of the model has beneficial effects on the main flow field, and flow separation is weakened and aerodynamic characteristics of the model are im-proved. The slope reversal angle of pitching moment also increases from 10◦to 12◦leading to improved performance in the turning region.
Fig.5shows the streamline distribution on the model surface before and after flow control (α=12◦). It can be seen that the jet disturbance has no significant influence on the inner wing sec-tion of the model and the pressure coefficient distribution displays no obvious changes before and after control. The reason is that the inner wing section is in a stable attached flow field and the jet disturbance will be quickly extinguished in the main flow field. The jet disturbance has a much larger influence on the outer wing sec-tion, which is located in the separation flow field that develops from the trailing edge to the leading edge. There is an amplifi-cation of disturbance within the sensitive position of the leading edge, hence, separation flow of the outer wing section is effectively weakened and the pitching moment performance of the model is improved by coupling and development with the main flow field.
Fig.6presents the arrangement of typical spanwise and chord stations on the model surface. Fig.7shows a comparison of the time-averaged pressure coefficient distribution of model at typi-cal spanwise stations before and after flow control (α=12◦). After flow control, the suction area of the upper wing surface at typi-cal jet locations is significantly enhanced and the influence of jet disturbance is extended from the leading edge of the wing to the trailing edge, with the jet locations moving to the wingtip. This produces beneficial effects downstream from the jet locations. The jet turbulence enhances the energy in the shear layer of the lead-ing edge, and decreases and weakens the flow separation of the outer wing section of the model. This not only improves the lift performance, but also provides part of the pitch down moment di-rectly as the jet impact area is located behind the center of gravity, thus improving the pitching moment performance.
In order to explain the effect of synthetic jet control on the global forces and moments, the velocity field was presented in the plane perpendicular to the leading edge of UAV. Fig.8shows time-averaged velocity vector fields near the actuator’s slot for the baseline and controlled configurations at station of 2y/b =0.326. The baseline data show signature of a typical leading edge vortex. Actually the flow resembles a detached shear layer with a mas-sive region of stagnant flow above the wing. With the synthetic jets activated, the reverse flow is completely eliminated at leading edge near the actuator slot. The activation of synthetic jet dramat-ically increases velocities near the wall making the profile fuller; the profile’s shape now is comparable to the attached flow form. Equally noteworthy is the elimination of the separation region due to control, indicating that aerodynamic performance has been im-proved at these conditions.
Fig.9(a–d) show the velocity fields near the actuator slot at dif-ferent times during one cycle beginning with zero blowing veloc-ity(a), progressing to maximum blowing at one-quarter cycle(b),