Aerodynamic Characteristics And Flow Control On A Rectangular Wing At Lowreynolds Numbers

It is found that the flight environment in thin atmosphere and at low Reynolds numbers leads to the drop of HALE UAV(high-altitude long-endurance unmanned aerial vehicles) aerodynamic performance, and as a result, how to improve it with flow control has drawn greater attention of researchers. An experimental investigation on the aerodynamic characteristics and flow control of a NACA633-421 rectangular wing at low Reynolds numbers is conducted in a low speed wind tunnel of NUAA with force measurement, pressure measurement, PIV(particle image velocimetry) technique and near-wall flow field hotwire test. It is shown from force measurement result that a sharp deterioration of aerodynamic characteristics of NACA633-421 wing occurs with the Reynolds number decreasing(Re<1.4E5), which includes severe losses of the maximum lift coefficient with respect to higher Reynolds numbers and the sharp decrease of stall angle. Pressure distribution on upper surface reveals that the emergence and development process of a laminar separation bubble(LSB) which moves forward to the wing leading edge and the length of which becomes shorter with the angles of attack increasing and bursts at the stall angle finally cause the sharp deterioration of aerodynamic characteristics.The SJ(synthetic jet) is used to control the flow structure of laminar separation bubbles as a result of which the maximum lift coefficient increases from 0.59 to 1.1 and the stall is delayed by 11 degrees and the maximum lift-drag ratio is improved by 13.6% meanwhile at Re=1.2E5. The active flow control of SJ possesses frequency selectivity in which case the SJ of which the frequencies are between 200 Hz to 400 Hz have the best flow control effects to promote the transition of the separated shear layer and to shorten the length of laminar separation bubbles.By contrasting the static and dynamic performance of separated shear layer in LSB structure before and after SJ control, the mechanism that how synthetic jet affects the laminar separation and manipulates the flow at low Reynolds numbers has been investigated. The result shows that free shear layer becomes highly unstable after separating from wing surface and induces the rolling up of big-scale vortices in the transition process. These vortices flap to the wall, leading to reattachment and time-averaged flow field structure keeps the same with classical two-dimensional LSB model. With SJ control, separated shear layer adjoins to the wall and sways within a very small range instead of flapping sharply, and LSB flow structure disappears. The main reason is the small artificial disturbance from SJ of which frequency is close to nature frequency of shear layer unstability is accepted and amplified in the flow, then locks the nature frequency to control frequency. As a result, the energy of shear layer gets a remarkable increase and big-scale LSB flow structure is restrained.