Effect of vortex dynamics and instability characteristics on the induced drag of trailing vortices

In this paper, trailing vortices generated by three wingtip configurations, namely the M6wing and the M6 wing with a blended or split winglet, are experimentally investigated using the Stereo Particle Image Velocimetry(SPIV) technology. Then, linear stability analysis is performed to investigate instability characteristics. Three corresponding trailing vortex patterns, including the isolated trailing vortex without wake(pattern v) and with wake(pattern v-w), co-rotating vortex pair(pattern v-v), are observed in experiments. The strength of trailing vortices, characterized by circulation, is reduced after installing winglets as expected, and the strength of pattern v-v can be further suppressed compared with pattern v-w. Moreover, instability characteristics, such as the eigenvalue spectrum and perturbation mode, are distinctive among these three vortex patterns.The distribution of eigenvalue spectrums indicates that pattern v and pattern v-w are temporally“marginally stable”, while pattern v-v is temporally “unstable.” The primary perturbation mode of pattern v and pattern v-w is the m =-1 helical mode, while |m|>1 for the case of pattern v-v.The effect of vortex dynamics and instability characteristics can be concluded in two aspects.Firstly, the value of induced drag is polluted by about 3% from vortex wandering since vortex wandering affects the tangential velocity and streamwise vorticity of trailing vortices. Secondly, the growth rate and penetration depth perturbation mode affect trailing vortex evolution and further affect induced drag. Specifically, the larger the growth rate and penetration depth are, the more turbulence injects inside the vortex core, thus leading to a quicker and more intense attenuation of trailing vortex, as well as a smaller induced drag. This finding can guide us to manipulate the induced drag in flow control.

利用翼尖减阻装置提高碟形飞行器性能(英文)

Saucer-shaped aircraft adopts a novel aerodynamic configuration of blending fuselage with wing. In contrast to the ordinary aircraft configurations, this kind of configuration can totally eliminate the drag resulted from fuselage, bringing many advantages such as simple structure, compact scale, high load capability. But its small aspeet ratio makes the induced drag higher. Through wind tunnel experiments, it is discovered that a type of sweepback fin-shaped winglet can efficiently reduce the induced drag of this kind of aircraft. When this winglet is mounted to a model in wind tunnel experiment, the maximal ratio of lift to drag of the model can be increased by 75% as compared with the model without winglet at the speed of 30 m/s, and reached 15 at the speed of 50 m/s. In order to investigate the performance of this aircraft with winglet at low speed, test flights were processed. The results of test flights not only verify the conclusions of experiments in wind tunnel but also indicate that the load capability of the aircraft with winglet is increased and its lateral stability is even better than that of the aircraft without winglet.

Drag reduction and flow structures of wing tip sails in ground effect

The hydrodynamics and flow structures of a base wing slotted with tip sails in proximity to the ground were studied experimentally in order to investigate the flow control efficiency of wing tip sails in ground effect.The experiment was conducted in a towing tank at a Reynolds number 1.5×10^5.The lift and drag forces were measured by a transducer,the velocity fields of the wing tip vortices were measured using a time-resolved particle image velocimetry system(TR-PIV).The tip-sails and ground clearance were both effective in reducing the total drag,the lift coefficients of the tip-sails wings were increased as compared with that of a base wing.The lift-drag ratios of the tip-sails wings were improved obviously in a range of angles of attack from 2°to stalling angle.The tip-sails played more important role in unwinding the concentrated wing tip vortices at higher angle of attack,the intensity of the tip vortices were much weaker than that of the base wing.The development of the wing tip vortices was suppressed as well due to the inhibition of the ground,the downwash speed was reduced and the induced drag was decreased.

Aerodynamic Cost of Flapping

A new modeling approach is presented for mathematically describing the drag due to wing flapping. It is shown that there is an aerodynamic cost of flapping in terms of an increase in the drag when compared with non-flapping. The drag increase con- cerns the induced drag which results from lift generation. There are two effects that yield the induced drag increase caused by flapping. The first effect is due to changes in the direction of the lift vectors at the left and right wings during the flapping cycle by tilting them according to the flapping angle of the wings. Because of tilting the lift vectors, more lift has to be generated than is required for the vertical force balance in flapping flight. This lift enlargement causes an increase of the induced drag. The second effect that increases the induced drag is due to changes in the magnitude of the lift vector in the course of the flapping cycle. Changes in the magnitude of the lift vector are necessary for generating thrust which is required for the longitudinal force balance. As a result, both effects of lift vector changes cause a drag increase when compared with non-flapping. Solutions on an analytical basis and as well as results using a computational fluid dynamics method are presented.