Investigation On The Evolution Of Instability Modes Of Large Aircraft Wingtip Vortices And Its Control Principle

The wingtip vortex of the large civil aircrafts is a crucial factor affecting the flight safety of the following aircrafts and the landing/take-off frequency of airports,which is also the main source of induced drag.To tackle the issue of the wingtip vortex,large civil aircrafts usually are equipped with winglets to weaken the strength of wingtip vortex to reduce its effect.However,such approaches cannot accelerate the decaying of wingtip vortex decay,hence airworthiness regulations nowadays still require a shortest interval for three minutes for landing/taking off,which greatly limits the airport's capacity.Therefore,without loss of performance of the winglets to reduce drag,the utilization of vortex instability to accelerate vortex breakdown is of great potential research and practical value to control aircraft wake.With this clear goal,the research herein approaches the problem of wingtip vortex from vortex instability.The wind tunnel and Stereo-PIV technique are utilized to capture the characteristics and evolution of the wingtip vortex,so that the mechanism underlying the temporal-spatial development of wingtip vortex instabiltily and the control principle of vortex instability based on vortex structure are proposed.The research covers three major aspects:(i)Elucidating the way the linear perturbation modes act on the motion of wingtip vortex wandering by investigating the temporal-spatial evolution of the instability of isolated wingtip vortex with SPIV and Local LSA,(ii)Studying the structures of the vortex systems and their evolution generated by three typical winglets as are cut-off winglet,blended winglet and split winglet,and quantifying their features of instability and perturbation modes,(iii)On basis of the influencing mechanism underlying the perturbation form on the wingtip vortex instability,the control principle on wingtip vortex instability by multi-vortex configuration will be proposed.The specific research contents and corresponding conclusions are as follows:(?)The mechanism underlying vortex wandering by the temporal/spatial evolution of wingtip vortex instability is revealed.The SPIV measurement of the wingtip vortex of a NACA0015 rectangular wing within its wake region of 1-6 chordlength demonstrates that the amplitude of vortex wandering increases along streamwise direction,and evolves into an anisotropic pattern,while its growth rate is increased at large Ao A conditions.The result of local linear stability analysis shows that such enlarged growth rate is due to larger growth rate of linear perturbation at large Ao A.The transverse velocity perturbation is confined within the vortex core with a distinct direction with periodic rotation.Such directivity and periodicity rules the features of vortex wandering,while the time/spatial growth rate dominates the development of wandering amplitude.(?)A universal influencing mechanism of perturbation form on the vortex instability is validated.The evolution of the structure and wandering behavior of the isolated vortex,vortex-wake system and co-rotating vortex pair by the three canonical wingtip configuration are obtained by SPIV,while Bi-Global LSA is implemented to analyze the instability and perturbation modes of these vortex structures.It is found that both isolated vortex and vortex-wake system is marginally stable,while the co-rotating vortex pair fully enters into the unstable plane,demonstrating significant instability.The instability of isolated vortex is dominated by a two-lobed mode with azimuthal wavenumber of m =-1;the vortex-wake system is dominated by the same perturbation mode from its primary vortex,while large m modes co-exist;in the co-rotating vortex pair,instability is ruled by the large m mode from its primary vortex.By comparing the various perturbation modes and its leading effect on vortex instability.It can be validated that the large m modes have stronger interferential effect on the vortex than the m =-1 mode;in addition,the essential condition for a perturbation mode to dominate the wingtip vortex is that it has to penetrate throught the wall of shear at the core boundary to act on the entire vortex core.(?)The optimized perturbation form of wingtip vortex and the control principle of vortex instability based on a multi-vortex configuration are proposed.Based on the universal influencial mechanism of the perturbation mode on the vortex instability,the m =-1 mode of isolated vortex is a low-efficiency perturbation to the vortex.In order to facilitate effective wake control of large civil aircraft,it is desirable to increase the azimuthal perturbation wavenumber while meeting the requirement of its influencial regime on the entire vortex core.The vortex structure of isolated vortex should be altered completely to realize such concept.A secondary vortex added to the original vortex configuration,with constant total circulation,is a more efficient strategy to control the wake behind large civil aircraft as it is inherent with large m modes with sufficient influencing region,thereby leading to remarkably enhanced instability.