Investigation On Vortex Drag Reduction Of Large Civil Aircrafts Based On Vortex Evolution Mechanisms

It is always the goal of designers to reduce resistance for aircrafts.Lower resistance means lower operating costs and lower carbon emissions to large civil aircrafts.Vortex drag(induced drag),which originates from aircraft wake vortices,is one of the major parts of aircraft flight resistance and contributes 30-50 percent to the total drag.Wingtip vortices and afterbody vortices are major wake vortex systems.It is one of the key methods to reduce drag by effectively controlling these vortices.In this thesis,vortex evolution mechanisms and drag reduction designs are combined.Wind tunnel tests are carried out to study wake vortices of large civil aircrafts and reduce drag by controlling evolution of vortices from perspective of vortex drag,in order to provide reference for drag reduction designs.Three aspects of research results are concluded as follows:(1)Co-rotating vortex pair dominated by winglet tip vortex is obtained during investigation on the rectangular wing with winglets.Vortex drag is affected by strength and scale of the vortex pair.A co-rotational twisted spiral double-vortex-tube structure consisting of winglet tip vortex and joint corner vortex induces movements of each other.The up-bent and raked winglets weaken core radius and circulation of tip vortex and reduce vortex drag.(2)Afterbody vortex system dominated by horizontal tail tip vortex pair is obtained during investigation on the simple upswept afterbody with horizontal tails.Vortex pair interactions influence vortex strength and vortex drag.Afterbody primary vortex pair forms on lower surface,and then separates,evolves and dissipates.After horizontal tails are installed,horizontal tail tip vortex pair significantly weakens circulation of afterbody primary vortex pair,accelerates its dissipation,and reduces vortex drag.The influence is stronger when the vortex pairs are closer to each other.(3)Drag reduction devices are added to the large civil aircraft model,and drag reduction is realized by controlling the strength and scale of vortices.Two up-bent winglets installed to half model of the aircraft effectively weaken circulation of tip vortex and reduce drag,while a split winglet breaks tip vortex up to two vortices but fails to reduce the total circulation and drag.Vortex generators are installed to afterbody model of the aircraft,and when they are installed at a more forward position,circulation of afterbody vortices decreases,and calculated value of vortex drag and measurements of total drag are reduced.In summary,when devices like winglets and vortex generators are added to large civil aircraft properly,strength(circulation)and scale(core radius)of wingtip vortices and afterbody vortices can be weakened by vortex interactions so that vortex drag is reduced.How to reduce vortex drag based on wake vortex evolution mechanisms is studied in this thesis,which can provide large civil aircrafts with a route for drag reduction designs.