Investigation Of Flow Mechanisms For Leading Edge Tubercles In Compressor Cascades

Flow separation is a typical flow phenomenon in a compressor cascade.It is an effective way to improve the cascade performanmce by suppressing it with appropriate methods.Inspired by the study on the humpback whale flippers,the application of leading edge tubercles is a potential passive flow control technique.However,to date there is no systematic study on the effects and flow mechanisms of leading edge tubercles in compressor cascades.Based on controlled diffusion airfoil,the author of this thesis constructs an “infinite” cascade without regards to endwall effects,and a linear cascade with endwalls.By analyzing and summarizing the research results,the flow mechanism of leading edge tubercles is clarified,which lay the foundation for the engineering application of tubercles in compressor cascades.Primary conclusions are as follows:(1)The influence laws of the crucial geometry parameters of tubercles,cascade solidity and Reynolds number on the effects of tubercles are as follows: tubercles will not obviously improve the cascade performance at the design condition,and achieve large loss reduction at a high incidence angle.Tubercles with a small wavelength(4% chord)and a large amplitude(3% camber length)may achieve a loss reduction of 20%.As the solidity increases,the resistence to separation is enhanced in a compressor cascade.Tubercles come into effect at a higher incidence angle.As the Reynolds number of incoming flow increases,the incidence angle corresponding to the best effect of tubercles,gets higher due to the low extent of flow separation.(2)An analysis method of vorticity transport is used to clarify the formation mechanism of tubercle-induced counter-rotating vortices.The stretching and turning due to velocity gradients explain the formation of counter-rotating vortices.Vortices behind the tubercle crests are mainly induced by the axial turning of pitchwise vorticity and the stretching of axial vorticity.Vortices behind the tubercle troughs are mainly induced by the axial turning of spanwise vorticity.(3)In a compressor cascade with high load and high inlet Mach number,the counter-rotating vortices induced by leading edge tubercles has an unsteady fluctuation with a high frequency.At a high incidence angle,counter-rotating vortices will energize the low-momentum fluids in the boundary layer,effectively delay the flow separation,greatly reduce the vortiex shedding,and finally improve the cascade performance.(4)In the modified cascade with uniform leading edge tubercles,there exists distinct spanwise aperiodic flow pattern,which is resulted from the tubercle-induced flow disturbances and dynamic instability of flow field.(5)In a linerar compressor cascade,counter-rotating vortices induced by leading edge tubercles energize the boundary layer on the blade suction surface,and subsequently achieve a great reduction of cascade losses at a high positive incidence angle.Tubercles with a smaller wavelength and a larger tubercled span have better effects.