Effect Of Leading-edge Protuberances Inspired By Humpback Whale Flipper On Airfoil Stall Control

When applying airfoils on aeroplanes,marine components or fluid machineries,the operation condition may reach or exceed the stall point,leading to the decline of the performance,the efficiency and the stability of the units.In recent years,humpback-whale-inspired leading-edge protuberances have attracted plenty of attentions as a new passive control method.However,the flow control mechanism of leading-edge protuberances has not been fully understood yet,impededing the optimization design and practical application of this method.In this research,through employing wind tunnel experiment,theorectical analysis and numerical simulation,the effect of a single leading-edge protuberance and multiple protuberances on the airfoil performance and flow pattern was studied.On this basis,the flow control mechanism of leading-edge protuberances was investigated.Aerodynamic forces were tested and tuft visualization experiments were performed in a wind tunnel,to investigate the effect of a single leading-edge protuberance on the airfoil performance.A special two-step stall process was discovered.When the first step of stall occured,one side of the protuberance was in stalled condition,while the other side kept non-stalled.A physical model was proposed to explain the generation mechanism of the one-sided stall phenomenon,including the instability of the flow condition in the hysteresis region,and the partition effect of the attached flow on the protuberance peak,which play a similar role to a wing fence.A theorectical model based on lifting line theory,which considered the amendment of the hysteresis and partition effect,was proposed for the performance prediction and analysis of the airfoils with a single leading-edge protuberance.The prediction results were well consistent with the aerodynamic performance and tuft visualization gained by the wind tunnel experiment.The therectical analysis result revealed that the first stall angle is mainly related to two elements,including the effective angle of attack around the protuberance root,and the hysteresis region of the baseline airfoil.The effect of protuberance parameters was further investigated,which revealed that the amplitude-to-wavelength ratio played a dominant role on the magnitude of the first stall angle of attack.On this basis,the flow regimes of the airfoil with a single leading-edge protuberance were concluded,which is related to the amplitude-to-wavelength ratio and the angle of attack.The effect of multiple leading-edge protuberances on airfoil performance was further investigated through wind tunnel experiment and numerical simulation.When some critical angle of attack was exceeded,the original periodic flow patterns suddenly become complicated aperiodic flow patterns.On the basis of the previous proposed mechanism of a single leading-edge protuberance,the generation mechamism of the complicated aperiodic flow patterns induced by multiple protuberances were discussed.On one hand,the spanwise extention of the local stalled region leads to slender high-momentum regions on neighboring protuberance peaks,which demonstrated compartmentalization effect on the suction surface.On the other hand,the occurrence of the local stalled region leads to a decline of local circulation,which will induce a strong downwash component on the regions with relatively high circulation,thus reducing the effective angle of attack and restraining stall on these regions.In conclusion,the effect of humpback-whale-inspired leading-edge protuberances on airfoil stall performance was investigated in this research.The flow control mechanism of leading-edge protuberance was proposed,which provides an important foundation for the optimization design and practical application of this method.