Study On Stall Control Mechanism Of Fin Segment Of Humpback Whale

In recent years,the operation safety of small rotor machinery,wind turbine and blade hydraulic machinery has attracted extensive attention from scholars and engineers.In the process of mechanical operation,when the lift coeffici ent exceeds a certain critical condition,the wing or blade will vibrate due to the rapid decrease of lift coefficient,and fracture will occur for a long time.In order to improve the stall,a new type of passive control method,the biomimetic leading edge convex structure,has been studied extensively.In this paper,numerical simulation was used to study the stall control mechanism of the fin segment of the humpback whale.The main research contents and conclusions are as follows:（1）NACA 63₄-021,NACA 0012 base airfoils and single convex airfoils were numerically simulated by numerical simulation method.The Transition SST model coefficient is modified by comparing with the results of silk thread visualization experiment.The modified turbulence model is used to study and analyze the basic airfoil and single convex airfoil.The causes of stall and the influence mechanism of the leading edge single convex on different stall airfoils are analyzed.It is found that under this Reynolds number,NACA 0012 airfoil belongs to the leading edge stall and NACA 63₄-021 airfoil belongs to the leading edge-trailing edge mixed stall.The influence of single convex structure on the leading edge stall airfoil is less than that of the leading edge-trailing edge mixed stall airfoil.The specific influence mechanism is as follows: with the increase of the Angle of attack,the addition of a single leading edge convex structure makes the separation zone formed by the edge develop asymmetrical,resulting in unilateral stall phenomenon,and the lift coefficient of the whole stall process presents a step-type change.The single projection on the leading edge has the effect similar to that of a wing blade,that is,it restricts the development of the separation zone to the othe r side of the convex structure within a certain Angle of attack.（2）The airfoil with different number of convex structure on the leading edge is numerically simulated.The influence law of the flow field among several convex structure and the influence mechanism of the number of convex structure on the stall process of airfoil are studied.It is found that a single leading edge convex structure forms a deflected high-energy flow zone,which provides energy for the rear fluid to resist the adverse pressure gradient and ensures the existence of adhesion flow behind the convex structure at a high attack Angle.When several convex structures are coupled to each other,the high energy flow region always changes from the expansive to the convergent.The expandable type replenishes the external energy and restricts the internal separation zone.The convergent type ensures the presence of adhesion flow in the middle of the convex structure at a high Angle of attack.The upper or lower washing of the leading edge convex structure causes the exchange between the fluid and the main flow in the boundary layer and causes different flow separation adhesion phenomena.When the Angle of attack is large,the lift coefficient of continuous thirteen convex airfoils increases b y about 30% more than that of base airfoils.（3）Numerical simulation method is used to study the influence mechanism of amplitude of single convex structure,spacing of double convex structure and amplitude of two convex structures on airfoil flow field.It is found that the bulge amplitude is independent of the occurrence of unilateral stall.The larger the convex amplitude is,the larger the flow field shadow range is,the smaller the initial attack Angle of unilateral stall phenomenon is,and the larger the adhesion flow range is at a large attack Angle.The double convex airfoil with single wavelength spacing has better external characteristics than other airfoils with different spacing.Different convex combinations of different sizes fixed the flow deflection from the large value side to the small value side so that the stall side of the one-way stall process is fixed.