Investigation On Boundary Layer Transition Judgment And Law Of Symmetrical Airfoil At The Condition Of Large Reynolds Number

Boundary layer transition is a highlight topic in fluid mechanics and its application in engineering.In turbine machinery such as aeroengine,Marine engine and energy machinery,the study of transition is of great significance for mechanical efficiency and heat exchange.In this paper,based onγ-Reθtransition model,under the condition of large Reynolds number（1.0×10⁶≤Re≤3.0×10⁶）,three groups of symmetrical airfoil with different thickness,NACA0012,NACA0015 and NACA0018,are taken as the research object to study the effect of improving the mesh around the airfoil on the accuracy of numerical simulation.At the same time,the transition determination method and transition rule of flow around airfoil are discussed.The main research contents of this paper are as follows:ⅠThe influence of the mesh around the airfoil on the numerical simulation accuracy of the flow field around the airfoil is studied.The accurate prediction of airfoil transition depends on the calculation accuracy of numerical simulation of airfoil,and the calculation accuracy of aerodynamic parameters and flow field of airfoil in numerical simulation is affected by the flow field grid.The mesh around the airfoil plays an important role in the flow field.By studying five important factors that affect the precision of numerical simulation of airfoil,namely,the number of mesh layers on airfoil surface,the number of mesh layers in0.1c,the height of mesh layer in the first layer of airfoil surface,the normal change rate of mesh layer in 0.1c and the normal growth rate of far-field mesh,it is found that the height of mesh layer in the first layer of airfoil surface has a more obvious influence on the precision of numerical simulation.It is mainly related to the flow information captured by the airfoil near the wall.In addition,the change rate of the grid along the normal direction of the airfoil surface is related to the transition between and within each grid block of the flow field in the computational domain.This parameter is directly related to the aspect ratio of the grid,and an appropriate grid normal change rate will make the numerical simulation process more stable and easier to converge.At the same time,according to the analysis,the precision of the numerical simulation results in this study scenario is the highest when the surface mesh number is 400,the inner mesh layer number is 140,the height of the first layer of the airfoil surface grid is 5.0×10^-6,the normal growth rate of the inner mesh is 1.1,and the far field mesh normal growth rate is 1.15.ⅡThe influence of different conditions on the flow around the airfoil is explored.Based on the high precision numerical simulation method obtained,the relevant numerical simulation is carried out,and the influence of different flow conditions and the change of airfoil thickness on the flow field around the airfoil is analyzed.Through analysis,it is found that with the increase of the Angle of attack,the low pressure area on the upper airfoil gradually shortens and moves forward,and the high pressure area on the lower airfoil gradually moves backward.The Reynolds number has little effect on the pressure distribution around the airfoil,but has obvious effect on the maximum flow pressure around the airfoil.As the thickness of the airfoil increases,the low pressure area of the upper airfoil expands gradually.ⅢA boundary layer transition judgment method based on turbulence intensity near the airfoil wall is proposed.By contrast with the transition determination method based on airfoil surface friction resistance coefficient,the transition process is more obvious when the turbulence intensity of airfoil near the wall takes a step of two orders of magnitude at the transition location,compared with the change of airfoil surface friction coefficient of less than or equal to one order of magnitude at the transition location.In addition,the step observation of turbulence intensity curve near the airfoil wall can effectively avoid the influence of flow disturbance before transition.Meanwhile,compared with the intermittent factor cloud map,the turbulence intensity cloud map around the airfoil can more clearly show the disturbance before transition position.It is proved that the transition judgment method based on turbulence intensity near the airfoil wall is effective and feasible.ⅣStudy on boundary layer transition law around airfoil.By comparing the transition of three groups of airfoils under different flow conditions and the transition of airfoils with different thickness under fixed flow conditions.It is found that for different airfoils,the transition location moves forward with the increase of attack Angle and Reynolds number,but the degree of transition moves forward is not completely the same.When the Angle of attack changes to a large,the transition forward speed presents a trend of acceleration.In addition,the unstable part appearing before transition is also gradually shortened in the process of increasing the Angle of attack of incoming flow.Increasing the thickness of airfoil can reduce the fluctuation of airfoil before transition.At the same time,as the thickness of the airfoil increases,the energy of the fluid along the flow line increases,which plays a stabilizing role on the flow and reduces the disturbance.Therefore,the length of the protruding part before transition of the airfoil gradually decreases,which proves that the flow around the airfoil is more stable when the relative thickness of the airfoil is higher under the same flow condition.On the one hand,this study provides the idea of improving the near-wall mesh quality in the process of high-precision numerical simulation of airfoil,and can also be carried out in the study of other airfoil based on this logic.At the same time,the transition determination based on turbulence intensity near the wall of airfoil proposed in the study of airfoil transition provides a new method for the study of transition.At the same time,this energy based transition determination method can also become another research idea in airfoil transition research.