Numerical Simulation On The Influence Of Vibration Of Rearview Mirror Bluff Body On Flow Field

When the vehicle is at high speed,the air flow in the rear of the rearview mirror outside the vehicle is complex,which has a significant influence on the aerodynamic noise of the vehicle.In the traditional numerical simulation of the external flow field of the rearview mirror and body,the rearview mirror is often regarded as static rigid bodies,and the results of analysis often need to be modified empirically when compared with those of wind tunnel and road test.In fact,many factors when the car is driving can cause the vibration of the convex structure of the rearview mirror to varying degrees.The vibration structure couples with the flow field and ultimately affects the aerodynamic characteristics outside the vehicle.Based on the theory of computational fluid dynamics and the dynamic grid technology of Fluent,this paper explores the influence of rearview mirror bluff bodies on the flow field at different vibration conditions.The main research contents of this paper are as follows:Firstly,the two-dimensional flow field simulation analysis at low Reynolds number and subcritical Reynolds number is carried out on the semi-cylindrical model that is close to the cross-sectional shape of the rearview mirror and has been neglected in the field of flow around the bluff body,to explore the aerodynamic variation law and flow field characteristics of the model at different vibration conditions.When the vibration frequency is locked with the natural vortex shedding frequency,the aerodynamic change curve which rises smoothly with the increase of vibration amplitude and vibration frequency appears a "pit".What’s more,the locking interval expands with the increase of amplitude.The wake flow of the semi-cylindrical model in the lock-in region is similar to that at rest,but the multi-frequency action outside the lock-in region increases the instability of the flow field.In addition,with the increase of amplitude,the boundary frequency of lock-in region extends to low frequency at low Reynolds number（Re=100）,and to high frequency at subcritical Reynolds number（Re=20000）.Secondly,considering the three-dimensional finite-length effect of the rearview mirror in the real flow field environment,three static models of cylinder,semi-cylinder and generic side-view（GSV）mirror with an aspect ratio of 2 are constructed to compare the aerodynamic and flow field characteristics of the three models,and deeply explore the rearview mirror bluff body flow mechanism at the subcritical Reynolds number.At the same time,the research results lay a foundation for the flow field analysis of threedimensional rearview mirror bluff bodies vibration model.The results show that all three models have obvious three-dimensional effect and downwash effect.The drag coefficient of the cylindrical model is slightly smaller than that of the semi-cylindrical model,and significantly larger than that of the GSV model.In addition,the cylinder model has the most obvious compression effect of downwash flow,and both the wake vortex and the free end vortex are the most complex.The flow transition of GSV model is more stable,and there is almost no backflow zone on the free end face,and the pressure drag in the direction of inflow is smaller.Finally,the flow field simulation of cylinder,semi-cylinder and GSV models at different vibration conditions is carried out.The results show that the boundary frequencies of the lock-in regions of the three models retain the characteristics of expanding to higher frequencies in the two-dimensional simulation at subcritical Reynolds number.But the variation laws of force coefficients are different.The lift coefficient increases sharply when the vibration frequency increases to the lock-in region,and the cylinder model has the largest increase due to the influence of vibration on the free end pressure field.The drag coefficient increases with the increase of the vibration frequency as a whole and is prominent in the lock-in region,with the semi-cylindrical performance being the most significant.In addition,the closer to the free end,the smaller the area of the rear recirculation area and the slower the airflow velocity.The vibration leads to the reduction of the area of the rear recirculation area of the three models,which is the smallest when locked,and the semi-cylindrical model is more affected by vibration than the other models.The influence of vibration on the tail flow field and aerodynamic performance of the three models is obvious and has certain differences,which should not be ignored in the relevant aerodynamic simulation analysis.It is important to consider the influence of the rearview mirror bluff body vibration on vehicle outflow field for the prediction and control of vehicle aerodynamic characteristics and NVH（Noise,Vibration,Harshness）performance.