Research On Aerodynamic Characteristics Of Rotating Isolated Wheel Based On Parametric Modeling

With the development of society,automobiles are also developing towards safety,energy saving and individualization.In the aerodynamics of the external flow field of a car,the wheel,as the main component of the automobile that rotates,will have a huge impact on the flow field around it.Therefore,studying the aerodynamic performance of the wheel,or studying the aerodynamic characteristics of the rotating isolated wheel,is of great importance whether it is the research of the flow field structure and aerodynamic forces around the wheel,or as a pilot study of the flow field structure in the wheel area of the vehicle.This research is based on the commercial computational fluid dynamics(CFD)software STAR-CCM+,using parametric modeling methodology and software redevelopment technology to carry out research on the flow field structure and aerodynamic force of rotating isolated wheel.The research are as follows:Firstly,the similarities and differences of various wheel rotation simulation methods were introduced.Based on the wind tunnel test data of the Fackrell A2 isolated wheel,the accuracy of the simulation method of the rotating isolated wheel was verified.The transient simulation method based on DES method was compared with the common steady-state RANS simulation method.The results showed that:(1)the hybrid rotation method composed of sliding mesh / MRF and rotating wall boundary conditions could effectively deal with the ground contact and wheel rotation process;(2)in terms of the aerodynamic drag coefficient,considering the test error,both the time-averaged value obtained by DES method and the steady-state RANS method were accurate enough,the steady-state RANS method had slightly higher accuracy;(3)in terms of the circumferential pressure coefficient distribution on the symmetrical plane,the DES method improved the ability of pressure prediction in the upper separation region of the wheel;(4)in terms of the total pressure contour of the wake separation region sections,the total pressure contour of the time-averaged flow field obtained by the DES method was close to that obtained by the steady-state RANS method,the error of the total pressure contour near the wheel side was very small,and increased gradually with the increase of distance;(5)the DES method could accurately predict the vortex shedding frequency at the upper separation region of the wheel and the development of vortex structure near the ground,while the steady-state RANS method had no prediction ability.In conclusion,the DES method and the hybrid rotation method used in this research were suitable for the research of aerodynamic characteristics of rotating isolated wheels.Secondly,based on parametric modeling,the software redevelopment technology and the verified wheel rotation simulation method,a new simplified modeling method for the wheel geometric model was proposed,which included three parts: the automatic parametric modeling of isolated wheel,the automatic geometric/mesh pre-processing of isolated wheel and the automatic setting of CFD software.In particular,it had strong compatibility for the wheel of novel-shaped fan-like spoke(spokes of attack angle).The object-oriented process of Python was used to merge the processes to develop a process automation platform,Iso Tire CFD,which was specially used to study the aerodynamic characteristics of rotating isolated wheels with different parameters.The results showed that the time consumption and the number of manual steps could be reduced by 69.7% and 88.9% respectively.Then,based on the Iso Tire CFD platform,a simplified five-spoke racing car wheel was used as the baseline wheel,and the aerodynamic research was carried out at a constant speed of 27.78m/s.The results showed that:(1)the flow field characteristics of baseline wheel could be divided into upper separation,wheel shoulder vortex structure,near ground vortex structure and wheel edge vortex structure,which were consistent with the existing research conclusions;(2)after removing the disc brake,the time-averaged aerodynamic force decreased slightly,and the disc brake hindered the air exchange between the free flow side and the spoke side;(3)based on baseline wheel,the flow field characteristics of baseline wheel were analyzed.It was found that the attack angle of spoke had an angle controlling effect on the flow field around the isolated wheel,but the air transport phenomena inside the rim cavity was mainly limited to the rear of the wheel;the time-averaged aerodynamic drag and aerodynamic moment of the rotating axis of the wheel with positive attack angle were larger,while the time-averaged aerodynamic lift was decreased;when the covering area of the spoke changes,the change of flow field and time-averaged aerodynamic forces were relatively not obvious;(4)considering the interaction effect of variables,a normalized general aerodynamic coefficient considering the time-averaged aerodynamic drag and aerodynamic lift was proposed,which was used as the objective function.Based on the Face-Centered Central Composite Design,the second-order response surface was constructed for these two parameters,and the contribution to each item of the response surface function was made.It was found that the spoke attack angle had the greatest influence on the normalized general aerodynamic coefficient of the isolated wheel.Finally,the Reduced Order Model(ROM)analysis method based on the Proper Orthogonal Decomposition(POD)was introduced in the research of aerodynamic characteristics of rotating isolated wheels.The isolated wheels with different attack angle spokes were taken as the research objects.The transient aerodynamic research was carried out based on the simulation data of positive/negative attack angle wheels and baseline wheel respectively,and the POD ROMs of seven typical two-dimensional sections near the wheels were constructed.The results show that:(1)the frequency range of the transient aerodynamic force of each wheel could be consistent with the dominant frequency range of the ROM’s mode coefficient of the typical two-dimensional section,which proved that the selection of the two-dimensional sections was representative;(2)the frequency of the main mode coefficient of typical two-dimensional flow field around the isolated wheel could be divided into low frequency range of 3-8Hz,medium frequency range of 19-33 Hz and high frequency range of 68.25 Hz,but different attack angles have different frequency ranges.The spoke attack angle could change the main mode coefficient of the ROMs of typical two-dimensional sections and the main frequency of the wheel transient aerodynamic forces;(3)compared with baseline wheel,the flow structure in the rim cavity changed from free flow shear layer to spoke periodic rotational motion control,that is,the main frequency of the corresponding mode coefficient was 68.25 hz,but the influence range of different attack angles on the flow field in the rim cavity was different.