Simulation And Experiment Of The Three Dimensional Flow Separation Characteristics About DrivAer Models

There are complicated phenomena such as flow separations and attachments in the vehicle external flow fields.Actually,these phenomena are the direct display of internal aerodynamic mechanism.So it has very important reference values for improving the vehicle performances and vehicle shape design to study the external flow field characteristics and try to explore the internal mechanism.The Driv Aer models with smooth chassis are studied in this paper.The Driv Aer models are the new standard models with vehicle details which are closed to the actual vehicles.Driv Aer models have three forms with estateback,notchback and fastback,and it is very convenient to make the comparative researches of the three forms.This paper is divided into three parts: Firstly,the three dimensional separation characteristics of Driv Aer models are studied by the transient simulation with Powerflow software;Secondly,the force measurement experiment,smoke experiment and PIV experiment are conducted to verify the simulation results;Thirdly,based on the notchback Driv Aer model,the flow field characteristics are studied for the rear spoilers with different chord lengths and angles of attack.The external flow field structures and characteristics in the above three parts are studied in depth in order to explain the flow mechanism.In the first part,the Driv Aer models are simulated under the speed of 10m/s,20m/s,30m/s and 40m/s.The simulation results show that the Cd value of estateback is bigger than the notchback and the notchback is bigger than the fastback.By analyzing the vorticity and streamlines of the planes in the direction of X,Y,Z about the three forms,it can be found that all the models show up A-pillar vortex,C-pillar vortex,D vortex caused by the chassis,E vortex caused by the side of vehicle,F vortex caused by the ground and wheels,L vortex within the wheel cavity.The biggest differences for the three forms are the G vortex caused by the similar rear spoiler for the estateback,the H vortex and I vortex separately caused by the tail of decklid for the notchback and fastback.However,the regions of wake flow caused by the back of the notchback and fastback are different to some extent.What's more,the upper edge,lower edge and the side of the mirrors also cause the three dimensional separations.After making the Fourier transform of the velocity from the planes in the Z direction and filtering,acquiring the values of 1-10 Hz,20-30 Hz and 40-50 Hz,the velocity shows the obvious positive and negative fluctuations.At the same height,the fluctuation scope is reduced and the fluctuation points are smaller and more as the increase of the frequency.The isosurfaces of Cpt=0 show that there are apparent turbulent structures with low frequency and large size around the wheels,mirrors,A-pillar,C-pillar and the wake.And there exist energy losses in these regions and the energy lost is equal to the dynamic pressure in the far field.The isosurfaces of ?2=-50 show that there are apparent turbulent structures with high frequency and small size around the front end of engine compartment,the front end of chassis,front and rear wheels,mirrors,A-pillar,B-pillar,C-pillar,the front and rear end of the vehicle top,the groove in the middle of engine compartment and windscreen.The scope caused by the C-pillar for the estateback is mainly around the C-pillar,but the scopes for the notchback and fastback are extended to the rear window and the top of the decklid.In the second part,the force measurement experiment,smoke experiment and PIV experiment are carried out for the Driv Aer models.The force measurement experiment show that the errors between the experiment values and simulation values are within reasonable limits.The flow conditions in the plane of Y=0 are analyzed qualitatively.The wake streamlines are obtained by the PIV experiment in the plane of Y=0,and the local turbulent structures are consistent with the simulation results.So all the experiment results have been verified the accuracy the simulation results.In the third part,the rear spoilers with different chord lengths and angles of attack are designed,and the notchback model is used as the research object.The simulation results show that the rear spoilers with different sizes all reduce the aerodynamic lift to a great extent.After adding the rear spoiler,the backflow phenomenon is intensified and the affected region is larger because the airflow is hindered.The positive and negative peak value for the original model is separately appeared around the tail of decklid and the tail of chassis.However,after adding the rear spoiler,the values are appeared around the front and rear edge of the spoiler.The turbulent kinetic energies in the wake are mainly focused on the separation regions where the turbulent kinetic energy is large and the turbulent dissipation is high.The large regions with large turbulent kinetic energy between the rear window and decklid are reduced after adding the rear spoiler and moved to the direction of the top,the corresponding peak values are also changed.