Experimental Study On The Flow Around A 3-D Ahmed Model And Drag Reduction Mechanism Of Combined Blowings

Under the dual pressure of huge fuel consumption and severe environmental prob-lems,it is particularly urgent to develop more effective aerodynamic drag reduction tech-nologies for ground vehicles.The simplified Ahmed car model,which has been widely studied,provides a good reference for studying highly three-dimensional and complex flow around a real vehicle.The Ahmed body wake may be divided into the high-and low-drag regimes where the rear slant angle()is in the ranges of 12.5°-30°and larger than 30°,respectively.On one hand,the previous understanding of the low-drag regime is still rather limited,especially on the unsteady structures and their frequencies.On the other hand,our previous study discovered that by combining actuations at different positions on the rear end of a high-drag Ahmed model,the aerodynamic drag can be significantly reduced by about 30%,approching that of a low-drag model.However,the drag reduction mechanism behind and its connection with the low-drag regime were still unclear.For the purpose of a thorough understanding of flow around a road vehicle and related drag reduction mechanisms,a systematic experimental study has been con-ducted on the unsteady flow around a low-drag Ahmed model and influences of combined blowings on the aerodynamics and wake of a high-drag model.This research is of high engineering significance and economic value for the development of aerodynamic drag reduction technology in my country's automobile industry.In addition,it is also of high theoretical significance for the wake of a similar bluff body and its control.This work firstly aims to gain a relatively thorough understanding of the unsteady flow around a low-drag Ahmed body.Extensive hotwire,wall pressure,flow visualization and particle image velocimetry(PIV)measurements have been conducted on a low-drag Ahmed body of=35°at Reynolds number0)?[0.3,2.7]×10⁵(based on the square root of the model frontal area).Firstly,predominent steady coherent structures including the upper and lower recirculation bubbles,a corner vortex and different streamwise vortices are studied.Among those,the characteristics and formation mechanisms of the spanwise corner vortex near the lower end of the slanted surface and a pair of longitudinal trailing vortices in the wake are carefully analyzed.Then,a total of five distinct frequenciesof unsteady flow structures have been identified in the wake.Four of those are the same as their counterparts in the high-drag regime.The research is focused on another distinct wake vortex structure captured behind the vertical base,whose frequency is?0.30and may vary rather wildly with flow regime and.Combining the hotwire and PIV data analysis,the characteristics of the wake vortices and the difference with their counterparts in the high-drag regime are studied,and their formation mechanism is further revealed.Based on the obtained understanding of the flow physics behind this structure,a new characteristic length,which reflects physically the bubble size,is proposed and thus a scaling law is developed to govern thisin different flow regimes and under different.Theis the vertical distance between the separation points of the two recirculation bubbles andis a deflection coefficient based on the effective separation angle of the upper bubble.Besides,the origins,interactions with other steady structures and spatial extent of the unsteady structures and the Reynolds number effect on their frequencies are also documented.Eventually,based on the present and previously reported data,a conceptual flow structure model is proposed for the low-drag Ahmed body,including both steady and unsteady coherent structures around the body.A thorough understanding on the low-drag regime and its difference from the high-drag regime may provide a guidance for studying aerodynamic drag reduction.This work aims to reveal the mechanism behind the significant drag reduction caused by combined control on a high-drag Ahmed model and its connection with the low-drag regime.Four separate actuations,including steady blowings along the upper and two side edges of the rear window and at the upper and lower edges of the vertical base,denoted as S1,S2,S3and S4,respectively,are applied on a high-drag Ahmed body of=25°.These actuations are combined in four different strategies,including(S1,S2),(S1,S2,S3),(S1,S2,S4)and(S1,S2,S3,S4),at0)=1.67×10⁵.Under the different optimal combinations,systematic wall pressure and PIV measurements are conducted to study the origin of drag reduction and effects of control on all predominant wake structures and the interactions among them.And the drag reduction mechanism is further revealed.After comparison,it is found that by effectively manipulating all predominant wake structures simultaneously,combined control change the wake of a high-drag Ahmed body into a pattern similar to the low-drag regime,and thus obtains a large drag reduction.In summary,this experimental research deepens the understanding of the unsteady flow around a low-drag Ahmed model,and reveals that a high drag reduction can be achieved by changing the wake of a high-drag Ahmed model to the low-drag regime using combined control.The results have contributed to the improvement of the theoretical system of the automobile aerodynamics,and also provide a good reference for the research on other bluff body problems.