Research Of Aerodynamic Drag Reduction Based On Wake Of Vehicle

As car ownership increased in recent years, the energy and environmental issues havebeen prominent. Confronted with the energy crisis and environmental degradation, it isnecessary for people to be aware of the need to reduce the resistance of automobile. It’sbecause aerodynamic drag, together with rolling resistance composing the resistance ofautomobile, take a larger proportion of the total resistance in high speed condition, thataerodynamic drag reduction is a approach to control fuel consumption.Aerodynamic drag of automobile is formed mainly on the rear body separated flow.In order to avoid the influence caused by the different rear styling of automobile, in thiswork, the Ahmed body model with a rear slant angle of35°, which has the typicalcharacteristics of the rear of automobile, is used for the object to conduct research ondrag reduction technology using flap or jet from circular holes by comparing previousstudies. A Π-shape flap is used for research on the separation on upper edge and sideedges of slant of the model in the study of drag reduction using flap. In the study ofdrag reduction with jet, jet from circular holes is used to control the separation on upperedge of slant of the model and investigate changes in aerodynamic drag. The mainresearch contents in this paper are the influences of the Π-shape flap with differentwidth and layout position and the jet with different layout position of jet hole andvelocity on aerodynamic drag. The changes of wake structure of the model in twomethods are analyzed and variations of laws of aerodynamic drag characteristics underdifferent disturbed conditions are summarized to provide a scientific theoretical basisfor the further development of the actual model of automobile.The five parts included in this paper is following:1) Research on aerodynamic characteristic and flow field of the Ahmed. Firstly, a bedstand was built in wind tunnel of Jilin University, according to previous results, whichcan support experiments for model with1m length. Comparing the aerodynamic dragand surface pressure on tail cant of25°and35°slant angle Ahmed with previous experimental data, and the result indicate that the repeatability of previous experimentand the availability of this experimental platform are verified by the analysis ofexperiment data. Secondly, based on comparison between simulation date andexperiment data of Lienhart et al, selecting35°slant angle Ahmed as object model withSST k-ω turbulence model, the influence of different boundary layer schemes on flowfield structure and aerodynamic drag will be studied in this part. Thirdly, SST-DES willbe used to obtain transient aerodynamic data of object model, what’s more, in order toanalyze the energetic structure in wake flow field, POD method was applied todismantle the data. Finally it turned out that wake separation vortex takes up a higherpercentage of total energy.2) The influence of Π-shape flap on aerodynamic drag characteristics of Ahmed body.The study on how a Π-shaped flap arranged on rear slant of the model impacts rear flowstructure and pressure distribution using low dray model of Ahmed body with a35°inclination as object, shows that flap is helpful to reduce the aerodynamic dragespecially the drag from rear slant, but little effect on rear vertical surface. The flap withwidth of w=50mm has the best effect of drag reduction among flaps studied in threedifferent values of width, and the greatest reduction of drag about8.39%is attainedwhile the mounting distance e=10mm. In all the condition of experimental verification,the efficiency of drag reduction is highest in this case, up to7.63%.3) The influence of jet flow on aerodynamic drag characteristics of Ahmed body. Theobject of study is the Ahmed model with a35°inclination. The jet holes whose diameteris3mm in numerical simulation research are lined up on the rear roof,20mm from theedge,and the rear slant of model,20mm from the top edge, while direction of the jetflow is normal of the surface or horizontal. The aerodynamic drag change of the modelin different speed of jet flow is observed. As a result, tail flow structure can becontrolled effectively by jet flow, which is able to reduce the drag of the model.5%drag is reduced when jet holes is placed on rear roof or top of slant with normal jet flow,but the reduction of drag can only be reached in low speed when horizontal jets comefrom top of slant. Placing normal jets on rear roof or top of slant, the coefficient of dragdecreases at first and then increases as the speed of jet flow rise, and when horizontal jets are set on top of slant, a similar rule can be observed, although the effect of dragreduction disappears and the coefficient of drag increases in a jet speed higher than32m/s.4) The research on the influence of dome and quadrate partition to the aerodynamicdrag on a truck model. This thesis primarily discussed the influence of two differentdome shapes to the characteristics of aerodynamic drag on the truck model, and foundthat the dome on roof has a larger significance to aerodynamic drag reduction of thistruck as the lateral dome makes less contribution. Refer to the result of research onaerodynamic drag reduction on the Ahmed type car, drag reduction research was carriedon the addition of quadrate partition in container tail of a middle truck model. Thecomparison of influences of partitions with different position and width on flowstructure and aerodynamic force of model tail, shows that the project with the partitionwidth w=50mm performs better drag reduction characteristics. The project of e=10mmperformed the best drag reduction result of3.88%,which was proved to be4.10%byexperiment, reaches the optimal valve of all partition projects.5) A research on drag reduction of a saloon model by jet flow. Based on the researchon drag reduction of Ahmed body by jet flow, jet holes are placed on the junction ofroof and rear window and steady jets are used to perturb the rear flow field of the saloonmodel to observe the variation of rear flow structure and pressure distribution indifferent jet speed. It’s found that the coefficient of drag decreases at first and thenincreases when the speed of jets ranges from8m/s to40m/s and the best effect of dragreduction is achieved with a jet speed of40m/s in both numerical simulation andexperimental study, up to3.51%and3.26%respectively.