Numerical and Experimental Investigations on the Aerodynamic Characteristic of Three Typical Passenger Vehicles

Journal of Applied Fluid Mechanics, Vol. 7, No. 4, pp. 659-671, 2014. Available online at www.jafmonline.net, ISSN 1735-3572, EISSN 1735-3645. Numerical and Experimental Investigations on the Aerodynamic Characteristic of Three Typical Passenger Vehicles Y. Wang1, 2†, Y. Xin1, Zh. Gu3, Sh. Wang3, Y. Deng1 and X. Yang4 1Hubei Key Laboratory of Advanced Technology of Automotive Parts, Wuhan University of Technology, Wuhan 430070, China 2State Key Laboratory of Automotive Simulation and Control, Changchun 130025, China 3State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Changsha 430082, China 4Wuhan Ordnance Noncommissioned officers School, Wuhan 430075, China † Corresponding Author Email: [email protected] (Received September 6, 2013; accepted November 11, 2013) ABSTRACT The numerical simulation and wind tunnel experiment were employed to investigate the aerodynamic characteristics of three typical rear shapes: fastback, notchback and squareback. The object was to investigate the sensibility of aerodynamic characteristic to the rear shape, and provide more comprehensive experimental data as a reference to validate the numerical simulation. In the wind tunnel experiments, the aerodynamic six components of the three models with the yaw angles range from -15 and 15 were measured. The realizable k-ε model was employed to compute the aerodynamic drag, lift and surface pressure distribution at a zero yaw angle. In order to improve the calculation efficiency and accuracy, a hybrid Tetrahedron-Hexahedron- Pentahedral-Prism mesh strategy was used to discretize the computational domain. The computational results showed a good agreement with the experimental data and the results revealed that different rear shapes would induce very different aerodynamic characteristic, and it was difficult to determine the best shape. For example, the fastback would obtain very low aerodynamic drag, but it would induce positive lift which was not conducive to stability at high speed, and it also would induce bad crosswind stability. In order to reveal the internal connection between the aerodynamic drag and wake vortices, the turbulent kinetic, recirculation length, position of vortex core and velocity profile in the wake were investigated by numerical simulation and PIV experiment. Keywords: Vehicle aerodynamic, Wind tunnel experiment, Numerical simulation, PIV aerodynamic drag is responsible for more than 50 1. INTRODUCTION percent of fuel consumption (McCallen et al. 1999). There is therefore much scope for improving It is undoubted that the improvement of fuel economy by reducing aerodynamic drag. However, efficiency in ground vehicles is currently, and will in considering the aerodynamic drag force, a continue to be, a significant issue in the auto thorough analysis of the airflow around the vehicle, industry. At present, there are mainly two is a prerequisite. After which, a shape optimization approaches to improve the fuel economy, one is to methodology can be utilized to reduce the improve the combustion process in the engine aerodynamic drag. (Abd-Alla 2002), and the other is to reduce the total drag force on the vehicle in motion (Jacques and The aerodynamic drag in ground vehicle includes Richard 2010, Frederique et al. 2004). In form drag, skin friction, interference drag, induced considering the latter, although the total drag force drag and cooling drag. The form drag due to the mainly consists of rolling resistance and flow separation around the vehicle body contributes aerodynamic drag, with a medium-size car, to 50 to 65 percent to the overall aerodynamic drag. aerodynamic drag accounts for nearly 80 percent of The flow around vehicles is highly three - the total drag force at 100km/h. Moreover, the dimensional, dominated by large separation regions, aerodynamic force is proportional to the square of large and small vortices, and complex recirculation the velocity, and the engine power required to regions. The main contributions to aerodynamic overcome the aerodynamic drag is a function of the drag in a bluff body type vehicle arise from cube of the velocity. At high speeds, overcoming separated flows in Y. Wang et al. / JAFM, Vol. 7, No. 4, pp. 659-671, 2014. the rear causing pressure recovery losses and the concluded from the wind tunnel testing that there creation of vorticity in the wake (Khalighi et al. existed two streamwise vortices that extend from 2001), and the contribution due to the rear part is the deck lid surface toward the center and often over 80%( Kourta and Gilieron 2009). The downstream. Gilhome (2001) proposed a new structure of the wake is dependent on the geometry topological structure for the wake that the wake of the vehicle upstream of the blunt base edge consisted of large hairpin and shear-layer vortices (Howell 1975, Morel 1978, Ahmed 1983). There which were regularly shed, a stable re-circulation are essentially three different vehicle vortex and the well-know C-pillar vortices. Gaylard configurations: squareback, fastback and notchback. et al. (2007) presented a series of observations of These geometries differ primarily in the angle of the time-averaged wake asymmetry for some notchback slanted rear window with the squareback essentially vehicle geometries. The mechanism that induces the a fastback with a zero angle of base slant and the asymmetry needs to be further researched; though it notchback a fastback with a boot lid. has been theorized that it may be related to the rear end shape. In order to explore the internal In response to the first fuel crisis of the 1970s，a connection between the wake and aerodynamic new focus on aerodynamics was instigated across characteristic, Sims-Williams et al. (2001) the automobile industry as a part of a strategy to investigated the links between notchback geometry, reduce fuel consumption, and part of this focus was aerodynamic drag, flow asymmetry and unsteady an investigation into the fundamental flow wake structure systematically. Without exception, characteristics of automobile bluff bodies. most of the previous studies were focused on the Likewise, it was realized that the aerodynamic drag aerodynamic drag while the other aerodynamic of road vehicles was dominated by the pressure forces were rarely mentioned. Actually, the other drag due to the flow separation at the rear end of the aerodynamic forces are closed to the overall vehicle body (Morel 1978), and there had been concern performance, such as the side force to the crosswind about the influence of the rear configuration on stability and so on. Therefore, in current research, aerodynamic characteristics (Hucho 1993, Song et the sensibility of the other aerodynamic force to the al. 2012). Generally, in the wake of road vehicle, rear shape will be investigated and more there exist a recirculation region which is comprehensive experimental data will be provide as characterized by two small inner vortices and a pair reference to investigate the change law between the of streamwise longitudinal vortices (Ahmed 1983). different rear shape and aerodynamic characteristic. For a squareback which is typical for SUV or mini- Van, a region recirculation is the dominant feature A lot of attention had been paid to the wake because there are no slanted edges around which a characteristic of squareback, fastback and longitudinal vortex may form (Richards 2002). notchback, but these researches were based on Moreover, because of the large separation region in individual models. Because the overall the wake region, the contribution to the configurations were difference, it was very difficult aerodynamic drag will be more. For fastback to verify the influence of rear configuration. In geometry, the works of Janssen and Hucho (1975), order to investigate the influence of the rear Morel (1978) and Ahmed (1984) were amongst the configuration on aerodynamic characteristics first to investigate the relation between the shape of systematic, and compare the wake topologies of a vehicles rear end and the aerodynamic drag, and different rear configuration intuitively. In current identify the significance of rear slanted angle on research, a group of models which named MIRA aerodynamic characteristics. Their results showed reference cars (Le Good and Garry 2004), which that the wake topology and aerodynamic drag was had the same configuration in front but the rear strongly depended on the angle of the slanted rear parts were squareback, fastback and notchback, window α, and there was a critical angle (α≈30°). were employed. The numerical simulation For angles less than the critical angle, the flow first combined with wind tunnel test was employed to separated from the roof of the body, then re- investigate the aerodynamic characteristics and the attached on the slanted surface, and then separated wake structure. again at the rear end of the model. As the angles In the past two decades, Computational Fluid increased and approached 30°, the form drag of the Dynamics (CFD) has been used widely in vehicle vehicle increases sharply. For larger angles the drag aerodynamic studies (Himeno and Fujitani 1993, drops again, and it remains almost constant with Jindal et al. 2005, Murad et al. 2004 and Basara et further increased in α. Compared to the other two al. 2012). Over this period major advances in CFD rear end geometries, for a notchback vehicle which codes, computational algorithms, physical models is more typical for passenger car, the complex wake and methods, high performance computing structure

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