Vehicle Engineering (VE) Volume 2, 2014 www.seipub.org/ve Aerodynamic Characteristics of Sedan with the Rolling Road Ground Effect Simulation System Yingchao Zhang 1, Linlin Ren 1, Kecheng Pan 2, Zhe Zhang* 1 1 State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130012, China 2 China FAW group corporation R&D center, Changchun 130012, China * zhangzhejlu@jlu.edu.cn Abstract The rolling road ground effect simulation system is one of the key factors affecting the accuracy and reliability of automotive wind tunnel test. It is necessary for wind tunnel experiments to reduce the influence of ground boundary layer effect in wind tunnel. In this paper, several CFD simulations were carried out to investigate the turbulence airflow and its differences with the wheel-rotation and the under-hood flow field. And the car model is one productive car model designed by Audi. It simulated the different flow field of automotive wind tunnel with or without ground effect rolling road system respectively based on Star - CCM+ CFD code. These works focus on the following three parts: first is setting up monitoring points in different positions of flow field, second is monitoring their velocity through vector and streamlines, and third is analyzing the two cases by comparing them. Rolling road system with belt width bigger than car width has great effect on accelerating the air velocity near the ground at the bottom of the model to control the ground boundary layer development around model. In the flow field behind the model, using the rolling road system with wide belt is a wise choice to reduce the velocity of the wake flow near the ground. Keywords Automotive Wind Tunnel; CFD; Aerodynamics of Road Vehicle; Ground Effect Introduction As one of the most important features on automobile, automotive aerodynamic characteristic has a close relationship with the related characteristics of the internal and external flow field near the automobile. Automotive wind tunnel is the main test site, in which we analyze and verify the aerodynamic theory and the corresponding parameters in experimental study, so a lot of important conclusions on automotive aerodynamics are derived from it. Nowadays there are many tunnels in use around the world, so Jilin University s building its own automotive wind tunnel is a substantial beginning of automotive wind tunnel test theory and technology development in China. Jilin University wind tunnel adopted the widely used rolling road system with belt to reduce the ground effect in wind tunnel test. In wind tunnel test, there are boundary layers near the test ground in flow field, which does not tally with the actual driving conditions, and will reduce the reliability of results. The above phenomenons are named as ground effect of Wind tunnel test. Many studies have shown that too thick boundary layer is to be generated near test ground due to the ground effect of Wind tunnel test, which will directly influence the measuring results of six-component force in wind tunnel test and reduce the measured drag coefficient of the car. In this paper, in order to supplement the domestic wind tunnel test techniques in theory and improve the domestic wind tunnel test techniques, we analyzed the influence of broadband rolling road system on wind tunnel test after simulation. Simulation Details In order to reflect the condition differences between wide belt rolling road and without rolling road more factually, the digital model is quite similar to a real one, as shown in Fig. 1. 31
www.seipub.org/ve Vehicle Engineering (VE) Volume 2, 2014 FIG. 1 DIGITAL MODEL OF A CAR USED IN CFD When building the model, its engine compartment structure was retained to ensure the validity and authenticity of the results, which can simulate the airflow s influence on ground effect when entering engine compartment from air-inlet grille and one kind of parts such as mirrors were also retained. As a result, the CFD simulation based on this digital model is more reliable and significant. some parameters of the model: Scale 1:2, overall length 2424mm, overall width 971mm, overall height 721mm, wheelbase 1427.5mm. model details: Main components in chassis: aisles exhaust pipe, fuel tank, spare tire compartment, air-inlet grille, engine, simplified engine nacelle, wheels with hub, wheel cover cavity, mirrors and doorknobs. The computational domain s width is about 5 times wider around each car model width. The distance from the domain s inlet to the front body is about 5 times vehicle length and that from domain s outlet to the rear body is about 10 times vehicle length meanwhile that from domain s top to the body top is about 5 times vehicle height. The parameters of the domain are displayed as follows: 37m long, 10m wide, 4.154m high, and all these are showed in Fig. 2. FIG. 3 FINAL VOLUME MESH According to the requirements of the simulation, the needed physical models were chosen as follows: three dimensional, steady, segregated flow, ideal gas and turbulent. The velocity in this simulation is respectively 80, 100, 120, 140 and 160. The front side of the domain was set as velocity inlet with 80, and the back side was set as pressure outlet with 0Pa. The other three sides were all slip boundaries and the model surface was wall. Table1 displays the initial conditions in this simulation. TABLE 1 INITIAL SIMULATE CONDITION Items Initial Condition Inlet Velocity Inlet Outlet Pressure Outlet Atmospheric Pressure Standard Atmospheric Pressure Air Density 1.205kg/m3 Turbulence Model Standard K-Epsilon State of Wheels Rotating CPU 8 cores Memory 16G Iterations 1800 Results and Discussions Values of drag coefficient and lift coefficient in each condition can be obtained from the simulation. After finishing 1800 iterations, the drag coefficient Cd and coefficient Cl have been stable, and this can be seen in Table 2 and Table 3. 80 TABLE 2 CALCULATION RESULTS OF CD 100 120 140 160 Slip 0.423 0.422 0.417 0.417 0.418 Sta 0.403 0.41 0.409 0.409 0.415 ΔCd 0.02 0.012 0.008 0.008 0.003 TABLE 3 CALCULATION RESULTS OF CL FIG. 2 COMPUTATIONAL DOMAIN In this paper, having considered factors such as the computer load, we eventually adopted core hexahedron mesh to obtain better grid quality. To emphatically analyze the ground effect, we increased the density of the flow field mesh at the bottom model, around the adjacent parts and near the rear model in simulation, which can improve the mesh quality around the model. The mesh conditions are showed in Fig. 3. 80 100 120 140 160 Slip 0.286 0.285 0.272 0.280 0.278 Sta 0.251 0.261 0.258 0.254 0.266 Sta in Table 2 refers to having not a mobile belt (the ground of mobile area is stationary); slip refers to having a mobile belt. It can be seen that Cd and Cl in mobile ground system are higher than those in ground system without moving belt, and this because the mobile ground system improved the flow field quality near ground surface under the model body. 32
Journal Title Volume XX Issue XX, Month Year Journal Website A. GROUND SYSTEM WITH MOVING BELT B. PRESSURE DISTRIBUTION AT TOP MODEL FIG. 5 PRESSURE ON MODEL MIDDLE PLANE B. GROUND SYSTEM WITHOUT MOVING BELT FIG.4 VELOCITY DISTRIBUTION ON MODEL MIDDLE PLANE Velocity distribution pictures can be obtained from the simulation, just like Fig. 4 under the condition of 160kph. From the velocity distribution in B, we can see that there is a boundary layer with a certain thickness caused by ground effect which can make the airflow near ground delays. While the ground system with moving belt increased the velocity near the ground surface under front seats, and generated more severe eddy current in the central aisle when compared with the ground system without moving belt. At the same time, the moving belt improved the velocity near the ground under model rear. In Fig. 5, the pressure distributions on middle plane of the model in different systems (ground system with moving belt and ground system without belt) are similar. However, the surface pressure at rear bottom of the model on moving belt is a little higher than that in ground system without moving belt. FIG. 6 PRESSURE ON MOVING BELT MIDDLE PLANE Comparing curves in Fig. 6, we find that the moving belt can reduce the surface pressure of the back moving belt and that on moving belt surface (Z=0m). Values of monitoring points on X=1.5, Y=0 are showed in Fig. 7. FIG. 7 VELOCITY DISTRIBUTION ON X=1.5, Y=0 The airflow s direction changed to opposite X direction on X=1.5, Y=0, in which situation the ground system with moving belt had multiple effects on the airflow velocity near ground: A. PRESSURE UNDER MODEL Increasing the minimum velocity of the opposite X direction at the above position; 33
www.seipub.org/ve Vehicle Engineering (VE) Volume 2, 2014 Decreasing the maximum velocity of the opposite X direction at the above position; As a whole, the moving belt makes the airflow distribution more uniform. In order to analyze the above phenomenon more deeply, we compared it with other results. The velocity distribution on section of Z=-0.13 at 160kph in ground system with moving belt is showed in Fig.8. It is not difficult to find that there is a large area near ground locating at the middle-rear part of the model, in which the velocity is opposite to X direction. In Fig. 9, there is one large and one small eddy current at the middle-rear part of the model, and the above two phenomenons can explain why the Cd values in system with moving belt is higher. Further analysing velocity scalar of region Z = -0.13 and streamlines at the model bottom, we can find the airflow in moving region along negative X axis is substantially located around the engine outlet, and scatters along the body edge, as shown in Fig. 10, in which the airflow also moving to wheels. Inferred from the above analysis, the airflow was forced by the high-pressure zone near the outlet and the wheels rolling disturbance.to be split along both sides at the model bottom. Conclusions Basing on the above analysis, we can draw the following conclusions: FIG. 8 VELOCITY ON HORIZONTAL SECTION (Z=-0.13) The ground system with moving belt can accelerate the airflow on ground surface to reduce the thickness of boundary layer, and disturb the viscous flow under the model. The moving belt can control the development of the ground boundary layer under the model. FIG. 9 VELOCITY STREAMLINES UNDER MODEL BODY Although the flow field condition near ground can be kept well at the back of the model and the moving belt as a result of the moving belt s accelerating effect, the moving belt can t inhibit the development of boundary layer so well as before. The faster the inflow velocity is, the more obvious the moving belt s accelerating effect is and the more effective the moving belt s improvement on boundary layer is. A.VELOCITY VECTOR ON SECTION OF Z=-0.13 Using moving belt can make the pressure in moving belt surface area and the back area of the moving belt lower. The pressure distribution on middle plane of the model in condition of using moving belt is similar with that in condition of not using moving belt. ACKNOWLEDGMENT B.VELOCITY VECTOR AT X=1.5 FIG. 10 VELOCITY VECTOR (Z=-0.13, X=1.5) Thanks for China's postdoctoral fund (2012M510874 & 2012T50314) and Hong Kong scholars program s support. 34
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