Pickup Trucks - Box Configuration and Drag Reduction

Transcription

1 28th AIAA Applied Aerodynamics Conference 28 June - 1 July 2010, Chicago, Illinois AIAA AIAA Pickup Trucks - Box Configuration and Drag Reduction Wael A. Mokhtar * Grand Valley State University, Grand Rapids, MI and Robert E. Camp ewpage Corporation, Rumford, ME Aerodynamics plays an important role in ground vehicle design and optimization. With the worldwide interest in saving energy, drag reduction is one area where aerodynamics can help. The aerodynamics of pickup trucks is studied. The main objective is to understand the effect of the box configuration on the generated aerodynamic drag. In addition to that, the flow structures around the vehicle are analyzed. The CFD simulations of a generic pickup truck are performed using STAR CCM+ package developed by CD-Adapco Inc. Several commonly used pickup truck configurations are presented including open-box, a box with tonneau cover, and a pickup truck with a flat cap. It was found that the tonneau cover reduced the generated drag significantly. The cap cover generates slightly more drag than the open-box case. I. Introduction and review nergy saving is a worldwide movement at all levels starting from the home equipment to the industrial Emachinery. Ground vehicle is one area where energy saving can be very effective. Pickup trucks have been one of the most popular vehicles in the market although they have relatively high gas mileage. They are attractive because of their good ground clearance, towing capability and four-wheel-drive systems. In other words, pickup trucks present a cheaper alternative to SUV s (Sports Utility Vehicle). Both pickup trucks and SUV s would have the same rolling resistance if they have the same weight and tire size. However, SUV s have much better aerodynamic performance in terms of drag and stability. The box region is one of the regions in the truck that effective improvements can be implemented. Several studies were presented including the use of experimental and numerical techniques. For example, Al-Garni et al. 1 presented an experimental study for pickup trucks aerodynamics 1. The focus of the study was to provide experimental data for a typical pickup truck with open-box. They used PIV to measure the flow near to the wake of a generic pickup truck. Zhigang and Khalighi 2 later presented a CFD study for the same generic pickup truck with open-box using a steady state solver and showed that the predicted flow structures compared well with the measured mean flow. The box length was the main focus in the study. Massarotti and Valarilli 3 also presented a CFD study for a generic pickup truck using a steady state solver. Their study included several modifications for the cab and the box for an open-box pickup truck. Both studies indicated that steady state solver presents a good and fast tool for the CFD analysis. Cooper 4 studied the effect of the tailgate on the generated drag for pickup trucks. He presented results from fullscale wind tunnel testing followed by some results from a CFD analyses to visualize the flow structures around the vehicle. He used an unsteady solver for the CFD study and the CFD part was limited to zero yaw angle pickup truck with tailgate up and tailgate off. Zhu and Yang 5 presented a CFD study for pickup trucks. The simulation was based on an unsteady formulation. The main focus of this study was to assess the use of RMS (Reynolds Stresses Model). The CFD results compared well with the wind tunnel results reported by Al-Garni et al 1 for a pickup truck with open-box. * PhD, Assistant Professor of Mechanical Engineering, AIAA senior member. ACE Engineer. 1 Copyright 2010 by Dr. Wael Mokhtar. Published by the, Inc., with permission.

2 II. Present work Recently, the current authors have studied the effect of the tailgate configurations on the aerodynamic drag and the flow structure around a pickup truck. The study included four configurations: flat bed (no walls for the box), tailgate off, tailgate down and tailgate up. 6 In the present work, the previous study is extended to include the effect of serveral configuations of bed covers. The main focus is to investigate the flow structures around several configurations of the pickup trucks and to study their effects on the aerodynamic drag. Three configurations are studied: open-box, a box with a tonneau cover, and a pickup truck with a flat cap. Figures 1 shows the studied pickup truck configurations. Four speeds are used to cover the effective range of aerodynamic drag, Wood and Bauer 7. The study is limited to zero yaw angle. Table 1 summarizes the studied parameters. Open-box Tonneau cover Cap Figure 1: Pickup truck configurations Vehicle configuration Yaw Angle (deg) Speed (mph) Open-box Tonneau cover Cap 0 45, 55, 65 & 75 Table 1: Studied Cases III. Computational method The present study is done using STAR CCM+ code. The code is a computational finite volume method for structured and unstructured grids developed by CD-Adapco Inc. In the current study, a segregated flow scheme is used to solve the three dimensional Reynolds Averaged Navier Stocks Equations (RANS). The code uses AMG SIMPLE implicit solver. Gauss-Seidel relaxation scheme was used in solving both the velocity and the pressure. The k-ε two-equation turbulence model is used. A generic extended cab pickup truck geometry is used in the study. The reference frontal area is m 2. The model external dimensions: length, width and height are 5.3, 1.78 and 1.9 m respectively. The cross sectional area of symmetrical numerical domain around the vehicle has a 0.78% area blockage ratio. The volume blockage ratio of the domain is 0.23%. The overall dimensions of the model match a typical 6.5-foot box pickup truck Unstructured polyhedral grid set is generated for each case with clustering near to the vehicle surfaces, corners and curved surfaces. In average, about six to seven hundred thousand cells were used in the domain. Figure 2 shows a close up for a sample of the grid sets used in the study. Wall boundary conditions were applied to the vehicle surfaces with no-slip conditions. For the moving ground, similar wall boundary conditions were used but with zero relative tangential velocity to the moving air. The rest of the domain boundaries are defined as far-field boundaries where the air speed direction can be adjusted for the yaw angle. 2

3 Figure 2: Sample of the unstructured grid used in the study. IV. Results and discussion For each one of the studied cases, a general description for the flow structure is presented in the following sections with more focus on the box region. All the cases are built from the same generic pickup truck geometry and all the changes are made in the box cover for consistency. The overall effect on the generated aerodynamic drag is discussed at the end of this part of the paper. All the cooling openings are sealed and the underbody effects are eliminated for simplicity. In other words, the cooling and part of the underbody drags are not represented in the current study. A. Open-box pickup truck Figure 3 shows the pressure contours for an open-box pickup truck at 75 mph. A major stagnation region can be seen at the truck front surface. Another high pressure region can observed between the hood and the windshield. The airflow is accelerated at the beginning of the hood surface around the corner. It is decelerated again at the beginning of the windshield due to the change in direction. Over the cab roof, the airflow is accelerated. High pressure regions can be observed on the front surfaces of the wheels. Figure 3: Pressure contours for an open-box pickup truck. Downstream to the cab, the flow separates and forms a recirculation region above the bed as shown by the streamlines plotted in Figure 4. Because of the reduced pressure in this recirculation region, side airflow is redirected toward the center of the truck to form a conical shape separation region above the bed. The shear layer around the separation region drives the circulating flow inside as shown in the figure. To visualize the wake shape, an iso-total-pressure surface is generated as shown in Figure 5. The conical-like shape of the wake behind the cab can be seen. Part of the box is not included in this separation region and it is in contact with the airflow that comes from the two sides of the truck around the sidewalls. Downstream to the tailgate, two identical conical-shape wakes are formed. The direction of the circulation flow inside these two wake regions can be seen in the tangentialvelocity vector field plotted on a plane downstream to the tailgate, Figure 6. 3

4 Figure 4: Streamlines around an open-box pickup truck. Figure 5: Wake represented by iso-total-pressure surface, open-box truck. Figure 6: Tangential-velocity vector field on a plane 0.5 m downstream to the tailgate, open-box truck. The air in the large separation region inside and above the open-box is driven by the moving air through a shear layer. This circulating flow decreases the pressure and adds to the generated drag. Figures 7 through 10 show the total pressure at the beginning, middle and end of the box and downstream to the tailgate. The shear layer enclosing the separation region can be seen clearly. Downstream to the cab, the size of the wake is the same to as the back surface of the cab, Figure 7. As the plotting plane is moved in the downstream direction, it is clear that the size of the wake is decreased by the flow coming from the sides of the tuck. Near to the end of the box and upstream to the tailgate, only a small volume outside the box is included in the wake as shown in Figure 9. It is clear that the flow coming from the sides of the vehicle plays an important role in sizing the wake above the box region. Downstream to the tailgate, Figure 10, it is 4

5 clear that the formation of the two separation regions generates strong circulating flow and decreases the total pressure significantly. It can be seen that several factors in the flow structures inside and above the open-box may affect the generated drag. The low pressure in the large separation region behind the cab and in the circulating flow downstream to the tailgate increase the drag. As shown in Figure 5, part the upstream surface of the tailgate is not covered by the wake and expected to have a high pressure region, stagnation-like effect, that increases the drag. The wakes downstream to the wheels, Figure 7, 9 and 10, also contribute to the generated drag. Covering the box seems to be one of the solutions that can reduce these effect and ultimate decrease the drag. In the following sections two methods are discussed. Figure 7: Total pressure on a plane located just downstream to the cap, open-box truck. Figure 8: Total pressure on a plane located at the middle of the box, open-box truck. Figure 9: Total pressure on a plane located upstream to the tailgate, open-box truck. Figure 10: Total pressure on a plane located 0.5 m downstream to the tailgate, open-box truck. 5

6 B. Pickup truck with a tonneau cover Figure 11 shows the static pressure contours for a pickup truck with a tonneau cover. Compared to the open-box case, no major difference can be observed. On the cover, a low pressure region can be seen near to the cab and a high pressure region can be seen near to the downstream end of the cover. The rest of the pressure distribution of the front part of the vehicle is the same. In other words, the only active factor is the box cover. Downstream to the cab, a separation region can be seen in the streamlines plot shown in Figure 12. Again no changes, compared to the open-box case, can be observed on the front part of the vehicle. The size of the separation region seems to be smaller than the open-box case. Because of this reduction in its volume, the strength of the circulating flow is expected to be higher. Figure 13 shows the size of the wake represented by an iso-total-pressure surface. The overall structure is similar to the open-box case. The flow that entrained from the sides decreases the size of the wake downstream to the cab. With the closed box, this airflow stretched the wake as shown in Figure 13 to be slightly longer that the bed. Downstream to the box, two conical shape wakes can be seen. All three wakes merge together and form a three-core wake downstream to the truck. Figure 14 through 17 shows section views in the flow downstream to the cab. The flow is represented by total pressure contours. Figure 11: Pressure contours for a pickup truck with a Tonneau cover. Figure 12: Streamlines for a pickup truck with a Tonneau cover. 6

7 Figure 13: Wake represented by iso- total-pressure surface for a pickup truck with a Tonneau cover. Figure 14: Total pressure on a plane located just downstream to the cab for a pickup truck with a Tonneau cover. Figure 15: Total pressure on a plane located at the middle of the box for a pickup truck with a Tonneau cover. Figure 16: Total pressure on a plane located upstream to the tailgate for a pickup truck with a Tonneau cover. Figure 17: Total pressure on a plane located 0.5 m downstream to the tailgate for a pickup truck with a Tonneau cover. 7

8 In this case, the wake is slightly elevated since the box is closed. Same starting size of the wake downstream to cab can be seen in Figure 14. Near to the middle of the box, a rectangular cross-section of the wake can be seen in Figure 15. At this location, it can also be observed that the wake is elevated and has smaller size compared to the open-box case. The strength of the circulating flow inside the wake, represented by negative pressure, is higher. Near to the end of the box, Figure 16, the shape of the wake cross-section is nearly a circle. This shows that the airflow that comes from the car sides and over the cab roof have similar effect on forming the wake. Downstream to the box, Figure 17, although the wake size is larger than the open-box case, the strength of the circulating flow is smaller. The generated drag for a pickup truck with the tonneau cover is expected to be smaller than the open-box case. It forms a smaller volume and longer wake. Downstream to the box the wake is much weaker. The stagnation-like effect on the upstream surface of the tailgate disappears. The skin friction between the cover surface and the airflow will add to the drag. This part is not expected to be significant because most of its area is covered by the wake, low speed flow, as shown in Figure 13. C. Pickup truck with a cap The flat cap case is totally different from the other two cases that were discussed in the previous sections. For this case, the separated flow over the box and downstream to the cab disappears. Figure 18 shows the pressure contours for a pickup truck with a flat cap. The front part of the truck has the same characteristics as the previous two cases. No flow separation can be observed over the surface of the cap as shown in the streamlines plotted in Figure 19. Also on the same figure, the wake downstream to the truck can be seen. Another representation of the wake can be seen in the iso-total-pressure surface presented in Figure 20. Figures 21 through 24 show section views of the flow around the cap and downstream to the truck represented by total pressure contours. The figures confirm that no flow separation occurs over the cap surface. The size of the wake downstream to the truck, Figure 24, is much larger than the open-box or the tonneau cover cases. The strength of the circulating flow is higher. Figure 18: Pressure contours for a pickup truck with a cap. Figure 19: Streamlines for a pickup truck with a cap. 8

9 Figure 20: Wake represented by iso- total-pressure surface for a pickup truck with a cap. Figure 21: Total pressure on a plane located at the beginning of the box for a pickup truck with a cap. Figure 22: Total pressure on a plane located at the middle of the box for a pickup truck with a cap. Figure 23: Total pressure on a plane located at the end of the box for a pickup truck with a cap. Figure 24: Total pressure on a plane located 0.5 m downstream to the back of the box for a pickup truck with a cap. 9

10 Comparing the cap case with the open-box and the tonneau cover truck, the drag is expected to be lower because the separated flow in the box region is replaced by the cap. However, the tonneau cover case also decreases the size of the separated flow and has the advantage of having a smaller wake downstream to the truck. Figure 25 shows the drag coefficient for the three studied cases. The tonneau cover case has the lowest drag and the cap cover as the highest drag. Two main factors seem to increase the drag for the cap case. The first is its increased surface that will increase the skin friction component of the drag. The other part is the large wake downstream to the truck which will increase the form drag. The tonneau cover case has the advantage of decreasing the size of the separated flow and at the same time its effect on the form drag seems to be minimal. All three configurations follow the same trend of dependency on the vehicle speed with a minimum drag around 60 mph. It is important to indicate that the drag coefficient is slightly lower than the expected value for a pickup truck of this size because the cooling drag and most of the underbody effects are not included in this study CD Open box Tonneau cover Cap Speed (MPH) Figure 25: Aerodynamic drag for several pickup truck configurations. V. Conclusions A CFD study of a pickup truck with several box configurations was presented. Three cases were discussed: open-box, tonneau cover, and flat cap trucks. Flow structures were presented. The main characteristic for this vehicle is the large separated flow over the box region that contributes to the form drag. It was found that the tonneau cover truck has the lowest drag. The difference between the drag generated by an open-box truck and a truck with a flat cap is not large. The first has a large separated flow over the box while the other has a large wake downstream to the vehicle. Also the truck with a flat cap has more surface area and generates more skin friction drag. References 1 Al-Garni, A., Bernal, L., and Khalighi, B., Experimental Investigation of the Near Wake of a Pick-up Truck, SAE Paper No ,

11 2 Yang, Z., and Khalighi, B., CFD Simulation of Flow over Pickup Trucks, SAE Paper No , Massarotti, M. and Valarelli, I, CFD Assessment of Aerodynamic Enablers for Pickup Trucks, 20 th International Congress of Mechanical Engineering, COBEM, ABCM, Gramado Brazil, Nov Cooper, K., Pickup Truck Aerodynamics Keep Your Tailgate Up, SAE Paper No , Zhu, H. and Yang, Z., Simulation of Flow around a Generic Pickup Truck with RSM Model, SAE Paper No , Wael A. Mokhtar, Colin P. Britcher, and Robert E. Camp Further Analysis of Pickup Trucks Aerodynamics, SAE World Congress, SAE paper no 09B-0197, April Wood, R. and Bauer, S., Simple and Low-Cost Aerodynamic Drag Reduction Devices for Tractor-Trailer Trucks, SAE Paper No ,