cfd study for a rear spoiler for sedan car

CFD STUDY OF A REAR SPOILER FITTED TO SEDAN CARMohd Aliff Bin Mohd Nor1, Aswatha Narayana2

1Bachelor of Engineering (Hons) (Mechanical)2 Faculty of Mechanical Engineering, Universiti Teknologi MARA (UiTM), Shah Alam

ABSTRACT

Aerodynamic forces are important aspects that need to be considered in the study of a road vehicle design. The present study focuses on the effect of rear spoiler fitted on a sedan car at different angles of attack with reference to drag and lift coefficient. The aim of this project is to compare the aerodynamic characteristics of a concept car between 2 different spoilers. The angles of attack of both spoilers have been varied. The method of study that was used in this project is simulation using Computational Fluid Dynamic (CFD), STAR-CCM+ software program. The car model is generated using CATIA V5R16 to create the 3-D geometry of the concept car together with rear mounted spoiler. Drag coefficient, (Cd) and Lift coefficient, (Cl) were obtained from the simulation processes. This thesis presents extensive discussion of numerical solution and the outcomes from the simulations. Comparisons of drag and lift coefficients were made with and without spoilers at different angles of attack.

INTRODUCTION

The performance, handling and comfort of an automobile are significantly affected by its aerodynamics properties. A low drag is a decisive prerequisite for good fuel economy. Increasing fuel prices and stringent legal regulations ensure that this long-established relationship becomes more widely acknowledged. But the other aspects of vehicle aerodynamics are no less important for the quality of an automobile such as side wind stability, wind noise, soiling of the body, the lights and the windows, cooling of the engine, the gear box and the brakes, and finally heating and ventilating of the passenger compartment all depend on the flow field around and through the vehicle.[1]

In the study of road vehicle aerodynamics design, the key concepts are the lift and drag components. A road vehicle must produce downforce in order to stay on the road. There are two ways of improving the vehicle’s downforce. The first method

is by increasing the car weight and second is by adding aerodynamics aids.

Aerodynamics aids are the components added to the vehicle in order to modify its aerodynamic properties. Aerodynamics aids such as spoilers and vortex generator are basically used to improve the vehicle’s performance without modifying the basic design of the vehicle itself. Today, almost all road vehicles are installed with aerodynamic aids. Among others, spoilers are one of the famous accessories for road vehicles. Even the new vehicles nowadays are already equipped with aerodynamics aids during manufacturing. However, some spoilers were merely installed for decoration purpose despite of its aerodynamics benefits.

Nowadays, with the high level of CAD prediction and pre-production evaluation coupled with a greater human understanding of aerodynamics, wind tunnel testing often comes into the design process later. The wind tunnel is the proving ground for the vehicle's form and allows engineers to obtain considerable advanced information within a controlled environment. Many elements of a vehicle's form only expose their behavior in air flow when carefully tested and cannot be estimated on computer. The reality of production, tolerances in components and accuracy of manufacturing can all play a part in affecting the aerodynamic behavior of a car.

DragDrag is determined by the amount that the air is pushed against the car's surface. It is generated by the interaction and contact of a solid body with a fluid. Drag is generated by the difference in velocity between the solid object and the fluid. LiftFor standard cars little or no lift is desired. .  In a racing car on the other hand both high downforce and little drag is needed.  Downforce enables the car to turn round corners at a very high speed because it effectively adds weight to the car and keeps the wheels firmly on the road at very high speeds.  Spoiler

A spoiler is an aerodynamics device attached to an automobile which its intended design function is to 'spoil' unfavourable air movement across a body of a vehicle To minimize the effect of lift force, spoiler is mounted at the rear of the car.  Spoilers work by blocking the flow of air as it comes off the rear windscreen.  This creates a high pressure area in front of the spoiler and therefore more downforce.

Different angles of attack of spoilers can give different effects to the car. The objective is to perform numerical simulation of rear mounted spoiler and to predict the performance of a concept car. Analysis has been made with rear spoilers at different angles of attack of 2 spoiler’s shapes using CFD. The chord length of the spoiler is 240 mm and the span is 1634 mm. Three angles of attack were chosen for the spoiler which is 0˚, +ve5˚ and –ve5˚ degrees angle of attack.

METHODOLOGY

CAD Model GenerationThe modeling generation is done by using CATIA V5R16 to create the 3-D geometry of the concept car and its rear wings (Flat plate and NACA 0012) at different angles of attack. The drawings were done in part drawing and saved by IGES (.igs) file. Drawings were later exported to the STAR-CCM+ and then the solver is run to get the result for drag and lift coefficients.

Figure 1: Concept car without spoiler

Flat plate spoiler with different angles of attack:

Figure 2: Concept car with 0˚ (angle of attack) flat plate spoiler

Figure 3: Concept car with +ve5˚ (angle of attack) flat plate spoiler

Figure 4: Concept car with -ve5˚ (angle of attack) flat plate spoiler

Figure 1 shows the drawing of a concept car without spoiler. Figure 2, 3, and 4 shows the concept car with flat plate spoiler at 0˚, +ve5˚, and -ve5˚ angles of attack at a height of 200 mm from the car’s boot.

NACA 0012 spoiler with different angles of attack:

Figure 5: Concept car with 0˚ (angle of attack) NACA 0012 spoiler

Figure 6: Concept car with +ve5˚ (angle of attack) NACA 0012 spoiler

Figure 7: Concept car with -ve5˚ (angle of attack) NACA 0012 spoiler

Figure 5, 6, and 7 shows the concept car with NACA 0012 spoiler at 0˚, +ve5˚, and -ve5˚ angles of attack at a height of 200 mm from the car’s boot. After that the drawings were exported to STAR-CCM+, just after the Boolean operation were done. Boolean operation is an operation to make a car model inside the wind tunnel.

Grid Independence StudyBefore the model is meshed, the grid independence study has been done. Although the types of meshing neither fine, coarse nor medium gave an acceptable results, best grid type must be chosen to minimize the time taken during the simulation processes.

Surface Boundar

y

Surface Mesh (cell)

Volume Mesh (cell)

Drag coeff.Cd

COARSE 47 13341 7911 0.575MEDIUM 51 13632 8325 0.565

FINE 57 13825 8537 0.560

Table1: Grid independent study

Table 1 shows the grid independent study for three tessellation density which is coarse, medium and fine. Medium tessellation density is chosen for the grid type as the results are almost the same with the fine grid condition. Besides, the time taken to complete the simulation is faster than the simulation with fine grid.

RESULTS AND DISCUSSION

The simulations were conducted to find the effects of different angles of attack (AoA) for different spoiler designs to the drag and lift coefficient of the concept car. The simulation is done at the speed of 110 km/h. The spoilers were mounted to the car for the simulation. From these two different spoilers the results were compared to choose the type of spoiler to be implemented to the passenger car. Figure 8 and 9 shows the results for drag and lift coefficients for the

case of cars without spoiler at 110km/h. Results for the simulations with flat plate spoiler are as shown in Table 2 and the results for the simulation with NACA 0012 spoiler are as shown in Table 3 respectively. The result shows that the spoiler with zero degree angle of attack has the lowest drag while the spoiler with positive angle of attack has the highest drag. It can be observed that the negative values of the angles of attack produce a positive value for the lift coefficient. The simulation results show that the NACA 0012 spoiler generates less drag coefficient, Cd compared to the flat plate spoiler. Therefore, it can be concluded that the NACA 0012 spoiler is better and suitable to be implemented to the passenger car in Malaysia. The normal drag force for the passenger car is in the range of 0.25 to 0.60. The negative value of angle of attack will produce the positive value for the lift coefficient that tends to lift up the car. Therefore, negative value of angle of attack should not be implemented to the spoiler AoA as it will ruin the stability of the car.

Figure 8: Drag coefficient

Figure 9: Lift coefficient

Spoiler’s AoA

Drag coefficient

Cd

Lift coefficient Cl

Without Spoiler

0.539 0.092

0˚ 0.571 -0.070+ve5˚ 0.604 -0.026-ve5˚ 0.573 0.014

Table 2: Result of Drag and Lift Coefficient for the flat plate spoiler

Spoiler’s AoA

Drag coefficient

Cd

Lift coefficient Cl

Without Spoiler

0.539 0.092

0˚ 0.550 -0.085+ve5˚ 0.564 -0.021-ve5˚ 0.563 0.012

Table 3: Result for Drag and Lift Coefficient for the NACA 0012 spoiler

CONCLUSION

The effect of rear mounted spoiler to concept car has been study in this report. The numerical solution of rear mounted spoiler and the comparisons of the aerodynamic characteristics of a concept car between 2 different spoilers have been made. From the result of the simulations, it can be concluded that the car with rear spoiler mounted on it will give better performance than the car without the rear spoiler. Besides, the simulation showed that NACA 0012 spoiler generates less drag coefficient, Cd compared to the flat plate spoiler. Therefore, NACA 0012 spoiler is more suitable to the passenger car compared to the flat plate spoiler. A greater angle of attack of the wing or spoiler creates more drag. Negative angle of attack for the spoiler is actually not favourable as it will tend to lift up the car.

REFERENCES

1. Douglas J. F., Gasiorex J. M. and Swaffield J.A.; “Fluid Mechanics” 4th edition Prentice Hall; 2001.

2. Munson B. R. Young D. F., and Okishi T. H.; “Fundamentals of Fluid Mechanics”; 5th

Edition John Willey and Son, Inc ; 2006.3. Wolf-Heinrich Hucho, Aerodynamics of

Road Vehicles, Butterworths, 1986.4. H.K Versteeg & W Malalasekera,

Introduction to Computational Fluid Dynamics, 1995.

5. McCabe, G., Explanation and Discovery in Aerodynamics, 2005.