Proceedings of MARTEC 2010
The International Conference on Marine Technology
11-12 December 2010, BUET, Dhaka, Bangladesh

COMPUTATIONAL DRAG ANALYSIS OVER A CAR BODY
M. M. Islam1 and M. Mamun2
1
Department of Mechanical Engineering, Bangladesh University of
Engineering and Technology, Dhaka, Bangladesh.
E-mail: engg_monir09@yahoo.com

2
Department of Mechanical Engineering, Bangladesh
University of Engineering and Technology, Dhaka, Bangladesh.
E-mail: mdmamun@me.buet.ac.bd

ABSTRACT
In the recent times, CFD simulations, with the advent of computer architectures with superfast processing capabilities are rapidly emerging as an attractive alternative to conventional wind tunnel tests which are either too restrictive or expensive, for aerodynamic styling of a car. In vehicle body development, reduction of drag is essential for improving fuel consumption thus protects the global environment and driving performance, and if an aerodynamically refined body is also aesthetically attractive, it will contribute much to increase the vehicle’s appeal to potential customers. This paper outlines the process taken to optimize the geometry of a vehicle.
Vertices and edges were imported into Gambit and a computational domain created. An unstructured triangular mesh was then applied. The CFD program Fluent was used to iterate toward a converged solution with the goal of obtaining a better flow around the car and drag force. The results are analyzed and only the drag force is compared with a recognized journal to validate the results. These practices were detailed in hopes that further research would use the ground work laid out in this paper to redesign existing vehicles in order to improve handling and increase fuel efficiency.
Key words: CFD, car, drag, velocity vector, pressure contour, Gambit, Fluent.

1. INTRODUCTION
The rapidly increasing fuel prices and the regulation of green house gasses to control global warming have given tremendous pressure on the design engineers to enhance the current designs of the automobile using minimal changes in the shapes. To full fill the above requirements, design engineers have been using the concepts of aerodynamics to enhance the efficiency of automobiles [2, 4]. A lot of emphasize is laid on the aerodynamics in car design as an aerodynamically well designed car spends the least power in overcoming the drag exerted by air and hence exhibits higher performance- cruises faster and longer, that too on less fuel (Figure 1).
Apart
from improved fuel economy, aerodynamically superior car offers better stability and handling at highway speeds and also minimization of harmful interactions with other vehicles on the roadway. In optimization of car aerodynamics, more precisely the reduction of associated drag coefficient (CD), which is mainly influenced by the exterior profile of car, has been one of the major issues of the automotive research centers all around the world. Average CD values have improved impressively over the time, from 0.7 for old boxy designs of car to merely 0.3 for the recent more streamlined ones [1].

Figure 1. Fuel economy with reduction in drag coefficient (CD)
The Figure 2 shows the description of the fuel energy used in a modern vehicle at urban driving and highway driving. The shape of the vehicle uses about
3 % of fuel to overcome the resistance in urban driving, while it takes 11% of fuel for the highway driving. This considerable high value of fuel usage in highway driving attracts several design engineers to enhance the aerodynamics of the vehicle using minimal design changes [2].
The effect of drag on the moving vehicle is proportional to the square of velocity, so with increase

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Proceedings of MARTEC 2010

in velocity (at approximately 50 km/h), aerodynamic drag becomes one of the most prominent factors contributing to the total drag experienced by the vehicle [5].

imported into Gambit and a computational domain created. An unstructured triangular mesh was then