Aerodynamic designing and position optimization of roof box

  • Bryant George

    Student thesis: Master's ThesisMaster of Science by Research


    This project examines the influence of aerodynamic factors of newly designed roof box at various positions over different car geometries. A baseline design of roof box was created and tested using MIRA reference cars in three different configurations at various positions to understand the behavior of the flow. A scaled design of roof box was also considered to understand the trends at various positions. Further investigation was conducted by varying the shape of the box. There was a lack of information in the literature on this subject. The existing data was insufficient to predict the optimum position of a roof box on a vehicle to reduce aerodynamic drag force.

    This project includes the analysis of a baseline design of roof box and its design variation which was inspired from Thule Motion 800 using CATIA V5 R20. It also includes the analysis at various positions in three perpendicular directions over MIRA vehicles. The key factor analyzed throughout the project was drag coefficient. In parallel, moment coefficients and flow behavior around the car and box were also evaluated.

    The numerical investigation of flow around MIRA cars and roof box were carried out using Computational Fluid Dynamics toolbox - Star-CCM+ V8.04. Due to the large number of experiments required to understand the influence of the position variation of roof box, Design of Experiments (DOE) using Taguchi method and general full factorial method in MINITAB were employed. Using Taguchi method, two-thirds of the simulations can be reduced.

    Out of the three geometric shape vehicles tested, the roof box has the highest influence on fastback car and least effect on squareback car. The average variation of drag due to roof box on squareback and fastback cars were 3% and 30% respectively. It was observed that there was a difference of 20% drag between the lowest and highest drag obtained with fastback car and that of squareback car was 5%. Nearly 6kW excess engine power was required to pull the fastback car along with fully loaded roof box. Due to the variation in design of roof box, 6% drag reduction was obtained on fastback car. Even if the individual drag due to roof box was higher, the overall drag remains less due to the interaction of flow between car and roof box. The results from various positions of roof box were compared using both methods in MINITAB. The result shows that Taguchi method was suitable to reduce the number of simulations in external aerodynamics.
    Date of Award2014
    Original languageEnglish
    Awarding Institution
    • Coventry University
    SupervisorRemus Cirstea (Supervisor)


    • Automotive
    • aerodynamics
    • car
    • vehicle

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