IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 14, Issue 6 Ver. IV (Nov. - Dec. 2017), PP 18-29 www.iosrjournals.org DOI: 10.9790/1684-1406041829 www.iosrjournals.org 18 | Page Effect of an Add-On Device on the Aerodynamic Characteristics of a 3-Dimensional Ahmed Body A.O. Muritala 1† , H.A. Fatokun 1 and S.O. Obayopo 1 1 Department of Mechanical Engineering, Obafemi Awolowo University, Ile-Ife, Osun State, 220005,Nigeria Abstract: This study investigated the effect of add-on-device (side-view mirrors) on the aerodynamic characteristics of a 3-Dimensional Ahmed body using both numerical and experimental methods. It aims at controlling aerodynamic drag on vehicles by determining the optimum mirror position for a simplified vehicle geometry known as Ahmed body model. The rear slant angle of 25 ° has been used as a benchmark. The geometry is generated in ANSYS ’14 design modeler with a single domain of air created surrounding the model after subtracting it from the air enclosure. The turbulent flows around the 3D Ahmed body model was solved using the realizable k-epsilon model with non-equilibrium wall function for near wall treatment with an inlet velocity of 40 m/s. For geometrical optimization, the following were used: distance between the attachment plate and the mirror ranges from 5-20 mm in an increment of 5 mm; height of the mirror foot ranges from 3-9 mm in an increment of 2 mm and the angle of inclination of the foot between -10 ° to +10 ° in an increment of 5 ° . The small scale prototype of Ahmed body was produced from a Prusa-i3 12 volts 3D printer. This was tested in a wind tunnel to determine the aerodynamic forces (drag and lift). The validation was done by comparing the small scale numerical modeling results with the experimental results obtained from the wind tunnel. The results from this study show that the position of the side-view mirror contributes to drag on the entire vehicle. Minimum drag coefficient C D = 0.3022 and the corresponding lift coefficient C L = 0.3410 was obtained at mirror position 20 mm foot length, 5 mm foot height and 10 ° angle of inclination. This position added 6% to the drag coefficient of the Ahmed body 25 ° model. The study concluded that the position of the side mirror contributes a significant effect when it comes to drag on the entire vehicle. Keywords: Ahmed body, Drag coefficient, Lift coefficient, Turbulent flow, Aerodynamic forces and side mirror. --------------------------------------------------------------------------------------------------------------------------------------- Date of Submission: 12 -12-2017 Date of acceptance: 05-01-218 --------------------------------------------------------------------------------------------------------------------------------------- I. Introduction Cars were mainly designed for high speed maneuver, comfort and safety before the oil crisis (Chien- Hsiung et al., 2009). However, automobile fuel efficiency standards have become more stringent due to the demand from governments and consumers leading to vast amounts of aerodynamic studies on vehicles (Jonathan et al., 2015).Automotive aerodynamics comprises of the study of aerodynamics of road vehicles aiming at reducing drag, minimizing noise emission, improving fuel economy, preventing undesired lift forces and minimizing other causes of aerodynamic instability at high speeds. A very important aerodynamic force is the drag which is caused by the pressure difference between the frontal and the rear end of the vehicle. It can be reduced by modification of the vehicle profile or systematic modification of the air flow system around the vehicle. It is necessary, at times, to generate down force to improve traction and thus cornering abilities. Another aerodynamic force is the lift which can be dangerous for an automobile, especially at high speeds. So, to maintain control by steering and braking, cars are designed so that the automobile exerts a downward force as their speed increases. However, increasing this downward force increases drag, which in turn, limits the top speed and increases fuel consumption. Hence, these two forces must be carefully balanced (Banga et al., 2015). The air flow around a ground vehicle can be classified into two categories, internal and external flows. The external flow includes the underbody flows, flow over body surface and wake behind body. A wake is the region of re-circulating flow immediately behind a moving or stationary solid body caused by the surrounding fluid around the body. The external flow is responsible for over 85% of the drag force on the bluff body, and research shows that about 50% of the mechanical energy of the vehicle is wasted on drag at highway speed of nearly 88.5 to 96.5 kph (Agarwal, 2013). Streamlined body design in a passenger car helps reducing the aerodynamics drag and eventually improves the engine mileage (Versteg and Malalasekera, 2007). However, in contrary, add-on devices or accessories attached to the body skin of a car cause unfavorable aerodynamics effects. A real-life automobile is a very complex shape to model or to study experimentally. However, the simplified vehicle shape employed by Ahmed et al. (1984) generates fully three-dimensional regions of