CFD modeling of a modified Ahmed Car Body for Reduced Drag

Abstract

The wake flow behind the car presents the major contribution to a car drag. The flow over a car body is very complex. Hence MOVA consortium partners agreed to study the vehicle shape employed by Ahmed and Ramm (1984), known as Ahmed body. The aerodynamic drag of the body has great impact on the fuel consumption by a car. So for economic and for environmental reasons also drag reduction is very important. The CFD model is used to investigate and for the better understanding of the aerodynamic behavior of the flow in the surrounding area of the vehicle. The development of a good CFD model depends firstly on investigating and selecting the best grid configurations. In this simulation experiment the total element count after final refinement was within an acceptable limit 1.7 to 2 million grids. The performance of a CFD model is not only depends on the number of grids but also on the turbulence model chosen for the simulation. Sometimes on the basis of the turbulence model chosen it is also required to select the roughness height. For the above mention purpose in this simulation study it has been found that a total of 1.7 million grids are suitable and acceptable. To select this number grid dependence test is performed. To choose the turbulence model a comparison among different turbulence models is done. On the basis of Azad et al. (2012) the k-a model, which is better suited, is chosen for the simulations. As k-a model has dependence on the roughness height it was required to choose also. A further study is done to find it and the findings are published. Azad et al. (2013) has shown the results and according to them the selection of the roughness height as 0.0002m is the best choice. Out of the two strategies active and passive, a passive strategy is chosen to reduce the drag over the Ahmed car body. To manipulate the flow insertion of grooves has been considered. The grooves are placed at the top, at the slant, at the top and slant, and at the rear surface of the body. The drag, after modeling the Ahmed body, is then calculated using ANSYS-11. The results thus obtained are compared with the results obtained by Lienhart et al. (2000). It has been found that overall drag has reduced.