CFD Investigation into the Flow Dynamics of the Ahmed Car Body In 1984 Ahmed, S. R., Et al, released a paper titled ‘Some Salient Features of the Time Averaged Ground Vehicle Wake’ in which a simplified generic land vehicle geometry was used to analyse the effect of a slanted rear end and the effect of the angle it carried. The geometry used in the experiment has since been used countless times in the field of automotive aerodynamics. The geometry contains all the important factors of land vehicles without any of the small intricate details that complicate CFD (Computational Fluid Dynamics) analysis. It is for this reason that it has been so widely used to analyse flow structures of land vehicles, as observations seen on the geometry (referred to as the Ahmed body) usually transfer to real world land vehicles. George Oates School of Engineering Yongmann Chung The objective of this research was to see if results, produced in a wind tunnel by a team of researchers in Hong Kong could be replicated using CFD simulations. The figure on the right shows the measuring points used in for their experiment being set up in the CFD program. After the case had been set up a series of tests was carried out to define a sensible and accurate mesh recipe. The mesh is the most crucial element of a CFD simulation and takes up a large amount of the simulation process. The graph on the right shows the results of the grid independence results. The chosen parameter to monitor was the drag force on the car body. It can be seen that this parameter appears to reach a state of grid independency at around 1.6 million cells. Below is a series of snapshots and a table showing the results from the CFD simulations carried out for this project. The pictures are labelled appropriately. Large Eddy Simulation modelling techniques were used and some encouraging agreement was found. The fluctuations observed on the car by the CFD however, often did not seem to have a particular dominant frequency unlike the experiments. Parameter Experiment CFD Strouhal number 0.265 0.14 – 0.22 Frequency 2.4 (Hz) 1.25-2 (Hz) Time Period 0.42 (s) 0.5-0.8(s) Velocity Isosurface Lambda 2Isosurface Lambda 2Isosurface Lambda 2Isosurface