Computational Investigations on the Aerodynamics of a Generic Car Model in Proximity to a Side-Wall 2018-01-0704

This paper discusses a realistic approach of simulating a generic idealized car model (Ahmed body) moving in close proximity to a Side-wall using transient CFD. This phenomenon is very important in motorsports where racing very close to the safety barrier is very common. Driving in close proximity to a Side-wall alters the aerodynamic characteristics of the vehicle significantly, however, only a handful of published work exists in this area. Additionally, the experimental studies conducted in the past suffer from certain inadequacies especially in properly emulating the Side-wall, which cast some uncertainty as to their applicability to the real world. As such, the present study attempted to imitate the real world flow phenomenon by taking a non-traditional CFD approach in which the body is translated relative to the stationary surrounding fluid and Side-wall instead of the classical method of flowing air over a stationary object. This was achieved by using a newer meshing technique for overlapping grids called the “Overset” or “Chimera” mesh. The initial challenging task was to predict accurately the flow over the rear slant of the 25^o slant angle Ahmed body model where previous studies struggled to achieve accurate enough predictions using the eddy-viscosity turbulence models. In the present study, the SST turbulence model with modified closure coefficients is utilized to accurately predict flow characteristics in the initial separated shear layer and, as well as, the flow reattachment over the rear slant. Compared to the eddy viscosity CFD simulations of an isolated 25-degree slant angle Ahmed body seen in existing literature, the results presented in this paper show significantly better correlations with the experiments in terms of overall aerodynamic characteristics, like drag and lift, and flow characteristics like pressure and velocity in the wake region. The wall proximity studies show a strong influence of the presence of the wall on the overall aerodynamic characteristics of vehicle body. When compared with the experimental studies, although both show similar trends, however, there exist significant differences between the experimental and CFD predicted results, which tend to worsen as one approaches the wall. These differences can be attributed to the fact that the CFD emulation of the flow around the side-wall is more realistic compared to the experimental implementation.