Use of the CFD for the analysis of the exhausts trajectory

The problem of the evaluation of the trajectory of the gas exhaust is important in a car project, because its particular impact on the comfort level. For the high performance cars the problem could be critical, because the high level of deceleration of the car during the braking phase. From the computational point of view the problem appears really challenging. In fact, it is necessary to made an unsteady evaluation, following the car in its velocity history. Furthermore, the geometry representation must be very refined, because the problem is highly related to the small details, and a corrected representation of all the geometrical elements is necessary to avoid an uncorrected analysis of the resulting flow. This imply really high requirements in computational capabilities, and HPC appears essential. In order to make an assessment of this problem, a numerical procedure, be presented in the present paper, was settled. The numerical procedure is based on the CFD code Ansys-Fluent 14.5, by using a RANS approach with a realizable k-ε model and non-equilibrium wall function. The reference car is the Ferrari production one. The geometry representation is very refined, and the volume grid is represented by about 60 millions of cells. Both the time history of the car velocity and the gas mass flow rate are given, defined by means of user-defined functions, and the evaluation was made following them. The total time of the simulation was 20 s, with a time step of 0.01 s and 10 internal iterations. In order to analyse the trajectory of the exhaust gas a particle tracking technique was used, by activating the discrete phase model. The injection zone was coincident with the exhausts. As an example, the particle traces at a given time step is shown in fig. 1. The evaluation was made on a 512 cores cluster in about 4 days. The results of the numerical evaluation has been confirmed by the analysis of the real behaviour, and the presented results given an important support to define a geometrical set-up suitable to avoid the problem of the interference between the car and the gas exhaust, with a consequent improvement in the comfort level.