Simulating External Vehicle Aerodynamics with Carflow

A zonal approach to simulate the incompressible and steady flow around automobiles is presented. The method incorporates two components for the inviscid and viscous domain of the flow field at high Reynolds numbers. Although in many details it is especially well suited for automobile aerodynamics, the code may also be applied to trains, submarines or aircraft. For the inviscid part of the flow field, a first order panel method is used, which is also able to simulate the separated flow downstream of the vehicle's base, by means of free shear layers. A constant source density is assigned to solid body panels, while a bilinear doublet distribution, equivalent to a panelwise constant vorticity, is used on the wake panels, which represent the free shear layers. The main objective of this wake model is to simulate the influence of separation on the vehicle's pressure distribution, rather than reproduce the wake structure in detail. Viscous effects are accounted for by a three-dimensional integral boundary layer code, working in general surface coordinates. The generation of a structured surface grid is part of the method. Besides friction drag and viscous results on the surface, the boundary layer analysis yields a three-dimensional separation line as the starting location for the wake simulation. As the present approach requires only a discretization of the vehicle's surface, the effort of model generation and data handling is reduced substantially compared to field methods. Depending on the level of model details, typical turnaround times of two or three days for the generation and analysis of five to ten variants may be achieved. Parametric modifications of the panel model are enabled by the code itself. Often, when conducting wind tunnel tests, it is not quite understood where differences between variants come from, and the success of shape modifications depends on the skills and experience of the experimental aerodynamicists. The example applications demonstrate some of the options to gain insight into details of the flow field. Comparisons with experimental results show a good agreement of pressure distributions in regions of attached flow. A larger difference arises between measured and computed absolute drag coefficients, due to the extreme accuracy requirements, discussed in chapter 1. However, satisfying results are obtained when the effect of shape modifications or the drag ranking of different vehicles or variants is concerned.