By Rajneesh Singh, General Motors Corporation, Detroit, MI

View the pdf of this article

Design variables used in the parametric study

The aerodynamic design of an automotive vehicle is an iterative process. It involves an interaction between the designer, who proposes a shape for the vehicle, and the aerodynamics engineer, who evaluates the shape for aerodynamic performance and provides enablers, or guidelines for drag minimization to the designer. The designer then incorporates these recommendations within the constraints of the design theme. After a number of iterations, the process gives rise to a vehicle with an improved drag profile. This process can take place using clay models in a wind tunnel or using CFD. While quicker and less costly, the CFD approach still takes time because of the need to create and mesh modified geometries several times. At General Motors, a new automated process has been developed using FLUENT and other software tools that together help reduce the time required for each geometric modification and therefore, each CFD solution.

Flow chart of the automatic analysis process

The automated process for aerodynamic optimization makes use of FLUENT and a "mesh morpher" (Meshworks/Morpher, from Detroit Engineering Products (DEP)). These codes are coupled via iSIGHT (from Engineous Software Inc.) for an automatic exchange of information and data. iSIGHT guides the process by selecting the design variable magnitude, executing Meshworks/ Morpher to modify the mesh, and providing this mesh to FLUENT for the next round of CFD computations. The process can be set up to conduct traditional design optimization or Design of Experiment (DOE) studies. It has a very quick turn-around time for multiple calculations, and contributes to the vehicle design in two ways. First, it can be used to develop the enablers for the most efficient design. Second, the process can be used to complement wind tunnel testing. The design space can be explored to identify the aerodynamically critical regions of the vehicle, and the wind-tunnel test engineer can use this information to reduce the number of wind-tunnel tests.

Flow chart of the automatic analysis process Pressure contours on the rear part of the vehicle for various designs; the aerodynamic drag is shown for each image

A remote, high performance, parallel computing machine is used to run the coupled calculations in a batch process mode. When FLUENT performs the aerodynamic simulation, it executes a sequence of commands listed in a journal file. The computation for each new design is started from the converged solution for the previous design, to reduce the computational effort required. Since the CFD computations are performed using a journal file, it is also possible to produce a set of images to visualize the flow for each design. When combined with averaged or integrated quantities (such as drag coefficient), these images help illustrate the flow characteristics of each case studied.

The automated analysis process for performing a DOE study was recently illustrated using a generic automobile shape in a rectangular tunnel. A base case and sixteen modified designs were created and analyzed in the study. The mesh for the baseline model was created with 1.3 million elements, with prism layers on all of the car surfaces for an accurate resolution of the boundary layer. The designs differed in the length and height of the rear deck and roof edge. It took less than 2 days of time to evaluate the 16 designs. Had more computational resources been available, more designs could have been tested and an exhaustive DOE study conducted to find the true optimal design. A conventional analysis procedure, in which the vehicle geometry is modified and a new mesh is constructed for each design, would require at least an order of magnitude more time to complete.

FluentNEWS