Accelerating CFD performance.

With the ever-increasing sophistication of computational fluid dynamics [CFD], rather dismissively known as "Color For Directors" by some aerodynamicists, the role of the wind tunnel has been called into question. The goal is to circumvent the traditional track test, wind tunnel-to-road approach and go straight from CFD design to the full model. Aston Martin, for example, is reputed not to have used the wind tunnel at all when developing the DBR9, the racing version of the DB9, opting instead to use "Sculptor," which Optimal Solutions Software (www.optimalsolutions.us/index.html), the developer claims is a revolutionary CFD design deformation tool. In 2004, Advantage CFD (www.advantagecfd.co.uk/index.htm), the company originally set up in late 1997 by Reynard Motorsport with the objective of becoming the leading CFD resource in motorsport (it now belongs to the Honda F1 team), became the European Union distributor of the Sculptor.

"The project was a unique opportunity for us to demonstrate that CFD really has come of age," says Dr. Rob Lewis, chief engineer at Advantage CFD. "Aston Martin Racing's decision to bypass the scale model wind tunnel and focus on going from CFD straight to the track was a major turning point for us. It demonstrated that we could offer a practical alternative to the wind tunnel for aerodynamic development." The project was rewarded with immediate results when the DBR9 took class victory in its race debut at the Sebring 12 Hours in 2005, beating the works Chevrolet Corvettes that had won its class the previous three Sebring races. The DBR9 then went on to feature in one of the all-time epic battles at Le Mans, where it raced wheel-to-wheel with the Corvettes for the first 22 hours of the race before hitting problems. The early success not only confirmed the performance of the Aston, but validated the brave decision to use CFD as the principal aerodynamic development tool.

CFD offers a number of key advantages over physical testing, claims its advocates. As well as providing the aerodynamicist with more information and understanding than a wind tunnel, CFD allows engineers to evaluate the aerodynamic performance of new designs without requiring the time and cost associated with building prototypes or model parts. Aspects of the vehicle which are notoriously difficult to study and develop in physical tests, such as managing and monitoring the air flow in the engine bay, require little additional effort in CFD as the engineer has access to the air pressure and velocity both inside and outside the vehicle.

While CFD can be likened to a "virtual wind tunnel", Sculptor is analogous to the "clay" used for shaping the desired component being tested in the tunnel. Created to help the CFD engineer get the best possible design results in the shortest time, it is an interactive, real-time design deformation software that can arbitrarily alter the shape of a computerized object whose shape is defined by the points of a grid.

The two major problems on shape optimization in automated CFD engineering design have been the difficulties in CAD parameterization and inadequate algorithms for automatically generating a quality CFD mesh. However, a solution has been provided by the Arbitrary Shape Deformation [ASD] technology used by Sculptor which offers the control and manipulation of the shape of any geometry. Essentially it writes exactly the same CFD file but with modified node coordinates so that none of the connectivity or boundary conditions associated with the volume mesh are changed. This allows the modified problem to be "run-on" from the converged solution of the baseline case. The deformations are smooth and volumetric to maintain cell quality for the CFD calculations. Additionally, the changes that can be made with the ASD technology are generally far more versatile and intuitive than those possible with CAD packages. Rather than being restricted to simple lengths and radii, the user can construct powerful parameters which can control the shape of the geometry.

By adding together arbitrary parametric control of the shape and the ability to make changes without re-meshing, Sculptor is a tool which makes design optimization in CFD not only a possibility but reality. Sculptor therefore saves hours by making the changes directly to the CFD input file and removing the need to re-CAD/re-mesh the CFD case file. Together, Sculptor and CFD can precisely "map" the airflow and predict the effect of a design modification on a car's aerodynamics quickly and with pinpoint accuracy.

When Sculptor was used to optimize the front windscreen and fairing geometry of a Yamaha R1 motorbike, sizable reductions in drag were found. Sculptor was used to deform the CFD case file directly, therefore removing the need to re-mesh. The fairing geometry was controlled using four parameters which were optimized using a response-surface method. Twenty-five runs were initially used to define the preliminary response surface; thereafter, a total of four cases were run attempting to optimize the fairing for low drag. Results from the study showed that the time taken for all of the Sculptor stages of the re-design process amounted to approximately one man-day where just a manual fairing modification would normally have taken three to four days for each run. The final design managed to reduce the total vehicle drag by 2.5%

By William Kimberley, European Correspondent, [e-mail]