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By Haakon Dahle Smith, Project Engineer, Advantage CFD, Brackley, United Kingdom
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Contours of static pressure on the go-kart fairing
When Los Angeles hot rodder Art Ingles built the first kart racer in 1956, he was simply trying to provide himself with an economical racecar. The earliest versions of what would become known as "go-karts" were fitted with surplus lawnmower engines and eventually sold as mail-order kits. Soon after, some of the first competitive kart races were held in the parking lot at the Rose Bowl. From such humble beginnings in a California muffler shop, kart racing quickly grew into a worldwide enterprise and today has national associations in over 100 countries. Like its big brother in Formula One, kart racing has an international governing body and several world championship series, and is a field of interest for design engineers.
The baseline fairing shape (top), the shape that resulted in the lowest drag (middle), and the shape that resulted in the highest downforce (bottom)
In late 2004, Advantage CFD completed shape optimization studies on the front fairing of a superkart, which competes in the European Superkart Championship governed by the CIK-FIA (Commission Internationale de Karting). The goals of the study were to reduce the overall drag and to increase the total downforce on the vehicle. The baseline case for the 250cc kart had been previously solved in FLUENT to produce steady forces on the vehicle. To then optimize the fairing shape, engineers used SculptorT, a software package developed by Optimal Solutions Software, LLC that allows for direct deformation of a CFD case file, in real-time, without the need to re-mesh.
Contours of downforce on the baseline fairing (left) and the maximum downforce design (right)
With the power of Sculptor working on the baseline 3.7 million-cell hybrid mesh, four fairing parameters were altered during the optimization process. These parameters included the outboard height, the height and width of the inboard, and the curvature of the leading edge in the X-Y plane. Using a response-surface method, 25 geometries were required to define the initial response surface for the four deformation parameters. After that, an additional 14 iterative geometries were run while attempting to optimize the fairing for either low drag or high downforce. Once the initial domain was meshed, each change in the fairing geometry took only 15 minutes to complete and save. Thus, the overall setup time for the 39 different runs was very small in comparison to the computation time.
The surface mesh for the baseline case
The results of the studies showed that narrowing the inboard section of the fairing caused a reduction in drag, but also a reduction in downforce. Lowering the inboard and outboard sections was found to increase the downforce because of the resulting wedge shape of the fairing, though a slight drag penalty was incurred as well. For the drag optimization study, the best run produced a 1.5% drag loss compared to the baseline, but also lost 2.9% of downforce. In this lowest-drag case, the fairing had a moderately narrower and lower inboard section, slightly higher outboard sections, and less curvature on the leading edge. When optimizing for downforce, the best run showed an additional 16% over the baseline, and only gained 1.1% of drag. This particular fairing design had a lowered and narrowed nose, a lowered outboard section, and greater curvature on the leading edge.
The most important aspect of this optimization study, however, is the amount of time required for the task. If the geometry had been modified and re-meshed manually, it would have taken weeks to complete all 39 cases. Using Sculptor, a true parametric optimization was performed within a few days, with most of the time spent on computation and analysis. At Advantage CFD, this process has been successfully applied to other aerodynamic structures, such as motorcycle fairings and airfoils. Future kart design work using Sculptor may include deforming different areas of the fairing, validation of the current work through manufacture and wind-tunnel testing, and optimization of other kart components such as the rear wing, sidepod, and radiator ducts. Sculptor is used in many other industries as well, such as aerospace, automotive, marine, healthcare, power generation, HVAC, and chemical and materials processing - anywhere internal or external fluid flow or heat/mass transfer is sensitive to shape.
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