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AUTOMOTIVESculptorShapesBy Haakon Dahle Smith, Project Engineer, Advantage CFD, Brackley, United KingdomWHEN LOS ANGELES HOT RODDER Art Ingles built the first kart racer in1956, he was simply trying to provide himself with an economical racecar. Theearliest versions of what would become known as “go-karts” were fitted with surpluslawnmower engines and eventually sold as mail-order kits. Soon after, someof 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 quicklygrew into a worldwide enterprise and today has national associations in over 100countries. Like its big brother in Formula One, kart racing has an internationalgoverning body and several world championship series, and is a field of interestfor design engineers.In late 2004, Advantage CFD completed shape optimization studies on the frontfairing of a superkart, which competes in the European Superkart Championshipgoverned by the CIK-FIA (Commission Internationale de Karting). The goals ofthe study were to reduce the overall drag and to increase the total downforce on16 <strong>Fluent</strong> News · Spring 2005Contours of static pressure on the go-kart fairing
AUTOMOTIVESuperkartthe vehicle. The baseline case for the 250cc kart had been previously solved inFLUENT to produce steady forces on the vehicle. To then optimize the fairingshape, engineers used Sculptor, a software package developed by OptimalSolutions Software, LLC that allows for direct deformation of a CFD case file, inreal-time, without the need to re-mesh.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 parametersincluded the outboard height, the height and width of the inboard, and thecurvature 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 fourdeformation parameters. After that, an additional 14 iterative geometries were runwhile attempting to optimize the fairing for either low drag or high downforce.Once the initial domain was meshed, each change in the fairing geometry tookonly 15 minutes to complete and save. Thus, the overall setup time for the 39 differentruns was very small in comparison to the computation time.The baseline fairing shape (top), the shape thatresulted in the lowest drag (middle), and the shapethat resulted in the highest downforce (bottom)The results of the studies showed that narrowing the inboard section of the fairingcaused a reduction in drag, but also a reduction in downforce. Lowering theinboard and outboard sections was found to increase the downforce because ofthe resulting wedge shape of the fairing, though a slight drag penalty wasincurred 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-dragcase, the fairing had a moderately narrower and lower inboard section,slightly higher outboard sections, and less curvature on the leading edge. Whenoptimizing 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 loweredand narrowed nose, a lowered outboard section, and greater curvature on theleading edge.Contours of downforce on the baseline fairing (left)and the maximum downforce design (right)The most important aspect of this optimization study, however, is the amount oftime required for the task. If the geometry had been modified and re-meshedmanually, it would have taken weeks to complete all 39 cases. Using Sculptor, atrue parametric optimization was performed within a few days, with most of thetime spent on computation and analysis. At Advantage CFD, this process hasbeen successfully applied to other aerodynamic structures, such as motorcyclefairings and airfoils. Future kart design work using Sculptor may include deformingdifferent areas of the fairing, validation of the current work through manufactureand wind-tunnel testing, and optimization of other kart components suchas the rear wing, sidepod, and radiator ducts. Sculptor is used in many otherindustries as well, such as aerospace, automotive, marine, healthcare, power generation,HVAC, and chemical and materials processing – anywhere internal orexternal fluid flow or heat/mass transfer is sensitive to shape. More.info@www.advantage-cfd.co.ukwww.optimalsolutions.usThe surface mesh for the baseline case<strong>Fluent</strong> News · Spring 2005 17
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