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LMP 1 REAR WINGS


we found were within reason. And while we were unable to extract answers as to how much downforce the major LMP efforts gained back, in hindsight it could be seen as tacit admission that the ACO’s 2009 regulations did very little to actually strip the cars of downforce. Indeed, with Dome’s


withdrawal from Le Mans competition, and subsequent release of aerodynamic figures for their S101 and S102 series


of LMPs, we had laid out in front of us what the net effect was to one competitor – between the 2008 S102 and the unraced 2011 S102i, Dome saw a 24lb gain in downforce for a 50lb drag increase in their Le Mans configuration. Dome’s Hiroshi Yucchi: ‘Due to the small rear wing, we initially lost around four per cent efficiency. Then we managed to recover three per cent by the rear fender, wing stay design and so on.’


THE CFD — PUTTING PRACTICE INTO THEORY


When all was said and done, Dome suffered a one per cent decrease in efficiency. ‘It was estimated around 0.5 sec per lap [at Le Mans].’ The cost? According to Yucchi, ‘between 20,000,000 and 25,000,000 yen ($239,500-$299,400 / £150,470-£188,100) to produce one car set. This does not include the aero development costs.’ This only covered tooling and one car set worth of update parts, not tunnel, CFD, or CAD time.


CONCLUSION So was upwards of $240,000 (£150,000) worth a one per cent reduction in efficiency, which equated to a two per cent increase in lap times and even less than that at Le Mans? When the stated goal was to reduce cornering speeds, no. Given that the 2011 regulations were coming on line, it made even less sense for the ACO to implement these changes when they did, especially given the


Figure 1: CAD model of half the 2008, 2m span wing (courtesy M Fuller)


Figure 3: static pressures and surface streamlines on the 2008 2m wing underside


Figure 5: a similar pattern appeared on the first 1.5m span model running modest camber and angle of attack


Figure 2: CAD model of the flow domain with the wing installed


Figure 4: the wakes from the wing mounting plates are clearly visible


Figure 6: with increased camber and angle of attack, the wakes from the wing mountings became much more pronounced


ourtesy of Ansys CFD-Flo, part of the Ansys 12 suite, this query from Mike Fuller was relatively easily addressed by accepting that we could only realistically evaluate the wings in isolation, there being no representative car model available to us.


C Full scale, half-wing CAD


models were imported into a 16m x 4m x 4m flow domain – the virtual wind tunnel, if you will – and attached to one sidewall in CAD. (Note: use of half a model in symmetric cases halves the computational


requirements.) The model was then subtracted from the flow domain’s volume (figure 2), which allowed the meshing software module to treat the wing as the item under test within the flow domain. Boundary layer prisms were incorporated into the mesh around the wing to capture near-surface flows and, hopefully, any flow separations reasonably realistically. Mesh settings were juggled until the size was of the order of 2.25 million elements, which was deemed adequate. CFD


78 www.racecar-engineering.com • January 2012


aficionados might like to know that the shear stress transport turbulence model, said to deal well with models such as this, was invoked. Boundary conditions included a 200mph inlet velocity but, because the wing was in isolation in the middle of the flow domain, the moving floor option was not utilised. The side to which the wing was attached was set as a frictionless wall. The options were worked


through sequentially and the forces on the wing models calculated (these have been


reported as downforce and drag in the main text). One of the other principle benefits of CFD is the ability to visualise what’s going on, and in this particular exercise this proved to be especially illuminating. Figure 3 shows the 2008 2m wing with static pressures and streamlines plotted on the surfaces of the wing (which has been mirrored so that the appearance of a whole wing is given). The relatively small wakes caused by the mounting plates can be seen in the centre of the wing, and show up more


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