AEROBYTES
Airflow passing over (and emerging from) the front louvres encounters the outer region of the rear wing further downstream
The engine bay inlet snorkel was replaced, and produced interesting results
The wool tufts in the centre of the wing’s trailing edge show how the engine inlet snorkel affected the airflow downstream
Remove snorkel -9 (-1.5%)
Δ -CL +47
It was hoped that opening the rear of the rear wheelarches and adding ramps at the back would collectively reduce drag and add rear downforce
Table 2: the effects of removing the snorkel intake, in counts and per cent Δ CD
Δ -CLfront Δ -CLrear Δ %front +19
+28 (+4.6%)
Rear arch modifications
-4 (-0.7%)
increased. The rear downforce increases, which in the first two cases were greater than the front downforce reductions. This may have been the result of improved flow to the outer portions of the rear wing, the converse being true were the louvres to be opened up. Certainly the airflow passing over (and emerging from) the front louvres encounters the outer region of the rear wing further downstream. Blanking the front three
louvres increased the efficiency more than taping over the rear three louvres by reducing drag slightly, the opposite again being the case if those louvres were to be opened up. But blanking off the rear three louvres also increased rear downforce slightly more, while making a similar
Δ -CL +6
(+5.6%) (+4.2%)
+0.31abs (+0.9%)
Δ -CLfront Δ -CLrear Δ %front +2
+4 (+0.6%) (+0.6%) (+0.6%)
difference at the front of the car as blanking off the front three louvres. So small changes to the numbers of louvres opened or closed could be used as balance adjustment tools, providing sufficient front-end downforce was available in the first place. In this case, the car really needed all the front louvres open to maximise front-end downforce.
REMOVING THE SNORKEL As mentioned briefly last month, the ADR’s intake snorkel on the rear body section fed into the engine bay rather than being directed into an airbox sealed to the engine. It was felt that this would be causing drag and possibly rear lift as well. The results of replacing the snorkel with a flat engine cover with no
22
www.racecar-engineering.com • September 2011
Table 3: the effects of modifications to the rear wheelarches, as changes Δ CD
+0.01abs (+0.03%)
Δ -L/D +106
(+6.2%)
Δ -L/D +23
(+1.3%)
feed into the engine bay are show in table 2. The results are again given as changes relative to the previous configuration. As hoped, this change did reduce the drag by a modest amount, and added more rear downforce. But it also added front downforce too, to the extent that there was barely any balance change. Efficiency was obviously well up as well. So how could such a change
yield more front, as well as more rear downforce? Well, there would seem to be two possibilities. First; rear wing performance, which could be seen to be impaired in the centre by the snorkel, as evidenced by the wool tufts on the wing’s trailing edge, probably improved and this may in turn have increased the interaction with the diffuser and
benefited the whole underbody’s downforce contribution, too. The other possibility might be that removing the snorkel allowed faster flow over the rear deck and into the wake, and this improved extraction from the diffuser to the betterment of the whole underbody, too.
REAR ARCH RAMPS Finally, the effect of truncating and opening up the rear of the rear wheelarches and simultaneously adding ramps to the rear of the wheelarches was examined. These modifications, it was hoped, would reduce drag and add some rear downforce, and what happened is shown in table 3. So, at this stage in the car’s
development, this was too small a benefit to justify parts manufacture, but at least the result was positive. Next month we’ll see what happened to the ADR3’s aerodynamics when we introduce small yaw angles.
Racecar Engineering’s thanks to ADR Engineering, Carbon Weezel and Simon Marsh.
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