Quote:
Originally Posted by California98Civic
Your box cavity pressure measurements are interesting and encouraging, but I think your specific build might be part of the reason you got quite different results than Ali. Ali observed in his article that the effect on drag was very sensitive to other mods. There was a lot of "interdependency" (his word) between the cavity, the difuser, and wheel well covers and such. You mention that in your video, too. But your tested design is radically different from Ali's in several ways that quite possibly matter a lot. Ali's final design was much smaller and was not an extension of the side, top or undercarriage body lines. Here is the final optimized box cavity from Ali's Audi A2 test subject:
Remember that he also claimed that the carefully optimized cavity was how he was able to get even more drag reduction through a 2° difuser.
Ali presented this research at Coventry University on October 19, 2011. I downloaded it on March 29, 2012 when it was still fully available on the conference website without a password. I don't see the 2.3% drag reduction claim in the slides detailing the box cavity. I am not saying he did not make the claim, but I don't see it. Which slide is it on? |
The percentage change can be worked out from the bar graph. Cd = 0.215 with mods up to box cavity, Cd = 0.210 after box cavity. A 2.3 per cent reduction.
It's a very small change in drag. What I measured was also a very small change in drag, so I can't see how you can say that I "got quite different results than Ali". Given the different size wakes, I'd say they're in the same ballpark.
Without further evidence, I'd say that box cavities - especially on cars without large wakes - probably aren't worth pursuing... certainly not as first steps in drag reduction, anyway.