Quote:
Originally Posted by ayau
I still wonder if the OEM "diffuser" is actually decreasing lift, or if it's just there for aesthetics. For a sub 30k car, I have a feeling it's the latter.
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What most people recognize as the "diffuser" isn't actually doing most of the diffuser-work. What's happening is that basically the muffler body combined with the "diffuser" is likely approximating the correct shape. Obviously, having exposed suspension, and a muffler with pipes sticking out in a few places, it's not an ideal situation.
The reason the diffuser isn't high is because there's not enough air going under the car. If you switch out the rear bumper and muffler and put on a typical aftermarket diffuser for aesthetics (yes, even the ones that are angled 10 degrees because someone thought that would always work), the diffuser will now sit in the wake and be more useless than the stock setup.
Quote:
Originally Posted by EarlQHan
My partner and I have been doing some CFD testing on the FR-S. Using the model we have, we estimate the CL to be .179, but our model has a smooth underbody. I don't expect the BRZ to have a different coefficient. A little more info is on our blog here: http://hanchagroup.wordpress.com/201...s-cfd/#more-27 |
Thanks for doing the testing. Your C_d is lower than the manufacturer claim most likely because you essentially have a sealed front grill (radiator causes drag), smooth bottom, and non rotating wheels.
There's no need to tuft test the car as this is essentially the usual fastback. Lesson number 1 about streamlining is it's all about the rear, not the nose. Pointy nose is (slightly) worse than blunt nose at subsonic speed, just look at a Boeing 747.
The "problems" with the rear end are as you can see the 2 giant blue vortices (this is because while the rear window is sloping downward at a nice gentle rate, the car's body isn't getting narrower laterally, so the air that gets pushed to the side of the car is still travelling fast while the air going over the top of the car is slowing down), and the green zone over the trunk (separated flow, read the aero streamlining template thread on ecomodder for more info).
The factory spoiler helps with these slightly by essentially raising the effective height of the trunk lid and bringing it closer to where it "should" be for that length of car. Lowering the car has the effect of blocking off more of the tire area "visible" to the flow, and it also affects the "fineness ratio" (in a positive way, as in reducing drag) which I understand to be a sort of height vs. length thing, I don't really understand it but I think the effect lowering has on this is because of the ground.