Interesting aerodynamics differences between USDM, JDM, etc.

[SubieNate]

making a smooth underbody with a nice diffuser is a great way to both increase downforce and reduce drag on any car. Many cars add a splitter to keep as much of the air possible going over the top.

In general, it's probably safe to say that you want as little air flowing under the car as possible, and what does flow under the car, you want it exiting as smoothly and quickly as possible. Hence, rear diffuser.

The hybrids that have the small lip spoiler are using a simple aerodynamic trick to reduce their Cd. The latest generation Prius is the easiest to demonstrate this with as it's the most drastic. The latest gen Prius doesn't just have a lip spoiler. The rear hatch actually pokes backwards a bit as seen in this pic:



(We can all make a collective "EEEWWWWWW!" noise now, let's get it out of our systems. )

What this does, is trip the flow. 99% of the time, the flow over the car will be turbulent by the time it reaches the rear of the car. Window trim, glass to sheetmetal transitions, all of these things are enough to cause turbulent flow. Aircraft use a serrated edge tape that's only ~.01" thick to trip flow to turbulent. BUT, this type of turbulent flow stays attached longer than a laminar flow would, leading to reduced pressure drag and a slight increase in viscous drag. This is why baseballs stitches help them fly faster, golf balls have dimples, tennis balls need fuzz, etc. All about tripping the flow. Let's not talk about spin right now haha.

This is all fine and good till you get to the back of the car. That attached flow gets to the end of the car and there's nothing nice to attach to anymore. So it freaks out and the low pressure void behind the car fills with vortices. You can see this on a flow past a cylinder here:

flow past cylinder - science experiment - YouTube

compare this with flow over an airfoil:

how wings work? Smoke streamlines around an airfoil - YouTube

As long as the angle of attack is reasonable, there is MUCH less turbulence.

What's happening with the little wings/lip spoilers on hybrids, is a forced detachment of the flow, the lip spoiler deflects the flow enough that it doesn't try to follow the surface back down around the the trunk and thus the magnitude of the vortexes in the car's wake is reduced, reducing drag.

This drag reduction is technically reducing what's called the "base drag" of the car, and in short, base drag is the drag caused by NOT having a shape that allows for the two flows (over, under) to rejoin cleanly at the rear of the car. In the simplest terms, the little lip spoilers etc. are all trying to "trick" the air into thinking the back of the car is shaped like the trailing edge of an airfoil.

This is one of the reasons a small spoiler can actually decrease drag, even if it doesn't (especially if it doesn't) increase the downforce significantly.

Dunno if that helped or just muddied the water but hey.

Nathan

[midenginebias]

Originally Posted by SkitterSkotter

Actually rear diffusers do not create drag, hence why they are so popular with supercar companies because they can make an exterior void of massive spoilers and achieve similar downforce. The diffuser just helps the higher velocity air traveling below the car to make a better transition with the air flowing atop.

Exactly my point, hence the counterargument of increasing drag increases downforce.

It's always true.

[fatoni]

Originally Posted by midenginebias

Disagree. Is drag created with a flat bottom and a rear diffuser? Does that not add downforce?

Pickup trucks have a lot of drag. I doubt that generates any significant downforce.

lift creates drag too. if you remove lift, you can remove drag

[fatoni]

Originally Posted by midenginebias

Exactly my point, hence the counterargument of increasing drag increases downforce.

It's always true.

who said increasing drag increases downforce?