Why the air dam lip?

[gone-ot]

...it's simple, the air dam conteracts the damn air (verbal punnage intended)!

[d0sitmatr]

Lol

[instarx]

Lips also provide mechanical stiffening to the dam.

[BeatUpRanger]

Originally Posted by wyatt

About 2/3 of the way down the page, there is a picture of the underbody of a car. at the car's back end, it should say "Slower Moving Lower Pressure Air" (not higher as it does in the picture).

Oops, my bad. I thought you were talking about the first graphic not the second one. But just to ask for clarity as I'm still learning this stuff, when the air speed slows down, doesn't the pressure increase. So if fast moving air (low pressure) slows down because of the diffusers at the back, doesn't the pressure increase?

I thought that's what they were illustrating.

[winkosmosis]

Originally Posted by BeatUpRanger

Oops, my bad. I thought you were talking about the first graphic not the second one. But just to ask for clarity as I'm still learning this stuff, when the air speed slows down, doesn't the pressure increase. So if fast moving air (low pressure) slows down because of the diffusers at the back, doesn't the pressure increase?

I thought that's what they were illustrating.

It's a rearward facing surface, so it's not higher pressure just because the airflow is slowing down. Like the top surface of a fastback isn't seeing high pressure.

[aerohead]

Originally Posted by BeatUpRanger

Oops, my bad. I thought you were talking about the first graphic not the second one. But just to ask for clarity as I'm still learning this stuff, when the air speed slows down, doesn't the pressure increase. So if fast moving air (low pressure) slows down because of the diffusers at the back, doesn't the pressure increase?

I thought that's what they were illustrating.

*If the diffuser angle is within the 2.5-4 degree upsweep,prescribed for diffusers,based on their point of 'origin',the air will remain attached as the air 'climbs' away from the road surface.
*As this air expands into the larger void,it will simultaneously decelerate and gain static pressure,raising the base pressure of the wake,cutting drag.
*If the diffuser angle exceeds the prescribed angles for attached flow,the air will immediately separate at the beginning of the diffuser,and the wake will do a 'reach-around',with it's low pressure communicating beneath the diffuser and acting upon its surface to create downforce,with a drag penalty to the car.
*If suction is applied to a perforated diffuser,as Ferrari is currently doing,the air can be made to follow a steeper ramp angle and burst into the wake region.

[wyatt]

Originally Posted by winkosmosis

It's a rearward facing surface, so it's not higher pressure just because the airflow is slowing down. Like the top surface of a fastback isn't seeing high pressure.

Exactly. The reason that performance cars want a diffuser is to increase their down-force. If it did the opposite and increased pressure (lifting the car off the ground) they would be avoided at all costs!
Diffusers can reduce the drag experienced by the vehicle. Drag is at it's lowest at ~2.66 (long, delta Cd 0.025) or 3.66 (short, delta Cd 0.02) degrees, and is still better drag-wise compared to having a 0 degree diffuser until at least after 11 degrees (short diffuser, delta Cd 0.007) or 6 degrees (long diffuser, delta Cd 0.013). In both cases, down-force (negative lift) is being increased all the way to the 6 and 11 degree numbers above, and in both cases they max out at delta Clr (coefficient of rear lift) at 0.26 (THIS IS HUGE!). I suppose I should quote Hucho on this one, figure 4.48 in his 4th edition.
What you can see is that a longer diffuser allows more downforce and lower drag than short diffusers.

[euromodder]

Originally Posted by wyatt

Drag is at it's lowest at ~2.66 (long, delta Cd 0.025) or 3.66 (short, delta Cd 0.02) degrees, and is still better drag-wise compared to having a 0 degree diffuser until at least after 11 degrees (short diffuser, delta Cd 0.007) or 6 degrees (long diffuser, delta Cd 0.013).

Unless the vehicle has a squarish rear end (think squared off hatchbacks, station wagons, vans) where any diffusor angle wil increase drag !

[BeatUpRanger]

I have soooo much to learn.

[wyatt]

Originally Posted by euromodder

Unless the vehicle has a squarish rear end (think squared off hatchbacks, station wagons, vans) where any diffusor angle wil increase drag !

Do you have a reference for this? I have never seen anything (to my recollection) that counters the work shown in Hucho's book. I am not saying you are wrong (since aerodynamics is often counterintuitive even to the experienced), I just can't think of a reason why a diffuser would make things worse. I would actually think the bigger the back end, the more of a positive effect it would have. Thanks.