That is a common misconception. The whole point of optimizing the underbody aerodynamics of a racecar is to create a low pressure area under the car, right? In order to do that, the most common approach is to create an undertray that forces the air to speed up under the car, thereby creating a low pressure area. If that high speed air is carried all the way back to the end of the car, it will create additional turbulence behind the car, creating additional drag. The primary function of the diffuser is to get the air slowed back down to the optimal velocity. Think about the underbody of the car as a pipe, like:

>===<

Slow moving air is funneled from the front of the car [>] (bumper, splitter) into a narrow area [===] (undertray), which forces the air to speed up and drops the air pressure (as documented by Bernoulli's and related equations) under the car. The diffuser [<] then allows the air to expand again in a controlled manner, slowing the air flow and increasing air pressure. As I understand it, the only potential area for a diffuser to actually assist in generating downforce is when it is perfectly balanced with the air flow coming off a rear wing, and I believe the only reason for the additional downforce in that scenario is the improved efficiency of the wing, not the diffuser.

This is also why a diffuser by itself doesn't really work very well. The air flowing into the diffuser must not be turbulent (i.e., standard car underbody), it should be smooth, and the transition from the undertray into the diffuser must also be smooth. Any kind of lip or gap between the undertray and diffuser will adversely impact the performance of the diffuser.

l8r)

 

A flat bottom won't do much for downforce, so I'd just cut a hole in it and dump the exhaust. If you were going to built a contoured undertray to create some venturi tunnels, then it might be an issue.

l8r)

 

A Diffuser does decrease drag, and that is one of its two main effects. But, the other is a low pressure area under the rear of the car. This happens at the point where the diffuser meets the underbody, and sees some of the lowest pressure under the car. It is that tiny area where the speed and direction of the air changes, that creates the "downforce". the rest of the diffuser is to decrease drag, as ludi said.

Take a look at jid's graphic on the first page; it is a clear, basic example of the amount of negative pressure under certain areas of the car.

 

i absolutely love this thread! there's so much I'd love to learn about aerodynamics.
http://www.f1technical.net/articles/10
a fun read!

 

There is a fair amount of downforce available from the diffuser, at least this is what I am told by the people building GT2 cars. It apparently serves a number of purposes. The name implies literally that it diffuses drag as mentioned above. If formed into the classic venturi shape, either by use of "tunnels" or by simply curving it upward, you can offset the lift created by the natural car shape. Here is a "venturi tunnel" design on the F430:



If used with a rear wing, this reduced lift can very easily turn into downforce. The idea of most modern race cars is to create a classic wing shape underneath the body of the car. Here is a classic example of the old F1 cars with very aggressive venturi tunnels with skirts to seal off the edges:



There are 2 different terms in the rear undertray of a ground effect car just as there is a difference in the splitter and the front wing. I was very quickly corrected when I said the Corvette I drove at Petit LeMans has a really effective SPLITTER which actually incorporated mini venturi tunnels which created a front wing......

The "ground effect" term that gets tossed around a lot is related to a theory that gets confusing when you go from positive lift as in an airplane to negative lift as in a race car. The idea in the race car, where downforce is the goal, is that when you force air between the ground and the negative-lift-inducing surface, the downforce effect is increased and drag is reduced. The effect is especially noticeable if you can get the ground in the laminar boundary layer (http://wright.nasa.gov/airplane/Images/boundlay.gif) of the lifting surface. One step better would be to create large, controlled vortexes under the car which is, more or less, what some past and more modern F1 cars attempt with their underbody tunnels. The biggest area of concern is bottoming out which removes all downforce VERY quickly-- likely the reason why Aytron Senna crashed and died.

This is one of my favorite books on the subject, "Race Car Aerodynamics" by Josef Katz (http://www.bentleypublishers.com/aut...y-768-8.html):

 

venturi/throat style diffusers are typically a by product of packaging around a rear mounted gearbox...

we considered it for ours, but with our contacts in the openwheel/lmp realm... we were instructed that the flat style like ours is more effective for a tin top type car.

cb

 

I picked up an APR rear carbon diffuser for free for my evo a year or so ago.. Whether it does a damn thing im gonna say probably not.. but it was free and it looks good.

what i would like to do it create a full underbody panel and front diffuser to try to make the rear more functional.. Im no engineer though

So thanks to the guys who know this stuff to post threads about our cars

it has helped out alot

 

Let's see some pictures of the front of yours and some from underneath! Did you get the undertray started/finished?

I wanted to build something a little more fancy, like yours. But I just don't have enough time. So mine is a bit of a compromise between optimal design and desire vs. available "I'll be in the garage" resources.

 

Getting ready for my very own flat underbody with front wing and rear aero aids using heat from my exhaust and rear-mounted radiator to diffuse drag:



Eventually I'd like to see somebody try this on a Time Attack Evo-- they have so much forward weight they may also benefit from moving things like the cooling system rearward.

 

except that cooling things behind the car is far harder then cooling it on the front. on our formula car with wings overheating was always an issue. proper sidepod shape, and opening size was critical and the radiators were always twice the size because there was so little airflow. granted that was at autocross speeds...

the porsches have tons of venting back there to keep it cool, so its not nearly as easy as slapping a radiator in the front of the car and calling it a day coming from under the car makes it harder yet, its a low pressure area and the lower the car is, the less airflow through the radiator.

 

[QUOTE=MATT@WORKS;7642428]There is a fair amount of downforce available from the diffuser, at least this is what I am told by the people building GT2 cars. It apparently serves a number of purposes. The name implies literally that it diffuses drag as mentioned above. If formed into the classic venturi shape, either by use of "tunnels" or by simply curving it upward, you can offset the lift created by the natural car shape. Here is a "venturi tunnel" design on the F430:





This is an Enzo ...........

 

My bad-- was going off the taillights... venturis nonetheless!

 

Finished up the diffuser, for now at least. I may go back and cut some more holes in the bumper at a later date - who knows.

Mounted 5 strakes.



A look from underneath.



And a look from behind.

 

Nice work

 

looks awesome!