Eric-
THe part I disagree with it his bit about the expanding air creating the downforce. Now we may all be saying the same ting in a different language, and of course we all know that happens a lot on forums
Nevertheless, we know when the air is fastest is where the pressure is lowest (and vise versa). We're creating a venturi, right. Simply put: Big space on the front (of the car), small in the center (under our seats), and big again in the back (the diffuser and behind the car)... like a carburator. The low pressure is in the smallest area (and fastest moving air). The lowest pressure (most DF) under the car is actually where the air turns to enter the diffuser due to the boundry layer at that point. Think of the air have to "go wide to hit the apex". It speeds up to do so, thus crating a spike in *low* pressure just at that point... Just like the front of the venturi, the rear of the system is where the slower (thus higher pressure) air is. The diffuser's job is essentially to "help" the air to continue a laminar path (laminar being the fastest path) under the car and to introduce it to the slow air behind the car. The introdcution that the diffuser makes is like a lead rope for the fast air. Without the introduction, the fast air hits a brick wall of sorts behind the car of higher pressure air. Tim did touch on this point, and then he gets off track
...
There's all sorts of theory behind diffusers. Some contraversial and some not. The above is widely accepted. There are certainly good arguments to be made about diffuser angles and such. Where the diffuser should start on the car dictates the angle it take to exit. That's a double edge sword since the diffuser starting point impacts the point of peak DF.
A couple other things we know... Some is just reiterating what Tim and others said... Deeper side strakes on the diffuser help keep dirty (aka high pressure) air out of the stream and help to promote laminar flow. Sharp angles on the diffuser strakes create vorticies which lower drag and stimulate velocity (notice that's much differnt than the very rounded corners on our diffusers). Side skirts help to keep high pressure air out of our low pressure stream of air under the car (thank you Mr. Chapman for figuring that out). Strakes that run down the underside of the car do similar...
The other reason I give TIm a hard time is becuase he's an advocate for the Lotus Engineers and it's just so fun to
at him
He knows it's all in good fun.
Best,
Phil