Take this with the grain of salt that I haven't designed any F1 diffusers, especially the ones pictured. As far as the step, I have a few guesses, I've done things like that for front diffusers to pick up a little velocity at an edge or reduce pitch sensitivity. There is a big gain in downforce from an additional degree of angle but if you start riding really close to stall you can get bad behavior when the car pitches if you wash out diffuser, sometimes little tricks like that will cause a "softer" stall and make that peak value window bigger. Then you get the car to live in the peak value range without making the car stiffer or losing rake options because they hurt aero. You gotta test, test, test because it could be any one of a few other possibilities as well.
About the concave shape: flow separation is the result of a rate of change that is too high in the pressure gradient, not any specific angle. If you look at that honda example above, the diffuser is contracting in plan view at the same time you see the concave shape in side profile. To target the maximum downforce value you need to ride the limit of that pressure gradient before the flow seperates over as much surface area as possible. One way to do that is to increase volume in third dimension even if rules (or packaging) constrain you in another.
Think of this like water flowing over a bump. You have a nice gradual bump and the fluid just rises and falls. If you have a very agressive curve on the bump, the water will start tumbling off the back side, however if you also start constricting the flow down from plan view at the same time as you start to recede on the back side of bump, it wont separate. Hope that helps.
Last edited by gixxer_drew
on Sat Dec 10, 2011 1:23 am, edited 1 time in total.