Quote:
Originally Posted by freebeard
What I'm fishing for is a description that contradicts aerohead's.
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I'll be blunt - Aerohead's description of how diffusers works is so ridiculous that no reference is ever going to mention his theories.
If you really want to know - and boy, am I giving you the benefit of the doubt - diffusers work like this. (And I am giving the normal workings of a diffuser, not odd ones where you suck away boundary layers, etc.)
1. They have attached flow. That attached flow, as it curves upwards, develops a low pressure.
2. That low pressure helps pull the car downwards.
3. That low pressure also helps the airflow under the car*, so giving high speed flow (and so lower pressure under a big area, so pulling the car even more downwards). *This is sometimes called 'diffuser pumping'.
4. The wake is made smaller as the airflow is directed partly upwards - just as on the upper surfaces, a fastback shape directs the air downwards. So drag can also be reduced.
5. On motorsport cars, the diffuser is much steeper and flow separates down the edges (and strakes), creating vortices. These just add to everything from 1-3, making downforce stronger but at the expense of a lot of drag.
6. There is a lot about diffusers that is not currently understood, eg diffusers work differently in drag and lift reduction on squarebacks versus notchbacks and fastbacks. The current lit suggests taking care in using diffusers for drag reduction on squarebacks.
As far as I can ascertain, Aerohead believes that lift/downforce comes from separated flow, so he assumes the diffuser is always in separated flow and it's wake pressures doing the work. But of course pressures from attached flow can be much greater than wake pressures, so no one would think that way. (Unless you'd read only a 1987 book where separation was common, of course.)
Seriously, Freebeard, if you want to learn about the topic, get away from the PC and read a book.
Barnard is cheap as dirt, he is a beautiful writer and he has no major errors I have ever found.