|
The diffuser part allows the flow to slow back down and the pressure to gradually head back towards the freestream value rather than having a kind of jet of high-speed air coming out from under the car. The loss of energy from such a jet mixing with near-stationary is much, much larger and means that the effective resistance of the underbody channel is very high.
As i gather you already know, the underbody airflow is very similar to flow through a normal pipe. Since the pressure at the front of the car is effectively fixed and the base pressure (general level of static pressure in the 'void' like region immediately behind the vehicle) normally only varies over a fairly narrow range, the pressure difference across the 'pipe' is pretty much fixed. This means the amount of airflow is essentially controlled by the resistance of the pipe. Low resistance means more flow. More flow for a fixed cross-sectional area means higher velocity. Higher velocity means lower pressure. Lower pressure leads to more downforce.
For maximum effectiveness you can try and reduce the pressure in the base area by careful location of a wing. This low pressure will increase the difference across the underfloor 'pipe' and either give you more flow for a given geometry or allow you to angle the diffuser more steeply by reducing the metephorical hill the air has to climb on it's way back to freestream static pressure. A steeper diffuser means you can have a longer flat section to the floor and/or exaggerates the suction peak you get at the very apex of the diffuser, where it joins onto the flat bit of the floor. Both mean more downforce, partly by increasing the size of the region exposed to the lowest pressure and partly because you're turning the flow upwards around a larger angle.
I'm sorry if any of this is teaching you to suck eggs (always thought that's a strange expression), but i hope some of it helps.
Cheers
Rob
|