Diffusers

[browney]

Hi,
I'm managing to get myself a bit confused with how diffusers work and why they are benificial. I understand that, to have what is called 'ground effects', you want to narrow the gap between the road and the underbody to increase the velocity of the air and reduce pressure. What I don't quite understand is why you then need to have a diffuser at the rear of the car for pressure recovery and why you don't just want the underbody-road gap to narrow, without getting wider again?

Is it becuase the boundry layer will get to large and choke the flow?

Is the main function of a diffuser more to channel the flow exiting the car upward to get downforce, rather than for pressure recovery?

Would having the narow gap continue all the way to the back of the car have a lower mass flow than with a diffuser?

I'd really love someone to clear this up for me.
Thanks