PRESSURE
I still say that pressure changes are the ultimate way to describe aerodynamic phenomena. OK other factors are involved but the vast majority of airflow changes are as a result of pressure, it has to be so as this is the only way to apply force and change momentum of the air. Yes many different explanations can be used without mentioning pressure, but pressure is still there.
Is the pressure increased in ground effect? This can and has been answered intwo ways:
1. In a steady state hover the average pressure difference over the blades must be the same to give the same force (=weight). So you can say there is no pressure difference.
2. Going back to the original question of the effect on an autorotation...a helo arriving close to the ground will produce a pressure build up which make the rotors more efficient reducing its rate of descent. So yes you get an increase in pressure.
We can all argue until we are blue in the face. I for one believe pressure explains an awful lot. We can all use different tools to help us visualise how the air moves, computer programs that predict air flows use different methods: sinks, sources and vortices, purely mathematical models from imaginary worlds, and even finite element blocks that use physical properties at the boundaries with neighbouring blocks. Non of these are 100% accurate, only the real world is, but it shows how the same thing can be described in many ways.
If nothing else this thread has got many of us looking at different viewpoints, and hopefully, given us new tools to look at other situations.
PS I have been out of this for a while due work commitments, glad it was still going when I got back.