A wind turbine receives wind at 10m/s, and slows it to 5m/s to generate electricity.
Does this mean that the wind around the turbine must be diverted to allow twice the cross-section area for the exiting wind?
If not, how does the exiting air escape?
To preserve continuity of mass flow across the propeller disc, the slower air exiting the disc downwind swells out to produce a slow-moving wake that is wider than the propeller disc itself. This gets mixed up with the surrounding air and eventually the wake dissipates as the air in it speeds up to match the surrounding velocity.
The air in the wake of the blades is turbulent. There will be a mixing of the air which interacted with the blades and the passing flow which did not.
