If you check out where some wind turbines have been located, valleys have been used as they are, in effect, a tapered concentrator.
Also wind turbines have been located on top of hills - some distance back from the edge as that improves the performance - the "compression" caused by the ground slope also improves the performance.
As I mentioned in the comment, mass flow between inlet and outlet is constant unless you have a leak. So the change in velocity will be according to the change between the diameters and there will be a discharge coefficient as there will be lost air.
There have been ventilation fans using shrouds to reduce the edge losses, but that becomes impractical for wind turbines.
You could set up an experiment using cones to evaluate the velocity increase based on the diameter change. One thing is to then use a pitot tube to measure the velocity profile across the outlet and inlet if you really want to find out what happens. Then you can use that velocity profile to check the mass flow etc and find out which parameters are mostly responsible for the errors. This was a really good lab experiment which was 3 hours work... The analysis and write-up took much longer.