Wind turbines / hydroelectric turbines - I think the same fundamental Physics applies to my question. The fluid (water or air) prior to meeting the turbine has kinetic energy, after the turbine the flow must be the same (or there would be a 'build up' of fluid) - so it has the same KE as before ????????, yet the turbine 'extracts' energy. Where has the energy which the turbine has produced come from ?