This shape obviously has the property that a fluid flowing through the middle of it will be accelerated at the central constriction. This is useful for concentrating the energy of flowing air or water so that more energy can be extracted from a higher-speed turbine. There are several advantages to this. According to mathematical analysis, wind or water power is proportional to the cube of the wind or water speed, or doubling the speed gives 8 times the power.
Additionally, this particular shape of a venturi has some very useful properties. Consider the shell to be paper-thin, with the fluid flowing past it both inside and outside. Because the curved surface is defined by straight lines, if the wind or water enters the turbine in a vortex spin, which it automatically does entering a turbine, if it follows the straightest path with least resistance, it will automatically flow along the generating straight lines, both inside and outside the shell, with minimal impedance to flow. Therefore, the ratio of the large opening to the small constriction will determine accelleration.