Archimedes' screw screws of decreasing pitch are used for the compaction of waste material. If water is fed into the top of an Archimedes screw, it will force the screw to rotate. The rotating shaft can then be used to drive an electric generator. Such an installation has the same benefits as using the screw for pumping: the ability to handle very dirty water and widely varying rates of flow at high efficiency. Settle Hydro and Torrs Hydro are two reverse screw micro hydro schemes operating in England. The screw works well as a generator at low heads, commonly found in English
Archimedes number Archimedes number In viscous fluid dynamics, the Archimedes number (Ar) (not to be confused with Archimedes' constant, π), named after the ancient Greek scientist Archimedes is used to determine the motion of fluids due to density differences. It is a dimensionless number defined as the ratio of external forces to internal viscous forces and has the form: where: When analyzing potentially mixed heat convection of a liquid, the Archimedes number parametrizes the relative strength of free and forced convection. When Ar » 1 natural convection dominates, i.e. less dense bodies rise and denser bodies sink, and when Ar « 1