A fan is a machine used to create flow within a fluid, typically a gas such as air.
The fan consists of a rotating arrangement of vanes or blades which act on the
fluid. The rotating assembly of blades and hub is known as an impeller, a rotor, or
a runner. Usually, it is contained within some form of housing or case. This may
direct the airflow or increase safety by preventing objects from contacting the fan
blades. Most fans are powered by electric motors, but other sources of power
may be used, including hydraulic motors and internal combustion engines.