forced flow, the fluid is forced to flow over a surface or in a tube by external means such as a pump or a
fan. In natural flow, any fluid motion is caused by natural means such as the buoyancy effect that manifests itself as the rise
of the warmer fluid and the fall of the cooler fluid. The flow caused by winds is natural flow for the earth, but it is
forced flow for bodies subjected to the winds since for the body it makes no difference whether the air motion is caused
by a fan or by the winds.