It is the role of friction, as an agent,
to convert a part of the mechanical energy into intermolecular
energy. This part of the mechanical energy converted into
the intermolecular energy is termed as the loss
of energy.
When the path of the fluid is suddenly
changed in course of its flow through a closed duct due
to any abrupt change in the geometry of the duct then apart
from the losses due to friction between solid surface and
fluid layer past it, the loss of mechanical energy is also
incurred. In long ducts, these losses are very small compared
to the frictional loss, and hence they are often termed as
minor losses.
But minor losses may, however, outweight
the friction loss in short pipes or ducts. The source of
these losses is usually confined to a very short length
of the duct, but the turbulence produced may persist for
a considerable distance downstream.