Answer: 4. Involves zero transverse components of flow
A one-dimensional flow is one which involves zero transverse components of flow.
It is a flow in which all of the flow parameters may be represented as functions of time and only one space coordinate. The distance along the fluid's flow centerline, which need not be straight, serves as the single space coordinate in most cases. For example, when differences in pressure and velocity occur along the length of a pipe, the flow is thought to be one-dimensional; however, any fluctuation over the cross-section is thought to be negligible. Hence it is clear that a one-dimensional flow is one which involves zero transverse components of flow.