Flow The so-called flow refers to the amount of fluid that flows through the effective cross section of a closed pipe or open channel per unit time, also known as instantaneous flow. When the amount of fluid is expressed by volume, it is called volume flow; When the amount of fluid is expressed by mass, it is called mass flow. The volume of fluid passing through a section in a flow tube per unit time is called the volume flow of the section. It's called flow, q