How is uniform flow defined in fluid mechanics?

Uniform flow in fluid mechanics is characterized by having the same velocity at every point in the fluid at a given instant. This means that if you were to measure the velocity of the fluid at any point in a specific cross-section or any part of the flow, the readings would yield the same value. This condition simplifies many analyses and equations in fluid dynamics because it allows assumptions to be made about the flow characteristics, especially in the context of conservation laws.

In contrast, if the velocity were to vary with time or position, as suggested by the other options, the flow would be classified as unsteady or non-uniform. Such variations would complicate the analysis and require different considerations and calculations. Thus, defining uniform flow as having a constant velocity at all points at a particular instant highlights its essential feature and provides a basis for further fluid dynamics studies.