However, in many cases, the flow variables depend only upon some of these four variables and not upon the remaining ones. Such flows are much easier to deal with both analytically and numerically because less information is needed to describe all that needs to be known about the flow. We use special names for flows that have this simplifying dependence upon space and time.
Steady flows are ones for which there is no variation of the flow variables as time progresses.
If we sit in an automobile travelling at a constant speed along an empty highway, the air flow relative to us and the vehicle is a steady flow. The partial time derivative of any flow variable is zero, and these variables depend only upon the distance measured from a fixed point in the vehicle. On the other hand, if the vehicle were accelerating or if it were to pass another vehicle on the road, then the flow observed by the vehicle's occupant would be unsteady . Sometimes an unsteady flow can be described as a steady flow by suitable choice of a reference frame. The flow observed by a pedestrian at the side of a highway who is passed by a bus moving at constant speed is an unsteady flow, but the same flow observed by a passenger in the bus is a steady flow.