In steady flow, which of the following characteristics remains constant over time?

• A. Depth
• B. Velocity
• C. Flow rate
• D. None of the above

Answer: (D)

In steady flow conditions, it is important to understand that the term "steady" implies that certain characteristics of the flow remain consistent over time. In this context, the flow rate is one characteristic that remains constant.

Flow rate, which is the volume of fluid that passes a certain point per unit of time, does not change during steady flow. This means that for any cross-section of the flow, the volume of fluid entering is equal to the volume of fluid exiting during that time frame, leading to a consistent flow rate.

In contrast, velocity and depth can vary within a flow field, particularly in open channel flows or when considering different cross-sections. Therefore, these variables may not remain constant even if the flow is steady overall.

This understanding clarifies the definition of steady flow and emphasizes the constancy of flow rate as a key characteristic under these conditions, making the correct conclusion that none of the other options represent characteristics that remain constant over time.