If properties are allowed to change within the control volume, the processes are called ______ flow.

If properties are allowed to change within the control volume, the processes are called unsteady flow.

Unsteady flow occurs when the fluid properties (such as velocity, pressure, density, etc.) change with time at a specific location. This means that conditions within the control volume are not constant over time, which can significantly affect the dynamics of the fluid. Unlike steady flow where properties are constant with respect to time at a given location, unsteady flow captures the transient behavior of fluids, which is crucial in many real-world applications, such as in the analysis of piping systems during start-up or shut-down phases, in natural water bodies, or during any situation where the fluid characteristics are fluctuating.

Understanding unsteady flow is essential in fluid mechanics and helps engineers design and analyze systems effectively, taking into account the variations that occur during different flow scenarios.