What does the term "entrain" refer to in fluid dynamics?

The term "entrain" in fluid dynamics specifically refers to the phenomenon of trapping or carrying along particles, bubbles, or droplets in a moving fluid. When a fluid flows, it can capture these small entities, which then move with the main body of the fluid. This process is essential in various applications, such as aeration in water treatment, where air bubbles become entrained in water, enhancing mixing and increasing oxygen transfer efficiency.

The other options do not accurately describe the concept of entrainment. Increasing pressure refers to a change in the state of a fluid, while decreasing flow rate is about the velocity or quantity of the fluid moving through a system. Adjusting temperature relates to the thermal properties of the fluid but does not involve any aspect of carrying or trapping particles or bubbles. Thus, the correct interpretation of "entrain" aligns with the capturing of bubbles in water.