What is the term used to describe when a fluid suddenly stops or changes direction?

The term "water hammer" specifically refers to the phenomenon that occurs when a fluid in motion suddenly stops or changes direction. This sudden change creates a pressure wave that can lead to a loud banging noise in pipes, as well as potential damage to the piping system if the pressure surge is strong enough. Water hammer is commonly associated with systems containing liquids, such as water supply lines, where valves are shut off quickly or pumps are turned off abruptly.

In contrast, while "fluid shock" and "pressure surge" relate to changes in pressure within a fluid system, they do not refer specifically to the sudden stop or redirection characteristic of water hammer. "Flow flutter" is not a commonly used term in this context and does not accurately describe this specific phenomenon. Therefore, the precise terminology of "water hammer" highlights both the mechanical implications of fluid dynamics in a piping system and the audible effect it generates, making it the correct choice.