I am trying to describe an end of a tube or the like that has a passage inside through which fluid flows. I think such an end can be described as an open end. However, when I googled "open end", a lot of images of spanners/wrenches appeared. Is it inappropriate to use "open end" to describe an end of a tube?
It certainly can be called an open end. A cylinder can have an open end as can a spanner. For me searching "open end" gives be lots of images of "open end spinning machines". Google sometimes doesn't read your mind.
Searching for "cylinder open at one end" find images of what you describe, with "sound waves" drawn inside (since the harmonics of an open, and half open cylinder are models of simple wind instruments)
Take care with the article: "the open end" implies that the other end is closed.