flow 用在机械上是什么意思?

flow 用在机械上是什么意思?以下这句想了半天也没翻译出来,哪位高人能帮帮忙?谢谢!two-way valves in the fully open position have the index marking on the top of the valve stem ,parallel with the direction of flow.two-way valves in the fully closed position have the index marking perpendicular to the direction of flow.