Hi all. When I read through text in fluid mechanics, I see various ways of non-dimensionlising the pressure. (Please see the figure) To me, non-dimensionlize a quantity is to make it of order unity to facilitate the mathematical processes needed afterwards. So, I can understand why density*gravity*H is used as a characteristic scale to non-dimensionlize the pressure since it is the maximum hydrostatic pressure. But I don't understand why density*U^2 is used to non-dimensionalize the pressure as well? In what situation is this nondimensionlization valid? What about the P(infinity), what is it?