## Dimensional Analysis

Chapter Overview and Goals Because the fundamental behavior of flows can change drastically as flow parameters are varied, it is customary to describe flows in terms of dimensionless parameters. For instance, when we say a flow is “fast,” a relative term, we mean that the flow speeds are large compared to some other quantity that has the dimensions of velocity. Another way to state this is to say that in equations such as our momentum and energy equations, the word “fast” is a comparison of the relative magnitudes of two terms in one or the other of these equations. Since all the terms in a given equation must have the same dimensions, a comparison of two terms gives a dimensionless ratio.