# How is the Froude number modified/used to describe shallow waves over a variable bottom?

I attended a seminar where the speaker was talking about dimensionless numbers that are critical to finding the resistance of objects in a given fluid with certain boundaries. I understood how Reynolds numbers worked, but the Froude numbers seemed a bit cryptic, and the speaker gave the example of using cross-section parameters to find the characteristics of a partially submerged object in shallow water. If the bounds are variable (unpredictably) at the bottom, how is it possible for an expression that works in terms of complete boundaries applicable to a partially inclusive situation?

• What do you mean by "complete boundaries" and "partially inclusive situation"?
– Deep
Feb 16, 2018 at 4:50
• Do you understand what the Froude number means? Or is that your question? Feb 16, 2018 at 9:34
• Yes, I understand that but how does it factor in variables form an unsmooth surface? I heard it was still applicable in conditions that were quite dynamic, like choppy waves. Sorry I can't really phrase that very well. Feb 16, 2018 at 17:02

From wikipedia:

The Froude number (Fr) is a dimensionless number defined as the ratio of the flow inertia to the external field (the latter in many applications simply due to gravity).

In naval architecture the Froude number is a very significant figure used to determine the resistance of a partially submerged object moving through water. Dynamics of vessels that have the same Froude number are easily compared as they produce a similar wake, even if their size or geometry are otherwise different.