#froude number

[fuckyeahfluiddynamics]

Why Unpaved Roads Washboard

As anyone who has regularly traveled unpaved roads knows, they have a tendency to develop regularly spaced corrugations, otherwise known as washboarding.  (Video and image credit: Practical Engineering; research credit: N. Taberlet et al. and I. Hewitt et al.)

[fuckyeahfluiddynamics]

Roll Waves in Debris Flows

When a fluid flows downslope, small disturbances in the underlying surface can trigger roll waves, seen above. Rather than moving downstream at the normal wave speed, roll waves surge forward -- much like a shock wave -- and gobble up every wave in their way. (Image credit: M. Malaska; research credit: X. Meng et al.; see also M. Glessmer; via APS) Read the full article

[realgarn]

Apparently Freud was really terrified of the number 62.

There's a pun here I want to make about fluid dynamics and resistance but I can't.

[fuckyeahfluiddynamics]

@spooferbarnabas asks: I was wondering what the difference is between Froude's number and Reynold's number? they seem very similar

Fluid dynamicists often use nondimensional numbers to characterize different flows because it's possible to find similarity in their behaviors this way. The Reynolds number is the most common of these dimensionless numbers and is equal to (fluid density)*(mean fluid velocity)*(characteristic length)/(fluid dynamic viscosity). The Reynolds number is considered a ratio of total momentum (or inertial forces) to the molecular momentum (or viscous forces). A small Reynolds number indicates a flow dominated by viscosity; whereas a flow with a large Reynolds number is considered one where viscous forces have little effect.

The Froude number, in contrast, focuses on resistance to flow caused by gravitational effects, not molecular effects. It is defined as (mean fluid velocity)/(characteristic wave propagation velocity). Initially, it was developed to describe the resistance of a model floating in water when towed at a given speed. As the boat's hull moves through the water, it creates a wave that travels forward (and backward in the form of the wake), carrying information about the boat--much like pressure waves travel before and behind a subsonic aircraft. The speed of the wave created by the boat depends on gravity (see shallow water waves). The closer the boat's speed comes to the water wave's speed, the greater the resistance the boat experiences. In this respect, the Froude number is actually analogous to the Mach number in compressible fluids.

I hope that helps explain some of the differences!