Science Heresy

April 2011


The break at Shipstern Bluff, Tasmania
Photo: Rodd Owen/Billabong, AAP

The AMC Towing Tank

Ocean Waves

Surface gravity waves (e.g. ocean waves) have an intersting property. They are "dispersive". This means that the group velocity (the speed of a "set" of waves) is exactly half of the phase velocity (the speed of an individual wave as it moves through the set). Both of these velocities depend on wavelength; the longer the wavelength, the faster the wave speed. Thus when swells from distant storms arrive at a coastline the longest waves travel fastest and arrive first. This is why swells shorten over time.

Waves slow down when the water becomes shallower. When they slow down they become shorter and steeper because the same amount of wave energy is contained in a shorter length interval just as cars cluster together on a freeway when the traffic stream slows down. When waves reach a certain critical steepness (wave height = wavelength / 7 approximately) they break.

Waves also break in the open ocean away from the shore. This is called "whitecapping" and characterises "wind seas" as distinct from swells.

All this has been known for more than a century. However there are some aspects of waves which are still not well understood. Chief among these is frequency downshifting., i.e. the way in which wind waves become longer in wavelength and lower in frequency with increasing fetch. The "fetch" is the distance over water that the wind has been blowing.

A modest experiment, completed in a single day using the Australian Maritime College towing tank (left), provided some insights into frequency down-shifting and the evolution of wind-waves.

Richard McCure pictured
in his natural habitat.
Are we too smart for our own good?

Richard McCure ponders the consequences of ever-increasing intelligence when viewed from an evolutionary perspective.

Is intelligent life always too brief to leave a significant trace in the fossil record?

Read his entertaining yet profound essay Evolution and Extinction.

The Future of Robotics

Self-navigating autonomous robots have been around for several decades now. These are not the same thing as the "robots" which paint and weld in car factories which are really machine tools. Nor do we mean the humanoid, radio-controlled toys sometimes seen in TV commercials and gee-whizz kids programs. These are really puppets since they require a human being to control their movements from behind the scenes.

Autonomous robots which fetch and carry and vacuum the house while you are at work sound like an attractive proposition and some have made it to the market place.

Why have they never taken off commercially?

Allan Branch, who has been involved with robots for several decades tells us about The 15 Year Cycle of Robotics (pdf 509 kB).