Wednesday, October 26, 2011

No zombies here

Do zombies eat popcorn with their fingers? No, they prefer to eat fingers separately.

This post has nothing to do with zombies, I just heard this joke the other day and found it bizarrely funny. I guess there is no accounting for my taste in humor. Instead of zombies, I’m going to write about lee waves as they are cool (in my mind) and I gave a talk about them recently. Lee waves can be found everywhere if you know what you are looking for. They lurk in your sink, form over mountains and even beneath the ocean’s surface (I wouldn’t be surprised if they can be found out in space too).

Topography, like mountains or under-sea ridges, affects the flow that passes over it. Fluid (air or water) in the lower layers is pushed up the windward or upstream side where it squeezes in with the upper layers causing the flow to speed up. On the lee side, flow slows down again and a disturbance to the flow is formed. This 'disturbance' is often a wave that travels in the opposite direction of the flow. When the speed of the flow and this wave are the same, the wave is stationary and called a lee-wave. (ever notice a bump in the water right below a weir? It’s a non-linear form of a lee wave called a hydraulic jump).

Lee waves were discovered by glider pilots in the 1930s. If a glider catches a lee wave in the right place, the unpowered aircraft can gain significant altitude. All these early papers are in German, so I don’t know what these early pilots had to say about lee waves, my guess is that they found it pretty exciting. As a teenager, I regularly flew in gliders – which typically included a full day of pushing the glider around on the ground and about 7 minutes of actual flight time. I loved it. We flew out of a field on the prairies in Alberta, too far from the Rockies to gain altitude through a lee-wave.

A soon as lee waves were discovered, scientists started looking at why and how they form. The theory requires looking at the Navier-Stokes equations – a mighty difficult task which only recently became doable with computers. As an alternative, lab experiments were conducted. These experiments (and there was lots of them) offered a straightforward way to look at the factors influencing lee wave formation – combinations of the obstacle height and width, and the fluid velocity and density. Once this parameter space was full, it could be used to predict real world phenomenon.

Ocean lee waves are common and in shallow waters and beneath them a significant amount of turbulence is created. Oceanographers can look at them in detail using current meters (I don’t think there is an equivalent measurement instrument for the atmosphere yet). Because of the augmented flows and turbulence, the sea floor under a lee wave makes great habitat for critters – especially stationary filter feeders, as a buffet of tasty treats is whooshed by.

Filter feeders are often also builders, such as coral reefs, glass sponge reefs or even mussel beds. Sometimes the structures they build can intensify the turbulent flows they moved there to take advantage of. They can add to the roughness of the bottom (thus creating more drag) or even make the slope steeper. A steeper slope will result in a steeper lee wave, steeper lee waves may even break (remember the hydraulic jump?) creating even more turbulence. More turbulence means more food can be churned through the water giving the filter feeders more to eat.

As a tangent: this is post number 100.

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