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Blowin' in the Wind, Issue #054 Acoustic boundary layers, what are they - May 1, 2008
May 01, 2008
Hello ,

# Introduction to Acoustic boundary layers

Ever had a window open in your home on a breezy day, with a wall of wind coming through – there's a flow of air from one window to another on the other side of your house – and outside the flow, there's a bit of a breeze?

You've just experienced a turbulent boundary layer condition, and they're an important part of modern physics and fluid dynamics. Boundary layer conditions regulate how hot and cold water (and air) mix in applications as diverse as plumbing (they're why your shower temperature drops from hot to lukewarm to cold, suddenly), cooking (they're how convection ovens cook the outside of the meal to seal in the juices) and aerodynamics (when a plane flies, it's the difference in pressure between two boundary layers forming around the wing that generates lift). Your car's engine runs on careful applications of different boundary layers in the pistons and compressor, as does your refrigerator's compressor.

# Layers of the earth's atmosphere

A realistic definition of a boundary layer is any set of circumstances where meteorological properties such as temperature or humidity change dramatically. For an acoustic boundary layer, these shifts affect compression and wavelengths and alter the propagation of waves, typically sound waves. The effects depend on both the atmosphere and the wave characteristics, such as frequency and whatnot. Now, that sounds like a lot of fiddly details, and the equations (the mathematical tools of Fourier analysis, for instance) that define boundary layers aren't for the faint of heart. They're also simplifications of the Navier-Stokes equations, which are much more difficult to solve.

These interactions between shock waves (longitudinal waves) and boundaries will often be the subject of grad school studies. Here is a bit more about boundary layers in general.

# What is a Boundary Layer?

In climate science, boundary layers are important, because they describe how different parts of the atmosphere behave and interact with one another. The most visible example of a boundary layer is clouds. If you ever see a cloud that seems to be "flat" on the bottom, that's because it's surfing the top of a boundary layer. Likewise, when you see thunderheads piling high just before a storm, you're seeing large scale boundary layer interactions in action.

The reason why boundary layers are of great concern to scientists who study climate is that they're the underpinning of the mathematical models that climate predictions are made off of, and they're not perfectly understood. Remember that boundary layer calculations are a simplification – but just like saying that a firecracker and an atom bomb are both explosions, there are simplifications, and there are causes for concern.

Most of the math and science used for boundary layers in science is based off of acoustic boundary layers - studies done by the Navy. In the ocean, an acoustic boundary layer is a zone where the water surface properties such as temperature changes abruptly (typically in the first hundred meters of depth) from warmer water heated by the sun, to cold waters. From the perspective of a submarine using sonar under the water, that boundary layer can act like a mirror, or it can cause sound waves to just vanish – or pull anomalous signals from a certain distance away. These get used a lot in submarine warfare.

Climatologists are finding similar bizarre effects when doing radar and laser mapping of water vapour in the air, and air mass rotations – things that happen in remote places trigger rainfall in other locales – often hundreds of miles away. Unfortunately, most of the acoustic boundary layer solutions are empirical, so there's always a need for more data.

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