What a Load of Hot Air!

Marion Cromb and Daniel Thomas reveal the invisible beauty of the world around us with schlieren imaging.

Look here, there’s something cool happening! No, not just on the page, but in the air in front of it! There are many interesting phenomena taking place all the time in the air surrounding us, but their transparency normally makes them difficult to admire. However, certain techniques are able to pick up on slight changes in the way materials bend light, allowing us to see the otherwise hidden beauty of air flow and much much more.

 

It all relies on a fundamental property of light. The speed of light is inversely proportional to the refractive index of the medium it is travelling through. Slight variations in the refractive index of a medium are often caused by variations in density. In air, cold regions will be denser, and light is bent towards those regions due to their higher refractive index. When light passes through a boundary between two materials of quite different refractivity, such as from water to air, the bending of light is obvious – objects are noticeably distorted. But subtler refractive index changes, such as those due to the heating of air, are too hard to observe with the naked eye unless something amplifies the deflection effect.

A simple way to amplify this effect is the shadowgraph method. This simply involves shining light through a transparent medium with varying refractive index, and observing the shadows. Light is bent away from less dense regions, and these areas will refract light outwards to form a bright fringe around an inner darker zone. This change in illumination is strongest at the boundaries of these areas. Shadowgrams occur frequently in everyday life, such as the dancing shadows cast by the air above a hot toaster or a candle flame, or the intricate shadows formed by light passing through a wine glass. Some ocean predators even use the shadowgrams cast onto the seafloor by transparent prey to identify their next meal!

Schlieren imaging is a more sensitive method which uses a boundary edge to view variations in refractive index (‘schlieren’). The boundary shows up ray refractions perpendicular to its edge. The simplest variation is ‘background distortion’, where light-dark boundaries are distorted by schlieren in front of them. This is often seen in the ‘wobbly air’ above hot roads. However, this method is only sensitive along the boundaries. A more reliable schlieren method requires lenses or a mirror to focus the schlieren image. Light passing through a schlieren source is focused to a point and a boundary is placed at this point. Light deflected by the schlieren misses the original focus point and moves across the boundary. Small differences in light direction are therefore translated into light amplitude or colour differences in the schlieren image. This process was used to produce the images on this page. By using a variety of cut-off filters, a huge range of different lighting effects can be achieved.

Shadowgrams occur frequently in everyday life, such as the dancing shadows cast by the air above a hot toaster

Different fluid flow phenomena are observed with these techniques. The mushroom-shaped plumes are examples of Rayleigh-Taylor instabilities which occur when a denser fluid sits above a less dense fluid. The mass of air heated by the lighter flame is buoyant and so moves upwards. Friction from the surrounding cold air slows the outer hot air relative to the air in the centre. This pulls the outer air around and down to create the mushroom shaped plume. The faster inner air drags in the outer air in its wake, creating a vortex ring that travels upwards. Rayleigh-Taylor instabilities often lead to KelvinHelmholtz instabilities: waveforms associated with boundaries between fluids moving at different velocities. These occur on the edges of the candle plume; as the buoyant air gains speed, the instabilities become larger and the plume turns turbulent.

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