Droplets in turbulence
In the turbulent clouds above our heads, small water droplets are whirred around. They collide and merge and in this way may become large enough to make rain. These collisions are enhanced by preferential concentration. Turbulent flows mix added material extremely efficiently. This is what we use if we stir milk in our cup of tea. However, when heavy particles (droplets) are dispersed in a turbulent flow, turbulence creates clusters: preferential concentration.
The research is about new experimental techniques to study clustering at the smallest turbulent length scales, to study collisions between droplets, to study the way they are torn apart in the flow, and to find new numerical techniques to compute their tortuous paths in turbulence.
We do experiments in a chamber in which strong turbulence is created through the pumping action of huge loudspeakers. The chamber is filled with a cloud of droplets with all the same size that are spun from a fast spinning disk. The droplets contain a solution of a Europium, which glows for a while after flashing them with an intense ultaviolet laser. The glowing droplets are followed with a fast intensified camera. They glow just long enough to see them spun form the smallest vortices in turbulence.
An exploding cloud
By focusing the laser beam to a line, we can prepare a thin pencil-like cloud in a strong turbulent flow. By following it with a fast camera, we can see it explode. The surprise is that it expands faster than a cloud of true tracer of light particles, although droplets are heavy and take a while to be accelerated to the velocity of the smallest vortices in turbulence.