Mikal Schlosser

Wind tunnels, soap bubbles, and super-fast cameras

Tuesday 23 Mar 21

Contact

Clara Marika Velte
Associate Professor
DTU Mechanical Engineering
+45 45 25 43 42

Turbulence Research Laboratory DTU

The lab has eight researchers and PhD students attached, as well as a retired researcher who participates because he finds it interesting.

Funding

• European Research Council: ERC Starting Grant project named UniEqTURB
• Poul Due Jensen Foundation – Turbulence Centre of Excellence

The centre is the only one of its kind in the world, and will also be open to international researchers.

trl.mek.dtu.dk 
The new turbulence research laboratory at DTU Mechanics will study turbulence through a strong combination of theory, experiments, and computer simulations.

The turbulence laboratory consists of two test rooms and a control room, from which the experiments are managed and the numerous data is collected. In one test room, the eddy currents will be measured macroscopically. Air will be blown in through a jet, which ensures that the current is smooth (laminar) and has the same speed across the aperture. At the same time, a soap bubble generator will feed a cloud of 15 micrometre-wide bubbles into the air jet, where they disperse like smoke. The bubbles are illuminated with laser light, and four high-speed cameras record how the light is spread.

The turbulence movements are divided into waves, because they can be described mathematically. It is like a jigsaw puzzle: when you know all the waves, you can combine them into a jet, and relate this to Navier-Stokes equations that describe the wave movement.

From eddy currents to heat

In the second test room the turbulence will be studied on the smallest relevant scales, to learn more about how the eddies exchange energy and are eventually converted into (friction) heat. In this room, the laser light is projected onto oil droplets 1 micrometre in size. They spread less light than the soap bubbles, but follow the airflow better, so the dissipation can be measured (how much kinetic energy is converted into heat, and how quickly).

Dissipation is very difficult to measure, but is a key parameter in all turbulence models used in universities and industry. Researchers use classic turbulence theory to predict what the dissipation will be, assuming that the small and medium-sized scales are in equilibrium. However, there is a suspicion that disequilibrium has a significant impact precisely on dissipation. If Clara’s team can also measure this, they will be able to calculate new parameters that science and industry can use in their computer simulations. 

High-speed cameras

Both types of experiment take place in a wind tunnel of sorts, in which the air circulates. The high-speed cameras can capture approx. 6,600 images in 7.8 seconds for the macroscopic measurements, and 250,000 images over 15 seconds for the measurements on the smallest relevant scales.

With five experiments a day and two parallel test rigs, a huge number of extremely detailed images need to be transferred to the computer. It takes one hour per test run, making this by far the most time-consuming part of the experiment. And once the data has been transferred, this is followed by a comprehensive analysis process. So the five years that have been allocated to the project at this stage will certainly be needed.

Read more in the article 'Making sense from the chaos of turbulence'

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