High-speed 3D imaging of liquid jets, surfaces and respiratory droplets

This thesis is intended for you with an interest in post-processing for 3D imaging and would not mind learning how some techniques can work on liquids. The thesis details and evaluates three measurement techniques applied on liquid jets, surfaces and respiratory droplets, respectively. They are connected to events found in among other combustion, food production and disease spreading.

On November 12, 1732, a man named Henri Pitot showed his new invention to the Royal Academy of Sciences in Paris. He showed two tubes. One tube was straight while the other was shaped like an L. What was it supposed to do? It was developed to measure the flow speed of water in rivers. At the time, the standard methodology was to put an object (typically an orange for its excellent floating abilities) in the river and observe its speed. It worked like a charm but there was a fundamental limitation. One could only measure the flow speed at the surface. Pitot’s tube worked in a unique way that made it possible to measure the velocity at different depths in the river. Suddenly a new dimension was available. With his new measurement device, Pitot challenged the prominent theory at the time that flow speed in a river increase with depth. He correctly found that the opposite was the case. This basic knowledge of fluid dynamics was discovered because of the groundbreaking measurement tool.

This thesis has quite some similarities with the novelty of the tube. Here, measurement techniques are presented that unlocks another dimension of liquids. This other dimension is the third dimension. The third dimension that we are all living in, the third dimension that you perceive in the distance, the third dimension in which all liquid phenomena take place. Currently many measurements on liquids use imaging which is a 2D measurement approach that has given significant insight into fluid dynamics. However, it can never give the complete picture since, as mentioned, all liquid phenomena take place in 3D. Therefore, 3D measurements are important. I present three different kind of 3D measurements in this thesis that have been performed on three different liquid phenomena: a water jet, a hollow cone water sheet and respiratory droplets. The first two are phenomena with major industrial applications. They are both closely connected to sprays and sprays are used in more places than you might think. Examples are painting, cooling, food processing, firefighting, combustion engines, and more. An improved understanding of the water jet and hollow cone water sheet can give a higher precision and efficiency for both applications in jets and sprays.

The third phenomenon was respiratory droplets who are connected to the way too well known COVID­19 pandemic. We can all probably say where we were when the world shut down (probably at home learning how to use zoom). To lower the risk of overfull hospitals, it was important to delay the spreading of the virus. The first suspect of this spreading was direct contact between people and contaminated surfaces such as keyboards and measures were taken to lower the probability of spreading the virus in this way. Later, research suggested that this path was not as probable as first suggested since a virus cannot survive on surfaces long enough. The next spreading suspect was then the infamous aerosols. Aerosols are small particles that can float for a long time in the air to then be inhaled and infect when containing viruses. In the fall of 2020, we wanted to help so we employed our laboratory skills in spray imaging to extract information of 3D velocity and size of these small droplets. This is essential information in understanding what measures should be taken to prevent the spreading of the virus.

In this thesis I present the 3D measurement techniques we have developed that are unlocking the third dimension of all these phenomena. Just like the Pitot tube did in its time, I hope that these measurement techniques will produce knowledge previously beyond our grasp and both improve sprays for specific applications and our ability to fight future pandemics.