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"Temperature imaging in gas and liquid flows using luminescent particles"
The visualization and quantification of thermal mixing in turbulent fluid flows is key to the development of accurate turbulence modelling. These models are needed to predict the behaviour of flows in industrially relevant applications as well as geophysical flows (ocean, atmosphere, earth’s mantle, etc.). This talk will describe thermometry techniques based on luminescent tracer particles which can be combined with particle-based velocimetry to image both temperature and velocity in laboratory flows. The emphasis will be mostly on inorganic luminescent crystals, typically referred to as thermographic phosphors. These particles exhibit a wide range of luminescence properties and can be chosen to match the application needs, e.g., for use in cryogenic flows, at physiological temperatures, or up to 1000 K. There are also various implementations of this measurement concept. We can exploit the temperature dependence of the luminescence emission spectrum or the decay time of the particles to measure the temperature. The velocity can be measured simultaneously using temporally separated images of the particles using light scattering as in traditional PIV, using luminescence light or even single images of phosphorescence streaks caused by the motion of the particles during their luminescence decay. Recent developments include high-resolution measurements in submillimeter boundary layers, a proof-of-concept study of 3D temperature and velocity measurements in gas flows, and 2D thermometry in water with sub-°C precision. Finally, we will show how, through chemical synthesis, we can tune the particle luminescence properties to exactly match the temperature range of our application, for example, around 100°C to study boiling.
Bio:
Dr. Benoît Fond is in charge of luminescence-based optical diagnostics at ONERA, the French Aerospace Lab, and Paris Saclay University. After a French engineering degree and an MSc from the University of Florida, he obtained his PhD in Mechanical Engineering from Imperial College London in 2014. In 2016, he was appointed Junior Professor of Experimental Thermofluids at the University of Magdeburg in Germany and joined ONERA in 2021 to lead the development of pressure- and temperature-sensitive paints (PSP/TSP). Dr. Fond’s works focus on experimental fluid mechanics, leveraging luminescent materials for thermometry or pressure sensing, whether on surfaces or in fluid flows, with various applications for batteries, combustion, chemical processes, hydrogeology, aerodynamics, and more. Dr. Fond is also a lecturer at the international PSP/TSP course organized by DLR Göttingen, Germany, and was invited to the Laser Diagnostics for Energy and Combustion Science Gordon Research Conference in 2023.
"High-speed imaging in multiphase flows: Fine sheets of air and liquids"
The splashing of a drop impacting on a liquid surface is emblematic of the complexity of multiphase flows. Despite the drop’s small size 5 mm, the ejecta dynamics are controlled by interactions on a wide range of scales. The first contact entraps a thin sheet of air under the bottom of the drop, which is about a micron thick, while the ejecta emerging from the neck between the drop and pool, is of the order of 5 microns, becoming thinner with increasing impact Reynolds numbers. Understanding the breakup of these fine structures is important in many industrial and natural phenomena. We use ultra-high-speed video, at up to 7 million fps, in combination with interferometry, to capture the breakup of these structures. This includes the generation of the double-crown driven by internal vortex rings [1] and the formation of spiky contacts for the impact of an emulsion droplet [2]. Finally, new results will be shown for high impact velocities at 20 m/s, by drops free-falling in a 25-m-long vacuum tube, revealing novel breakup dynamics of the dancing ejecta [3].
Figures: Left: The double crown formed when a viscous drop impacts a low-viscosity liquid film [1].
Right: Bottom-view of an air-disc entrapped under an emulsion drop impacting on a glass surface, showing localized spikes from the heavier emulsion droplets [2].
1. Aljedaani, A. B., Afzaal, M. F., Langley, K. R., Yang, Z. Q. & Thoroddsen, S. T. Double crown during drop impact on an immiscible shallow pool, in press at J. Fluid Mech. (2025).
2. Raja, K., Daniel, D., Aguirre-Pablo, A. A. & Thoroddsen, S. T., Local spiky contacts during impact of an emulsion drop on a solid surface. J. Fluid Mech. 1001, A60 (2024).
3. Tian, Y. S., Aljedaani, A. B., Alghamdi, T. & Thoroddsen, S. T., Dancing ejecta, J. Fluid Mech. 981, A4 (2024).
Bio:
Dr. Sigurdur Thoroddsen is a Professor in the Physical Sciences and Engineering Division at King Abdullah University of Science and Technology (KAUST), which he joined as a founding faculty in 2009. Thoroddsen received a BS in ME from his home-country at University of Iceland, followed by MS in Civil Engineering from Colorado State University and PhD in Applied Mechanics from UC San Diego in 1991.
He started as an Assistant Professor in Theoretical and Applied Mechanics at the University of Illinois, Urbana-Champaign. Then became an Associate Professor in ME at the National University of Singapore, before joining KAUST to set up the High-Speed Fluids Imaging Laboratory, with a focus on experimental fluid mechanics with high-speed imaging of free-surface flows, such as drops and bubbles, focusing on splashing, coalescence and singular jetting.
Thoroddsen has authored 200 publications in international scientific journals, describing his work on turbulence, coating flows and granular jets, in addition to his studies of the dynamics of drops and bubbles. His images and videos of flow phenomena have received seven separate Gallery of Fluid Motion Awards. He was elected a Fellow of the American Physical Society in 2012.
