High Reynolds Number Turbulence Research in Cryogenic Helium

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Many flows in nature, including those generated by aircraft, ships, and the atmosphere, have extremely high Reynolds numbers. The Reynolds number is the ratio of flow's inertial forces relative to viscous forces, and it determines important flow properties such as turbulence. Understanding flows with extremely high Reynolds numbers will increase efficiency in practical vehicle applications and improve understanding of the climate. However, high Reynolds-number flow in conventional fluids like air and water occurs on a relatively large scale thus is difficult to study on the laboratory scale. Fortunately, high Reynolds number flows can be achieved using lower-density fluids such as cryogenic helium-4 in, for example, a small-scale laboratory pipe flow experiment. But to unlock the full potential of cryogenic helium-4, suitable flow measurement tools to quantify the turbulence are also required. The proposed research will therefore develop advanced molecular tagging velocity measurement techniques and apply the technique to high Reynolds number cryogenic helium pipe flow. In terms of educational opportunities, the joint College of Engineering operated by Florida State University and Florida A&M University will be leveraged to recruit under-represented minorities to participate in the project. The PI and the co-PI's groups will also contribute educational demonstrations for public open-house events at the National High Magnetic Field Laboratory and at the Florida Center for Advanced Aero-Propulsion.The scientific goal of this project is to develop a state-of-the-art molecular tagging velocimetry technique for cryogenic helium and to demonstrate its usefulness by applying it in the study of high Reynolds number cryogenic helium pipe flow. Sophisticated patterns of the molecular tracer lines will be created by splitting and focusing a femtosecond laser beam in liquid helium. Advanced pattern-tracking algorithms will be incorporated, and the spatial and temporal resolutions of the tracer imaging process will be optimized. These developments will unlock the full potential of cryogenic helium in turbulence research and model testing. The planned study on high Reynolds number pipe flows in helium will allow an independent examination of the near-wall velocity field and the associated von K?rm?n coefficient, which may help resolve existing controversies regarding the universality of this coefficient. Also, new knowledge about the near-wall spatial velocity correlations will be produced by tracking the tracer lines created perpendicular to the wall. Furthermore, by measuring the pressure drop along the pipe, reliable friction factor data for high Reynolds number flows will be obtained, which will benefit the design of various engineering systems that exhibit high Reynolds numbers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.