Researchers from the FAMU-FSU College of Engineering, in conjunction with the team from Oak Ridge National Laboratory, recently developed a novel method for producing tracer particles for turbulence research in liquid helium at temperatures just 1-2 degrees above absolute zero. The technique, which used neutrons to produce molecular tracers in liquid helium, was outlined in a paper in the April edition of Physical Review Letters and was selected as the Editor’s Suggestion.
Among the paper’s co-authors are Shiran Bao, Ph.D. and Wei Guo, Ph.D. of the National High Magnetic Field Laboratory (Maglab). Guo, the program director of the Maglab’s cryogenics research group and a formost authority in the field of cryogenics, proposed the research project with together with the Oak Ridge team (led by Dr. Fitzsimmons). Bao, a postdoctoral researcher in Guo’s group, participated the experimental work at the Oak Ridge National Lab.
As expounded by Guo, “The key point of this paper is that we have developed a promising novel technique that enables visualization measurement of the full-space velocity field in liquid helium, which can possibly break the ground for high Reynolds number turbulence research in a compact laboratory facility,” he said.
Their paper described the process by which long-lived molecular tracers are produced in liquid helium via neutron capture on helium isotopic impurities. These molecular tracers were then visualized through laser-induced fluorescence.
On its face, the finding is notable for several reasons.
First, flow visualization in the low-temperature and low-density liquid helium is notoriously difficult. Many conventional methods for tracer particle injection in water and air are not applicable. In the past, Guo’s team demonstrated the creation of the molecular tracers in liquid helium using focused laser beams, but only limited information about the flow field can be obtained. The new method allows the generation of a large number of small clouds of molecular tracers. These clouds disperse throughout the entire fluid body and each can be easily tracked, making full-space velocity-field measurements possible.
“Since Stokes drag dominates other forces, these microscopic tracer particles are expected to follow faithfully the motion of the fluid,” Bao noted, citing the small molecular clouds the researchers worked with among their chief assets.
The research also lays the foundation enabling next-generation measurements of turbulence with extremely large Reynolds numbers (Re, ratio of velocity times object dimension to kinematic viscosity of the medium). Due to the vanishingly small viscosity of liquid helium, especially in its superfluid phase (He II), extreme values of Re (above to 107) can be obtained for miniature objects, e.g., scaled replicas of airplanes, submarines, etc., immersed in flowing liquid helium at moderate speeds. These Re numbers are far in excess of the capabilities of the most powerful wind tunnels.
“The conventional methods to achieve a high Reynolds number include using highly compressed gases in high-pressure facilities or using large-scale flow facilities. These facilities are usually very expensive both in their construction and operational costs,” Bao said.
“It's definitely a big progress if we can produce high Reynolds number flows in a compact flow facility that fits in our laboratory space.”
By unlocking the potential of liquid helium in high Re turbulence research, the researchers expect that “systematic study of high Re flows will deepen our knowledge of many phenomena that occur in our everyday life such as ocean waves, windstorms, et cetera. The technique may also benefit the design and prototype test of high speed aircraft, large naval vessels.”
Read the paper, titled "Imaging Fluorescence of He2 Excimers Created by Neutron Capture in Liquid Helium II."