The superfluid phase of helium-4, known as He~II, is predominantly used to cool low-temperature devices. It transfers heat by a unique
thermally driven counterflow of its two constituents, a classical normal fluid and an inviscid superfluid devoid of entropy. It also has
potential use for economical reproduction and study of high Reynolds number turbulent flow due to the extremely small kinematic viscosity and
classical characteristics exhibited by mechanically driven flow. A number of diagnostic techniques have been applied in attempts to better
understand the complex behavior of this fluid, but one of the most useful, flow visualization, remains challenging because of complex
interactions between foreign tracer particles and the normal fluid, superfluid, and a tangle of quantized vortices that represents turbulence in
the superfluid. An apparatus has been developed that enables application of flow visualization using particle tracking velocimetry (PTV) in
conjunction with second sound attenuation, a mature technique for measuring quantized vortex line density, to both thermal counterflow and
mechanically-driven towed-grid turbulence in He~II. A thermal counterflow data set covering a wide heat flux range and a number of different
fluid temperatures has been analyzed using a new separation scheme for differentiating particles presumably entrained by the normal fluid ("G2")
from those trapped on quantized vortices ("G1"). The results show that for lower heat flux, G2 particles move at the normal fluid velocity vn,
but for higher heat flux all particles move at roughly vn/2 ("G3"). Probability density functions (PDFs) for G1 particle velocity vp are
Gaussian curves with tails proportional to |vp|⁻³, which arise from observation of particles trapped on reconnecting vortices. A probable link
between G1 velocity fluctuations and fluctuations of the local vortex line velocity has been established and used to provide the first
experimental estimation of c₂, a parameter related to energy dissipation in He~II. Good agreement between the length of observed G2 tracks and a
simple model for the mean free path of a particle traveling through the vortex tangle suggests that flow visualization may be an alternative to
second sound attenuation for measurement of vortex line density in steady-state counterflow. Preliminary PTV and second sound data in decaying
He~II towed-grid turbulence shows agreement with theoretical predictions, and enables reliable estimation of an effective kinematic viscosity
and calculation of longitudinal and transverse structure functions, from which information about the energy spectrum evolution and intermittency
enhancement can be obtained. / A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements
for the degree of Doctor of Philosophy. / Fall Semester 2018. / August 21, 2018. / Includes bibliographical references. / Wei Guo, Professor Directing Dissertation; Jorge Piekarewicz, University Representative; William Oates,
Committee Member; Kunihiko Taira, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_661204 |
| Contributors | Mastracci, Brian (author), Guo, Wei (professor directing dissertation), Piekarewicz, Jorge (university representative), Oates, William (committee member), Taira, Kunihiko (committee member), Florida State University (degree granting institution), College of Engineering (degree granting college), Department of Mechanical Engineering (degree granting departmentdgg) |
| Publisher | Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text, doctoral thesis |
| Format | 1 online resource (115 pages), computer, application/pdf |
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