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Title: Exploration of thermal counterflow in He II using particle tracking velocimetry

Flow visualization using particle image velocimetry (PIV) and particularly particle tracking velocimetry (PTV) has been applied to thermal counterflow in He II for nearly two decades now, but the results remain difficult to interpret because tracer particle motion can be influenced by both the normal fluid and superfluid components of He II as well as the quantized vortex tangle. For instance, in one early experiment it was observed (using PTV) that tracer particles move at the normal fluid velocity v n, while in another it was observed (using PIV) that particles move at v n/2. Besides the different visualization methods, the range of applied heat flux investigated by these experiments differed by an order of magnitude. To resolve this apparent discrepancy and explore the statistics of particle motion in thermal counterflow, we apply the PTV method to a wide range of heat flux at a number of different fluid temperatures. In our analysis, we introduce a scheme for analyzing the velocity of particles presumably moving with the normal fluid separately from those presumably influenced by the quantized vortex tangle. Our results show that for lower heat flux there are two distinct peaks in the streamwise particle velocity probability density functionmore » (PDF), with one centered at the normal fluid velocity v n (named G2 for convenience) while the other is centered near v n/2 (G1). For higher heat flux there is a single peak centered near v n/2 (G3). Using our separation scheme, we show quantitatively that there is no size difference between the particles contributing to G1 and G2. We also show that nonclassical features of the transverse particle velocity PDF arise entirely from G1, while the corresponding PDF for G2 exhibits the classical Gaussian form. The G2 transverse velocity fluctuation, backed up by second sound attenuation in decaying counterflow, suggests that large-scale turbulence in the normal fluid is absent from the two-peak region. We offer a brief discussion of the physical mechanisms that may be responsible for our observations, revealing that G1 velocity fluctuations may be linked to fluctuations of quantized vortex line velocity, and suggest a number of numerical simulations that may reveal the underlying physics in detail.« less
Authors:
 [1] ;  [1]
  1. Florida State Univ., Tallahassee, FL (United States). National High Magnetic Field Lab. (MagLab). Dept. of Mechanical Engineering
Publication Date:
Grant/Contract Number:
FG02-96ER40952
Type:
Accepted Manuscript
Journal Name:
Physical Review Fluids
Additional Journal Information:
Journal Volume: 3; Journal Issue: 6; Journal ID: ISSN 2469-990X
Publisher:
American Physical Society (APS)
Research Org:
Florida State Univ., Tallahassee, FL (United States). National High Magnetic Field Lab. (MagLab)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Superfluid Helium-4; Thermal Counterflow; Quantum Turbulence; Particle Tracking Velocimetry; Flow Visualization
OSTI Identifier:
1455435
Alternate Identifier(s):
OSTI ID: 1456299

Mastracci, Brian, and Guo, Wei. Exploration of thermal counterflow in He II using particle tracking velocimetry. United States: N. p., Web. doi:10.1103/PhysRevFluids.3.063304.
Mastracci, Brian, & Guo, Wei. Exploration of thermal counterflow in He II using particle tracking velocimetry. United States. doi:10.1103/PhysRevFluids.3.063304.
Mastracci, Brian, and Guo, Wei. 2018. "Exploration of thermal counterflow in He II using particle tracking velocimetry". United States. doi:10.1103/PhysRevFluids.3.063304.
@article{osti_1455435,
title = {Exploration of thermal counterflow in He II using particle tracking velocimetry},
author = {Mastracci, Brian and Guo, Wei},
abstractNote = {Flow visualization using particle image velocimetry (PIV) and particularly particle tracking velocimetry (PTV) has been applied to thermal counterflow in He II for nearly two decades now, but the results remain difficult to interpret because tracer particle motion can be influenced by both the normal fluid and superfluid components of He II as well as the quantized vortex tangle. For instance, in one early experiment it was observed (using PTV) that tracer particles move at the normal fluid velocity vn, while in another it was observed (using PIV) that particles move at vn/2. Besides the different visualization methods, the range of applied heat flux investigated by these experiments differed by an order of magnitude. To resolve this apparent discrepancy and explore the statistics of particle motion in thermal counterflow, we apply the PTV method to a wide range of heat flux at a number of different fluid temperatures. In our analysis, we introduce a scheme for analyzing the velocity of particles presumably moving with the normal fluid separately from those presumably influenced by the quantized vortex tangle. Our results show that for lower heat flux there are two distinct peaks in the streamwise particle velocity probability density function (PDF), with one centered at the normal fluid velocity vn (named G2 for convenience) while the other is centered near vn/2 (G1). For higher heat flux there is a single peak centered near vn/2 (G3). Using our separation scheme, we show quantitatively that there is no size difference between the particles contributing to G1 and G2. We also show that nonclassical features of the transverse particle velocity PDF arise entirely from G1, while the corresponding PDF for G2 exhibits the classical Gaussian form. The G2 transverse velocity fluctuation, backed up by second sound attenuation in decaying counterflow, suggests that large-scale turbulence in the normal fluid is absent from the two-peak region. We offer a brief discussion of the physical mechanisms that may be responsible for our observations, revealing that G1 velocity fluctuations may be linked to fluctuations of quantized vortex line velocity, and suggest a number of numerical simulations that may reveal the underlying physics in detail.},
doi = {10.1103/PhysRevFluids.3.063304},
journal = {Physical Review Fluids},
number = 6,
volume = 3,
place = {United States},
year = {2018},
month = {6}
}