Producing and imaging a thin line of He2∗ molecular tracers in helium-4

Intermittency is a hallmark of turbulence, which exists not only in turbulent flows of classical viscous fluids but also in flows of quantum fluids such as superfluid 4 He. Despite the established similarity between turbulence in classical fluids and quasi-classical turbulence in superfluid 4 He, it has been predicted that intermittency in superfluid 4 He is temperature dependent and enhanced for certain temperatures, which strikingly contrasts the nearly flow-independent intermittency in classical turbulence. Experimental verification of this theoretical prediction is challenging since it requires well-controlled generation of quantum turbulence in 4 He and flow measurement tools with high spatial and temporal resolution. Here, we report an experimental study of quantum turbulence generated by towing a grid through a stationary sample of superfluid 4 He. The decaying turbulent quantum flow is probed by combining a recently developed He * 2 molecular tracer-line tagging velocimetry technique and a traditional second sound attenuation method. We observe quasi-classical decays of turbulent kinetic energy in the normal fluid and of vortex line density in the superfluid component. For several time instants during the decay, we calculate the transverse velocity structure functions. Their scaling exponents, deduced using the extended self-similarity hypothesis, display non-monotonic temperature-dependent intermittency enhancement, in excellent agreement with recent theoretical/numerical study of Biferale et al. [Phys. Rev. Fluids 3, 024605 (2018)].

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“…The experiment utilizes the Tallahassee He * 2 tracerline visualization setup [35] as shown schematically in Fig. 1 (a).…”

Section: Experimental Methodsmentioning

confidence: 99%

Intermittency enhancement in quantum turbulence in superfluidHe4

et al. 2018

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“…Since tracer particles interact with both fluid components, a major challenge when applying PTV to thermal counterflow is determining what influences the motion of an observed particle at a given time, so that the behavior of the underlying flow field can be interpreted 2 correctly [9,[12][13][14]. Until recently analysis was confined to qualitative characterizations: evolution of particle motion in response to applied heat flux [15], or of statistical distributions of particle kinematics in response to image acquisition rate [16,17].…”

Section: Introductionmentioning

confidence: 99%

Characterizing vortex tangle properties in steady-state He II counterflow using particle tracking velocimetry

Mastracci

Guo

2019

Phys. Rev. Fluids

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Historically, there is little faith in particle tracking velocimetry (PTV) as a tool to make quantitative measurements of thermal counterflow in He II, since tracer particle motion is complicated by influences from the normal fluid, superfluid, and quantized vortex lines, or a combination thereof.Recently, we introduced a scheme for differentiating particles trapped on vortices (G1) from particles entrained by the normal fluid (G2). In this paper, we apply this scheme to demonstrate the utility of PTV for quantitative measurements of vortex dynamics in He II counterflow. We estimate ℓ, the mean vortex line spacing, using G2 velocity data, and c 2 , a parameter related to the mean curvature radius of vortices and energy dissipation in quantum turbulence, using G1 velocity data. We find that both estimations show good agreement with existing measurements that were obtained using traditional experimental methods. This is of particular consequence since these parameters likely vary in space, and PTV offers the advantage of spatial resolution. We also show a direct link between power-law tails in transverse particle velocity probability density functions (PDFs) and reconnection of vortex lines on which G1 particles are trapped.

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“…Some other investigations have been focused on counterflow [35][36][37] and forced flow [38] around cylinders; velocity profile in mechanically driven pipe flow [39]; dynamics of quantized vortices [40][41][42]; and flow induced by oscillating [43] and towed grids [44]. More recently, a different approach to He II flow visualization has been introduced, making use of metastable He 2 * molecules as tracer particles [45]. Measurements of the turbulent normal fluid velocity with these particles have lead to non-classical forms of the second order transverse structure function [8], effective kinematic viscosity in decaying counterflow turbulence [46], and the energy spectrum in a sustained thermal counterflow [14].…”

Section: Introductionmentioning

confidence: 99%

“…Measurements of the turbulent normal fluid velocity with these particles have lead to non-classical forms of the second order transverse structure function [8], effective kinematic viscosity in decaying counterflow turbulence [46], and the energy spectrum in a sustained thermal counterflow [14]. These measurements are free of the ambiguity associated with PIV and PTV methods since the He 2 * molecules strictly trace the normal fluid for temperatures above about 1 K [45].…”

Section: Introductionmentioning

confidence: 99%

Exploration of thermal counterflow in He II using particle tracking velocimetry

Mastracci

Guo

2018

Phys. Rev. Fluids

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Flow visualization using PIV (particle image velocimetry) and particularly PTV (particle tracking velocimetry) 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 have applied 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 PDF (probability density function), 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 non-classical 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.

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Producing and imaging a thin line of He2∗ molecular tracers in helium-4

Cited by 27 publications

References 60 publications

Intermittency enhancement in quantum turbulence in superfluidHe4

Intermittency enhancement in quantum turbulence in superfluidHe4

Characterizing vortex tangle properties in steady-state He II counterflow using particle tracking velocimetry

Exploration of thermal counterflow in He II using particle tracking velocimetry

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