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 Title
 Exploration Of Thermal Counterflow In He Ii Using Particle Tracking Velocimetry.
 Creator

Mastracci, Brian, Guo, Wei
 Abstract/Description

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...
Show moreFlow 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 function (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 largescale turbulence in the normal fluid is absent from the twopeak 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.
Show less  Date Issued
 20180622
 Identifier
 FSU_libsubv1_wos_000436043200001, 10.1103/PhysRevFluids.3.063304
 Format
 Citation
 Title
 Dissipation In Quantum Turbulence In Superfluid He4 Above 1 K.
 Creator

Gao, J., Guo, W., Yui, S., Tsubota, M., Vinen, W. F.
 Abstract/Description

There are two commonly discussed forms of quantum turbulence in superfluid He4 above 1 K: in one there is a random tangle of quantized vortex lines, existing in the presence of a nonturbulent normal fluid; in the second there is a coupled turbulent motion of the two fluids, often exhibiting quasiclassical characteristics on scales larger than the separation between the quantized vortex lines in the superfluid component. The decay of vortex line density, L, in the former case is often...
Show moreThere are two commonly discussed forms of quantum turbulence in superfluid He4 above 1 K: in one there is a random tangle of quantized vortex lines, existing in the presence of a nonturbulent normal fluid; in the second there is a coupled turbulent motion of the two fluids, often exhibiting quasiclassical characteristics on scales larger than the separation between the quantized vortex lines in the superfluid component. The decay of vortex line density, L, in the former case is often described by the equation dL/dt = chi 2 (kappa/2 pi)L2, where kappa is the quantum of circulation and chi(2). is a dimensionless parameter of order unity. The decay of total turbulent energy, E, in the second case is often characterized by an effective kinematic viscosity, v', such that dE/dt = v'kappa L2(2). We present values of chi(2 )derived from numerical simulations and from experiment, which we compare with those derived from a theory developed by Vinen and Niemela. We summarize what is presently known about the values of v' from experiment, and we present a brief introductory discussion of the relationship between chi(2 )and v', leaving a more detailed discussion to a later paper.
Show less  Date Issued
 20180529
 Identifier
 FSU_libsubv1_wos_000433287200005, 10.1103/PhysRevB.97.184518
 Format
 Citation
 Title
 Intermittency Enhancement In Quantum Turbulence In Superfluid He4.
 Creator

Varga, Emil, Gao, Jian, Guo, Wei, Skrbek, Ladislav
 Abstract/Description

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 He4. Despite the established similarity between turbulence in classical fluids and quasiclassical turbulence in superfluid He4, it has been predicted that intermittency in superfluid He4 is temperature dependent and enhanced for certain temperatures, which is in striking contrasts to the nearly flowindependent intermittency...
Show moreIntermittency 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 He4. Despite the established similarity between turbulence in classical fluids and quasiclassical turbulence in superfluid He4, it has been predicted that intermittency in superfluid He4 is temperature dependent and enhanced for certain temperatures, which is in striking contrasts to the nearly flowindependent intermittency in classical turbulence. Experimental verification of this theoretical prediction is challenging since it requires wellcontrolled generation of quantum turbulence in He4 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 He4. The decaying turbulent quantum flow is probed by combining a recently developed He*(2) molecular tracerline tagging velocimetry technique and a traditional secondsound attenuation method. We observe quasiclassical 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 selfsimilarity hypothesis, display nonmonotonic temperaturedependent intermittency enhancement, in excellent agreement with a recent theoretical and numerical study
Show less  Date Issued
 20180904
 Identifier
 FSU_libsubv1_wos_000443685600007, 10.1103/PhysRevFluids.3.094601
 Format
 Citation