arXiv subjects

L. Skrbek

Publications and source records attributed to L. Skrbek.

14 recordsLinked to original sources

arXiv2026

Experimental Challenges in Determining Heat Transfer Efficiency Scaling in Highly Turbulent Cryogenic Rayleigh-Benard Convection

Cryogenic Rayleigh-Benard convection (RBC) at very high Rayleigh numbers (Ra) serves as a key system for understanding buoyancy-driven industrial and large scale natural flows and for testing theories of turbulent convective heat transport. Cryogenic helium experiments allow one to reach extremely high Ra under well-controlled laboratory conditions; however, interpretation of the resulting heat-transfer scalings remains sensitive to non-Oberbeck-Boussinesq (NOB) effects, experimental uncertainties, as well as a number of corrections that ought to be applied to raw data, including corrections for the adiabatic temperature gradient, parasitic heat leaks, or finite thermal conductivity of plates and sidewalls of RBC cells. We present an analysis of experimental uncertainties and data corrections procedures applicable to cryogenic RBC experiments, specifically to those performed in cylindrical RBC cells in Brno: measurement uncertainties, parasitic effects, choice of 4He working points in the p-T diagram and evaluation of relevant properties of the particular working fluid in connection with the available thermophysical property databases. In particular, our study highlights the necessity of rigorous uncertainty analysis for assessing experimental evidence suggesting either transition to the ultimate regime of RBC due to intrinsic ultimate-regime dynamics or as a manifestation of NOB effects and experimental imperfections.

physics.flu-dyn↗

arXiv2021

Thermal Waves and Heat Transfer Efficiency Enhancement in Harmonically Modulated Turbulent Thermal Convection

We study turbulent Rayleigh-Bénard convection over four decades of Rayleigh numbers 4E8 < Ra < 2E12, while harmonically modulating the temperatures of the plates of our cylindrical cell. We probe the flow by temperature sensors placed in the cell interior and embedded in the highly conducting copper plates and detect thermal waves propagating at modulation frequency in the bulk of the convective flow. We confirm the recent numerical prediction [PRL 125, 154502 (2020)] of the significant enhancement of Nusselt number and report its dependence on the frequency and amplitude of the temperature modulation of plates.

physics.flu-dyn↗

arXiv2020

Thermal Radiation in Rayleigh-Bénard Convection Experiments

An important question in turbulent Rayleigh-Bénard convection (RBC) is the effectiveness of convective heat transport, which is conveniently described via the scaling of the Nusselt number (${\rm{Nu}}$) with the Rayleigh (${\rm{Ra}}$) and Prandtl (${\rm{Pr}}$) numbers. In RBC experiments, the heat supplied to the bottom plate is also partly transferred by thermal radiation. This heat transport channel, acting in parallel with the convective and conductive heat transport channels, is usually considered insignificant and thus neglected. Here we present a detailed analysis of conventional far-field as well as strongly enhanced near-field radiative heat transport occurring in various RBC experiments, and show that the radiative heat transfer partly explains differences in ${\rm{Nu}}$ measured in different experiments. A careful inclusion of the radiative transport appreciably changes the ${\rm{Nu}}={\rm{Nu}}({\rm{Ra}})$ scaling inferred in turbulent RBC experiments near ambient temperature utilizing gaseous nitrogen and sulphur hexafluoride as working fluids. On the other hand, neither the conventional far-field radiation nor the strongly enhanced near-field radiative heat transport appreciably affects the heat transport law deduced in cryogenic helium RBC experiments.

physics.flu-dyn↗

arXiv2016

Transition to turbulence and streamwise inhomogeneity of vortex tangle in thermal counterflow

We report preliminary results of the complementary experimental and numerical studies on spatiotemporal tangle development and streamwise vortex line density (VLD) distribution in counterflowing $^4$He. The experiment is set up in a long square channel with VLD and local temperature measured in three streamwise locations. In the steady state we observe nearly streamwise-homogeneous VLD. Experimental second sound data as well as numerical data (vortex filament method in a long planar channel starting with seeding vortices localized in multiple locations) show that the initial build up pattern of VLD displays complex features depending on the position in the channel, but the some tangle properties appear uniform along its length.

cond-mat.other↗

arXiv2016

Coexistence and interplay of quantum and classical turbulence in superfluid He-4: Decay, velocity decoupling and counterflow energy spectra

We report complementary experimental, numerical and theoretical study of turbulent coflow, counterflow and pure superflow of superfluid He-4 in a channel, resulting in a physically transparent and relatively simple model of decaying quantum turbulence that accounts for interactions of coexisting quantum and classical components of turbulent superfluid He-. We further offer an analytical theory of the energy spectra of steady-state quantum turbulence in the counterflow and pure superflow, based on algebraic approximation for the energy fluxes over scales. The resulting spectra are not of the classic Kolmogorov form, but strongly suppressed by the mutual friction, leading to the energy dissipation at all scales, enhanced by the counterflow-induced decoupling of the normal- and superfluid velocity fluctuations.

cond-mat.other↗

arXiv2015

Anisotropic behaviour of transmission through thin superconducting NbN film in parallel magnetic field

Transmission of terahertz waves through a thin layer of the superconductor NbN deposited on an anisotropic R-cut sapphire substrate is studied as a function of temperature in a magnetic field oriented parallel with the sample. A significant difference is found between transmitted intensities of beams linearly polarized parallel with and perpendicular to the direction of applied magnetic field.

cond-mat.supr-con↗

arXiv2015

Backreaction of tracer particles on vortex tangle in helium II counterflow

We report computer simulations of the interaction of seeding particles with quantized vortices and with the normal fluid flow in thermal counterflow of superfluid $^4$He. We show that if the particle concentration is too large, the vortex tangle is significantly affected, posing problems in the interpretation of visualization experiments. The main effects are an increase in vortex line density and a change in polarization of the vortex tangle, caused by the action of the Stokes drag of the viscous normal fluid on the trapped particles. We argue that in the case of large particle concentration, typically used for the particle image velocimetry technique, the tangle properties might become significantly changed. On the contrary, the particle tracking velocimetry technique that uses smaller particle concentration should not be appreciably affected.

cond-mat.other↗

arXiv2011

Viscosity of liquid He-4 and quantum of circulation: Why and how are they related?

The relationship between the apparently unrelated physical quantities -- kinematic viscosity of liquid He-4, $ν$, and quantum of circulation, $κ=2π\hbar/m_4$, where $\hbar$ is the Planck constant and $m_4$ denotes the mass of the $^4$He atom -- is examined in the vicinity of the superfluid transition occurring due to Bose-Einstein condensation. A model is developed, leading to the surprisingly simple relation $ν\approx κ/6$. We critically examine the available experimental data for $^4$He relevant to this simple relation and predict the kinematic viscosity for the stretched liquid $^4$He along the $λ$-line at negative pressures.

physics.flu-dyn↗

arXiv2006

Quartz Tuning Fork: Thermometer, Pressure- and Viscometer for Helium Liquids

Commercial quartz oscillators of the tuning-fork type with a resonant frequency of ~32 kHz have been investigated in helium liquids. The oscillators are found to have at best Q values in the range 10^5-10^6, when measured in vacuum below 1.5 K. However, the variability is large and for very low temperature operation the sensor has to be preselected. We explore their properties in the regime of linear viscous hydrodynamic response in normal and superfluid 3He and 4He, by comparing measurements to the hydrodynamic model of the sensor.

cond-mat.supr-con↗

arXiv2006

Energy Spectra of Developed Turbulencein Helium Superfluids

We suggest a "minimal model" for the 3D turbulent energy spectra in superfluids, based on their two-fluid description. We start from the Navier-Stokes equation for the normal fluid and from the coarse-grained hydrodynamic equation for the superfluid component (obtained from the Euler equation for the superfluid velocity after averaging over the vortex lines) and introduce a mutual friction coupling term, proportional to the counterflow velocity, the average superfluid vorticity and to the temperature dependent parameter $q=α/(1+α')$, where $α$ and $α'$ denote the dimensionless parameters characterizing the mutual friction between quantized vortices and the normal component of the liquid. We then derive the energy balance equations, taking into account the cross-velocity correlations. We obtain all asymptotical solutions for normal and superfluid energy spectra for limiting cases of small/big normal to superfluid density ratio and coupling. We discuss the applicability limits of our model to superfluid He II and to $^3$He-B and compare the model predictions with available experimental data.

nlin.CD↗

arXiv2004

Flow Phase Diagram for the Helium Superfluids

The flow phase diagram for He II and $^3$He-B is established and discussed based on available experimental data and the theory of Volovik [JETP Letters {\bf{78}} (2003) 553]. The effective temperature - dependent but scale - independent Reynolds number $Re_{eff}=1/q=(1+α')/α$, where $α$ and $α'$ are the mutual friction parameters and the superfluid Reynolds number characterizing the circulation of the superfluid component in units of the circulation quantum are used as the dynamic parameters. In particular, the flow diagram allows identification of experimentally observed turbulent states I and II in counterflowing He II with the turbulent regimes suggested by Volovik.

cond-mat.other↗

arXiv2004

Flow of He II due to an Oscillating Grid in the Low Temperature Limit

The macroscopic flow properties of pure He II are probed in the limit of zero temperature using an oscillating grid. With increasing oscillation amplitude the initially pure superflow changes abruptly: the resonant frequency decreases and the response becomes strongly nonlinear, attributable to the growth of a boundary layer of quantized vortices that increases the effective mass of the grid. On further increase of oscillation amplitude, the flow undergoes a transition to turbulence.

cond-mat.stat-mech↗

arXiv2003

Transition to superfluid turbulence governed by an intrinsic parameter

Hydrodynamic flow in both classical and quantum fluids can be either laminar or turbulent. To describe the latter, vortices in turbulent flow are modelled with stable vortex filaments. While this is an idealization in classical fluids, vortices are real topologically stable quantized objects in superfluids. Thus superfluid turbulence is thought to hold the key to new understanding on turbulence in general. The fermion superfluid 3He offers further possibilities owing to a large variation in its hydrodynamic characteristics over the experimentally accessible temperatures. While studying the hydrodynamics of the B phase of superfluid 3He, we discovered a sharp transition at 0.60Tc between two regimes, with regular behaviour at high-temperatures and turbulence at low-temperatures. Unlike in classical fluids, this transition is insensitive to velocity and occurs at a temperature where the dissipative vortex damping drops below a critical limit. This discovery resolves the conflict between existing high- and low-temperature measurements in 3He-B: At high temperatures in rotating flow a vortex loop injected into superflow has been observed to expand monotonically to a single rectilinear vortex line, while at very low temperatures a tangled network of quantized vortex lines can be generated in a quiescent bath with a vibrating wire. The solution of this conflict reveals a new intrinsic criterion for the existence of superfluid turbulence.

cond-mat↗

arXiv2002

Shear flow and Kelvin-Helmholtz instability in superfluids

The first realization of instabilities in the shear flow between two superfluids is examined. The interface separating the A and B phases of superfluid He-3 is magnetically stabilized. With uniform rotation we create a state with discontinuous tangential velocities at the interface, supported by the difference in quantized vorticity in the two phases. This state remains stable and nondissipative to high relative velocities, but finally undergoes an instability when an interfacial mode is excited and some vortices cross the phase boundary. The measured properties of the instability are consistent with a modified Kelvin-Helmholtz theory.

cond-mat↗