The interplay between viscous and frictional dissipation is key to understanding quantum turbulence dynamics in superfluid 4He. Based on a coarse-grained two-fluid description, an original scale-by-scale energy budget that identifies each scale's contribution to energy dissipation is derived. Using the Hall-Vinen-Bekharevich-Khalatnikov (HVBK) model to further characterize mutual friction, direct numerical simulations at temperatures 1.44 K ≲ T ≲ 2.16 K indicate that mutual friction promotes intense momentum exchanges between the two fluids to maintain a joint energy cascade despite their viscosity mismatch. However, the resulting overall frictional dissipation remains small (compared to the viscous dissipation) and confined to far-dissipative scales. This remarkable feature allows us to define an effective Reynolds number for the turbulence intensity in a two-fluid system, helping to disentangle the effects of Reynolds number and temperature in quantum turbulence. Thereby, simple physical arguments predict that the distance ℓ between quantized vortices (normalized by the turbulence integral scale L0) should behave as [Formula: see text] with the Reynolds number based on the quantum of circulation κ. This law is well supported by a large set of experimental and numerical data within the temperature range of the HVBK model. Finally, this approach offers the possibility of revisiting the ongoing controversy on intermittency in quantum turbulence. It is shown that observed changes in intermittency arise from Reynolds number effects rather than from temperature variations, as proposed in recent studies.

The carbon footprint of academia has become a prominent concern and a burgeoning research area, with a notable focus on greenhouse gas emissions (GHG) from research-related travels. Mitigation strategies often promote alternatives, such as developing virtual communication or adopting sustainable transportation modes for short distances. While more ambitious strategies involving the transformation of research practices are increasingly discussed, these mitigation solutions are rarely subjected to rigorous quantitative assessments or meaningful comparisons. This study analyzes a unique database of about 130 000 travel segments by car, train and plane in 159 research entities across a wide array of disciplines in France. We investigate the patterns and associated carbon footprint of these research travels and explore a diversity of mitigation options. Our analysis shows that air travel overwhelmingly outweighs the carbon footprint of research travel, representing more than 96% of GHG emissions. Intercontinental flights are infrequent (less than 10% of all plane trips) but dominate GHG travel emissions, accounting for over 64% of total emissions. In contrast, domestic and continental flights are the most common but their mitigation potential by modal shift to train is limited (e.g. less than 15% for trips under 1000 km). Similar reductions can be achieved by targeting a small subset of travels, for example by modulating the frequency of conference attendance. The greatest and possibly most robust mitigation potential lies in combining modal shift with moderating air mileage (e.g. reducing travelled distance or number of flights). Strategies focusing on electrification or modal shifts for cars, proposed in official guidelines, are found to have negligible impact. In the absence of low-carbon alternatives for long-haul flights, we contend that only comprehensive strategies and policies which include moderating air travel distance or frequency can achieve a robust significant reduction in the GHG emissions from academic travel.

An analytical model of open-cavity second sound resonators is presented and validated against simulations and experiments in superfluid helium using a new resonator design that achieves unprecedented resolution. The model incorporates diffraction, geometrical misalignments, and flow through the cavity and is validated using cavities operated up to their 20th resonance in superfluid helium. An important finding is that resonators can be optimized to selectively sense either the quantum vortex density carried by the throughflow-as typically done in the literature-or the mean velocity of the throughflow. We propose two velocity probing methods: one that takes advantage of misalignments between the tweezers' plates and other that drives the resonator non-linearly, beyond a threshold that results in the self-sustainment of a vortex tangle within the cavity. A new mathematical treatment of the resonant signal is proposed to adequately filter out parasitic signals, such as temperature and pressure drift, and accurately separate the quantum vorticity signal. This elliptic method consists in a geometrical projection of the resonance in the inverse complex plane. Its effectiveness is demonstrated over a wide range of operating conditions. The resonator model and elliptic method are being utilized to characterize a new design of resonators with high resolution, thanks to miniaturization and design optimization. These second-sound tweezers are capable of providing time-space resolved information similar to classical local probes in turbulence, down to sub-millimeter and sub-millisecond scales. The principle, design, and microfabrication of second sound tweezers are being presented, along with their potential for exploring quantum turbulence.

<p>In the midst of climate change, academic travels - one salient aspect of the carbon footprint of research activities - are at the center of a growing concern. Mitigation options often focus on two dimensions : (i) decreasing the frequency of attendance to conferences and (ii) modal shift in transport. Here, we analyze professional travel in academia from a unique database compiling about 100 000 travels from about 150 research labs across a large array of disciplines and localities in France to detail the structure, patterns and heterogeneity of national and international research travels for research purposes. We estimate the mitigation potential of a series of options encompassing but not limited to institutional options. We show that, if short distance traveling (typically below 1000 km) are largely dominant in number, their relative mitigation potential via modal shift is small (i.e., below 15%). On the other hand, long distance traveling, which is often associated with international collaborations or field work hold a much larger mitigation potential but question the very nature of research activities. We propose ambitious sobriety options to robustly decrease travel-induced GHG emissions in academia and discuss their acceptability in the context of the French public research system.</p>

Miniature heaters are immersed in flows of quantum fluid and the efficiency of heat transfer is monitored versus velocity, superfluid fraction, and time. The fluid is He-4 helium with a superfluid fraction varied from 71% down to 0% and an imposed velocity up to 3 m/s, while the characteristic sizes of heaters range from 1.3 mu m up to a few hundreds of microns. At low heat fluxes, no velocity dependence is observed, in agreement with expectations. In contrast, some velocity dependence emerges at larger heat flux, as reported previously, and three nontrivial properties of heat transfer are identified. First, at the largest superfluid fraction (71%), a new heat transfer regime appears at non-null velocities and it is typically 10% less conductive than at zero velocity. Second, the velocity dependence of the mean heat transfer is compatible with the square-root dependence observed in classical fluids. Surprisingly, the prefactor to this dependence is maximum for an intermediate superfluid fraction or temperature (around 2 K). Third, the heat transfer time series exhibit highly conductive short-lived events. These cooling glitches have a velocity-dependent characteristic time, which manifest itself as a broad and energetic peak in the spectrum of heat transfer time series, in the kHz range. After showing that the velocity dependence can be attributed to the breaking of superfluidity within a thin shell surrounding heaters, an analytical model of forced heat transfer in a quantum flow is developed to account for the properties reported above. We argue that large scale flow patterns must form around the heater, having a size proportional to the heat flux (here two decades larger than the heater diameter) and resulting in a turbulent wake. The observed spectral peaking of heat transfer is quantitatively consistent with the formation of a Von Karman vortex street in the wake of a bluff body nearly two decades larger than the heater but its precise temperature and velocity dependence remains unexplained. An alternative interpretation for the spectral peaking is discussed, in connection with existing predictions of a bottleneck in the superfluid velocity spectra and energy equipartition.

The density fluctuations of quantum vortex lines are measured in a turbulent flow of superfluid He, at temperatures corresponding to superfluid fractions of 16%, 47% and 81%. The probe is a micro-fabricated second sound resonator allowing for local and small-scale measurements in the core of the flow, downstream a grid at a 10-mesh size. Remarkably, all the vortex power spectra collapse on a single master curve, independently from the superfluid fraction and the mean velocity. By contrast with previous measurements, we report a peculiar shape of the power spectra. The vortex density probability distributions are found to be strongly skewed, similarly to the vorticity distributions observed in classical turbulence. Implications of those results are discussed.

The long-standing puzzle of diverging heat transport measurements at very high Rayleigh numbers (Ra) is addressed by a simple model based on well-known properties of classical boundary layers. The transition to the ‘ultimate state’ of convection in Rayleigh–Bénard cells is modeled as sub-critical transition controlled by the instability of large-scale boundary-layer eddies. These eddies are restricted in size either by the lateral wall or by the horizontal plates depending on the cell aspect ratio (in cylindrical cells, the cross-over occurs for a diameter-to-height ratio around 2 or 3). The large-scale wind known to settle across convection cells is assumed to have antagonist effects on the transition depending on its strength, leading to wind-immune, wind-hindered or wind-assisted routes to the ultimate regime. In particular winds of intermediate strength are assumed to hinder the transition by disrupting heat transfer, contrary to what is assumed in standard models. This phenomenological model is able to reconcile observations from more than a dozen of convection cells from Grenoble, Eugene, Trieste, Göttingen and Brno. In particular, it accounts for unexplained observations at high Ra, such as Prandtl number and aspect ratio dependences, great receptivity to details of the sidewall and differences in heat transfer efficiency between experiments.

We present the principle, modeling, and the first implementation of a new type of high resolution hot-wire anemometer designed to operate at cryogenic temperatures and very high Reynolds numbers. Its spatial resolution of a few micrometers is comparable to the most spatially resolved hot-wires reported in the literature. Compared to existing designs, its fabrication involves a limited number of steps, essentially the shaping at nanoscales of a superconducting NbTi wire of submicron diameter. The velocity spectra in the far wake of a centimeter-sized grid are measured as a proof of concept in stringent flow conditions.

We present the principle for a micro-sensor aimed at measuring local correlations of turbulent velocity and temperature. The operating principle is versatile and can be adapted for various types of flow. It is based on a micro-machined cantilever, on the tip of which a platinum resistor is patterned. The deflection of the cantilever yields an estimate for the local velocity, and the impedance of the platinum yields an estimate for the local temperature. The velocity measurement is tested in two turbulent jets: one with air at room temperature which allows us to compare with well-known calibrated reference anemometers, and another one in the GReC jet at CERN with cryogenic gaseous helium which allows a much larger range of resolved turbulent scales. The recording of temperature fluctuations is tested in the Barrel of Ilmenau which provides a controlled turbulent thermal flow in air. Measurements in the wake of a heated or cooled cylinder demonstrate the capability of the sensor to display the cross correlation between temperature and velocity correctly.

We report the first evidence of an enhancement of the heat transfer from a heated wire to an external turbulent coflow of superfluid helium. We used a standard Pt-Rh hot-wire anemometer and overheat it up to 21 K in a pressurized liquid helium turbulent round jet at temperatures between 1.9 K and 2.12 K. The null-velocity response of the sensor can be satisfactorily modeled by the counterflow mechanism, while the extra cooling produced by the forced convection is found to scale similarly as the corresponding extra cooling in classical fluids. We propose a preliminary analysis of the response of the sensor and show that—contrary to a common assumption—such sensor can be used to probe local velocity in turbulent superfluid helium.

The concept of "effective viscosity" v(eff) of superfluid helium, widely used to interpret decaying turbulence, is tested in the steady-state case. We deduce.eff from measurements of the vortex line density, L, in a grid flow. The scaling of L with velocity confirms the validity of the heuristic relation defining v(eff), epsilon =v(eff) (kappa L)(2), where epsilon is the energy dissipation rate and. the circulation quantum. Within 1.17-2.16K, v(eff) is consistent with that from decays, allowing for uncertainties in flow parameters. Numerical simulations of the two-fluid equations yield a second estimation of v(eff) within an order of magnitude with all experiments. Its temperature dependence, more pronounced in numerics than experiments, shows a crossover from a viscous-dominated to a mutual-friction-based dissipation as temperature decreases, supporting the idea that the effective viscosity of a quantum turbulent flow is an indicator of the dissipative mechanisms at play. Copyright (C) EPLA, 2014

In this paper, we report the first evidence of an enhancement of the heat transfer from a heated wire by an external turbulent flow of superfluid helium. We used a standard Pt-Rh hot-wire anemometer and overheat it up to 21 K in a pressurized liquid helium turbulent round jet at temperatures between 1.9 K and 2.12 K. The null-velocity response of the sensor can be satisfactorily modeled by the counter flow mechanism while the extra cooling produced by the forced convection is found to scale similarly as the corresponding extra cooling in classical fluids. We propose a preliminary analysis of the response of the sensor and show that -contrary to a common assumption- such sensor can be used to probe local velocity in turbulent superfluid helium.

Turbulence in superfluid helium is unusual and presents a challenge to fluid dynamicists because it consists of two coupled, interpenetrating turbulent fluids: the first is inviscid with quantized vorticity, and the second is viscous with continuous vorticity. Despite this double nature, the observed spectra of the superfluid turbulent velocity at sufficiently large length scales are similar to those of ordinary turbulence. We present experimental, numerical, and theoretical results that explain these similarities, and illustrate the limits of our present understanding of superfluid turbulence at smaller scales.

Superfluid Turbulence is unusual and presents a challenge to fluid dynamicists because it consists of two coupled, inter penetrating turbulent fluids: the first is inviscid with quantised vorticity, the second is viscous with continuous vorticity. Despite this double nature, the observed spectra of the superfluid turbulent velocity at sufficiently large length scales are similar to those o ordinary turbulence. We present experimental, numerical and theoretical results which explain these similarities, and illustrate the limits of our present understanding of superfluid turbulence at smaller scales.

A new regime of turbulent convection has been reported nearly one decade ago, based on global heat transfer measurements at very high Rayleigh numbers. We examine the signature of this "Ultimate Regime" from within the flow itself. A systematic study of probe-size corrections shows that the earlier temperature measurements within the flow were altered by an excessive size of thermometer, but not according to a theoretical model proposed in the literature. Using a probe one order of magnitude smaller than the one used previously, we find evidence that the transition to the Ultimate Regime is indeed accompanied with a clear change in the statistics of temperature fluctuations in the flow.

The equations of motion describing buoyant fluids are often simplified using a set of approximations proposed by J. Boussinesq one century ago. To resume, they consist in assuming constant fluid properties, incompressibility and conservation of calories during heat transport. Assuming fulfilment of the first requirement (constant fluid properties), we derive a set of 4 criteria for assessing the validity of the two other requirements in turbulent Rayleigh-Bénard convection. The first criterion αΔ≪ 1 simply results from the incompressibility condition in the thermal boundary layer (α and Δ are the thermal expansion coefficient and the temperature difference driving the flow). The 3 other criteria are proportional or quadratic with the density stratification or, equivalently with the temperature difference resulting from the adiabatic gradient across the cell Δ_h. Numerical evaluations with air, water and cryogenic helium show that most laboratory experiments are free from such Boussinesq violation as long as the first criterion is fulfilled. In ultra high Rayleigh numbers (Ra>10^16) experiments in He, one of the stratification criteria, scaling with αΔ_h, could be violated. This criterion garanties that pressure fluctuations have a negligible influence both on the density variation and on the heat transfer equation through compression/expansion cycles. Extrapolation to higher Ra suggests that strong violation of Boussinesq approximation could occur in atmospheric convection.

In a recent preprint (cond-mat/0601398), D. Funfschilling and G. Ahlers describe a new effect, that they interpret as non-Boussinesq, in a convection cell working with ethane, near its critical point. They argue that such an effect could have spoiled the Chavanne {\it et al.} (Phys. Rev. Lett. {\bf 79} 3648, 1997) results, and not the Niemela {\it et al.} (Nature, {\bf 404}, 837, 2000) ones, which would explain the differences between these two experiments. We show that:-i)Restricting the Chavanne's data to situations as far from the critical point than the Niemela's one, the same discrepancy remains.-ii)The helium data of Chavanne show no indication of the effect observed by D. Funfschilling and G. Ahlers.

Cryogenic gaseous helium gives access to extreme turbulent experimental conditions. The very high cooling helium flow rates available at CERN have been used to reach Reynolds numbers up to Re ~ 10 7 in a round jet experiment. First results are discussed. Turbulence is a highly challenging problem both for fundamental understanding and industrial applications. Present capabilities of the most powerful computer simulations remain far away from common situations, and experimental data are needed to test the validity of theoretical predictions. Large air wind tunnels (≥ 20 m), for example, have been used for many years; however in such large experiments the Re hydrodynamic parameter cannot vary on a wide range. The physical idea is to understand how the energy injected at the large scale L of the flow cascades down through the inertial scales to the smallest scale η where it is dissipated by viscosity. In air this behavior is investigated in large systems : L is in the range of several meters and η is in the millimetric or submillimetric range. Turbulence studies in gaseous helium at cryogenic temperatures have been achieved in the last twenty years and have opened a very attractive way to vary Re on several orders of magnitude in a single geometry and reach very high Re values. They take advantage of the very low kinematic viscosity ν of gaseous helium (ν = 7×10-8 m 2 /s at 4.2 K and 1 bar, 230 times lower than air in STP conditions). In a typical (φ = 10 cm) laboratory jet experiment, Re = VL/ν (V is the mean velocity and L the width of the jet) may easily vary over three orders of magnitude by varying pressure and flow rate [1]. A large experiment with low temperature helium flow rates is today available at CERN, in the framework of the Large Hadron Collider development. We use a large scale refrigerator producing 6 kW at 4.5 K, with flow rates from 20 g/s up to 300 g/s [2]. A specific 6 m long φ 22 cm transfer line, with concentric tubes, feeds the experiment. The inner tube drives liquid and gaseous helium out of the refrigerator; the mixture is turned to gas at controlled temperature by a powerful heater (up to 10 kW) before entering a specific flow meter, a honeycomb and finally the φ = 25 mm nozzle. The round jet develops inside the 2.5 m high experimental chamber. The investigation of the expected micrometric dissipative η scale requires an original 5 µm superconducting anemometer [3], located axially 1.25 m down the nozzle : it gives access to the velocity measurements. The helium flows back to the refrigerator through the transfer line outer tube. The grid at the nozzle entrance is a honeycomb, φ 240 mm and 20 mm thickness, with hexagons of 4 mm side dimension. The nozzle diameter has been determined to maintain the flow velocity significantly lower than the sound velocity : at 300 g/s the output velocity is half the sound velocity. The nozzle height is 280 mm. The surface contraction factor in the nozzle is 92. Thus the flow in the experimental chamber is independent from the incoming flow characteristics as confirmed by first tests with water: they showed that the outgoing flow from the nozzle is laminar for several cm and is aligned with the nozzle axis before turning to the turbulent state. At the level of the detectors the flow diameter is about 40-50 cm. The mean temperature is 4.8 K. Special care has been taken to avoid spurious heat leaks in order to limit the temperature difference ∆T between the incoming and outgoing He in the experimental chamber. For heat leaks ranging around 30 W, ∆T, including the temperature fluctuations of the flow induced by the turbulent regime, does not exceed 100 mK at a 50 g/s flow rate. With such a low ∆T, the outgoing He feeds directly the last stage of the liquefier and the He flow is in constant average temperature conditions.