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A Lagrangian experimental study of an axisymmetric turbulent water jet is performed to investigate the highly anisotropic and inhomogeneous flow field. Measurements are conducted within a Lagrangian exploration module, an icosahedron apparatus, to facilitate optical access of three cameras. Stereoscopic particle tracking velocimetry results in three-component tracks of position, velocity and acceleration of the tracer particles within the vertically oriented jet with a Taylor-based Reynolds number ${\\textit {Re}}_\\lambda \\simeq 230$. Analysis is performed at seven locations from 15 diameters up to 45 diameters downstream. Eulerian analysis is first carried out to obtain critical parameters of the jet and relevant scales, namely the Kolmogorov and large (integral) scales as well as the energy dissipation rate. Lagrangian statistical analysis is then performed on velocity components stationarised following methods inspired by Batchelor (J. Fluid Mech., vol. 3, 1957, pp. 67–80), which aim to extend stationary Lagrangian theory of turbulent diffusion by Taylor to the case of self-similar flows. The evolution of typical Lagrangian scaling parameters as a function of the developing jet is explored and results show validation of the proposed stationarisation. The universal scaling constant $C_0$ (for the Lagrangian second-order structure function), as well as Eulerian and Lagrangian integral time scales, are discussed in this context. Constant $C_0$ is found to converge to a constant value (of the order of $C_0 = 3$) within 30 diameters downstream of the nozzle. Finally, the occurrence of finite particle size effects is investigated through consideration of acceleration-dependent quantities.

An experimental Lagrangian study based on particle tracking velocimetry has been completed in an incompressible turbulent round water jet freely spreading into water. The jet is seeded with tracers only through the nozzle: inhomogeneous seeding called nozzle seeding. The Lagrangian flow tagged by these tracers therefore does not contain any contribution from particles entrained into the jet from the quiescent surrounding fluid. The mean velocity field of the nozzle seeded flow, $\\langle \\boldsymbol {U}_{\\boldsymbol {\\varphi }} \\rangle$, is found to be essentially indistinguishable from the global mean velocity field of the jet, $\\langle \\boldsymbol {U} \\rangle$, for the axial velocity while significant deviations are found for the radial velocity. This results in an effective compressibility of the nozzle seeded flow for which $\\boldsymbol {\\nabla }\\boldsymbol {\\cdot } \\langle \\boldsymbol {U}_{\\boldsymbol {\\varphi }} \\rangle \\neq 0$ even though the global background flow is fully incompressible. By using mass conservation and self-similarity, we quantitatively explain the modified radial velocity profile and analytically express the missing contribution associated with entrained fluid particles. By considering a classical advection–diffusion description, we explicitly connect turbulent diffusion of mass (through the turbulent diffusivity $K_T$) and momentum (through the turbulent viscosity $\\nu _T$) to entrainment. This results in new practical relations to experimentally determine the non-uniform spatial profiles of $K_T$ and $\\nu _T$ (and hence of the turbulent Prandtl number $\\sigma _T = \\nu _T/K_T$) from simple measurements of the mean tracer concentration and axial velocity profiles. Overall, the proposed approach based on nozzle seeded flow gives new experimental and theoretical elements for a better comprehension of turbulent diffusion and entrainment in turbulent jets.

We report experimental results on the dynamics of heavy particles of the size of the Kolmogorov scale in a fully developed turbulent flow. The mixed Eulerian structure function of two-particle velocity and acceleration difference vectors $\\langle {\\delta }_{r} \\mathbi{v}\\boldsymbol{\\cdot} {\\delta }_{r} {\\mathbi{a}}_{\\mathbi{p}} \\rangle $ was observed to increase significantly with particle inertia for identical flow conditions. We show that this increase is related to a preferential alignment between these dynamical quantities. With increasing particle density the probability for those two vectors to be collinear was observed to grow. We show that these results are consistent with the preferential sampling of strain-dominated regions by inertial particles.

Thermal counterflow of superfluid $^4$He is investigated experimentally, by employing the particle tracking velocimetry technique. A flat heater, located at the bottom of a vertical channel of square cross-section, is used to generate this unique type of thermally driven flow. Micronic solid particles, made in situ, probe this quantum flow and their time-dependent positions are collected by a digital camera, in a plane perpendicular to the heat source, away from the channel walls. The experiments are performed at relatively large heating powers, resulting in fluid velocities exceeding $10\\ \\textrm {mm}\\,\\textrm {s}^{-1}$, to ensure the existence of sufficiently dense tangles of quantized vortices. Within the investigated parameter range, we observe that the particles intermittently switch between two distinct motion regimes, along their trajectories, that is, a single particle can experience both regimes while travelling upward. The regimes can be loosely associated with fast particles, which are moving away from the heat source along almost straight tracks, and to slow particles, whose erratic upward motion can be said to be significantly influenced by quantized vortices. We propose a separation scheme to study the properties of these regimes and of the corresponding transients between them. We find that particles in both regimes display non-classical, broad distributions of velocity, which indicate the relevance of particle–vortex interactions in both cases. At the same time, we observe that the fast particles move along straighter trajectories than the slow ones, suggesting that the strength of particle–vortex interactions in the two regimes is notably different.

Dimensionality is known to play an important role in many compounds for which ultrathin layers can behave very differently from the bulk. This is especially true for the paramagnetic metal LaNiO 3 , which can become insulating and magnetic when only a few monolayers thick. We show here that an induced antiferromagnetic order can be stabilized in the [111] direction by interfacial coupling to the insulating ferromagnet LaMnO 3 , and used to generate interlayer magnetic coupling of a nature that depends on the exact number of LaNiO 3 monolayers. For 7-monolayer-thick LaNiO 3 /LaMnO 3 superlattices, negative and positive exchange bias, as well as antiferromagnetic interlayer coupling are observed in different temperature windows. All three behaviours are explained based on the emergence of a (¼,¼,¼)-wavevector antiferromagnetic structure in LaNiO 3 and the presence of interface asymmetry with LaMnO 3 . This dimensionality-induced magnetic order can be used to tailor a broad range of magnetic properties in well-designed superlattice-based devices. Oxide materials can be combined to create heterostructures exhibiting complex properties not found in either substance individually. Here, the authors observe antiferromagnetic interlayer exchange coupling between ferromagnetic lanthanum manganite and nominally paramagnetic lanthanum nickel oxide.

Velocity measurements in turbulent superfluid helium between co-rotating propellers are reported. The parameters are chosen such that the flow is fully turbulent, and its dissipative scales are partly resolved by the velocity sensors. This allows for the first experimental comparison of spectra in quantum versus classical turbulence where dissipative scales are resolved. In some specific conditions, differences are observed, with an excess of energy at small scales in the quantum case compared to the classical one. This difference is consistent with the prediction of a pileup of superfluid kinetic energy at the bottom of the inertial cascade of turbulence due to a specific dissipation mechanism.

We investigate thin-film resistive thermometry based on metal-to-insulator transition (niobium nitride) materials down to very low temperature. The variation of the NbN thermometer resistance has been calibrated versus temperature and magnetic field. High sensitivity in temperature variation detection is demonstrated through efficient temperature coefficient of resistance. The nitrogen content of the niobium nitride thin films can be tuned to adjust the optimal working temperature range. In the present experiment, we show the versatility of the NbN thin-film technology through applications in very different low-temperature use cases. We demonstrate that thin-film resistive thermometry can be extended to temperatures below 30 mK with low electrical impedance.

Recently, CEA Grenoble SBT has designed, built and tested three liquid helium facilities dedicated to turbulence studies. All these experiments can operate either in HeI or HeII within the same campaign. The three facilities utilize moving parts inside liquid helium. The SHREK experiment is a von Kármán swirling flow between 0.72 m diameter counterrotating disks equipped with blades. The HeJet facility is used to produce a liquid helium free jet inside a 0.200 m I.D., 0.47 m length stainless steel cylindrical testing chamber. The OGRES experiment consists of an optical cryostat equipped with a particle injection device and an oscillating grid. We detail specific techniques employed to accommodate these stringent specifications. Solutions for operating these facilities without bubbles nor boiling cavitation are described. Control parameters as well as Reynolds number and temperature ranges are given.

In this paper we present a novel hydrodynamic experiment using liquid \\(^4\\)He. The flow is forced inertially by a canonical oscillating grid using either its normal (He~I) or superfluid (He~II) phase, generating a statistically stationary turbulence. We characterise the turbulent properties of the flow using 2D Lagrangian Particle tracking on hollow glass micro-spheres. As expected for tracer particles, the Vorono\"{i} tessellation on particle positions does not show a significant departure from a random Poisson process neither in He~I nor He~II phase. Particles' positions are tracked with high temporal resolution, allowing to resolve velocity fluctuations at integral and inertial scales while properly assessing the noise contribution. Additionally, we differentiate the particles' positions (by convolution with Gaussian kernels) in order to access small scale quantities like acceleration. Using these measured quantities and the formalism of classical Homogeneous Isotropic Turbulence (HIT) to perform an energy budget across scales we extract the energy injection rate at the large scale, the energy flux cascading through inertial scales, down to small scales at which it is dissipated. We found that in such inertially driven turbulence, regardless of the normal or superfluid state of the fluid, estimates of energy at the different scales are compatible with each other and consistent with oscillating grid turbulence results reported for normal fluids in the literature. The largest discrepancy shows up at small scales where the signal to noise ratio is harder to control and where the 2D measurement is contaminated by the 3D nature of the flow. This motivates to focus future experimental projects towards small scales, low noise and 3D measurements.

The study of quantum vortex dynamics in HeII offers great potential for advancing quantum-fluid models. Bose-Einstein condensates, neutron stars, and even superconductors exhibit quantum vortices, whose interactions are crucial for dissipation in these systems. These vortices have quantized velocity circulation around their cores, which, in HeII, are of atomic size. They have been observed indirectly, through methods such as second sound attenuation or electron bubble imprints on photosensitive materials. Over the past twenty years, decorating cryogenic flows with particles has become a powerful approach to studying these vortices. However, recent particle visualization experiments often face challenges with stability, initial conditions, stationarity, and reproducibility. Moreover, most dynamical analyses are performed in 2D, even though many flows are inherently 3D. We constructed a rotating cryostat with optical ports on an elongated square cupola to enable 2D2C, 2D3C, and 3D3C Lagrangian and Eulerian studies of rotating HeII flow. Using this setup, individual quantum vortices have been tracked with micron-sized particles, as demonstrated by Peretti et al., Sci. Adv. 9, eadh2899 (2023). The cryostat and associated equipment -- laser, cameras, sensors, and electronics -- float on a 50 \\(\\)m air cushion, allowing for precise control of the experiment's physical parameters. The performance during rotation is discussed, along with details on particle injection.