arXiv subjects

Jérôme Valentin

Publications and source records attributed to Jérôme Valentin.

4 recordsLinked to original sources

arXiv2023

Second sound resonators and tweezers as vorticity or velocity probes : fabrication, model and method

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 with aspect ratios close to unity, operated up to their 20th resonance in superfluid helium.An important finding of this study 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 geometrical misalignments between the tweezers plates, and another 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 second-sound resonator 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 micro-fabrication of second sound tweezers are being presented in detail, along with their potential for exploring quantum turbulence.

cond-mat.other↗

arXiv2021

Local measurement of vortex statistics in quantum turbulence

The density fluctuations of quantum vortex lines are measured in a turbulent flow of superfluid He, at temperatures corresponding to superfluid fraction of 16%, 47% and 81%. The probe is a micro-fabricated second sound resonator that allows for local and small-scale measurements in the core of the flow, at a 10-mesh-size behind a grid. 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 an 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.

physics.flu-dyn↗

arXiv2021

Cooling with a subsonic flow of quantum fluid

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 $^4$He 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 $μ$m up to few hundreds of microns. At low heat fluxes, no velocity dependence is observed. In contrast, some velocity dependence emerges at larger heat flux, as reported previously, and three non-trivial 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 \textit{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 Kármán vortex street in the wake of a bluff body.

physics.flu-dyn↗

arXiv2013

Extended point defects in crystalline materials: Ge and Si

B diffusion measurements are used to probe the basic nature of self-interstitial 'point' defects in Ge. We find two distinct self-interstitial forms - a simple one with low entropy and a complex one with entropy ~30 k at the migration saddle point. The latter dominates diffusion at high temperature. We propose that its structure is similar to that of an amorphous pocket - we name it a 'morph'. Computational modelling suggests that morphs exist in both self-interstitial and vacancy-like forms, and are crucial for diffusion and defect dynamics in Ge, Si and probably many other crystalline solids.

cond-mat.mtrl-sci↗