arXiv subjects

Nicolas Plihon

Publications and source records attributed to Nicolas Plihon.

16 recordsLinked to original sources

arXiv2023

Emissive cathode immersed in a plasma: plasma-cathode interactions, operation and stability

Thermionic emission from a polycrystalline tungsten emissive cathode immersed in a magnetized plasma column is investigated experimentally and numerically. Electrical and optical measurements of the cathode temperature show a highly inhomogeneous cathode temperature profile due to plasma-cathode interactions. The spatially and temporally resolved cathode temperature profile provides an in-depth understanding of the thermionic electron current, in excellent agreement with experimental data. The plasma-cathode coupling leads to a sharp and heterogeneous rise in temperature along the cathode, which can eventually lead to unstable cathode operation, with divergent current growth. A detailed thermal modeling accurately reproduces the experimental measurements, and allows to quantify precisely the relative importance of heating and cooling mechanisms in the operation of the cathode immersed in the plasma. Numerical resolution of the resulting integro-differential equation highlights the essential role of heterogeneous ohmic heating and the importance of ion bombardment heating in the emergence of unstable regimes. Detailed thermal modelling enables operating regimes to be predicted in excellent agreement with experimental results.

physics.plasm-ph↗

arXiv2023

Influence of the porosity pattern on the aerodynamics of a square plate

The evolution of the normal aerodynamic coefficient of 19 configurations of square plates with various porosity patterns, ranging from solid plate to homogeneous porous plate, is experimentally characterized. The variation of the porosity pattern is obtained by partially covering the holes of a commercial fly-swatter using adhesive tape. Evolution of the normal aerodynamic coefficient is assessed from the measurement of the angular position of the porous plate, placed as a freely rotating pendulum swept by a flow in a wind tunnel. These angular measurements are also supported by PIV measurements of the structure of the wake. We show that the porosity pattern determines whether or not an abrupt stall occurs. In particular, the details of the porosity pattern on the edges of the plate are decisive for the existence of abrupt stall.

physics.flu-dyn↗

arXiv2023

Path instabilities and drag in the settling of single spheres

The settling behavior of individual spheres in a quiescent fluid was studied experimentally. The dynamics of the spheres was analyzed in the parameter space of particle-to-fluid density ratio ($Γ$) and Galileo number ($\mathrm{Ga}$), with $Γ\in (1.1, 7.9)$ and $\mathrm{Ga} \in (100, 340)$. The experimental results showed for the first time that the mean trajectory angle with the vertical exhibits a complex behavior as $\mathrm{Ga}$ and $Γ$ are varied. Numerically predicted regimes such as Vertical Periodic and Planar Rotating were validated at high $Γ$ values. In particular, for the denser spheres, a clear transition from planar to non-planar trajectories was observed, accompanied by the emergence of semi-helical trajectories corresponding to the Planar Rotating Regime. The spectra of trajectory oscillations were also quantified as a function of $\mathrm{Ga}$, confirming the existence of oblique oscillating regimes at both low and high frequencies. The amplitudes of the perpendicular velocities in these regimes were also quantified and compared with numerical simulations in the literature. The terminal velocity and drag of the spheres were found to depend on the particle-to-fluid density ratio, and correlations between the drag coefficient and particle Reynolds number ($Re_p$) as a function of Ga were established, allowing for the estimation of drag and settling velocity using $\mathrm{Ga}$, a control parameter, rather than the response parameter $Re_p$.

physics.flu-dyn↗

arXiv2023

How dirt cones form on glaciers: field observation, laboratory experiments and modeling

Dirt cones are meter-scale structures encountered at the surface of glaciers, which consist of ice cones covered by a thin layer of ashes, sand or gravel, and which form naturally from an initial patch of debris. In this article, we report field observations of cone formation in the French Alps, laboratory-scale experiments reproducing these structures in a controlled environment, and two-dimensional discrete-element-method-finite-element-method numerical simulations coupling the grains mechanics and thermal effects. We show that cone formation originates from the insulating properties of the granular layer, which reduces ice melting underneath as compared to bare ice melting. This differential ablation deforms the ice surface and induces a quasi static flow of grains that leads to a conic shape, as the thermal length become small compared to the structure size. The cone grows until it reaches a steady state in which the insulation provided by the dirt layer exactly compensates for the heat flux coming from the increased external surface of the structure. These results allowed us to identify the key physical mechanisms at play and to develop a model able to quantitatively reproduce the various field observations and experimental findings.

physics.flu-dyn↗

arXiv2023

Nonlinear interactions of ion acoustic waves explored using fast imaging decompositions

Fast camera imaging is used to study ion acoustic waves propagating azimuthally in a magnetized plasma column. The high speed image sequences are analyzed using Proper Orthogonal Decomposition and 2D Fourier Transform, allowing to evaluate the assets and differences of both decomposition techniques. The spatio-temporal features of the waves are extracted from the high speed images, and highlight energy exchanges between modes. Growth rates of the modes are extracted from the reconstructed temporal evolution of the modes, revealing the influence of ion-neutral collisions as pressure increases. Finally, the nonlinear interactions between modes are extracted using bicoherence computations, and show the importance of interactions between modes with azimuthal wave numbers $m$, $m-1$ and $-1$, with $m$ an integer.

physics.plasm-ph↗

arXiv2021

High-speed imaging of magnetized plasmas: when electron temperature matters

High speed camera imaging is a powerful tool to probe the spatiotemporal features of unsteady processes in plasmas, usually assuming light fluctuations to be a proxy for the plasma density fluctuations. In this article, we systematically compare high speed camera imaging with simultaneous measurements of the plasma parameters -- plasma density, electron temperature, floating potential -- in a modestly magnetized Argon plasma column at low pressure (1 mTorr, magnetic fields ranging from 160 to 640~G). The light emission was filtered around $488\pm5$~nm, $750\pm5$~nm, $810\pm5$~nm. We show that the light intensity cannot be interpreted as a proxy for the plasma density and that the electron temperature cannot be ignored when interpreting high speed imaging, both for the time-averaged profiles and for the fluctuations. The features of plasma parameter fluctuations are investigated, with a focus on ion acoustic waves (at frequency around 70 kHz) at low magnetic field and low-frequency azimuthal waves (around a few kHz) at larger magnetic fields. An excellent match is found between the high speed images fluctuations and an Arrhenius law functional form which incorporates fluctuations of the plasma density and of the electron temperature. These results explain the discrepancies between ion saturation current and narrow-band imaging measurements previously reported in the literature.

physics.plasm-ph↗

arXiv2021

Dynamo enhancement and mode selection triggered by high magnetic permeability

We present results from consistent dynamo simulations, where the electrically conducting and incompressible flow inside a cylinder vessel is forced by moving impellers numerically implemented by a penalization method. The numerical scheme models jumps of magnetic permeability for the solid impellers, resembling various configurations tested experimentally in the von-Karman Sodium experiment. The most striking experimental observations are reproduced in our set of simulations. In particular, we report on the existence of an axisymmetric dynamo mode, self-consistently generated when the magnetic permeability of the impellers exceeds a threshold. We describe a possible scenario involving both the turbulent flow in the vicinity of the impellers and the high magnetic permeability of the impellers.

physics.flu-dyn↗

arXiv2021

Onset of glacier tables

A glacier table consists of a rock supported by a slender column of ice and form naturally on glaciers. We investigate the onset of their formation at a smaller scale in a controlled environment. Depending on the size and thermal conductivity of a cap, it can either form of a table standing on an ice foot, or sink into the ice block. A one-dimension conduction model shows that the differential ice melting is controlled by a competition between two effects: a geometrical amplification, and a heat flux reduction due to the higher temperature of the cap as compared to the ice. Our model captures the transition between the two regimes and identifies a dimensionless number which controls the onset of glacier tables formation.

physics.flu-dyn↗

arXiv2020

Rare event-triggered transitions in aerodynamic bifurcation

The transitions between two states of a bistable system are investigated experimentally and analyzed in the framework of rare-event statistics. Considering a disk pendulum swept by a flow in a wind tunnel, bistability between two aerodynamic branches is observed, with spontaneous transitions from one branch to the other. The waiting times before spontaneous transition are distributed following a double-exponential as a function of the control parameter, spanning four orders of magnitude in time, for both transitions. Inspired by a model originally applied to the transition to turbulence, we show that, for the disk pendulum, both transitions are controlled by rare events of the aerodynamic forces acting on the disk which we propose to link in particular to the vortex shedding-induced fluctuations. Beyond the aerodynamic aspects, this work has interesting fundamental outcomes regarding the broad field of rare events in out-of-equilibrium systems.

physics.flu-dyn↗

arXiv2019

Sedimentation of a suspension of paramagnetic particles in an external magnetic field

We investigate the sedimentation of initially packed paramagnetic particles in presence of a homogeneous external magnetic field, in a Hele-Shaw cell filled with water. Although the magnetic susceptibility of the particles is small and the particle-particle induced magnetic interactions are significantly smaller compared to the gravitational acceleration, we do observe a measurable reduction of the decompaction rate as the amplitude of the applied magnetic field is increased. While induced magnetic dipole-dipole interactions between particles can be either attracting or repulsive depending on the particles relative alignment, our observations reveal an effective overall enhancement of the cohesion of the initial pack of particles due to the induced interactions, very likely promoting internal chain forces in the initial pack of particles. The influence of the magnetic field on the particles once they disperse after being decompacted is on the other hand found to remain marginal.

cond-mat.soft↗

arXiv2017

Synthetic Schlieren -- application to the visualization and characterization of air convection

Synthetic schlieren is an digital image processing optical method relying on the variation of optical index to visualize the flow of a transparent fluid. In this article, we present a step-by step, easy-to-implement and affordable experimental realization of this technique. The method is applied to air convection caused by a warm surface. We show that the velocity of rising convection plumes can be linked to the temperature of the warm surface and propose a simple physical argument to explain this dependence. Moreover, using this method, one can reveal the tenuous convection plumes rising from ounce's hand, a phenomenon invisible to the naked eye. This spectacular result may help student realize the power of careful data acquisition combined with astute image processing techniques.

physics.flu-dyn↗

arXiv2014

Flow dynamics and magnetic induction in the von-Karman plasma experiment

The von-Karman plasma experiment is a novel versatile experimental device designed to explore the dynamics of basic magnetic induction processes and the dynamics of flows driven in weakly magnetized plasmas. A high-density plasma column (10^16 - 10^19 particles.m^-3) is created by two radio-frequency plasma sources located at each end of a 1 m long linear device. Flows are driven through JxB azimuthal torques created from independently controlled emissive cathodes. The device has been designed such that magnetic induction processes and turbulent plasma dynamics can be studied from a variety of time-averaged axisymmetric flows in a cylinder. MHD simulations implementing volume-penalization support the experimental development to design the most efficient flow-driving schemes and understand the flow dynamics. Preliminary experimental results show that a rotating motion of up to nearly 1 km/s is controlled by the JxB azimuthal torque.

physics.plasm-ph↗

arXiv2014

Robust estimate of dynamo thresholds in the von Kármán sodium experiment using the Extreme Value Theory

We apply a new threshold detection method based on the extreme value theory to the von Kármán sodium (VKS) experiment data. The VKS experiment is a successful attempt to get a dynamo magnetic field in a laboratory liquid-metal experiment. We first show that the dynamo threshold is associated to a change of the probability density function of the extreme values of the magnetic field. This method does not require the measurement of response functions from applied external perturbations, and thus provides a simple threshold estimate. We apply our method to different configurations in the VKS experiment showing that it yields a robust indication of the dynamo threshold as well as evidence of hysteretic behaviors. Moreover, for the experimental configurations in which a dynamo transition is not observed, the method provides a way to extrapolate an interval of possible threshold values.

physics.flu-dyn↗

arXiv2014

Long-term memory in experiments and numerical simulations of hydrodynamic and magnetohydrodynamic turbulence

We analyze time series stemming from experiments and direct numerical simulations of hydrodynamic and magnetohydrodynamic turbulence. Simulations are done in periodic boxes, but with a volumetric forcing chosen to mimic the geometry of the flow in the experiments, the von Kármán swirling flow between two counter-rotating impellers. Parameters in the simulations are chosen to (within computational limitations) allow comparisons between the experiments and the numerical results. Conducting fluids are considered in all cases. Two different configurations are considered: a case with a weak externally imposed magnetic field, and a case with self-sustained magnetic fields. Evidence of long-term memory and $1/f$ noise is observed in experiments and simulations, in the case with weak magnetic field associated with the hydrodynamic behavior of the shear layer in the von Kármán flow, and in the dynamo case associated with slow magnetohydrodynamic behavior of the large scale magnetic field.

physics.flu-dyn↗

arXiv2009

Induction in a von Karman flow driven by ferromagnetic impellers

We study magnetohydrodynamics in a von Kármán flow driven by the rotation of impellers made of material with varying electrical conductivity and magnetic permeability. Gallium is the working fluid and magnetic Reynolds numbers of order unity are achieved. We find that specific induction effects arise when the impeller's electric and magnetic characteristics differ from that of the fluid. Implications in regards to the VKS dynamo are discussed.

physics.flu-dyn↗

arXiv2009

Large scale fluctuations and dynamics of the Bullard - von Kármán dynamo

A synthetic fluid dynamo built in the spirit of the Bullard device [E. C. Bullard, Proc. Camb. Phil. Soc., 51, 744 (1955)] is investigated. It is a two-step dynamo in which one process stems from the fluid turbulence, while the other part is an alpha effect achieved by a linear amplification of currents in external coils [M. Bourgoin et al., New J. Phys., 8, 329 (2006)]. Modifications in the forcing are introduced in order to change the dynamics of the flow, and hence the dynamo behavior. Some features, such as on-off intermittency at onset of dynamo action, are very robust. Large scales fluctuations have a significant impact on the resulting dynamo, in particular in the observation of magnetic field reversals.

physics.flu-dyn↗