High-resolution Density Measurements Research Articles

Study of Stable Stratification in HiRJET Facility With Direct Numerical Simulation

Stable density stratification in a large enclosure could significantly hamper the effectiveness of natural convection cooling in pool-type liquid metal or gas-cooled advanced reactors. In addition, accurate prediction of stratified front behavior remains to be a challenging task for turbulence modeling. With the rapid growth of high-performance-computing capabilities in recent years, conducting high-fidelity simulations for a large-timescale transient has become more affordable and hence a valuable data source to support turbulence modeling as well as to gain further physical insights. In this work, direct numerical simulation is performed at experiment-consistent conditions to simulate the density stratification transient High-Resolution Jet (HiRJET) facility. Specifically, we focus on the case where an injected aqueous sugar solution has 1.5% density higher than that in the enclosure. In the early stage of the transient, the impingement of the denser jet to the bottom surface of the enclosure promoted turbulent mixing locally. This rendered the establishment of the mixture layer, formation and swift upward propagation of the stratified front, and elevation with (locally) the highest vertical concentration gradient. As the front rose, the diminishing turbulent mass flux slowed down the propagation, and a larger vertical concentration gradient was established. In this stage, both the velocity and the concentration scalar showed large-timescale fluctuation behavior around the stratified front. For the concentration time signal, the characteristic frequency in the power spectral density was found to agree well with the Brunt-Väisällä frequency. The preliminary validation endeavor showed that the stratified front location and the corresponding concentration gradient magnitude in the simulation agreed well with the experiment data. Further validation will mainly revolve around benchmarking against high-resolution density measurement and high-order flow statistics.

Multi-tracer study of gas trapping in an East Antarctic ice core

Abstract. We study a firn and ice core drilled at the new “Lock-In” site in East Antarctica, located 136 km away from Concordia station towards Dumont d'Urville. High-resolution chemical and physical measurements were performed on the core, with a particular focus on the trapping zone of the firn where air bubbles are formed. We measured the air content in the ice, closed and open porous volumes in the firn, firn density, firn liquid conductivity, major ion concentrations, and methane concentrations in the ice. The closed and open porosity volumes of firn samples were obtained using the two independent methods of pycnometry and tomography, which yield similar results. The measured increase in the closed porosity with density is used to estimate the air content trapped in the ice with the aid of a simple gas-trapping model. Results show a discrepancy, with the model trapping too much air. Experimental errors have been considered but do not explain the discrepancy between the model and the observations. The model and data can be reconciled with the introduction of a reduced compression of the closed porosity compared to the open porosity. Yet, it is not clear if this limited compression of closed pores is the actual mechanism responsible for the low amount of air in the ice. High-resolution density measurements reveal the presence of strong layering, manifesting itself as centimeter-scale variations. Despite this heterogeneous stratification, all layers, including the ones that are especially dense or less dense compared to their surroundings, display similar pore morphology and closed porosity as a function of density. This implies that all layers close in a similar way, even though some close in advance or later compared to the bulk firn. Investigation of the chemistry data suggests that in the trapping zone, the observed stratification is partly related to the presence of chemical impurities.

Arbitrary-order Hilbert Spectral Analysis and Intermittency in Solar Wind Density Fluctuations

Abstract The properties of inertial- and kinetic-range solar wind turbulence have been investigated with the arbitrary-order Hilbert spectral analysis method, applied to high-resolution density measurements. Due to the small sample size and to the presence of strong nonstationary behavior and large-scale structures, the classical analysis in terms of structure functions may prove to be unsuccessful in detecting the power-law behavior in the inertial range, and may underestimate the scaling exponents. However, the Hilbert spectral method provides an optimal estimation of the scaling exponents, which have been found to be close to those for velocity fluctuations in fully developed hydrodynamic turbulence. At smaller scales, below the proton gyroscale, the system loses its intermittent multiscaling properties and converges to a monofractal process. The resulting scaling exponents, obtained at small scales, are in good agreement with those of classical fractional Brownian motion, indicating a long-term memory in the process, and the absence of correlations around the spectral-break scale. These results provide important constraints on models of kinetic-range turbulence in the solar wind.

Multifractal analysis of high resolution solar wind proton density measurements

The solar wind is a highly turbulent medium, with a high level of field fluctuations throughout a broad range of scales. These include an inertial range where a turbulent cascade is assumed to be active. The solar wind cascade shows intermittency, which however may depend on the wind conditions. Recent observations have shown that ion-scale magnetic turbulence is almost self-similar, rather than intermittent. A similar result was observed for the high resolution measurements of proton density provided by the spacecraft Spektr-R. Intermittency may be interpreted as the result of the multifractal properties of the turbulent cascade. In this perspective, this paper is devoted to the description of the multifractal properties of the high resolution density measurements. In particular, we have used the standard coarse-graining technique to evaluate the generalized dimensions Dq, and from these the multifractal spectrum f(α), in two ranges of scale. A fit with the p-model for intermittency provided a quantitative measure of multifractality. Such indicator was then compared with alternative measures: the width of the multifractal spectrum, the peak of the kurtosis, and its scaling exponent. The results indicate that the small-scale fluctuations are multifractal, and suggest that different measures of intermittency are required to fully understand the small scale cascade.

Layering of surface snow and firn at Kohnen Station, Antarctica: Noise or seasonal signal?

The density of firn is an important property for monitoring and modeling the ice sheet as well as to model the pore close-off and thus to interpret ice core-based greenhouse gas records. One feature, which is still in debate, is the potential existence of an annual cycle of firn density in low-accumulation regions. Several studies describe or assume seasonally successive density layers, horizontally evenly distributed, as seen in radar data. On the other hand, high-resolution density measurements on firn cores in Antarctica and Greenland showed no clear seasonal cycle in the top few meters. A major caveat of most existing snow-pit and firn-core based studies is that they represent one vertical profile from a laterally heterogeneous density field. To overcome this, we created an extensive dataset of horizontal and vertical density data at Kohnen Station, Dronning Maud Land on the East Antarctic Plateau. We drilled and analyzed three 90 m long firn cores as well as 160 one meter long vertical profiles from two elongated snow trenches to obtain a two dimensional view of the density variations. The analysis of the 45 m wide and 1 m deep density fields reveals a seasonal cycle in density. However, the seasonality is overprinted by strong stratigraphic noise, making it invisible when analyzing single firn cores. Our density dataset extends the view from the local ice-core perspective to a hundred meter scale and thus supports linking spatially integrating methods such as radar and seismic studies to ice and firn cores.

Developments toward hard X-ray radiography on heavy-ion heated dense plasmas

AbstractWe have studied the potential of hard X-ray radiography as a diagnostic in high energy density experiments, proposed for the future Facility for Antiproton and Ion Research (FAIR). We present synthetic radiographic images generated from hydrodynamic simulations of the target evolution. The results suggest that high-resolution density measurements can be obtained from powerful hard X-ray sources driven by a PW-class high-energy laser system. Test measurements of a prototype hard X-ray imaging detector for photon energies above 100 keV are presented.

Critical behavior of dielectric permittivity in the vicinity of nematic–isotropic and smectic–nematic phase transitions in smectogenic binary mixtures

We report the phase diagram of a binary mixture composed of compounds, one having NCS terminal group (5DBT, showing smectic A1 phase) and the other with CN terminal group (10OCB, showing smectic Ad phase), exhibiting induced nematic phase in a certain concentration range (0.05<x5DBT<0.95). Measurement of static dielectric parameters on this binary system within the whole mesomorphic range is being reported. Pretransitional behavior at the nematic–isotropic and the smectic A–nematic phase transitions has been studied by measuring the static dielectric parameters in the entire mesomorphic range. Precise determination of discontinuity (ΔT) and the critical exponent (α) has been carried out for the nematic–isotropic phase transition. Effective molecular dipole moment and the angle of inclination with respect to the molecular long axis have been determined. High resolution density measurement is also carried out for one concentration x5DBT=0.301 and from the molar volume so obtained, the discontinuity (ΔT) of the transition and the critical exponent are measured. Additionally, the nature of the N–SmA phase transition has been studied for a few mixtures from dielectric anisotropy measurements to explore the critical behavior of the N–SmA phase transition.

Relaxation Kinetics of Bulk Metallic Glasses below Glass Transition Temperature

The free volume relaxation just under the glass transition region was investigated by the high-resolution density measurement using the bulk metallic glasses with the compositions of Pd40Ni40P20 and Zr55Cu30Ni5Al10. The relaxation process was well described by a stretched exponential function with Kohlrausch exponent values less than unity. The reduced free volumes in an as-quenched state were estimated as 0.0117 and 0.0353 for Pd40Ni40P20 glass and Zr55Cu30Ni5Al10 glass, respectively. The specific heat curves Cp(T) for Pd42.5Cu30Ni7.5P20 alloy were obtained for the supercooled liquid, the equilibrium liquid and the crystallized alloy. The isenthalpic Kauzmann temperature TKH and isentropic Kauzmann temperature TKS were estimated as 471 K and 522 K, respectively, from the specific heat data.

Edge density fluctuation characterization in H -mode and polarimetry measurement via the FIReTIP system on NSTX

The edge interferometry channel of the far infrared tangential interferometer polarimeter (FIReTIP) is used to characterize the localized edge density fluctuations during the L/H transition in conjunction with other edge diagnostic systems on the National Spherical Tokamak Experiment (NSTX). With the high IF frequency employing a “Stark-tuned” laser as local oscillator, it is possible to measure the absolute level of fluctuations at high time resolution. Density fluctuation measurements using FIReTIP are feasible at both the high field side and the low field side boundary throughout the plasma shot. The normalized level of measured density fluctuations was in the range of 0.01–0.3 depending upon the channel location and the plasma confinement. The experimental results showed that the sampling rate of 500 KHz was sufficiently high to produce valuable frequency spectral information during the L/H transition, ELMs period and TAE/f.b.s activities. The improved polarimetry measurement was introduced for the para/diamagnetic study of the NSTX plasma. Multichannel operation of FIReTIP with high resolution density measurement provides promising opportunities as a density fluctuation diagnostic system for turbulence/transport and boundary physics studies as well as the robust density measurement for plasma operation. In this article, a brief description of the upgraded portion of the FIReTIP system will be given together with the experimental results of spectral analysis of the edge fluctuations and assessments on polarimetry signals.

Mode transitions in helicon discharges

High resolution density measurements in a conventional helicon source show sudden jumps during a rf power ramp. The downwards power ramp enters all three known discharge modes, capacitive, inductive, and helicon wave sustained mode. For an upwards power ramp the discharge jumps directly from the capacitive into the helicon mode. This observation stands in contrast to models that are based on the plasma density as the critical parameter for the transition to the helicon mode.

A study of the omega equation for diagnosing vertical motions at ocean fronts

Estimation of vertical velocity is a key issue for understanding ocean physics and transport of biogeochemical tracers. We examine the accuracy of estimating vertical velocity in fronts with the omega equation. The diagnostic performance of the omega equation is evaluated by using vertical velocities obtained from simulation of frontal instabilities in a primitive equation model as a reference. We use two traditional quasigeostrophic methods in which the flow is either a geostrophic flow computed from density or a nondivergent flow derived from vorticity and also test two new formulations: a quasigeostrophic method using the total flow field and the semigeostrophic omega equation. Results show that all four formulations correctly diagnose the vertical velocity pattern. However, estimates provided by the traditional quasi-geostrophic formulations have systematic bias. In contrast, the two new techniques, which are practically equivalent, produce unbiased vertical velocity diagnostic at fronts. These results point out the importance of including higher order dynamics than quasigeostrophy to take into account the ageostrophic advection in the front. Since adequate filtering of ADCP data is not yet available to obtain a suitable total flow, the semigeostrophic omega equation is proposed as the most valuable tool to compute vertical velocities from high resolution density measurements.

Influence of tonicity on the viscoelastic properties of blood during isovolemic dilution.

The influence of isotonic or hypotonic dilution on viscoelastic properties of blood is examined. The viscous, as well as the elastic, properties of blood samples diluted with isotonic saline or pure water, respectively, and of undiluted whole blood samples are compared by means of a dynamic capillary viscosimeter (OCR-D, A. Paar K.G., Austria). The dilution (approximately 17% of the total blood volume) was performed isovolemically and retained the same rbc count. The rbc swelling observed as a consequence of changes in plasma osmolarity was tracked by the high resolution density measurement, according to the mechanical oscillator technique. Since no significant rbc swelling was found in the dilution with isotonic saline, viscoelastic resistance of blood was efficiently reduced in the observed range of shear rates (2 s-1-100 s-1). This decrease is due to reduced plasma protein concentration, which also lowers plasma viscosity by approximately 18%. Although plasma viscosity is significantly decreased in hypotonic dilutions (-12%), flow properties of the rbc suspensions are, in general, significantly impaired. This is due to the osmotic rbc swelling (hematocrit = +8%), which increases viscous resistance within the suspending fluid, as well as elastic resistance of the rbc due to a loss in rbc deformability. It can be concluded that isotonic dilution leads to a decrease in the viscosity of blood, whereas hypotonic dilution--in an order of magnitude which may occur during resorption of water--leads to increased viscous and elastic resistance of the blood.