Velocity Cross-correlation Research Articles

Dominant heat transfer mechanism with conical roughness in a cubical box in turbulent Rayleigh–Bénard convection

In the present study, we numerically investigate the effect of Prandtl number on the heat transfer mechanism in turbulent Rayleigh–Bénard convection inside a cubical box endowed with conical roughness elements. The Rayleigh number is kept fixed at Ra=108, while the Prandtl number (Pr) varies from 1 to 50. In contrast to the monotonic increasing trend of the Nusselt number (Nu∼Pr0.27) in the two-dimensional (2D) roughness explored previously [Sharma et al., “Influence of Prandtl number in turbulent Rayleigh-Bénard convection over rough surfaces,” Phys. Rev. Fluids 7, 104609 (2022)], it assumes an invariant behavior (∼Pr0.012) for three dimensional (3D), though it is approximately 50% higher than its smooth counterpart. Flow intensity, measured in terms of Reynolds number (Re), drops with increasing Pr showing consistently lower magnitude for the 3D configuration. The addition of roughness elements is observed to disrupt the preferred orientation of large-scale circulation (LSC). The effect is predominant for lower Pr, where the roughness interferes most with the natural bias of LSC toward the diagonal planes of the cubical box. The analysis of plume statistics reveals that both coverage and intensity of plumes are augmented for the roughened cell. Increased homogeneity in the flow at higher Pr is reflected by the emergence of a more pronounced and distinguishable peak in probability density functions of temperature and velocity. Temporal spectra and variance data substantiate augmented intensity of fluctuations in the rough cell, while behavioral differences in the flow at different Pr are elucidated by using cross correlation of vertical velocity and temperature fluctuations.

Effect of Relative Mass on Ion Velocity Cross-Correlations in Ionic Liquids and Molten Salts: Different Perspectives in Different Reference Frames.

In electrolytes, the self- and interdiffusion coefficients, transport numbers, and electrical conductivity can be used to determine velocity cross-correlation coefficients (VCC) that are also accessible through molecular dynamics simulations. In an ionic liquid or molten salt, there are only three, corresponding to correlations between the velocities of distinct ion pairs (cation-anion, cation-cation, and anion-anion) averaged over both the ensemble and time, calculable from experimental ion self-diffusion coefficients and the electrolyte conductivity. Most usually, the mass-fixed frame of reference (with velocities relative to that of the center of mass of the system) is used to discuss the VCC and the distinct diffusion coefficients (DDC) derived from them. Recent work in the literature has suggested a dependence of the DDC on the ratio of the anion to cation mass. Here, we demonstrate, using our own and literature transport property data for a large number of ionic liquids and molten salts, that the trends observed depend on the particular choice of velocity reference frame, mass-, number-, or volume-fixed. The perception of ion-ion interactions may be distorted in the mass- and volume fixed frames when the co-ions have very different masses or volumes, particularly for systems containing light lithium ions.

Efficient Molecular Dynamics Simulations of Deep Eutectic Solvents with First-Principles Accuracy Using Machine Learning Interatomic Potentials.

In recent years, deep eutectic solvents emerged as highly tunable and ecofriendly alternatives to common organic solvents and liquid electrolytes. In the present work, the ability of machine learning (ML) interatomic potentials for molecular dynamics (MD) simulations of these liquids is explored, showcasing a trained neural network potential for a 1:2 ratio mixture of choline chloride and urea (reline). Using the ML potentials trained on density functional theory data, MD simulations for large systems of thousands of atoms and nanosecond-long time scales are feasible at a fraction of the computational cost of the target first-principles simulations. The obtained structural and dynamical properties of reline from MD simulations using our machine learning models are in good agreement with the first-principles MD simulations and experimental results. Running a single MD simulation is highlighted as a general shortcoming of typical first-principles studies if the dynamic properties are investigated. Furthermore, velocity cross-correlation functions are employed to study the collective dynamics of the molecular components in reline.

On the binary diffusion coefficients of n-alkanes in He/N2

The binary diffusion coefficients of fuel molecules in bath gasses are key parameters for accurate predictions of flame properties such as extinction strain rates. In combustion simulations, they are often calculated using the Hirschfelder-Bird-Spotz (HBS) equation, which requires the availability of effective Lennard-Jones parameters, i.e. collision diameters and well depths, which can be obtained from the intermolecular potentials of two interacting species by applying an orientation-averaging rule. Besides the HBS equation, the Green-Kubo formula can be used to obtain binary diffusion coefficients, using velocity autocorrelation and cross-correlation functions from molecular dynamics simulations. In this work, we propose a new orientation-averaging rule, called VolW-σ-ε, based on the characteristic of intermolecular potentials, and evaluate different approaches to computing the diffusion coefficients of n-alkanes in He/N2 mixtures. Several orientation-averaging rules are evaluated and compared with experimental data of n-alkanes in He and N2 and the results of molecular dynamics simulations. Finally, using the binary diffusion coefficients of n-dodecane in N2 evaluated by the VolW-σ-ε method, we reproduce previously reported experimental extinction strain rates reported for counterflow n-dodecane/N2 versus O2 jets.

Temperature and Pressure Dependence of the Transport Properties of the Ionic Liquid Triethyloctylphosphonium Bis(trifluoromethanesulfonyl)amide, [P222,8][Tf2N

Viscosities of triethyloctylphosphonium bis(trifluoromethanesulfonyl)amide, [P222,8][Tf2N], are reported as a function of temperature [(273 to 363) K] and pressure (maximum 302 MPa) with a falling-body viscometer together with electrical conductivities (κ) measured by impedance spectroscopy at (283 to 348) K, to 250 MPa maximum pressure. pVT data were determined with a vibrating tube densimeter from (298 to 353) K to 50 MPa. Ion self-diffusion coefficients (DSi) were measured by steady-gradient spin-echo NMR [(313 to 365) K], and densities [(273 to 363) K] were determined with a vibrating tube densimeter, both at atmospheric pressure. The results were correlated with Walden and Stokes–Einstein–Sutherland relations. Velocity cross-correlation coefficients (VCC), distinct diffusion coefficients (DDC) and Laity resistance coefficients (LRC) were calculated from DSi and κ at 0.1 MPa. The DDC and LRC for [P222,8][Tf2N] and the pentyl-substituted analogue, [P222,5][Tf2N], showed differences for cation–cation and cation–anion velocity anti correlations, presumably due to the different cation structures. The high-pressure viscosities were used to predict the pressure dependence of the glass-transition temperature for [P222,5][Tf2N] and [P222,8][Tf2N]. Density scaling was applied to the high-pressure viscosities and conductivities of [P222,8][Tf2N] for comparison with [P222,5][Tf2N]. The scaling parameters are consistent with the theoretical treatment of Knudsen et al. for ionic liquids.

Experimental measurements of fluid flow in an 84-pin hexagonal rod bundle with spacer grid for a gas-cooled fast modular reactor

A 50 MWe helium-cooled fast modular reactor (FMR) is under development, and as part of the Department of Energy Integrated Research Project (IRP), Texas A&M University is conducting the thermal-hydraulic characterization of its baseline core configuration. We experimentally investigated the axial and cross flow fields characteristics in the hexagonal fuel rod bundle composed of 84 rods, a central rod, and spacer grids at a Reynolds number of 12,000. A fully transparent experimental facility resembling of one unit of the fuel assembly was constructed. Time-resolved particle image velocimetry (TR-PIV) measurements were performed to characterize hydraulic behavior downstream the spacer grid. Velocity measurements were conducted in the axial and radial direction of the rod bundle. From the PIV velocity vector fields, the full-field flow statistics were computed for the mean velocity, vorticity, and Reynolds stresses. An analysis of the energy decay downstream of the spacer grid was conducted with calculations of secondary-flow intensities, turbulent kinetic energy, power spectrum, and spatial-temporal velocity cross correlations. The vorticity field was obtained and pairs of counter-rotating vortexes were identified using a 3D reconstruction of several measurement planes. The data from this experimental campaign will be used to validate numerical models based on Computational Fluid Dynamics and other codes.

Propagating Alfvénic Waves Observed in the Chromosphere around a Small Sunspot: Tales of 3-minute Waves and 10-minute Waves

Recent observations provided evidence that the solar chromosphere of sunspot regions is pervaded by Alfvénic waves—transverse magnetohydrodynamic (MHD) waves (Alfvén waves or kink waves). In order to systematically investigate the physical characteristics of Alfvénic waves over a wide range of periods, we analyzed the time series of line-of-sight velocity maps constructed from the Hα spectral data of a small sunspot region taken by the Fast Imaging Solar Spectrograph of the Goode Solar Telescope at Big Bear. We identified each Alfvénic wave packet by examining the cross-correlation of band-filtered velocity between two points that are located a little apart presumably on the same magnetic field line. As result, we detected a total of 279 wave packets in the superpenumbral region around the sunspot and obtained their statistics of period, velocity amplitude, and propagation speed. An important finding of ours is that the detected Alfvénic waves are clearly separated into two groups: 3-minute period (<7 minutes) waves and 10-minute period (>7 minutes) waves. We propose two tales on the origin of Alfvénic waves in the chromosphere; the 3-minute Alfvénic waves are excited by the upward-propagating slow waves in the chromosphere through the slow-to-Alfvénic mode conversion, and the 10-minute Alfvénic waves represent the chromospheric manifestation of the kink waves driven by convective motions in the photosphere.

Experimental analysis of a non-isothermal confined impinging single plume using time-resolved particle image velocimetry and planar laser induced fluorescence measurements

In this study, the experimental results of the flow behavior of a non-isothermal confined impinging single plume were analyzed. Experimental measurements were performed in the upper plenum of a high-temperature gas-cooled reactor (HTGR) that was scaled, designed, and assembled at Texas A&M University. The goal was to investigate flow mixing, thermal stratification, and plume impingement in the upper plenum of the HTGR under a loss-of-forced-coolant accident and provide a high-fidelity experimental database for validating computational fluid dynamics (CFD) and system codes. Time-resolved particle image velocimetry (TR-PIV) and planar laser-induced fluorescence (P-LIF) measurements were performed in the central plane of the plume flow in the upper plenum. A total power of 0.4 kW of heat was applied to the system. Convergence was achieved for the statistical results from the TR-PIV and P-LIF snapshots. The TR-PIV velocity vectors presented statistical characteristics of the plume flow, such as the mean velocity, root-mean-square fluctuating velocity, and Reynolds stress. The results revealed that the flow reached the maximum velocity far from the jet inlet at y/Dj=2. Furthermore, two-point velocity cross-correlation was performed on the TR-PIV velocity vector fields. Finally, the statistical results were calculated from the P-LIF temperature snapshots and the mean temperature. Furthermore, the temperature profiles were plotted. It was observed that the flow reached the maximum temperature near the plume inlet and decreased downstream.

Effect of pressure on the transport properties of 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide and 1-hexyl-3-methylimidazolium tetrafluoroborate

High-pressure self-diffusion and electrical conductivity data are reported for the ionic liquid reference material 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide. The data are used to calculate velocity cross-correlation coefficients and Laity resistance coefficients. High-pressure self-diffusion coefficients are also reported for 1-hexyl-3-methylimidazolium tetrafluoroborate. Comparisons are made with the viscosities using the Stokes-Einstein-Sutherland relation for the two ionic liquids. Density scaling is applied to the transport property measurements and comparison made with earlier studies on ionic liquid systems.

Dynamical Splitting of Spot-producing Magnetic Rings in a Nonlinear Shallow-water Model

Abstract We explore the fundamental physics of narrow toroidal rings during their nonlinear magnetohydrodynamic evolution at tachocline depths. Using a shallow-water model, we simulate the nonlinear evolution of spot-producing toroidal rings of 6° latitudinal width and a peak field of 15 kG. We find that the rings split; the split time depends on the latitude of each ring. Ring splitting occurs fastest, within a few weeks, at latitudes 20°–25°. Rossby waves work as perturbations to drive the instability of spot-producing toroidal rings; the ring split is caused by the “mixed stress” or cross-correlations of perturbation velocities and magnetic fields, which carry magnetic energy and flux from the ring peak to its shoulders, leading to the ring split. The two split rings migrate away from each other, the high-latitude counterpart slipping poleward faster due to migrating mixed stress and magnetic curvature stress. Broader toroidal bands do not split. Much stronger rings, despite being narrow, do not split due to rigidity from stronger magnetic fields within the ring. Magnetogram analysis indicates the emergence of active regions sometimes at the same longitudes but separated in latitude by 20° or more, which could be evidence of active regions emerging from split rings, which consistently contribute to observed high-latitude excursions of butterfly wings during the ascending, peak, and descending phases of a solar cycle. Observational studies in the future can determine how often new spots are found at higher latitudes than their lower-latitude counterparts and how the combinations influence solar eruptions and space weather events.

A Thermal Transport Study of Branched Carbon Nanotubes with Cross and T-Junctions

This study investigated the thermal transport behaviors of branched carbon nanotubes (CNTs) with cross and T-junctions through non-equilibrium molecular dynamics (NEMD) simulations. A hot region was created at the end of one branch, whereas cold regions were created at the ends of all other branches. The effects on thermal flow due to branch length, topological defects at junctions, and temperature were studied. The NEMD simulations at room temperature indicated that heat transfer tended to move sideways rather than straight in branched CNTs with cross-junctions, despite all branches being identical in chirality and length. However, straight heat transfer was preferred in branched CNTs with T-junctions, irrespective of the atomic configuration of the junction. As branches became longer, the heat current inside approached the values obtained through conventional prediction based on diffusive thermal transport. Moreover, directional thermal transport behaviors became prominent at a low temperature (50 K), which implied that ballistic phonon transport contributed greatly to directional thermal transport. Finally, the collective atomic velocity cross-correlation spectra between branches were used to analyze phonon transport mechanisms for different junctions. Our findings deeply elucidate the thermal transport mechanisms of branched CNTs, which aid in thermal management applications.

Experimental Measurements of the Wake of a Sphere at Subcritical Reynolds Numbers

Abstract This work experimentally investigated the flow phenomena and vortex structures in the wake of a sphere located in a water loop at Reynolds numbers of Re = 850, 1,250, and 1,700. Velocity fields in the wake region were obtained by applying the time-resolved stereoscopic particle image velocimetry (TR-SPIV) technique. From the acquired TR-SPIV velocity vector fields, the statistical values of mean and fluctuating velocities were computed. Spectral analysis, two-point velocity–velocity cross-correlation, proper orthogonal decomposition (POD) and vortex identification analyses were also performed. The velocity fields show a recirculation region that decreases in length with an increase of Reynolds numbers. The power spectra from the spectral analysis had peaks corresponding to a Strouhal number of St = 0.2, which is a value commonly found in the literature studies of flow over a sphere. The two-point cross-correlation analysis revealed elliptical structures in the wake, with estimated integral length scales ranging between 12% and 63% of the sphere diameter. The POD analysis revealed the statistically dominant flow structures that captured the most flow kinetic energy. It is seen that the flow kinetic energy captured in the smaller scale flow structures increased as Reynolds number increased. The POD modes contained smaller structure as the Reynolds number increased and as mode order increased. In addition, spectral analysis performed on the POD temporal coefficients revealed peaks corresponding to St = 0.2, similar to the spectral analysis on the fluctuating velocity. The ability of POD to produce low-order reconstructions of the flow was also utilized to facilitate vortex identification analysis, which identified average vortex sizes of 0.41D for Re1, 0.33D for Re2, and 0.32D for Re3.

Improving estimates of the growth rate using galaxy–velocity correlations: a simulation study

ABSTRACT We present an improved framework for estimating the growth rate of large-scale structure, using measurements of the galaxy–velocity cross-correlation in configuration space. We consider standard estimators of the velocity autocorrelation function, ψ1 and ψ2, the two-point galaxy correlation function, ξgg, and introduce a new estimator of the galaxy–velocity cross-correlation function, ψ3. By including pair counts measured from random catalogues of velocities and positions sampled from distributions characteristic of the true data, we find that the variance in the galaxy–velocity cross-correlation function is significantly reduced. Applying a covariance analysis and χ2 minimization procedure to these statistics, we determine estimates and errors for the normalized growth rate fσ8 and the parameter β = f/b, where b is the galaxy bias factor. We test this framework on mock hemisphere data sets for redshift z &amp;lt; 0.1 with realistic velocity noise constructed from the l-picola simulation code, and find that we are able to recover the fiducial value of fσ8 from the joint combination of ψ1 + ψ2 + ψ3 + ξgg, with 15 per cent accuracy from individual mocks. We also recover the fiducial fσ8 to within 1σ regardless of the combination of correlation statistics used. When we consider all four statistics together we find that the statistical uncertainty in our measurement of the growth rate is reduced by $59{{\ \rm per\ cent}}$ compared to the same analysis only considering ψ2, by $53{{\ \rm per\ cent}}$ compared to the same analysis only considering ψ1, and by $52{{\ \rm per\ cent}}$ compared to the same analysis jointly considering ψ1 and ψ2.

Numerical investigation of flow instability and heat transfer characteristics inside pulsating heat pipes with different numbers of turns

Pulsating heat pipes (PHPs) are multiphase devices with relatively well known properties. Like other heat pipes, PHPs provide high heat transfer rates over long distances due to the fluid phase changes that take place within them. Several studies focused on improving our understanding of the processes driving PHPs, but limited advances have been made to date. For instance, no comprehensive theory about the processes driving PHPs has been developed. This severely restricts PHP use because of uncertainties in predictions of the behaviors of new PHP designs, particularly for sub-optimal conditions such as non-vertical orientations. Computational fluid dynamics can be used to inspect PHPs non-intrusively, but have only been used for a few PHPs with simple geometries. Here, Volume Of Fluid and Lee phase-change models of PHPs with seven, 16, and 23 turns were compared with experimental data and a semi-empirical correlation and used to analyze flow instability and heat transfer. The flow structure, velocity, and temperature profiles at predefined locations and periods were assessed to describe the PHP operating ranges. Cross-correlations of the internal velocity and pressure signals helped explain the effects of the number of turns on PHP thermo-hydrodynamics. These techniques and additional experimental and numerical analyses can improve our understanding of PHPs.

An experimental investigation of a square supersonic jet and impinging jet on an inclined plate

Supersonic free jets and impinging jets are found in many engineering applications, such as short and vertical take-off and landing vehicles, cold gas dynamic spray processes, hot surface cooling mechanisms, and turbomachinery systems. The flow characteristics of a supersonic square jet discharging into the ambient and a supersonic jet impinging on a 45° inclined surface were experimentally investigated for nozzle-pressure-ratios (NPRs) of 4.8 and 5.9. Experimental measurements of impinging jets were acquired for nozzle-to-plate distances of 0.82Dj and 1.8Dj, where Dj is the jet hydraulic diameter. The velocity fields in the central plane of the jet were obtained using planar particle image velocimetry. The flow characteristics of the supersonic jets, including mean velocity and turbulent kinetic energy, were computed from the acquired two-dimensional two-component velocity vector fields, and statistical profiles were compared for different NPRs and nozzle-to-plate distances. For supersonic free jets, the acquired statistical results revealed the presence of multiple shock cells along the streamwise direction. Impinging jet measurements revealed the presence of shock cells in the vicinity of the nozzle outlet, oblique plate shocks near the impingement location, and several tail shocks along the streamwise direction. Spatial turbulent velocity cross correlations were calculated for various points located along the shear layers to investigate the characteristics of turbulent features, such as the shape, orientation, and integral length scales of the studied configurations. In addition, a proper orthogonal decomposition analysis was applied to the instantaneous velocity fields to identify the statistically dominant flow structures that play an important role in the flow field characteristics of supersonic free jets and supersonic impinging jets.

Reconstruction of Taylor Bubbles in Slug Flow Using a Direct-Image Multielectrode Conductance Sensor

Gas-liquid two-phase flows are widely encountered in industrial processes. To realize the dynamic detection of gas-liquid slug flows, a direct-image multi-electrode conductance sensor (DMCS) is optimized using the finite element method to access high measurement sensitivity. Through carrying out an experiment of vertical gas-liquid two-phase flows, the gas-liquid interfaces in slug flows are detected by the DMCS and compared to the results from a planar laser-induced fluorescence (PLIF) system. The interfaces detected by the DMCS and the PLIF system indicate a good agreement. By selecting an optimal threshold, the liquid film thicknesses around Taylor bubbles (TBs) are derived, and the asymmetry of circumferential film thickness distribution is investigated. Besides, the local cross-correlation velocities at circumferential pipe wall are measured by the DMCS. Finally, the TBs in slug flows are reconstructed based on the thickness and the velocity of the TBs. The reconstructed TBs allow for a clear indication of gas-liquid slug flow structures.

Experimental investigation of turbulent wake flows in a helically wrapped rod bundle in presence of localized blockages

In nuclear sodium fast reactors, bundles of rods are tightly packed into a triangular lattice, enclosed in a hexagonal duct, and each pin is spirally wrapped with a thin wire. Flow blockages can potentially impact the local flow characteristics and heat transfer mechanisms in the bundle due to its small subchannel size. The effects of the blockage on the flow structures and heat transfer mechanisms are important aspects that require an accurate investigation. In this study, the flow-field characteristics in the vicinity of a blockage located in the exterior subchannel of rod bundles with helically wrapped wires were experimentally investigated. The velocity fields in the exterior subchannel were acquired by applying matched-index-of-refraction and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers of Re1 = 4000 and Re2 = 17 000, i.e., equivalent to Rew1 = 19 600 and Rew2 = 83 200, respectively, based on the blockage width. The results from the TR-PIV measurements revealed an arch-shaped vortex with a large flow recirculation and a pair of counter-rotating vortices in the wake region downstream of the blockage, which is commonly observed in the wake flow of bluff bodies. The relative lateral distance and angle between the two vortices decreased when the Reynolds numbers increased. Profiles of maximum turbulence intensity along the shear layers illustrated the transition process including the growth, peak, and decay along the flow direction. From the spectral analysis of the turbulent velocities extracted at points along the shear layer, the Strouhal numbers (St) representing the vortex shedding frequency were found to be St = 0.25 and St = 0.56 for the left and right shear layers, respectively. Characteristics of shear layers generated by the blockage in the exterior subchannel were investigated via the two-point cross correlation of fluctuating velocities. The spatiotemporal cross correlations of turbulent velocities, computed at points in the region where the left shear layer exhibited rolling effects and vortex breakdowns, were considerably wider and longer. The convection velocity Uc was estimated to be ∼0.82Um to 0.93Um. Proper orthogonal decomposition (POD) analysis was applied to the instantaneous velocity fields to extract the statistically dominant flow structures. It was found that POD modes 2–3 and 4–5 formed the pair modes when the corresponding POD temporal coefficients depicted sinusoidal shapes and exhibited nearly circular orbits in the phase space. Spectral analysis of the POD temporal coefficients confirmed the vortex shedding frequencies detected in the analysis of turbulent velocities.

Cross-Correlation Sensitivity-Based Electrostatic Direct Velocity Tomography

Electrostatic tomography (EST) has been widely used in gas–solid two-phase flows to measure particle velocity in many industrial processes. However, EST is a passive measurement technique, and this results in rare independent measurements compared with other tomographic modalities. In this article, an electrostatic direct velocity tomography (EDVT) method is proposed to increase the number of independent measurements and to improve the accuracy of velocity estimation. For a two-plane electrostatic sensor, the cross-correlation (CC) velocities between the electrodes from the two planes at different circumferential positions can represent the particle velocities in different regions. First, the CC velocities between the electrodes on different planes are calculated and used as new measurements. In this way, the number of independent measurements of a 16-electrode EST sensor can be easily increased from 16 to 120. Second, by analyzing the relationship between the CC velocities and the particle charge and particle velocity distribution, a CC sensitivity matrix is established. Finally, a tomographic model to represent the velocity distribution of charged particles can be established based on the new measurements and the CC sensitivity matrix. Simulation and experimental results show that the velocity distribution reconstructed by EDVT can represent the particle velocity distribution in the pipeline and is promising to provide a method to study the mechanism of gas–solid two-phase flows.

Spectral Broadening of Tonal Sound Propagating Through an Axisymmetric Turbulent Shear Layer

In aeroacoustics, spectral broadening refers to the scattering of tonal sound fields by turbulent shear layers, whereby the interaction of the sound with turbulent flow results in power lost from the tone and distributed into a broadband field around the tone frequency. Fan and turbine tone spectral broadening is known colloquially as “haystacking”. Recently a new analytical model has been derived to predict weak spectral broadening of a tone radiated through a circular jet. A key part of the modeling is the choice of the two-point turbulent velocity cross-correlation function, which is used to provide a statistical description of the turbulence in the shear layer. A new cross-correlation function for an axisymmetric turbulent shear layer formed by a circular jet, based on the theory for homogeneous axisymmetric turbulence, has been developed. Validation results of weak-scattering calculated using this correlation function show better agreement with measurements when compared with the results calculated using a correlation function based on the theory for homogeneous isotropic turbulence.

Electrical conductivity, ion pairing, and ion self-diffusion in aqueous NaCl solutions at elevated temperatures and pressures.

We have performed classical molecular dynamics (MD) simulations of aqueous sodium chloride (NaCl) solutions from 298 to 674 K at 200 bars to understand the influence of ion pairing and ion self-diffusion on electrical conductivity in high-temperature/high-pressure salt solutions. Conductivity data obtained from the MD simulation highlight an apparent anomaly, namely, a conductivity maximum as temperature increases along an isobar, which has been also observed in experimental studies. By examining both velocity autocorrelation and cross-correlation terms of the Green-Kubo integral, we quantitatively demonstrate that the conductivity anomaly arises mainly from a competition between the single-ion self-diffusion and the contact ion pair formation. The velocity autocorrelation function in conjunction with structural analysis suggests that diffusive motion of ions is suppressed at high temperatures due to the persistence of an inner hydration shell. The contribution of velocity cross-correlation functions between oppositely charged ions becomes significant at the onset of the conductivity decrease. Structural analysis based on Voronoi tessellation and pair correlation functions indicates that the fraction of contact ion pairs increases as temperature increases. Spatial decomposition of the electrical conductivity also indicates that the formation of contact ion pairs significantly decreases the electrical conductivity compared to Nernst-Einstein conductivity, but the contribution of distant opposite charges cannot be ignored except at the highest temperature due to unscreened long-range interactions.