Time-averaged Field Research Articles

Qualitative and quantitative measurements of horseshoe vortex formation in the junction of horizontal and vertical plates

Horseshoe vortex formation around a vertical flat plate mounted in the water channel is experimentally investigated. In order to demonstrate the development of the horseshoe vortex system in the junction of the horizontal and vertical plates a Particle Image Velocimetry (PIV) and dye visualization techniques were used. Instantaneous and time-averaged velocity vector fields, corresponding streamline topology, vorticity values were analyzed. Boundary layer separation, developing vortices, secondary or counter-rotating vortices, merging of developing and counter-rotating vortices which results in a primary horseshoe vortex system and corner vortex were demonstrated qualitatively and quantitatively.

Parameters of a collisional radio-frequency sheath and dust characteristics resulting from the microparticle levitation

The screening length, the time-average electric field, and the particle charge as well as the local vertical gradients of these quantities are determined experimentally within a sheath of a capacitively coupled rf, 13.56 MHz, discharge at enhanced argon gas pressures of 30, 55, and 100 Pa. The parameters are derived directly from comparative measurements of levitation positions of the particles of different sizes and variations in the levitation heights caused by formation of new dust layers. The electrostatic effect of the horizontally extended dust layers on the sheath electric field is investigated.

Relativistic Quantization of Cooper Pairs and Distributed Electrons in Rotating Superconductors

Relativistic time synchronization along closed integral lines maintains magnetic flux quantization independently from gravitation. All Fermi-volume electrons form time-averaged electromagnetic fields within rotating conductors, while Fermi-surface superelectrons enable flux quantization in SQUID experiments. Inertia is not related to instantaneous self-coherent states of the distributed electric charge and, therefore, the Cooper pair mass cannot be measured in principle from magnetic flux quantization.

Large-scale electromagnetic modelings based on high-order methods: Nanoscience applications

This paper presents large-scale computations and theoretical or computational aspects of the spectral element methods for solving Maxwell's equations that have potential applications in nanoscience for surface-enhanced Raman scattering (SERS) and solar cell devices. We study the surface-enhanced electromagnetic fields near the surface of metallic nanoparticles using spectral element discontinuous Galerkin method. We solve Maxwell's equations in time-domain and provide accuracy and efficiency of our method compared to the conventional finite difference method. We demonstrate light transmission properties for nanoslab and nanoslits, and time-averaged electric fields over the cross sections of nanoholes in a hexagonal array.

Sensitivity of aerosol optical thickness and aerosol direct radiative effect to relative humidity

Abstract. We present a sensitivity study of the effects of spatial and temporal resolution of atmospheric relative humidity (RH) on calculated aerosol optical thickness (AOT) and the aerosol direct radiative effects (DRE) in a global model. We carry out different modeling experiments using the same aerosol fields simulated in the Global Modeling Initiative (GMI) model at a resolution of 2° latitude by 2.5° longitude, using time-averaged fields archived every three hours by the Goddard Earth Observation System Version 4 (GEOS-4), but we change the horizontal and temporal resolution of the relative humidity fields. We find that, on a global average, the AOT calculated using RH at a 1°×1.25° horizontal resolution is 11% higher than that using RH at a 2°×2.5° resolution, and the corresponding DRE at the top of the atmosphere is 8–9% and 15% more negative (i.e., more cooling) for total aerosols and anthropogenic aerosol alone, respectively, in the finer spatial resolution case. The difference is largest over surface escarpment regions (e.g. >200% over the Andes Mountains) where RH varies substantially with surface terrain. The largest zonal mean AOT difference occurs at 50–60° N (16–21%), where AOT is also relatively larger. A similar impact is also found when the time resolution of RH is increased. This increase of AOT and aerosol cooling with the increase of model resolution is due to the highly non-linear relationship between RH and the aerosol mass extinction efficiency (MEE) at high RH (>80%). Our study is a specific example of the uncertainty in model results highlighted by multi-model comparisons such as AeroCom, and points out one of the many inter-model differences that can contribute to the overall spread among models.

Experimental study of reactive mixing in a mini-scale mixer by laser-induced fluorescence technique

The liquid–liquid mixing process coupled with chemical reactions in a mini-scale jet mixer was visualized by reactive laser-induced fluorescence (reactive-LIF) technique for a deep understanding of the interplay between the mixing and the simultaneous reactions. A novel approach for implementing the reactive-LIF measurements was advanced in this work, where the principle was based on the quenching of the fluorescence signal emitted from the Rhodamine-B dye using the mechanism of Fenton reaction. The purely physical mixing and the reactive mixing processes were investigated extensively by comparing the concentration fields under different operating conditions, i.e. the different momentum ratios between the jet and the bulk flows, and the different Reynolds numbers in the mixing channel of a mini-scale mixer. The results revealed that the transient dynamics in the reactive mixing process cannot be precisely understood based on the time-averaged concentration fields from either the physical or the reactive mixing measurements. The coupled mixing and the reaction processes occurred simultaneously, unless the rapid mixing was achieved by tuning the operating conditions so that the processes can be decoupled into two isolated ones.

Paleomagnetic behavior of volcanic rocks from Isla Socorro, Mexico

Abstract The direction and magnitude of the geomagnetic field vary both spatially and temporally and undergo significant departures from that of a geocentric axial dipole. In order to properly characterize persistent behaviors, time-averaged field models must be based on the highest quality data. Here we present full-vector paleomagnetic data for volcanic units exposed in the southeast quadrant of the island of Socorro, Mexico. We carried out a joint expedition between the Scripps Institution of Oceanography and the Universidad Nacional Autónoma México to Isla Socorro in January of 2005 during which we collected oriented paleomagnetic samples from 21 sites, representing as many as 10 different volcanic units (the oldest of which is ~540 ka). We subjected over 100 specimens to the most up-to-date paleointensity methods, and included the standard reliability checks. In an earlier study, Bohrson et al. (1996) proposed a series of widespread eruptive events, based on similarities of argon/argon dates. Paleointensity from specimens that conform to the strictest acceptance criteria are available from both the (unoriented) original sample collection and our fully oriented (but as yet undated) new collection. Correlation between the two collections is however problematic. The time-averaged direction from Socorro is consistent with that expected from a geocentric axial dipole, and the time-averaged intensity is 30.0±7.1 μT, equivalent to a virtual axial dipole moment (VADM) of 67.6±16.0 ZAm2.

Paleomagnetic field variation with strong negative inclination during the Brunhes chron at the Banda Sea, equatorial southwestern Pacific

We reconstruct the earth magnetic field in the Brunhes epoch at the Banda Sea by studying the paleomagnetic data from core MD012380, collected during the International Marine Global Change Study (IMAGES) VII Cruise in 2001. Magnetic analysis is carried out for whole core with a sampling spacing of 1cm by using u-channel. Magnetic susceptibility (χ), nature remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) are measured in our paleomagnetic experiment. Results show the low latitude geomagnetic field variation at the Banda Sea during the last ∼820kyr. Except for the Brunhes/Matuyama boundary (BMB), there is no clear signal of reverse events in paleo- inclination and paleo-declination patterns. However, the synthetic paleointensity curve displays the asymmetrical saw-tooth pattern that can be used for determining reverse events, and shows a maximum intensity drop at the BMB. The characteristics of paleointensity provide a useful tool to identify reverse signals and improve the difficulties from only using inclination and declination patterns, especially at low latitude. With the help of paleointensity, inclination and declination, we have identified five reverse events. Furthermore if we consider the secular variation effect, we think that the strong negative inclination observed in our study may be the zonal time-averaged field with paleo secular variation, rather than non-dipole effect within the Brunhes epoch.

Measurement of pressure field around a NACA0018 airfoil from PIV velocity data

In solving these equations numerically, Eqs. (1) and (2) are transformed into general coordinates and they are solved by SOR scheme to obtain the time-averaged static pressure field around the airfoil. Note that the numerical grids are given by O type in the present study and the grid spacing is controlled to be fine near the airfoil and outer boundary, which is set at 5 times of the chord length C of the airfoil from the airfoil center. The number of grids is 600 x 300 in circumferential and radial direction, respectively. In solving these equations, all velocities on the right hand side of Eq. (1) are evaluated from the experimental velocity data measured by PIV. 3. Experiment The velocity field around a NACA0018 airfoil is measured using a standard PIV system, which consists of Nd:YAG laser (532 nm, 30 mJ/pulse), CCD camera with frame straddling function (1280 x 1024 pixels with 12 bits in gray level) and the pulse controller. The experiment is carried out in an { } ( ) j i j u u

Comparison of fluid and particle-in-cell simulations on atmospheric pressure helium microdischarges

Microdischarges at atmospheric pressure were studied by two computational methods. The first method is a typical one-dimensional fluid model in which the electron velocity distribution function is assumed to be Maxwellian and the energy equation is solved to determine the spatial profile of the electron temperature. The second method is a particle-in-cell (PIC) model with Monte-Carlo collisions (MCC). We compared the time-averaged density, electric field and power consumption profiles of helium microdischarges driven at 13.56 MHz and 2.45 GHz obtained with the two models. The agreement between the two models depends on the driving frequency. The kinetic information obtained from the PIC-MCC model indicates that the improved agreement at higher frequency is due to the evolution of the electron energy distribution function from a three-temperature distribution at 13.56 MHz to a close-to-Maxwellian distribution at 2.45 GHz.

Turbulent flow between counter-rotating concentric cylinders: a direct numerical simulation study

We report three-dimensional direct numerical simulations of the turbulent flow between counter-rotating concentric cylinders with a radius ratio 0.5. The inner- and outer-cylinder Reynolds numbers have the same magnitude, which ranges from 500 to 4000 in the simulations. We show that with the increase of Reynolds number, the prevailing structures in the flow are azimuthal vortices with scales much smaller than the cylinder gap. At high Reynolds numbers, while the instantaneous small-scale vortices permeate the entire domain, the large-scale Taylor vortex motions manifested by the time-averaged field do not penetrate a layer of fluid near the outer cylinder. Comparisons between the standard Taylor–Couette system (rotating inner cylinder, fixed outer cylinder) and the counter-rotating system demonstrate the profound effects of the Coriolis force on the mean flow and other statistical quantities. The dynamical and statistical features of the flow have been investigated in detail.

Simulation of dust voids in complex plasmas

In dusty radio-frequency (RF) discharges under micro-gravity conditions often a void is observed, a dust free region in the discharge center. This void is generated by the drag of the positive ions pulled out of the discharge by the electric field. We have developed a hydrodynamic model for dusty RF discharges in argon to study the behaviour of the void and the interaction between the dust and the plasma background. The model is based on a recently developed theory for the ion drag force and the charging of the dust.With this model, we studied the plasma inside the void and obtained an understanding of the way it is sustained by heat generated in the surrounding dust cloud. When this heating mechanism is suppressed by lowering the RF power, the plasma density inside the void decreases, even below the level where the void collapses, as was recently shown in experiments on board the International Space Station. In this paper we present results of simulations of this collapse.At reduced power levels the collapsed central cloud behaves as an electronegative plasma with corresponding low time-averaged electric fields. This enables the creation of relatively homogeneous Yukawa balls, containing more than 100 000 particles. On earth, thermophoresis can be used to balance gravity and obtain similar dust distributions.

Paleomagnetic directions from mid-latitude sites in the southern hemisphere (Argentina): Contribution to time averaged field models

Back-arc volcanism located to the east of the Andean Cordillera was sampled in the Argentina provinces of Mendoza and Neuquen for paleomagnetic time average field and paleosecular investigations. The activity ranges from 2 Ma to very recent time, with a large variety of products, from basalts to highly differentiated lavas. After removal of sites affected by lightning, those with α 95 higher than 10°, and combining of nearby sites displaying close directions, we present new paleomagnetic results from 31 flows units belonging to two volcanic massifs: the Payun Matru and the Cerro Nevado. Previous and new K–Ar age determinations constrain the volcanic activity of these massifs from 300 to 0 ka, and from 1.9 to 0.9 Ma, respectively. Most paleomagnetic samples have NRM intensities between about 1 and 20 A/m and depict progressive removal of magnetization components in a consistent fashion during stepwise AF or thermal demagnetization. Nineteen flows yielded a normal direction (declination = 354.8°, inclination = −53.0°, α 95 = 6.8°) and 12 flows a reverse direction (declination = 181.0°, inclination = 52.3°, α 95 = 5.9°). The combined data yielded a mean direction (declination = 357.3°, inclination = −52.8°, α 95 = 4.6°), which is not statistically different from the axial dipole field ( g 1 0 ) expected at this latitude (36°S). The angular dispersion of virtual geomagnetic poles calculated from flows with normal directions (ASD = 16.5°) compares well with the observed value from global datasets for this site latitude, but flows with reverse directions display a surprisingly low dispersion (ASD = 12.5°). Since most reverse directions were sampled from flows ranging between 1.9 and 0.9 Ma, this can be interpreted as an interval of low paleomagnetic secular variation. Additional data, also with accurate time constraints, are obviously needed to better support this observation. Finally, no convincing evidence for a complex time average field significantly different from the axial dipole can be supported by this study for the last 2 Myr.

Thermochemical flows couple the Earth's inner core growth to mantle heterogeneity

Seismic waves sampling the top 100 km of the Earth's inner core reveal that the eastern hemisphere (40 degrees E-180 degrees E) is seismically faster, more isotropic and more attenuating than the western hemisphere. The origin of this hemispherical dichotomy is a challenging problem for our understanding of the Earth as a system of dynamically coupled layers. Previously, laboratory experiments have established that thermal control from the lower mantle can drastically affect fluid flow in the outer core, which in turn can induce textural heterogeneity on the inner core solidification front. The resulting texture should be consistent with other expected manifestations of thermal mantle control on the geodynamo, specifically magnetic flux concentrations in the time-average palaeomagnetic field over the past 5 Myr, and preferred eddy locations in flows imaged below the core-mantle boundary by the analysis of historical geomagnetic secular variation. Here we show that a single model of thermochemical convection and dynamo action can account for all these effects by producing a large-scale, long-term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle. The main feature of this thermochemical 'wind' is a cyclonic circulation below Asia, which concentrates magnetic field on the core-mantle boundary at the observed location and locally agrees with core flow images. This wind also causes anomalously high rates of light element release in the eastern hemisphere of the inner core boundary, suggesting that lateral seismic anomalies at the top of the inner core result from mantle-induced variations in its freezing rate.

Time-averaged paleomagnetic field and secular variation: Predictions from dynamo solutions based on lower mantle seismic tomography

We compare three dynamo solutions incorporating laterally varying boundary heat flux with paleomagnetic models and data. The boundary condition is defined by the D ″seismic shear-wave velocity and the three solutions have boundary anomalies with different amplitudes. The generated fields appear to divide into a stationary, boundary-locked part and a time-varying part with persistent centres of activity. Both parts contribute to the time average. A very long averaging time can be needed for nearly-locked solutions, but a rough time average that remains within the threshold set by the accuracy of paleomagnetic data is achieved in a few diffusion times. The locked part dominates for larger amplitude boundary anomalies. In previous work the locked field was shown to have strong similarities with the modern geomagnetic field. Previous dynamo solutions that were not locked to the boundary show similarities with our solutions with weak boundary forcing. The axisymmetric time average has small g 2 0 and larger g 3 0 components and peaks in inclination anomaly in high latitudes (associated with the locked field) and low latitudes (associated with the time average of the time-varying fields). The non-axisymmetric time average displays a striking longitudinal variation in inclination anomaly, with a large negative anomaly in the Pacific region in agreement with observations. None of the dominant geomagnetic coefficients are axisymmetric and g 2 0 negligible in all three models. Secular variation is concentrated in equatorial latitudes, as in some recent paleomagnetic models. The locked field agrees with the inclination difference found between Hawai’i and Réunion, in agreement with paleomagnetic averages. The locked field agrees with the paleomagnetic time average rather better than the fields with less boundary variations. We conclude that, because the locked field agrees with the modern field as well as some aspects of the long-term time average, the geomagnetic field spends a considerable time in the present four-lobe configuration: it is not a coincidence that the present field resembles the time average. Longitudinal variations are likely to be at least as important as latitudinal variations in the paleomagnetic time average. This presents a challenge for dynamo theory, since a move to more geophysically realistic parameters would appear to destroy the locked solutions.

Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows

We present a synthesis of 0–5 Ma paleomagnetic directional data collected from 17 different locations under the collaborative Time Averaged geomagnetic Field Initiative (TAFI). When combined with regional compilations from the northwest United States, the southwest United States, Japan, New Zealand, Hawaii, Mexico, South Pacific, and the Indian Ocean, a data set of over 2000 sites with high quality, stable polarity, and declination and inclination measurements is obtained. This is a more than sevenfold increase over similar quality data in the existing Paleosecular Variation of Recent Lavas (PSVRL) data set, and has greatly improved spatial sampling. The new data set spans 78°S to 53°N, and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time‐averaged field (TAF) and paleosecular variation (PSV) for the Brunhes and Matuyama chrons, and for the 0–5 Ma interval combined. The Brunhes and Matuyama chrons exhibit different TAF geometries, notably smaller departures from a geocentric axial dipole field during the Brunhes, consistent with higher dipole strength observed from paleointensity data. Geographical variations in PSV are also different for the Brunhes and Matuyama. Given the high quality of our data set, polarity asymmetries in PSV and the TAF cannot be attributed to viscous overprints, but suggest different underlying field behavior, perhaps related to the influence of long‐lived core‐mantle boundary conditions on core flow. PSV, as measured by dispersion of virtual geomagnetic poles, shows less latitudinal variation than predicted by current statistical PSV models, or by previous data sets. In particular, the Brunhes data reported here are compatible with a wide range of models, from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude. Discriminating among such models could be helped by increased numbers of low‐latitude data and new high northern latitude sites. Tests with other data sets, and with simulations, indicate that some of the latitudinal signature previously observed in VGP dispersion can be attributed to the inclusion of low‐quality, insufficiently cleaned data with too few samples per site. Our Matuyama data show a stronger dependence of dispersion on latitude than the Brunhes data. The TAF is examined using the variation of inclination anomaly with latitude. Best fit two‐parameter models have axial quadrupole contributions of 2–4% of the axial dipole term, and axial octupole contributions of 1–5%. Approximately 2% of the octupole signature is likely the result of bias incurred by averaging unit vectors.

Signatures of Dynamical Heterogeneity in the Structure of Glassy Free-Energy Minima

From numerical minimization of a model free-energy functional for a system of hard spheres, we show that the width of the local peaks of the time-averaged density field at a glassy free-energy minimum exhibits large spatial variation, similar to that of the "local Debye-Waller factor" in simulations of dynamical heterogeneity. Molecular dynamics simulations starting from a particle configuration generated from the density distribution at a glassy free-energy minimum show similar spatial heterogeneity in the degree of localization, implying a direct connection between dynamical heterogeneity and the structure of glassy free-energy minima.

Control of diffuser jet flow: turbulent kinetic energy and jet spreading enhancements assisted by a non-thermal plasma discharge

An axisymmetric air jet exhausting from a 22-degree-angle diffuser is investigated experimentally by particle image velocimetry (PIV) and stereo-PIV measurements. Two opposite dielectric barrier discharge (DBD) actuators are placed along the lips of the diffuser in order to force the mixing by a co-flow actuation. The electrohydrodynamic forces generated by both actuators modify and excite the turbulent shear layer at the diffuser jet exit. Primary air jet velocities from 10 to 40 m/s are studied (Reynolds numbers ranging from 3.2 to 12.8 × 104), and baseline and forced flows are compared by analysing streamwise and cross-stream PIV fields. The mixing enhancement in the near field region is characterized by the potential core length, the centreline turbulent kinetic energy (TKE), the integrated value of the TKE over various slices along the jet, the turbulent Reynolds stresses and the vorticity fields. The time-averaged fields demonstrate that an effective increase in mixing is achieved by a forced flow reattachment along the wall of the diffuser at 10 m/s, whereas mixing enhancement is realized by excitation of the coherent structures for a primary velocity of 20 and 30 m/s. The actuation introduces two pairs of contra-rotating vortices above each actuator. These structures entrain the higher speed core fluid toward the ambient air. Unsteady actuations over Strouhal numbers ranging from 0.08 to 1 are also studied. The results suggest that the excitation at a Strouhal number around 0.3 is more effective to enhance the turbulence kinetic energy in the near-field region for primary jet velocity up to 30 m/s.

Computational fluid dynamic (CFD) simulation of snowdrift in alpine environments, including a local weather model, for operational avalanche warning

AbstractA new continuum approach to snowdrift modelling is introduced. In addition, numerical studies are carried out to identify the influence of time-varying wind conditions on snowdrift simulations. We compare the snowdrift patterns at Grimming mountain, Austria, derived using a time-averaged wind field and a time-varying wind field obtained from the numerical weather prediction model INCA. The results show significant differences in the deposition patterns and snow depth even after a 6 hour drift period. Using time-averaged boundary conditions leads to an underprediction of the resulting snow depth caused by averaging the wind speed, which lets gusts of wind disappear while snow transport is a non-linear function of the wind speed. Using numerical weather prediction models for snowdrift simulation therefore provides enhanced knowledge of the snow depth for local avalanche warning services.

Gas–liquid flow around an obstacle in a vertical pipe

This paper presents a novel technique to study the two-phase flow field around an asymmetric obstruction in a vertical pipe with a nominal diameter of DN200. Main feature of the experiments is the shifting of a half-moon shaped diaphragm causing the obstruction along the axis of the pipe. In this way, the 3D void field is scanned with a stationary wire-mesh sensor that supplies data with a spatial resolution of 3 mm over the cross-section and a measuring frequency of 2.5 kHz. Besides the measurement of time-averaged void fraction fields and bubble-size distributions, novel data evaluation methods were developed to extract estimated liquid velocity profiles as well as lateral components of bubble velocities from the wire-mesh sensor data. The combination of void fraction fields and velocity profiles offer the opportunity to analyse a two-phase flow in a geometry that owns a series of features characteristic for complex components of power and chemical plant equipment. Such characteristics are sharp edges with flow separation, recirculation areas, jet formation, stagnation points and curved stream-lines. The tests were performed with an air–water flow at nearly ambient conditions and with a saturated steam–water mixture at 6.5 MPa. The superficial velocities of liquid and gas or, respectively, vapour were varied in a wide range. The flow structure upstream and downstream of the obstacle is characterized in detail. Bubble size dependent effects of bubble accumulation and migration are discussed on basis of void-fraction profiles decomposed into bubble-size classes. A pronounced influence of the fluid parameters was found in the behaviour of bubbles at the boundary of the jet coming from the non-obstructed part of the cross-section. In case of an air–water flow, bubbles are restrained from entering the jet, a phenomenon which was not observed in high-pressure steam–water flow. A detailed uncertainty analyse of the velocity assessments finishes the presented paper. A blind pre-test calculation with CFX-10 based on the assumption of a mono-disperse bubbly flow has reproduced the overall void and velocity profiles. The results are used for the assessment of the influence of local accelerations on the liquid velocity measurement.