Time-averaged Field Research Articles

Computed versus measured ion velocity distribution functions in a Hall effect thruster

We compare time-averaged and time-varying measured and computed ion velocity distribution functions in a Hall effect thruster for typical operating conditions. The ion properties are measured by means of laser induced fluorescence spectroscopy. Simulations of the plasma properties are performed with a two-dimensional hybrid model. In the electron fluid description of the hybrid model, the anomalous transport responsible for the electron diffusion across the magnetic field barrier is deduced from the experimental profile of the time-averaged electric field. The use of a steady state anomalous mobility profile allows the hybrid model to capture some properties like the time-averaged ion mean velocity. Yet, the model fails at reproducing the time evolution of the ion velocity. This fact reveals a complex underlying physics that necessitates to account for the electron dynamics over a short time-scale. This study also shows the necessity for electron temperature measurements. Moreover, the strength of the self-magnetic field due to the rotating Hall current is found negligible.

Multi-scale temporal stability analysis of surface and subsurface soil moisture within the Upper Cedar Creek Watershed, Indiana

Soil moisture plays a significant role in determining the amount of energy exchange between the atmosphere and the earth's surface and is highly variable in space and time. Temporal stability analysis (TSA) is a statistical approach for describing the persistence of spatial patterns and characteristic behavior of soil moisture. Using TSA, this study is aimed at determining the adequacy of long term point-scale surface and subsurface soil moisture (θ v ) measurements in representing field and watershed scale averages that serve as in situ ground truth locations for remotely sensed soil moisture calibration and validation programs, as well as applications for hydrologic modeling . In two agricultural fields, twenty temporary frequency-domain reflectometry (FDR) soil moisture sensors, spaced 70 m apart, were installed at depths of 5 and 20 cm in each field with measurements transmitted every 30 min from June 29 through September 21, 2010. Soil moisture data were also obtained from FDR sensors permanently installed at depths of 5 and 20 cm at seven sites located within the USDA, Upper Cedar Creek Watershed (UCCW) monitoring network in northeastern Indiana. Additionally, meteorological data (i.e., rainfall, air temperature, humidity) were obtained from existing UCCW network weather stations . Spatiotemporal analysis revealed persistent patterns in surface soil moisture and identified sites that were temporally stable at both study scales. However, soil water patterns differed between preferred states (wet/dry) and were primarily controlled by lateral and vertical fluxes. At the field scale, locations that were optimal for estimating the area-average water contents were different from the permanent sensor locations. However, minimum offset values could be applied to the permanent sensor data to obtain representative field average values of surface θ v . TSA of 20 cm θ v showed little correlation with surface θ v TSA results in terms of comparable stable sites or vertical transferability at either scale. The results are of relevance for the interpretation, scaling, or in describing the variability of coarser resolution soil moisture data such as that retrieved from remotely sensed active and passive microwave platforms and in terms of modeling field and watershed scale soil moisture based on point measurements. ► Evaluation of multi-scale in situ soil monitoring network. ► Investigations are needed to address the influence of scale both spatially and temporally. ► Permanent 5 and 20 cm soil moisture sites did not represent local time-averaged field scale conditions. ► Development of enhanced geospatial scaling methods and measurement technologies and strategies.

Study of modeling unsteady blade row interaction in a transonic compressor stage part 1: code development and deterministic correlation analysis

The average-passage equation system (APES) provides a rigorous mathematical framework for accounting for the unsteady blade row interaction through multistage compressors in steady state environment by introducing deterministic correlations (DC) that need to be modeled to close the equation system. The primary purpose of this study is to provide insight into the DC characteristics and the influence of DC on the time-averaged flow field of the APES. In Part 1 of this two-part paper, firstly a 3D viscous unsteady and time-averaging flow CFD solver is developed to investigate the APES technique. Then steady and unsteady simulations are conducted in a transonic compressor stage. The results from both simulations are compared to highlight the significance of the unsteady interactions. Furthermore, the distribution characteristics of DC are studied and the DC at the rotor/stator interface are compared with their spatial correlations (SC). Lastly, steady and time-averaging (employing APES with DC) simulations for the downstream stator alone are conducted employing DC derived from the unsteady results. The results from steady and time-averaging simulations are compared with the time-averaged unsteady results. The comparisons demonstrate that the simulation employing APES with DC can reproduce the time-averaged field and the 3D viscous time-averaging flow solver is validated.

Heat transfer processes in parallel-plate heat exchangers of thermoacoustic devices – numerical and experimental approaches

This paper addresses the issues of heat transfer in oscillatory flow conditions, which are typically found in thermoacoustic devices. The analysis presented concerns processes taking place in the individual “channels” of the parallel-plate heat exchangers (HX), and is a mixture of experimental and numerical approaches. In the experimental part, the paper describes the design of experimental apparatus to study the thermal-fluid processes controlling heat transfer in thermoacoustic heat exchangers on the micro-scale of the individual channels. Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) techniques are applied to obtain spatially and temporally resolved temperature and velocity fields within the HX channels. The temperature fields allow obtaining the local and global, phase-dependent heat transfer rates and Nusselt numbers, and their dependence on the Reynolds number of the oscillating flow. The numerical part of the paper deals with the implementation of CFD modelling capabilities to capture the physics of thermal-fluid processes in the micro-scale and to validate the models against the experimental data. A two-dimensional low Mach number computational model is implemented to analyse the time-averaged temperature field and heat transfer rates in a representative domain of the HXs. These are derived by integrating the thermoacoustic equations of the standard linear theory into a numerical calculus scheme based on the energy balance. The comparisons between the experimental and numerical results in terms of temperature and heat transfer distributions suggest that the optimal performance of heat exchangers can be achieved when the gas displacement amplitude is close to the length of hot and cold heat exchanger. Heat transfer coefficients from the gas-side can be predicted with a confidence of about 40% at moderate acoustic Reynolds numbers.

Internal energy flows and instantaneous field of a monochromatic paraxial light beam

It is known that the orbital angular momentum of a paraxial light beam is related to the rotational features of the instantaneous optical-frequency oscillation pattern within the beam cross section [J. Opt. A 11, 094004 (2009)]. Now this conclusion is generalized: any identifiable directed motion of the instantaneous two-dimensional pattern of the field oscillations ("running" behavior of the instant oscillatory pattern) corresponds to the transverse energy flow in the experimentally observable time-averaged field. The transverse orbital flow density can be treated as a natural geometric and kinematic characteristic of this running behavior.

A calibration scheme for quantitative concentration measurements using simultaneous PIV and PLIF

A new approach for calibration of planar laser-induced fluorescence (PLIF) measurements is presented. The calibration scheme is based on the fact that there is a constant concentration flux through each cross-section of a fluorescent plume in a given flow field and makes use of simultaneous measurements of particle image velocimetry (PIV) and PLIF. The following are the advantages of the current technique: (1) it is experimentally less demanding and (2) it does not require in situ calibration for generating the calibration curves. The technique can be implemented in many experimental setups (both in water and gaseous flows) provided the geometry of the time-averaged scalar field is known. Using the calibration scheme, an analysis is carried out on the measurements of concentration fields in grid turbulence to validate the proposed technique. To demonstrate the feasibility of the scheme, the distributed second-order moments (μ2), and concentration and velocity correlations (\( \left\langle {u^{\prime}c^{\prime}} \right\rangle \) and \( \left\langle {v^{\prime}c^{\prime}} \right\rangle \)) are computed. Good agreement is found with previous studies. In addition, a quantitative appraisal of a simple closure approximation of the moment-based transport equation is also presented using simultaneous PIV and PLIF.

A Combined Measurement Method with PIV and Schlieren Technology on under Expanded Supersonic Jet

A combined measurement method with PIV and schlieren technology is described in this article. The velocity field and the shock wave structure can be obtained simultaneously with this combined measurement method by a synchronous time controller and the post-processing. The color schlieren can capture the instantaneous image effectively and clearly, while the PIV can calculate the time average velocity field and put out with the visual image. Both of them are ripe technologies, they can setup easily and even get some commercial groupwares. In this article, this combined measurement method study the under expanded supersonic jet successfully. Not only limit in the under expanded supersonic jet, this combined measurement method also can apply in the other supersonic flow. They are complementary advantaged to the investigation of supersonic flow.

Paleosecular variation during the Kiaman Superchron: a 1150 year record from glacial varves of the Seaham Formation, New South Wales

Glacial varves of the Seaham Formation at Abercrombie Quarry in eastern New South Wales, Australia, preserve a reverse-polarity characteristic remanence acquired at high paleolatitude during the early stages of the Kiaman Superchron, the longest known interval of constant geomagnetic polarity. Normal polarity overprinting, including the pervasive Cretaceous overprint commonly observed on the southeastern Australian seaboard, is present but does not obscure the primary magnetisation. Ferrimagnetic grains are in the pseudo-single domain size range, confirming the reliability of the varves as paleo-remanence recorders. Rip-up clasts maintain the original direction of the chararacteristic remanence despite their random orientation following redeposition, a form of conglomerate test that restricts the timing of remanence acquisition to within at most a few years of original sedimentation. Characteristic remanence directions from 11 sites spanning 1150 years of annual layers move through a simple, systematic cycle, resembling patterns of secular variation observed for the modern geomagnetic field. Mean virtual geomagnetic poles (VGPs) calculated for the sites trace part of a secular variation cycle around the Upper Carboniferous paleomagnetic pole for the region. The mean of site poles at this locality is at longitude 132.8°E, latitude 31.1°S, with A 95 = 21.3°, although this pole does not span sufficient secular variation to represent a true paleomagnetic pole. The angular standard deviation of the site VGPs exceeds both the standard Model G phenomenological description of the modern field and the Time-Averaged Field Initiative results for comparable latitudes, and appears to agree with the original interpretation of non-suppressed paleosecular variation from late Kiaman, low latitude sites in France. Large scatter in the Abercrombie Quarry poles supports the interpretation that the primary (dipole) family dominates the geomagnetic field during a superchron, and that this condition was already established within the first 11 million years of its 52 million year extent.

Large-Eddy Simulation of Turbulent Natural Convection in Vertical Parallel-Plate Channels

A numerical investigation examining natural convection in a vertical parallel-plate channel with the simultaneous presence of laminar, transitional, and turbulent regimes is conducted using large-eddy simulation. The compressible three-dimensional Favre-filtered mass, momentum, and energy conservation equations are closed using the Smagorinsky and the Vreman subgrid-scale models. A two-stage predictor-corrector numerical methodology for low-Mach-number compressible flows is adopted. Time-averaged wall temperature and field profiles are well captured by the Vreman model, while the Smagorinsky model underpredicts wall behavior considerably. It is demonstrated that the present code is capable of capturing the flow development which is unachievable by conventional Reynolds-averaged Navier-Stokes approaches.

Spin-Motive Force Caused by Vortex Gyration in a Circular Nanodisk with Holes

Spin-motive force has drawn attention because it contains a fundamental physical property. Spin-motive force creates effective electric and magnetic fields in moving magnetization; a vortex is a plausible system for observing the spin-motive force because of the abrupt profile of magnetization. However, the time-averaged value of a spin-motive force becomes zero when a vortex core undergoes gyroscopic motion. By means of micromagnetic simulation , we demonstrates that a non-zero time-averaged electric field induced by spin-motive force under certain conditions. We propose an experimental method of detecting spin-motive force that provides a better understanding of spin transport in ferromagnetic system.

Study of Characteristics of the Radio-Frequency Sheath over a Substrate with a Circular Trench

Since processed substrates usually exhibit nonplanar surface structures in micro-electro-mechanical-systems (MEMS) etching, a two-dimensional (2D) fluid model is developed to simulate the characteristics of the sheath near a conductive substrate with a circular trench, which is placed in an argon discharge powered by a radio-frequency (RF) current source. The model consists of 2D time-dependent fluid equations, the Poisson equation, and a current balance equation that can self-consistently determine the instantaneous voltage on the substrate placed on a powered electrode. The effects of both the aspect ratio (depth/width) and the structure of the trench on the characteristics of the sheath are simulated. The time-averaged potential and electric field in the sheath are calculated and compared for different discharge parameters. The results show that the radial sheath profile is not uniform and always tends to adapt to the contour of the substrate, which is believed to be the moulding effect. Affected by the structure of the substrate surface, the potential and electric field near the inner and outer sidewalls of the trench exhibit obvious non-uniformity, which will inevitably lead to non-uniformity in etching, such as notching. Furthermore, with a fixed amplitude of the RF current source, the potential drops and the sheath thickness decrease with an increase in aspect ratio.

Development of a nonlinear eddy-viscosity closure for the triple-decomposition stability analysis of a turbulent channel

The analysis of the instabilities in an unsteady turbulent flow is undertaken using a triple decomposition to distinguish between the time-averaged field, a coherent wave and the remaining turbulent scales of motion. The stability properties of the coherent scale are of interest. Previous studies have relied on prescribed constants to close the equations governing the evolution of the coherent wave. Here we propose an approach where the model constants are determined only from the statistical measures of the unperturbed velocity field. Specifically, a nonlinear eddy-viscosity model is used to close the equations, and is a generalisation of earlier linear eddy-viscosity closures. Unlike previous models the proposed approach does not assume the same dissipation rate for the time- and phase-averaged fields. The proposed approach is applied to a previously published turbulent channel flow, which was harmonically perturbed by two vibrating ribbons located near the channel walls. The response of the flow was recorded at several downstream stations by phase averaging the probe measurements at the same frequency as the forcing. The experimentally measured growth rates and velocity profiles, are compared to the eigenvalues and eigenvectors resulting from the stability analysis undertaken herein. The modes recovered from the solution of the eigenvalue problem, using the nonlinear eddy-viscosity model, are shown to capture the experimentally measured spatial decay rates and mode shapes of the coherent scale.

Equatorial paleosecular variation of the geomagnetic field from 0 to 3 Ma lavas from the Galapagos Islands

Complete progressive thermal demagnetization of nearly 400 oriented samples from 58 sites (lava flows) from the Galapagos Islands of Santa Cruz, San Cristobal and Floreana provide data for the statistical characterization of the time-averaged geomagnetic field near the Equator for the past few million years. Estimates of VGP dispersion due to paleosecular variation range from 9.2° to 11.8° depending on site selection criteria; our preferred estimate based on 64 site VGPs (51 accepted from this study and 13 from the 1971 study by Cox) is 11.4° (95% confidence interval 10.2–13.0°), consistent with previous estimates from the Galapagos Islands as well as paleosecular variation Model G, and confirming that angular dispersion of VGPs near the Equator is relatively low. The mean direction is not significantly different from a geocentric axial dipole field when account is taken of southward plate motion over the Galapagos hotspot. Preliminary paleointensity results from a comparison of the natural remanence with a total thermal remanence produced in a lab field of 15 μT on a subset of 321 samples from 48 sites that had relatively small changes in magnetic susceptibility after laboratory heating suggest that the time-averaged field was about 21 μT, or only two-thirds the present strength, in agreement with some other recent estimates.

Axial impeller selection for anchorage dependent animal cell culture in stirred bioreactors: Methodology based on the impeller comparison at just-suspended speed of rotation

Animal cells, which are nowadays essential for the industrial production of proteinic compounds, are commonly cultivated inside stirred tank bioreactors. In case of anchorage dependent cells, they are usually fixed on microcarriers. The choice of agitation conditions (impeller type, rotational speed…) in this type of process is not an easy task as it has to fulfil three potentially conflicting goals: (1) maintaining microcarriers in complete suspension, (2) homogenizing the culture medium, and (3) limiting mechanical constraints generated by the hydrodynamics on the cells. The aim of this study is to present an original methodology to select the most appropriate axial impeller for this specific application. Seven propellers are preselected on basis of their characteristics available in the literature. Instead of comparing impellers at a given rotational speed or a given power input, they are compared at their respective minimum impeller rotational speed that leads to a complete microcarrier suspension, i.e. at their respective just-suspended speed N js . They are then compared at higher rotational speeds N, expressed as multiples of N js . The impeller classification is based on the intensity of mechanical constraints they produced, evaluated from: (1) the macro-shear rate quantified by the spatial derivative of time average velocity fields measured by P.I.V., (2) the micro-shear rate characterized by the ratio between the microcarrier diameter to the average Kolmogorov scale computed from power input measurements, and (3) the impact of microcarrier collisions on cells described via the turbulent collision severity index also computed from power input measurements. Results show that the 125 mm diameter TTP impeller (Mixel) and the 150 mm diameter Elephant Ear impeller (Applikon) produce the smallest mechanical constraints at their just-suspended speed (50 and 20 rpm, respectively). Moreover, the mechanical constraints they produce increase more slowly with the N/ N js ratio than the mechanical constraints produced by other impellers. These propellers are thus even more advantageous if rotational speeds higher than the just-suspended speed have to be used.

Observations and Models of the Long-Term Evolution of Earth’s Magnetic Field

The geomagnetic signal contains an enormous temporal range—from geomag- netic jerks on time scales of less than a year to the evolution of Earth's dipole moment over billions of years. This review compares observations and numerical models of the long-term range of that signal, for periods much larger than the typical overturn time of Earth's core. On time scales of 10 5 -10 9 years, the geomagnetic field reveals the control of mantle ther- modynamic conditions on core dynamics. We first briefly describe the general formalism of numerical dynamo simulations and available paleomagnetic data sets that provide insight into paleofield behavior. Models for the morphology of the time-averaged geomagnetic field over the last 5 million years are presented, with emphasis on the possible departures from the geocentric axial dipole hypothesis and interpretations in terms of core dynamics. We discuss the power spectrum of the dipole moment, as it is a well-constrained aspect of the geomagnetic field on the million year time scale. We then summarize paleosecular varia- tion and intensity over the past 200 million years, with emphasis on the possible dynamical causes for the occurrence of superchrons. Finally, we highlight the geological evolution of the geodynamo in light of the oldest paleomagnetic records available. A summary is given in the form of a tentative classification of well-constrained observations and robust numerical modeling results.

Finite-bias electronic transport of molecules in a water solution

The effects of water wetting conditions on the transport properties of molecular nano-junctions are investigated theoretically by using a combination of classical molecular dynamics and first principles electronic transport calculations. These are at the level of the non-equilibrium Green's function method implemented for self-interaction corrected density functional theory. We find that water effectively produces electrostatic gating to the molecular junction, with a gating potential determined by the time-averaged water dipole field. Such a field is large for the polar benzene-dithiol molecule, resulting in a transmission spectrum shifted by about 0.6 eV with respect to that of the dry junction. The situation is drastically different for carbon nanotubes (CNTs). In fact, because of their hydro-phobic nature the gating is almost negligible, so that the average transmission spectrum of wet Au/CNT/Au junctions is essentially the same as that in dry conditions. This suggests that CNTs can be used as molecular interconnects also in water-wet situations, for instance as tips for scanning tunnel microscopy in solution or in biological sensors.

Electromagnetic field energy in dispersive materials

A general expression for the electromagnetic energy density in a lossy dispersive medium, applicable for a field having a narrow temporal frequency bandwidth, is derived and compared with exact results for an example dielectric constant. Consequently, the possibility of negative time-averaged stored field energy is shown to have physical meaning. This observation is of interest in the study of dispersive metamaterials, such as those which can exhibit a negative refractive index.

Climate model errors, feedbacks and forcings: a comparison of perturbed physics and multi-model ensembles

Ensembles of climate model simulations are required for input into probabilistic assessments of the risk of future climate change in which uncertainties are quantified. Here we document and compare aspects of climate model ensembles from the multi-model archive and from perturbed physics ensembles generated using the third version of the Hadley Centre climate model (HadCM3). Model-error characteristics derived from time-averaged two-dimensional fields of observed climate variables indicate that the perturbed physics approach is capable of sampling a relatively wide range of different mean climate states, consistent with simple estimates of observational uncertainty and comparable to the range of mean states sampled by the multi-model ensemble. The perturbed physics approach is also capable of sampling a relatively wide range of climate forcings and climate feedbacks under enhanced levels of greenhouse gases, again comparable with the multi-model ensemble. By examining correlations between global time-averaged measures of model error and global measures of climate change feedback strengths, we conclude that there are no simple emergent relationships between climate model errors and the magnitude of future global temperature change. Algorithms for quantifying uncertainty require the use of complex multivariate metrics for constraining projections.

Uniform-field transverse electroreflectance using a mode-locked laser and a radio-frequency bias

We describe a technique for transverse electroreflectance that uses broadband optical pulses from a mode-locked laser and a rapidly oscillating bias with a frequency near half the laser repetition rate. Since the electrodes are electrically isolated from the sample, highly nonuniform trap-enhanced fields do not occur. Because the time-averaged electric field is zero, space charges do not build up. Experimental results are presented for semi-insulating GaAs. The technique is also useful in ultrafast experiments that require a uniform transverse electric field.

Palaeomagnetism of the Western and Central sectors of the Trans-Mexican volcanic belt-implications for tectonic rotations and palaeosecular variation in the past 11 Ma

SUMMARY New palaeomagnetic data from the central and western sectors of the Trans-Mexican volcanic belt (TMVB) are presented and analysed for palaeosecular variation (PSV) and tectonic rotations. Fifty-one sites with geochronological control were collected from selected volcanic lava flows, which cover the temporal and spatial activity in the magmatic arc for the past ∼11 Ma. Rock magnetic experiments reveal that generally Ti-poor titanomagnetites, sometimes maghemitized, are the magnetic carriers of the characteristic remanent magnetizations. After analysis of detailed progressive demagnetization data, 47 mean-site directions are determined. In addition, data from previous regional palaeomagnetic study in the eastern TMVB sector (53 sites) are re-analysed. Palaeomagnetic data are grouped according to geographic distribution (three structural sectors) and according to age (late Miocene, Pliocene or Quaternary). To avoid discarding tectonic effects, the less stringent criterion (i.e. a fixed cut-off angle to the mean equal to 45°) was applied to identify transitional data, affecting four sites. Palaeomagnetic results, backed by positive reversal tests, indicate no palaeomagnetically detectable vertical-axis block rotations in the study areas. The nearly geocentric axial dipole (GAD) inclinations suggest no significant tilting effects and negligible quadrupolar contributions. Since the mean palaeomagnetic directions for the study areas do not differ from reference directions from the North American polar wander path, two data sets for 11–5 and 5–0 Ma are tested for geomagnetic purposes. Virtual geomagnetic poles (VGPs) were selected using both fixed −45°- and optimum, variable cut-off angles. This resulted in VGP dispersions that increase back on time and are consistent with those obtained from globally distributed observations at the site latitude for their respective age ranges and cut-off criteria. Palaeomagnetic data from late Miocene and Pliocene TMVB rocks can be considered in the databases for time averaged field (TAF) and palaeosecular variation from lavas (PSVL) analyses and geomagnetic field geometry characterization during the past ∼11 Ma.