Time-averaged Field Research Articles

Computation of instantaneous and time-averaged active acoustic intensity field around rotating source

A vector aeroacoustics method is developed to analyze the acoustic energy flow path from the rotating source. In this method, the instantaneous and time-averaged active acoustic intensity vectors are evaluated from the time-domain and frequency-domain acoustic pressure and acoustic velocity formulations, respectively. With the above method, the acoustic intensity vectors and the acoustic energy streamlines are visualized to investigate the propagation feature of the noise radiated from the monopole and dipole point sources and the rotor in subsonic rotation. The result reveals that a portion of the acoustic energy spirals many circles before moving towards the far field, and another portion of the acoustic energy firstly flows inward along the radial direction and then propagates along the axial direction. Further, an acoustic black hole exists in the plane of source rotation, from which the acoustic energy cannot escape once the acoustic energy flows into it. Moreover, by visualizing the acoustic intensity field around the rotating sources, the acoustic-absorption performance of the acoustic liner built in the casing and centerbody is discussed.

Insights from geodynamo simulations into long-term geomagnetic field behaviour

Detailed knowledge of the long-term spatial configuration and temporal variability of the geomagnetic field is lacking because of insufficient data for times prior to 10 ka. We use realisations from suitable numerical simulations to investigate three important questions about stability of the geodynamo process: is the present field representative of the past field; does a time-averaged field actually exist; and, supposing it exists, how long is needed to define such a field. Numerical geodynamo simulations are initially selected to meet existing criteria for morphological similarity to the observed magnetic field. A further criterion is introduced to evaluate similarity of long-term temporal variations. Allowing for reasonable uncertainties in the observations, observed and synthetic axial dipole moment frequency spectra for time series of order a million years in length should be fit by the same power law model. This leads us to identify diffusion time as the appropriate time scaling for such comparisons. In almost all simulations, intervals considered to have good morphological agreement between synthetic and observed field are shorter than those of poor agreement. The time needed to obtain a converged estimate of the time-averaged field was found to be comparable to the length of the simulation, even in non-reversing models, suggesting that periods of stable polarity spanning many magnetic diffusion times are needed to obtain robust estimates of the mean dipole field. Long term field variations are almost entirely attributable to the axial dipole; nonzonal components converge to long-term average values on relatively short timescales (15–20 kyr). In all simulations, the time-averaged spatial power spectrum is characterised by a zigzag pattern as a function of spherical harmonic degree, with relatively higher power in odd degrees than in even degrees. We suggest that long-term spatial characteristics of the observed field may emerge on averaging times that are within reach for the next generation of global time-varying paleomagnetic field models.

Stability of the laminar flow on a body of revolution at incidence

An asymmetric incompressible gas flow past a body of revolution was studied in a subsonic wind tunnel. Characteristics of the velocity field at the stern of the experimental model, where a laminar boundary-layer separation and concomitant destabilization of the flow occur under axisymmetric conditions, were determined. Declination of the axis of symmetry of the body within several angular degrees results in a radical change of the time-average velocity field, the amplitude distribution of growing hydrodynamic disturbances, and their spectral distribution.

A new model for the (geo)magnetic power spectrum, with application to planetary dynamo radii

We propose two new analytical expressions to fit the Mauersberger–Lowes geomagnetic field spectrum at the core–mantle boundary. These can be used to estimate the radius of the outer liquid core where the geodynamo operates, or more generally the radius of the planetary dynamo regions. We show that two sub-families of the geomagnetic field are independent of spherical harmonics degree n at the core–mantle boundary and exhibit flat spectra. The first is the non-zonal field, i.e., for spherical harmonics order m different from zero. The second is the quadrupole family, i.e., n+m even. The flatness of their spectra is motivated by the nearly axisymmetric time-average paleomagnetic field (for the non-zonal field) and the dominance of rotational effects in core dynamics (for the quadrupole family). We test our two expressions with two approaches using the reference case of the Earth. First we estimate at the seismic core radius the agreement between the actual spectrum and the theoretical one. Second we estimate the magnetic core radius, where the spectrum flattens. We show that both sub-families offer a better agreement with the actual spectrum compared with previously proposed analytical expressions, and predict a magnetic core radius within less than 10 km of the Earth's seismic core radius. These new expressions supersede previous ones to infer the core radius from geomagnetic field information because the low degree terms are not ignored. Our formalism is then applied to infer the radius of the dynamo regions on Jupiter, Saturn, Uranus and Neptune. The axisymmetric nature of the magnetic field of Saturn prevents the use of the non-zonal expression. For the three other planets both expressions converge and offer independent constraints on the internal structure of these planets. These non-zonal and quadrupole family expressions may be implemented to extrapolate the geomagnetic field spectrum beyond observable degrees, or to further regularize magnetic field models constructed from modern or historical observations.

Full counting statistics of quantum dot resonance fluorescence.

The electronic energy levels and optical transitions of a semiconductor quantum dot are subject to dynamics within the solid-state environment. In particular, fluctuating electric fields due to nearby charge traps or other quantum dots shift the transition frequencies via the Stark effect. The environment dynamics are mapped directly onto the fluorescence under resonant excitation and diminish the prospects of quantum dots as sources of indistinguishable photons in optical quantum computing. Here, we present an analysis of resonance fluorescence fluctuations based on photon counting statistics which captures the underlying time-averaged electric field fluctuations of the local environment. The measurement protocol avoids dynamic feedback on the electric environment and the dynamics of the quantum dot's nuclear spin bath by virtue of its resonant nature and by keeping experimental control parameters such as excitation frequency and external fields constant throughout. The method introduced here is experimentally undemanding.

입자영상유속계를 이용한 자항상태 모형선의 프로펠러 후류 계측

A two-dimensional particle image velocimetry (2D PIV) system in a towing tank is employed to measure a wake field of a very large crude oil carrier model with rotating propeller in self propulsion condition, to identify characteristics of wake of a propeller working behind a ship. Phase-averaged and time-averaged flow fields are measured for a horizontal plane. Scale ratio of the model ship is 1/100 and Froude number is 0.142. By phase-averaging technique, trajectories of tip vortex and hub vortex are identified and characteristic secondary vortex distribution is observed in the hub vortex region. Propeller wake on the starboard side is more accelerated than that on the port side, due to the difference of inflow of propeller blades. The hub vortex trajectory tends to face the port side. With the fluctuation part of the phase-averaged velocity field, turbulent kinetic energy (TKE) is also derived. In the center of tip vortex and hub vortex region, high TKE concentration is observed. In addition, a time-averaged vector field is also measured and compared with phase-averaged vector field.

Extended order parameter and conjugate field for the dynamic phase transition in a Ginzburg-Landau mean-field model in an oscillating field.

We present numerical evidence for an extended order parameter and conjugate field for the dynamic phase transition in a Ginzburg-Landau mean-field model driven by an oscillating field. The order parameter, previously taken to be the time-averaged magnetization, comprises the deviations of the Fourier components of the magnetization from their values at the critical period. The conjugate field, previously taken to be the time-averaged magnetic field, comprises the even Fourier components of the field. The scaling exponents β and δ associated with the extended order parameter and conjugate field are shown numerically to be consistent with their values in the equilibrium mean-field model.

Development of a Turning Mid Turbine Frame With Embedded Design—Part I: Design and Steady Measurements

The paper presents a new setup for the two-stage two-spool facility located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) of Graz University of Technology. The rig was designed in order to simulate the flow behavior of a transonic turbine followed by a counter-rotating low pressure (LP) stage like the spools of a modern high bypass aeroengine. The meridional flow path of the machine is characterized by a diffusing S-shaped duct between the two rotors. The role of turning struts placed into the mid turbine frame is to lead the flow towards the LP rotor with appropriate swirl. Experimental and numerical investigations performed on the setup over the last years, which were used as baseline for this paper, showed that wide chord vanes induce large wakes and extended secondary flows at the LP rotor inlet flow. Moreover, unsteady interactions between the two turbines were observed downstream of the LP rotor. In order to increase the uniformity and to decrease the unsteady content of the flow at the inlet of the LP rotor, the mid turbine frame was redesigned with two zero-lifting splitters embedded into the strut passage. In this first part of the paper the design process of the splitters and its critical points are presented, while the time-averaged field is discussed by means of five-hole probe measurements and oil flow visualizations. The comparison between the baseline case and the embedded design configuration shows that the new design is able to reduce the flow gradients downstream of the turning struts, providing a more suitable inlet condition for the low pressure rotor. The improvement in the flow field uniformity is also observed downstream of the turbine and it is, consequently, reflected in an enhancement of the LP turbine performance. In the second part of this paper the influence of the embedded design on the time-resolved field is investigated.

0605 アブレシブファンジェットの速度場計測

Water jets issuing from a fan jet nozzle (fan jets) are used in radiological decontamination, coating removal, and cleaning. However, the flow characteristics of fan jet have not been clarified sufficiently. An experimental study is conducted to clarify the flow structures and the velocity fields of fan jet and abrasive suspension fan jet issuing from a fan jet nozzle. The velocity fields of the fan jets and the abrasive fan jets are calculated using PIV (Particle image velocimetry). It is found that the time average velocity field of the abrasive fen jet is lower than that of the fan jet.

Approximating hemodynamics of cerebral aneurysms with steady flow simulations

Computational fluid dynamics (CFD) simulations can be employed to gain a better understanding of hemodynamics in cerebral aneurysms and improve diagnosis and treatment. However, introduction of CFD techniques into clinical practice would require faster simulation times. The aim of this study was to evaluate the use of computationally inexpensive steady flow simulations to approximate the aneurysm's wall shear stress (WSS) field. Two experiments were conducted. Experiment 1 compared for two cases the time-averaged (TA), peak systole (PS) and end diastole (ED) WSS field between steady and pulsatile flow simulations. The flow rate waveform imposed at the inlet was varied to account for variations in heart rate, pulsatility index, and TA flow rate. Consistently across all flow rate waveforms, steady flow simulations accurately approximated the TA, but not the PS and ED, WSS field. Following up on experiment 1, experiment 2 tested the result for the TA WSS field in a larger population of 20 cases covering a wide range of aneurysm volumes and shapes. Steady flow simulations approximated the space-averaged WSS with a mean error of 4.3%. WSS fields were locally compared by calculating the absolute error per node of the surface mesh. The coefficient of variation of the root-mean-square error over these nodes was on average 7.1%. In conclusion, steady flow simulations can accurately approximate the TA WSS field of an aneurysm. The fast computation time of 6min per simulation (on 64 processors) could help facilitate the introduction of CFD into clinical practice.

Development of a Pulsed-Field Differential Mobility Analyzer: A Method for Measuring Shape Parameters for Nonspherical Particles

For a nonspherical particle, a standard differential mobility analyzer (DMA) measurement yields a mobility-equivalent spherical diameter, but provides no information about the degree of sphericity. However, given that the electrical mobility for nonspheres is orientation-dependent, and that orientation can be manipulated using electric fields of varying strength, one can, in principle, extract some type of shape information through a systematic measurement of mobility as a function of particle orientation. Here, we describe the development of a pulsed-field differential mobility analyzer (PFDMA) which enables one to change the peak E-field experienced by the particle to induce orientation, while still maintaining the same time-averaged field strength as a standard DMA experiment. The instrument is validated with polystyrene latex (PSL) spheres with accurately known size, and gold rods with dimensions accurately determined by transmission electron microscopy (TEM). We demonstrate how the instrument can be used for particle separation and extraction of shape information. In particular, we show how one can extract both length and diameter information for rod-like particles. This generic approach can be used to obtain dynamic shape factors or other multivariate dimensional information (e.g., length and diameter). Copyright 2014 American Association for Aerosol Research

Ensembles of low degree archeomagnetic field models for the past three millennia

We introduce ensembles of time-varying archeomagnetic field models, consisting of a reference model, a mean model and a thousand individual models. We present a set of three such ensembles, built from archeomagnetic, volcanic and sedimentary data sets, that cover the past three millennia. These ensembles can be used to describe the field at any location from the core surface to the magnetosphere, and assess the way uncertainties due to the limited distribution and quality of the data affect any of its component or parameter, such as individual Gauss coefficients. They provide alternative – and, we argue, more complete – descriptions of the archeomagnetic field to those provided by previously published archeomagnetic field models, being better suited to existing and emerging needs, such as those of geomagnetic data assimilation. We present the data sets we rely on – essentially the same as those used by other recent archeomagnetic field models – and describe how errors affecting the data, and errors due to non-modelled small spatial scales of the field, are taken into account. We next explain our modeling strategy and motivation for building low degree spherical harmonic degree ensembles of models. We carry on a number of end-to-end simulations to both illustrate the usefulness of such ensembles and point at the type of errors one should expect. Practical illustrations of what can be done with these three ensembles of models, with examples of geomagnetic inferences, are also described. Northern high-latitude flux patches, for instance, appear to be the most robust features of all. These patches tend to fluctuate, but clearly have some favored locations, resulting in the same clear signature with three tongues (over Northern America, Europe and Asia) in the time-averaged field at the core-mantle boundary, similar to what had been found in earlier models. Inferences about the field behavior in the Southern hemisphere are more difficult to draw. Still, some suggestions that the well-known present South Atlantic reversed patch could have arisen as early as in 1500 A.D. are found in some of the ensembles. We otherwise confirm that most of the current archeomagnetic field model limitations are related to a number of sediment cores, identified as producing frequent outliers in the modeling process. We provide evidence that such cores are likely affected by timing errors and timing delays between magnetization lock-in and sediment deposition, that future more advanced treatment should be able to handle. All source files for the three ensembles of models, together with appropriate Matlab applications can be downloaded from http://geomag.ipgp.fr/download/ARCHEO_FM.zip.

Large-eddy simulation of mixing and combustion in a premixed swirling combustor with synthesis gases

A lean-premixed swirling combustor with synthesis gases is studied in non-reacting and reacting cases using large-eddy simulation. Code validation and grid dependence test are performed to validate the models and mesh resolution. With the introduction of unmixedness and correlation coefficients in the non-reacting cases, the influence of Reynolds number on recirculation, vorticity breakdown and mixing is studied. In reacting cases, instant and time-averaged scalar fields are examined to study the flame dynamics for a varied fuel composition and operating conditions. The results reveal the effects of hydrogen concentration, Reynolds number, equivalence ratio and pressure on the combustion processes. Conclusions for syngas combustion operation from this work are expected to provide useful information for gas turbine combustor design.

Resolving the stratification discrepancy of turbulent natural convection in differentially heated air-filled cavities. Part III: A full convection–conduction–surface radiation coupling

The present study concerns an air-filled differentially heated cavity of 1m×0.32m×1m (width×depth×height) subject to a temperature difference of 15K and is motivated by the need to understand the persistent discrepancy observed between numerical and experimental results on thermal stratification in the cavity core. An improved experiment with enhanced metrology was set up and experimental data have been obtained along with the characteristics of the surfaces and materials used. Experimental temperature distributions on the passive walls have been introduced in numerical simulations in order to provide a faithful prediction of experimental data. By means of DNS using spectral methods, heat conduction in the insulating material is first coupled with natural convection in the cavity. As heat conduction influences only the temperature distribution on the top and bottom surfaces and in the near wall regions, surface radiation is added to the coupling of natural convection with heat conduction. The temperature distribution in the cavity is strongly affected by the polycarbonate front and rear walls of the cavity, which are almost black surfaces for low temperature radiation, and also other low emissivity walls. The thermal stratification is considerably weakened by surface radiation. Good agreement between numerical simulations and experiments is observed on both time-averaged fields and turbulent statistics. Treating the full conduction–convection–radiation coupling allowed to confirm that experimental wall temperatures resulted from the coupled phenomena and this is another way to predict correctly the experimental results in the cavity.

Experimental and numerical study of solids circulation in gas-vibro fluidized beds

The effects of vertical, sinusoidal vibrations on the fluidization behavior of a granular bed are studied, where we focus on the solid circulation rate and its dependence on the vibration parameters. To this end, an experimental study was performed in a pseudo-2D bed, and the optical non-invasive measurement technique particle image velocimetry was employed to obtain whole field velocity information. Additionally, discrete element model (DEM) simulations were performed and the time-averaged fields were compared. In both simulations and experiments, it was found that the solid circulation rate at constant vibration frequency increases with acceleration, in agreement with other experimental findings reported in literature. Gas-vibro fluidization generally exhibits larger circulation rates as compared to conventional gas-fluidization.

PROPAGATION OF LASER BEAMS IN AN ARRAY OF SEMICONDUCTOR CARBON NANOTUBES

In this paper, we consider propagation of a monochromatic laser beam in an array of semiconductor carbon nanotubes. Initial distribution of the beam intensity is taken in the form of a Gaussian profile in the plane perpendicular to the wave vector. The electromagnetic field in an array of nanotubes is described by Maxwell equations, reduced to a multidimensional wave equation. With an approximation of the slowly varying amplitudes and phases, we derive the effective equation describing the time-averaged field intensity distribution of the laser beam in a medium. Numerical solution of the derived equations allows us to analyze the dependence of the diffractive spreading of the beam on its frequency and initial amplitude. Furthermore, the influence of the nanotube radius on the diffractive spreading of the laser beam is investigated.

Thermal boundary layers in turbulent Rayleigh–Bénard convection at aspect ratios between 1 and 9

We report highly resolved temperature measurements in turbulent Rayleigh–Bénard convection in air at a fixed Prandtl number Pr = 0.7. Extending our previous work (du Puits et al 2007 J. Fluid Mech.572 231–54), we carried out measurements at various aspect ratios while keeping the Rayleigh number constant. We demonstrate that the temperature field inside the convective boundary layers of both horizontal plates is virtually independent on the global flow pattern accompanying the variation in the aspect ratio. Thanks to technical upgrades of the experimental facility as well as a significant improvement of the accuracy and reliability of our temperature measurement—and unlike in our previous work—we find that the measured profiles of the time-averaged temperature field neither follow a clear power-law trend nor fit a linear or a logarithmic scaling over a significant fraction of the boundary-layer thickness. Analyzing the temperature data simultaneously acquired at both horizontal plates, various transitions in the cross-correlation and the auto-correlation function of the temperature signals are observed while varying the aspect ratio Γ. These transitions might be associated with a change in the global flow pattern from a single-roll mode at Γ = 1 toward a double- or a multi-roll mode pattern at higher aspect ratios.

Controls on geomagnetic reversals and core evolution by mantle convection in the Phanerozoic

Numerical dynamos driven by non-uniform heat flux at the core-mantle boundary are used to investigate the connections between geomagnetic field structure, geomagnetic reversal frequency, core evolution, and mantle convection through Phanerozoic time. Polarity reversal sequences and time average magnetic field structures are calculated using dynamos driven by two representations of lower mantle history: a reconstruction of mantle convection with plate motions by Zhang and Zhong (2011) that produces time variable core-mantle boundary (CMB) heat flux and an irregular evolution of the core, and a second model based on hotspot locations with a time independent pattern of CMB heat flux derived from the present-day seismic shear wave heterogeneity of the lower mantle that produces a monotonic evolution of the core. For both mantle histories, present-day values of the dynamo control parameters are tuned to match Geomagnetic Polarity Time Scale reversal statistics for 0–5Ma, and the time dependences of the dynamo control parameters are determined from the thermal evolution of the core, including time variability of CMB heat flow, inner core size, inner core chemical buoyancy flux, and rotation rate. The dynamo with time independent CMB heat flux shows minor fluctuations in reversal frequency with age, whereas the dynamo with time variable CMB heat flux shows reversal rate fluctuations including stable polarity at 275 and 475Ma and frequent reversals at other times. This dynamo also produces departures from geocentric axial dipole symmetry during the time of supercontinent Pangaea and a heterogeneous growth history of the inner core.

Algorithm Base on Gamma-Ray Computed Tomography Method for Gas-Solid Flow Time Average Concentration in Cyclone

A new approach to the gamma-ray densitometry tomography(GDT) technique was proposed for measurement of gas-solid flow in cyclone, where the time average concentration field can be simplified to continuous function and symmetry system, based on only one bunch of through the regional center’s ray and another distance the center d position’s ray data, through the measurement of body. So it realized faster data acquisition and image reconstruction, and possessed both the continuous function model’s high gradient of radial concentration distribution and highly accuracy. Due to the collected data were severely diminished, the method sharply reduced the cost of image scans mobile system. The numerical simulation proved the accuracy of that algorithm base on gamma-ray computed tomography method for gas-solid time average concentration field in cyclone

Potential for small scale added value of RCM’s downscaled climate change signal

In recent decades, the need of future climate information at local scales have pushed the climate modelling community to perform increasingly higher resolution simulations and to develop alternative approaches to obtain fine-scale climatic information. In this article, various nested regional climate model (RCM) simulations have been used to try to identify regions across North America where high-resolution downscaling generates fine-scale details in the climate projection derived using the “delta method”. Two necessary conditions were identified for an RCM to produce added value (AV) over lower resolution atmosphere-ocean general circulation models in the fine-scale component of the climate change (CC) signal. First, the RCM-derived CC signal must contain some non-negligible fine-scale information—independently of the RCM ability to produce AV in the present climate. Second, the uncertainty related with the estimation of this fine-scale information should be relatively small compared with the information itself in order to suggest that RCMs are able to simulate robust fine-scale features in the CC signal. Clearly, considering necessary (but not sufficient) conditions means that we are studying the “potential” of RCMs to add value instead of the AV, which preempts and avoids any discussion of the actual skill and hence the need for hindcast comparisons. The analysis concentrates on the CC signal obtained from the seasonal-averaged temperature and precipitation fields and shows that the fine-scale variability of the CC signal is generally small compared to its large-scale component, suggesting that little AV can be expected for the time-averaged fields. For the temperature variable, the largest potential for fine-scale added value appears in coastal regions mainly related with differential warming in land and oceanic surfaces. Fine-scale features can account for nearly 60 % of the total CC signal in some coastal regions although for most regions the fine scale contributions to the total CC signal are of around ∼5 %. For the precipitation variable, fine scales contribute to a change of generally less than 15 % of the seasonal-averaged precipitation in present climate with a continental North American average of ∼5 % in both summer and winter seasons. In the case of precipitation, uncertainty due to sampling issues may further dilute the information present in the downscaled fine scales. These results suggest that users of RCM simulations for climate change studies in a delta method framework have little high-resolution information to gain from RCMs at least if they limit themselves to the study of first-order statistical moments. Other possible benefits arising from the use of RCMs—such as in the large scale of the downscaled fields– were not explored in this research.