Variation Of Momentum Research Articles

Determination of the Earth's pole tide Love number k2 from observations of polar motion using an adaptive Kalman filter approach

The geophysical interpretation of observed time series of Earth rotation parameters (ERP) is commonly based on numerical models that describe and balance variations of angular momentum in various subsystems of the Earth. Naturally, models are dependent on geometrical, rheological and physical parameters. Many of these are weakly determined from other models or observations. In our study we present an adaptive Kalman filter approach for the improvement of parameters of the dynamic Earth system model DyMEG which acts as a simulator of ERP. In particular we focus on the improvement of the pole tide Love number k2. In the frame of a sensitivity analysis k2 has been identified as one of the most crucial parameters of DyMEG since it directly influences the modeled Chandler oscillation. At the same time k2 is one of the most uncertain parameters in the model. Our simulations with DyMEG cover a period of 60 years after which a steady state of k2 is reached. The estimate for k2, accounting for the anelastic response of the Earth's mantle and the ocean, is 0.3531 + 0.0030i. We demonstrate that the application of the improved parameter k2 in DyMEG leads to significantly better results for polar motion than the original value taken from the Conventions of the International Earth Rotation and Reference Systems Service (IERS).

Uncertainties in relative atmospheric angular momentum computed from zonal winds in reanalysis data

Numerous fields of computational Earth system research require a precise account of variations in the relative atmospheric angular momentum (AAM). Since zonal winds from reanalysis data sets of ERA‐40 (ECMWF) and NCEP1 (NCEP/NCAR) are frequently used to compute relative AAM estimates it is critically important to understand uncertainties in these data products. Here, we discuss the varying degree of structural and internal uncertainties associated with ERA‐40 and NCEP1 zonal wind data in the tropical Pacific troposphere and lower stratosphere. We assess these uncertainties by quantifying differences in relative AAM computed from reanalyses wind data (1958–2000), that are subsequently compared to a 10‐year record of radiosonde observations carried out in the tropical Pacific where reanalyses data differ most. While our analysis suggests that in general differences between reanalysis data and observed tropical winds in the Pacific are not significantly different from each other, we found varying degrees of discrepancies at each radiosonde site. Zonal winds in NCEP1 data (RMS = 1.84 ms−1) reproduce less bias in tropospheric easterly winds than ERA‐40 (RMS = 3 ms−1), while for the lower stratosphere ERA‐40 data fit better westerly winds (RMS = 3.95 ms−1) than NCEP1 (RMS = 5.03 ms−1). Anomalies in zonal tropical wind fields can be induced by large mountain and friction torques that efficiently affect Earth's rotation rate. Hence, a precise account of the wind term in reanalysis data directly determines the accuracy of computed estimates of relative AAM.

Spin-to-Orbital Angular Momentum Conversion and Spin-Polarization Filtering in Electron Beams

We propose the design of a space-variant Wien filter for electron beams that induces a spin half-turn and converts the corresponding spin angular momentum variation into orbital angular momentum of the beam itself by exploiting a geometrical phase arising in the spin manipulation. When applied to a spatially coherent input spin-polarized electron beam, such a device can generate an electron vortex beam, carrying orbital angular momentum. When applied to an unpolarized input beam, the proposed device, in combination with a suitable diffraction element, can act as a very effective spin-polarization filter. The same approach can also be applied to neutron or atom beams.

Effect of Unsteadiness and Nozzle Asymmetry on Thrust of a Microthruster

Due to the small size and short propulsion pulses of solid propellant microthrusters, unsteady characteristics and/or fabrication errors may cause appreciable effects on the total thrust. The present work simulates a 2D planar and conical nozzle of 200 μm throat diameter using the traditional momentum equations and turbulence closure model. Unsteady characteristics were found to be insensitive to the inlet mass flux but sensitive to the chamber size. The characteristic time of transient momentum variations in the nozzle was found to be on the order of 0.1 ms, and that for decay was smaller than that for development by about 25%. This work confirmed that the magnitude of thrust was not sensitive to distortions in the nozzle geometry. A mismatch of the nozzle throat changed the position of sonic plane, but its effect on thrust was almost negligible.

SOFT SENSORS FOR SPLITTER PRODUCT PROPERTY ESTIMATION IN CDU

Soft sensor application for properties estimation of splitter bottom product in a crude distillation unit (CDU) is investigated. Based on continuous temperature, pressure, and flow measurements, two soft sensors are developed as estimators of the initial boiling point and end boiling point of splitter product. Soft sensor models are developed using multiple regression techniques and neural networks. After performing multiple linear regression analysis, it was concluded that linear models are not sufficiently accurate for the implementation in the real plant. Within multilayer perceptron (MLP) and radial basis function (RBF) neural networks, different learning algorithms are used (back propagation with variations of learning rate and momentum, conjugate gradient descent, Levenberg-Marquardt) as well as pruning and Weigend regularization techniques. Statistics and sensitivity analysis are provided for both models. Two developed soft sensors will be used as on-line estimators of heavy naphtha properties and for control purposes.

Design of an ejector cycle refrigeration system

A design procedure, based on a one-dimensional simulation, is presented for an ejection refrigeration system. Heat exchangers are included in the calculation, accounting for temperature differences between the fluids and for pressure losses. The ideal gas assumption, which is quite common in the literature concerning ejector systems, is avoided. Furthermore, the supersonic diffuser is designed with a continuous profile, without cylindrical piece, controlling the variation of momentum along the flow path and accounting for friction. At design conditions, this should reduce the irreversibility due to the normal shock. A comparison between different operating fluids is presented and R245fa is selected. The results of the design procedure and the expected performance, in terms of first and second law efficiency, are presented.

The axial dipole strength and flow in the outer core

From 1590 to 1833, although extensive measurements of the geomagnetic field were made, no direct measurements of its intensity are available; field models for this period either make use of indirect (palaeomagnetic) intensity determinations or assume a similar change in dipole strength to that seen presently. Here, we examine the impact of different choices of field model strength on models of the core-surface flow, derived from the geomagnetic secular variation. We find that for a reasonable range of possible changes in the field strength, the variability that this unknown imposes on the flow structure is small compared with other choices in the modelling strategy, such as the degree to which the geomagnetic models are fit, the smoothness of the flows, and a priori information included to reduce non-uniqueness in the flow calculation. The flows predict variations in core angular momentum of similar magnitude to those seen in recent times. These variations are larger than seen in models from geodetic data, but do not seem inconsistent with the geodetic data themselves. We therefore suggest that it is reasonable to study core flows back at least until 1650, thereby doubling the time interval for which flows can be considered to constrain the behaviour of the geodynamo.

해안지형의 복잡성에 따른 해풍침투 변화가 대기경계층에 미치는 영향 분석

In order to clarify the relation between sea breeze penetration and Planetary Boundary Layer development in southeastern part of the Korean Peninsula, several numerical assessments were carried out using atmospheric numerical model WRF(Weather Research and Forecasting). Compared with onset time of sea breeze at eastern coast area(Uljin), the time at southern coast region(Masan) with complex costal line tend to delay for several hours. The penetration patterns of sea breeze between two coastal regions are some different due to the shape of their coastal line and back ground topography. Intensified valley wind due to high topography of lee side of Uljin can help penetration of sea breeze at early time. So penetration of sea breeze at early time often prevent PBL to develop at Uljin and lower PBL height last for a day time. But because of late penetration of sea breeze at Masna, PBL Height dramatically decrease after 1500LST. The distribution of front genesis function based on the heat and momentum variation are explained obviously the sea breeze penetration patterns and agreed well with the PBL height distribution.

The electric dipole form factor of the nucleon in chiral perturbation theory to sub-leading order

The electric dipole form factor (EDFF) of the nucleon stemming from the QCD θ¯ term and from the quark color-electric dipole moments is calculated in chiral perturbation theory to sub-leading order. This is the lowest order in which the isoscalar EDFF receives a calculable, non-analytic contribution from the pion cloud. In the case of the θ¯ term, the expected lower bound on the deuteron electric dipole moment is |dd|≳1.4⋅10−4θ¯e fm. The momentum dependence of the isovector EDFF is proportional to a non-derivative time-reversal-violating pion-nucleon coupling, and the scale for momentum variation—appearing, in particular, in the radius of the form factor—is the pion mass.

Relation between length‐of‐day variation and angular momentum of geophysical fluids

The remarkable relation well observed between variation in the length of day (ΔLOD) and variation in the axial atmospheric angular momentum (ΔAAM) (plus the oceanic counterpart, to a much lesser extent) is a consequence of the conservation of angular momentum on planet Earth. Quantification of the exact ΔLOD‐ΔAAM relation, which we seek in the present study, depends significantly on the extent to which the core participates, or is dynamically coupled with the mantle, in transmission of the axial ΔAAM from the mantle to the core. If, after conversion to the equivalent ΔLOD assuming core‐mantle decoupling (as in current standard practice), the calculated axial ΔAAM (according to atmospheric general circulation models [GCMs]) is systematically greater than the observed ΔLOD, then we can conclude the presence, and, furthermore, estimate the strength, of the said dynamic core‐mantle coupling. However, in this study we find the opposite instead, that the calculated ΔAAM (plus the small oceanic counterpart) is smaller than the observed ΔLOD by 10%–20% consistently across the intraseasonal and seasonal time scales. Our main logical conclusion is that the atmospheric GCMs in general underpredict the ΔAAM, by at least 10%–20% at the mentioned time scales, a fact of importance with respect to the assessment of GCMs. Therefore, the systematic discrepancy found between the ΔAAM‐predicted and the observed ΔLOD masks the relevant information on core‐mantle coupling that we seek.

Atmospheric angular momentum variations of Earth, Mars and Venus at seasonal time scales

Atmospheric angular momentum variations of a planet are associated with the global atmospheric mass redistribution and the wind variability. The exchange of angular momentum between the fluid layers and the solid planet is the main cause for the variations of the planetary rotation at seasonal time scales. In the present study, we investigate the angular momentum variations of the Earth, Mars and Venus, using geodetic observations, output of state-of-the-art global circulation models as well as assimilated data. We discuss the similarities and differences in angular momentum variations, planetary rotation and angular momentum exchange for the three terrestrial planets. We show that the atmospheric angular momentum variations for Mars and Earth are mainly annual and semi-annual whereas they are expected to be “diurnal” on Venus. The wind terms have the largest contributions to the LOD changes of the Earth and Venus whereas the matter term is dominant on Mars due to the CO 2 sublimation/condensation. The corresponding LOD variations (Δ LOD) have similar amplitudes on Mars and Earth but are much larger on Venus, though more difficult to observe.

The 52-week high momentum strategy in international stock markets

We study the 52-week high momentum strategy in international stock markets proposed by George and Hwang [George, T., Hwang, C.Y., 2004. The 52-week high and momentum investing. Journal of Finance 59, 2145–2176.]. This strategy produces profits in 18 of the 20 markets studied, and the profits are significant in 10 markets. The 52-week high momentum profits exist independently from the Jegadeesh and Titman [Jegadeesh, N., Titman, S., 1993. Returns to buying winners and selling losers: implications for market efficiency. Journal of Finance 48, 65–91.] individual stock and Moskowitz and Grinblatt [Moskowitz, T.J., Grinblatt, M., 1999. Do industries explain momentum? Journal of Finance 54, 1249–1290] industry momentum strategies. These profits do not show reversals in the long run. We find that the 52-week high is a better predictor of future returns than macroeconomic risk factors or the acquisition price. The individualism index, a proxy to the level of overconfidence, has no explanatory power to the variations of the 52-week high momentum profits across different markets. However, the profits are no longer significant in most markets once transaction costs are taken into account.

Gauging Alignments: An Ethnographically Informed Method for Process Evaluation in a Community-Based Intervention

Abstract: Community-based projects feature multidimensional interventions and interactions within unpredictable contexts. Process evaluations can shed light on variability in outcomes across sites and the reasons why some project outcomes fall short of expectations. The authors present an ethnographically informed study of the interactive project components in a pilot community-based falls prevention project that was implemented in 4 communities across Canada. Ethnographic descriptions and analyses of alignments between multilevelled project components allowed the researchers to better understand the mechanisms of project evolution at each site and variations in project momentum, mobilization, and sustainability across sites. Primary data sources consisted of project teleconference transcripts triangulated with log notes, field notes, and interviews. Descriptions and analyses of alignments may be instrumental to process evaluation. Project adjustments could then be made accordingly in propelling progress toward program objectives, informing program decisions, and in making sense of variability in program outcomes. Further exploration and operationalization of the alignment concept is recommended to advance knowledge about how to conduct process evaluations of complex interventions.

Altimeter-Based Estimates of Eddy Variability and Eddy Transports in the Subpolar North Atlantic

Geostrophic surface velocity anomalies, inferred from the tandem Jason-1- TOPEX/Poseidon (JTP) altimeter mission, are used to analyze space-time variations of eddy kinetic energy (EKE), eddy momentum, and heat transports in the subpolar North Atlantic. The geostrophic currents are estimated using the “parallel-track approach” with a 10 km along-track resolution. The analysis covers the entire three-year period of the tandem mission (109 repeat cycles) from September 2002 to September 2005. In this paper, we used the three-year record of geostrophic surface velocities available from the JTP tandem mission to estimate eddy transport processes in the subpolar North Atlantic and interpret them in combination with similar results available from a high-resolution numerical simulation of the North Atlantic with a 1/12° horizontal resolution for a 25-year period. Due to the sampling characteristics of the JTP tandem mission, only transports of large-scale eddies can be observed. Moreover, the time series are too short to get statistically stable estimates of eddy processes. Nevertheless, distinct estimates of spatially varying eddy processes can be diagnosed from the novel data set, especially when interpreted in combination with the longer model results. Results suggest that eddy forces accelerate the mean flow over large fractions of the subpolar North Atlantic. There is valuable information present in the JTP velocity time series that contribute to an enhanced understanding of eddy dynamics and the representation of eddy heat and momentum transports in numerical models.

Laboratory study of differential rotation in a convective rotating layer

The evolution of a large-scale azimuthal velocity field in a rotating cylindrical layer of fluid (radius 150 mm, depth 30 mm, free upper surface) with meridional convective circulation was studied experimentally. Two cases were considered: inward upper level circulation provided by a rim heater at the periphery and outward upper level circulation provided by a central heater. The heating rate is characterized by the Grashoff number Grf defined through the heat flux. The detailed 3D structure of the mean large-scale velocity field is reconstructed using the particle image velocimetry technique for 105 < Grf < 4 × 107. It was shown that the energy of meridional circulation grows with the Grashoff number as in both directions of circulation. Due to the action of the Coriolis force, the meridional flow results in differential rotation (DR). DR is characterized by values of radial D r and vertical D z gradients of azimuthal velocity. Mean values of D r and D z for different Grf were obtained. Meridional circulation leads to substantial variation of the integral angular momentum. Inward circulation results in the growth of the integral angular momentum and outward circulation causes it to decrease. At the same heating power, the increase of angular momentum at inward circulation is much stronger than its decrease at outward circulation.

On a formulation of meteor physics problems

The general equation describing the motion of a meteoroid in the Earth’s atmosphere is considered. It is shown that, in this case, the momentum variation of the system consisting of the meteoroid and its separating mass is due to the velocity variation of the meteoroid. The projection of Newton’s equation onto the tangent to the meteoroid trajectory is analyzed. It is also shown that, because of large geocentric velocities (between 11 and 72 km/s), the dominant force is the drag force proportional to the square of the incident air flow velocity. Some numerical results obtained for the Bene>sov bolide registered by the European Fireball Network (May 7, 1991) are given as an illustrative example.

Accurate Capture of Propeller-Rudder Interaction using a Coupled Blade Element Momentum-RANS Approach

This paper compares experimental results with numerical simulations using three different body-force propeller models. The results demonstrate that as long as the radial variation of axial and tangential momentum generated by the propeller are included, the influence of the unsteady propeller flow can be removed and calculations with a steady propeller model can be performed for the evaluation of the influence of the propeller on the rudder.

Transition-State Spectroscopy of Partial Wave Resonances in the F + HD Reaction

Partial wave resonances, quasi-bound resonance states with well-defined rotation in the transition state region of a chemical reaction, play a governing role in reaction dynamics but have eluded direct experimental characterization. Here, we report the observation of individual partial wave resolved resonances in the F + HD --> HF + D reaction by measuring the collision energy-dependent, angle- and state-resolved differential cross section with extremely high resolution, providing a spectroscopic probe to the transition state of F + HD --> HF + D. The agreement of the data with the high-level theoretical calculations confirms the sensitivity of this probe to the subtle quantum mechanical factors guiding this benchmark reaction.

Study of singular surfaces of systems of unsupported gyrodynes by the parameter continuation method

A system of two-degree-of-freedom force gyroscopes (gyrodynes) is considered to be used for spacecraft attitude control. Possible values of its total angular momentum form some finite region P in the frame of reference rigidly connected with the spacecraft. Near the boundary of this region and singular surfaces located inside it the control of the angular momentum is complicated or impossible. The program angular momentum of the gyrodynes, realizing the law of variation of the spacecraft orientation, should lie inside P and outside some singular surfaces, and due to this fact the boundary and internal singular surfaces should be studied. This work is dedicated to the numerical construction of region P and its internal singular surfaces by the method of parameter continuation. Using the results by E.N. Tokar’ we formalize sufficient conditions which in some cases allow one to determine the type of the singular surface. As an example, a system of six gyrodynes is considered, for which the regions of variations of the intrinsic angular momentum and singular surfaces are constructed. The possibilities of the system are demonstrated for the case when one gyrodyne fails.

Collisions between spinning and nonspinning co-axial three-dimensional Ginzburg-Landau solitons

We report results of the first analysis of collisions between stable fundamental (alias spinless) and vortical (spinning) three-dimensional dissipative solitons in a model of a laser cavity. The systematic analysis is carried out for values S=1 and S=2 of the vorticity of the latter soliton. With the increase of the collision momentum, Χ, the same generic scenarios are observed in either case: merger into a single fundamental soliton at both small and relatively large values of Χ, and the formation of two fundamental solitons in an intermediate interval of variation of the collision momentum Χ. At very large values of Χ, the collision seems quasi-elastic, but the vortex soliton eventually splits into two nonspinning fragments.