Variation Of Momentum Research Articles

The angular momentum of dark haloes: merger and accretion effects

We present new results on the angular momentum evolution of dark matter haloes. Haloes, from N-body simulations, are classified according to their mass growth histories into two categories: the accretion category contains haloes whose mass has varied continuously and smoothly, while the merger category contains haloes which have undergone sudden and significant mass variations (greater than 1/3 of their initial mass per event). We find that the angular momentum grows in both cases, well into the non-linear regime. For individual haloes we observe strong correlation between the angular momentum variation and the mass variation. The rate of growth of both mass and angular momentum has a characteristic transition time at around z ∼ 1.5-1.8, with an early fast phase followed by a late slow phase. Haloes of the merger catalogue acquire more angular momentum even when the scaling with mass is taken into account. The spin parameter has a different behaviour for the two classes: there is a decrease with time for haloes in the accretion catalogue whereas a small increase is observed for the merger catalogue. When the two catalogues are considered together, no significant variation of the spin parameter distribution with the redshift is obtained. We have also found that the spin parameter neither depends on the halo mass nor on the cosmological model. From our simple model developed for the formation of a disc galaxy similar to the Milky Way, we conclude that our own halo must have captured satellites in order to acquire the required angular momentum and to achieve most of the disc around z ∼ 1.6. The distribution of the angular momentum indicates that at z ∼ 1.6 only 22 per cent of the haloes have angular momentum of magnitude comparable to that of disc galaxies in the mass range 10 10 -5 x 10 11 M ○. , clearly insufficient to explain the present observed abundance of these objects.

Decaying quasi-two-dimensional turbulence in a rectangular container: laboratory experiments

Results from a new series of experiments on quasi-two-dimensional turbulence decaying in a rectangular container are presented. The flows are generated electromagnetically in a thin layer of conducting fluid when different configurations of permanent magnets are used to specify the initial characteristics of the flow. The particle image velocimetry method is used to determine the velocity and vorticity fields. These fields are further used to determine the global characteristics of the flow such as the energy, enstrophy and the net angular momentum with respect to the center of the container. The experimental results demonstrate that variations of net angular momentum occur due to the interactions of coherent vortex dipoles/jets with the walls of the container. The variation of the angular momentum of the system, which occurs due to the action of stresses at the solid boundary, can be obtained quantitatively if the initial linear impulse exerted by the magnets on the fluid is known. An analysis of external forces and torques allows one to predict that during the intermediate state of the flow evolution the pattern of the flow will be a rotating quadrupole. The energy spectra of the flow demonstrates upscale energy transfer and corresponding growth of the energy-weighted mean scale. Power law exponents are obtained for both the low and high wave number regions of the spectra. Growth of the Reynolds number of the flow was observed during the intermediate phase of the flow evolution.

Superfluid in the Space Between Rotating Spheres II: The Shape of Vortices and Nonequilibrium States

Vortex dynamics is used to consider the following problems concerning vortices in the space between concentric spheres: the equilibrium shape of vortices (they are almost rectilinear if their number is large), the metastable clusters with nonequilibrium number of vortices, the processes of vortex breaking and connection in the vicinity of the inner sphere equator, the annihilation in the vicinity of the outer sphere equator, and the behavior of an almost freely rotating double-sphere device during the variation of the superfluid angular momentum caused by the changes of vortex configuration (similar phenomena are among the possible reasons of pulsars self-acceleration).

Contribution of Oceanic and Atmospheric Excitations to the Chandler Wobble

AbstractBy using the data series of oceanic and atmospheric angular momentum during 1985~1995, mean energies of the oceanic and atmospheric excitation in Chandler wobble band (CWB) are estimated and compared with that of the geodetically observed excitation. Results show that the variation of oceanic angular momentum (OAM) can provide about 64% of the excitation energy in CWB with a dominant contribution of the oceanic bottom pressure, while the excitation energies provided by atmospheric angular momenta (AAM (NCEP/NCAR) and AAM (JMA)) are about 23% and 214% in comparison with the observed one. Squared coherence, 0.52 (0.32 and 0.37), are also estimated between the observed excitation and the OAM (AAM (NCEP/NCAR) and AAM (JMA) excitations. Coherent phases show a lagging phase about 19° for OAM excitation, leading phases about 47° for AAM (NCEP/NCAR) and 19° for AAM (JMA), relative to the observed excitation, respectively. Analyses of some shorter (9 and 6 years.) and longer (16 and 39 years.) data series indicate that either the excitation energies, or the squared coherence are temporally variable, and their variability are of, in some extent, stochastic property.

Two-dimensional hydrodynamic model including inertia effects in carrier momentum for power millimetre-wave semi-conductor devices

A two dimensional-hydrodynamic model is carried out to predict the performance of short gate length power field effect transistors based on III–V semi-conductor systems. The model is based on the conservation equations, deduced from the Boltzmann transport equation, solved in their whole form by taking temporal and spatial variations in carrier momentum into account. This model is well suited to get accurate predictions for power devices having various gate recess topologies. Results are performed for devices with different gate lengths and compared with those obtained from simplified models. For gate lengths shorter than 0.5 μm, a drop in the overshoot velocity phenomenon involves a better estimation of the characteristics of the device.

Temporal Variation of Angular Momentum in the Solar Convection Zone

We derive the angular momentum as a function of radius and time with the help of the rotation rates resulting from inversions of helioseismic data obtained from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) and the density distribution from a model of the Sun. The base of the convection zone can be identified as a local maximum in the relative angular momentum after subtracting the contribution of the solid-body rotation. The angular momentum as a function of radius shows the strongest temporal variation near the tachocline. This variation extends into the lower convection zone and into the radiative interior and is related to the 1.3 yr periodicity found in the equatorial rotation rate of the tachocline. In the upper convection zone, we find a small systematic variation of the angular momentum that is related to torsional oscillations. The angular momentum integrated from the surface to a lower limit in the upper convection zone provides a hint that the torsional oscillation pattern extends deep into the convection zone. This is supported by other quantities such as the coefficients of a fit of Legendre polynomials to the rotation rates as a function of latitude. The temporal variation of the coefficient of P4, indicative of torsional oscillations, suggests that the signature of these flows in the inversion results extend to about r ≈ 0.83 R☉. With the lower limit of integration placed in the middle or lower convection zone, the angular momentum fluctuates about the mean without apparent trend, i.e., the angular momentum is conserved within the measurement errors. However, when integrated over the layers slightly below the convection zone (0.60-0.71 R☉), the angular momentum shows the 1.3 yr period and hints at a long-term trend that might be related to the solar activity cycle.

Features of the Observed Annual Ocean–Atmosphere Flux Variability on the West Florida Shelf

The annual cycle of sea surface temperature and ocean–atmosphere fluxes on the west Florida shelf is described using in situ measurements and climatology. Seasonal reversals in water temperature tendency occur when the net surface heat flux changes sign in boreal spring and fall. Synoptic-scale variability is also important. Momentum and heat flux variations result in successive water column stratification and destratification events, particularly at shallower depths during spring. Fall is characterized by destratification of the water column and a series of steplike decreases in the temperature. These are in response to both tropical storms and extratropical fronts. Tropical storms are responsible for the largest momentum fluxes, but not necessarily for the largest surface heat fluxes. A one-dimensional analysis of the temperature equation suggests that surface heat flux is primarily responsible for the spring and fall seasonal ocean temperature changes, but that synoptic-scale variability is also controlled by the ocean circulation dynamics. During summer, the situation is reversed and the major influence on water temperature is ocean dynamics, with the heat flux contributing to the synoptic-scale variability. There is also evidence of interannual variability: the wintertime temperatures get increasingly colder from 1998 to 2000, and the greatest stratification and coldest subsurface temperatures occur in 1998. NCEP–NCAR reanalysis fields do not reproduce the high spatial flux variability observed in situ or with satellite measurements. Reconciling these differences and their impacts on the climate variability of this region provides challenges to coupled ocean–atmosphere models and their supporting observing systems.

Systems of Gyrodynes with an Additional Degree of Freedom

The systems of gyrodynes installed on a spacecraft body on the one-axis controlled platform are considered. An algorithm to determine the special states and the special surfaces corresponding to them in the region of variation of the total angular momentum is obtained, as well as an algorithm to study the permeability of special surfaces. The results of calculation of the special surfaces for two types of gyro systems are presented.

Atmospheric, hydrological and oceanic comprehensive contributions to seasonal polar wobble of Earth Rotation

The geophysical quantitative excitation on seasonal polar wobble of Earth Rotation has not been well achieved so far. The atmospheric, hydrologic and oceanic angular momentum variations are investigated from monthly values simulated by a coupled ocean-atmosphere general circulation model. The simulated equatorial AAM functions agree well with that from the JMA operational analysis in 90°E direction, but disagree along Greenwich meridian. As for the annual cycle, not only the hydrologic and oceanic excitations partly match the residuals between geodetic functions of polar wobble and JMA AAM functions, but also the combinations with NCEP and JMA analysis AAM functions are better than those estimated from NCAR-CSM1 climate model.

Energy and angular momentum radiated for non-head-on binary black hole collisions

We investigate the possible total radiated energy produced by a binary black hole system containing non-vanishing total angular momentum. For the scenearios considered we find that the total radiated energy does not exceed 1%. Additionally we explore the gravitational radiation field and the variation of angular momentum in the process.

Pacific warm pool excitation, earth rotation and El Niño southern oscillations

The interannual changes in the Earth's rotation rate, and hence in the length of day (LOD), are thought to be caused by the variation of the atmospheric angular momentum (AAM). However, there is still a considerable portion of the LOD variations that remain unexplained. Through analyzing the non‐atmospheric LOD excitation contributed by the Western Pacific Warm Pool (WPWP) during the period of 1970–2000, the positive effects of the WPWP on the interannual LOD variation are found, although the scale of the warm pool is much smaller than that of the solid Earth. These effects are specifically intensified by the El Niño events, since more components of the LOD‐AAM were accounted for by the warm pool excitation in the strong El Niño years. Changes in the Earth's rotation rate has attracted significant attention, not only because it is an important geodetic issue but also because it has significant value as a global measure of variations within the hydrosphere, atmosphere, cryosphere and solid Earth, and hence the global changes.

S Equulei: A Low Mass Ratio Algol‐Type Eclipsing Binary at the Active Phase of Mass Transfer

Orbital period change of a low mass ratio (q = 0.13) Algol-type eclipsing binary, S Equulei, is presented based on the analysis of its century-long times of light minimum. Alternate period changes are discovered to superimpose on a rather rapid period increase with rate of dP/dt = +1.27 x 10(-6) days yr(-1). The alternate period changes can be explained either by the variation in the gravitational quadruple momentum via the cyclic magnetic activity of the G-type secondary or by the light-time effect via the presence of a third body. Since the third-body assumption is in good agreement with Zola's photometric solution, S Equ may be a truly triple system. The third body is rotating in an eccentric orbit (e' = 0.37) with an orbital inclination of i' - 16degrees, which is rather smaller than that of the eclipsing binary system (i = 88.degrees3). This indicates that the third body may be captured by the eclipsing pair. New accurate photometric and spectroscopic data over the next decade are very important to verify this conclusion. The rapid period increase suggests that S Equ is a low mass ratio Algol-type binary system in an active phase of mass exchange. To satisfy such a period increase, a conservative mass transfer from the less massive to the more massive components would be of the order dm/dt = 4.94 x 10(-8) M. yr(-1).

The special features of the ground state in CeAl3

The temperature (3–60 K) and transferred momentum (0.3–2.3 A−1) dependences of the intensity of quasi-elastic magnetic neutron scattering were studied for the polycrystalline heavy-fermion CeAl3 compound to elucidate the special features of its ground state. Transferred momentum variations caused oscillations of the intensity of quasi-elastic magnetic neutron scattering, which was evidence of magnetic correlations in the f-electron subsystem occurring in a fairly wide temperature range.

Oceanic torques on solid Earth and their effects on Earth rotation

We calculate that the oceanic torque vectors (x, y, z components) acting on the solid Earth using the Parallel Ocean Climate Model (POCM) output data at 3‐day intervals for the period 1988–1995. This study examined three distinct types of torque, with different physical mechanisms: pressure, gravitational, and frictional. The pressure torque is further divided into two parts: that due to Earth's ellipticity and that due to the ocean bottom topography. According to resultant time series the ellipticity pressure torque causes the largest effect for the x and y components. The gravitational torque and the ellipticity pressure torque are shown numerically to be exactly proportional by a factor of −0.49. The topographic pressure torque is somewhat smaller in overall amplitude than the ellipticity‐induced torque (the sum of the ellipticity pressure torque and gravitational torque). The ocean bottom frictional torque is found to be negligible in all three components. To validate the torque calculations, we compare the calculated oceanic torques with effective torque derived from the ocean angular momentum (OAM) variation, both from POCM. It is shown that the effective torque associated with the mass term of the OAM agrees remarkably well with the ellipticity‐induced torque as required by theory. For the motion term the effective torque can be explained mostly by the topographic pressure torque in the intraseasonal periods, but the seasonal and longer‐period components show significant differences between the two. The difference should theoretically be attributed to the exclusion of the wind frictional torque on the ocean. However, a comparison based on the NASA GEOS‐1 atmospheric model shows that the wind frictional torque obtained is insufficient in power in the x and y components. Finally, in the application to the Earth rotation excitation it is found that both the ellipticity and topographic torques have comparable contributions to polar motion excitation for both seasonal and nonseasonal variations. For the length of day variation, our torque approach is presently inadequate because the oceanic contribution is relatively too small to be estimated in the presence of data and model errors.

Influence of the seasonal winds and the CO2 mass exchange between atmosphere and polar caps on Mars' rotation

The Martian atmosphere and the CO2 polar ice caps exchange mass. This exchange, together with the atmospheric response to solar heating, induces variations of the rotation of Mars. Using the angular momentum budget equation of the system solid‐Mars–atmosphere–polar ice caps, the variations of Mars' rotation can be deduced from the variations of the angular momentum of the superficial layer; this later is associated with the winds, that is, the motion term, and with the mass redistribution, that is, the matter term. For the “mean” Martian atmosphere, without global dust storms, total amplitudes of 10 cm on the surface are obtained for both the annual and semiannual polar motion excited by the atmosphere and ice caps. The atmospheric pressure variations are the dominant contribution to these amplitudes. Length‐of‐day (lod) variations have amplitudes of 0.253 ms for the annual signal and of 0.246 ms for the semiannual signal. The lod variations are mainly associated with changes in the atmospheric contribution to the mass term, partly compensated by the polar ice cap contribution. We computed lod variations and polar motion for three scenarios having different atmospheric dust contents. The differences between the three sets of results for lod variations are about one order of magnitude larger than the expected accuracy of the NEtlander Ionosphere and Geodesy Experiment (NEIGE) for lod. It will thus be possible to constrain the global atmospheric circulation models from the NEIGE measurements.

Low‐frequency intraseasonal variations of the wintertime very high latitude mesopause regions

We have performed a spectral analysis of three fall and winter airglow data sets each for South Pole Station and for Eureka, Canada (80°N). These analyses show significant spectral features in the mesopause region at very high latitudes that are similar in period to low‐frequency intraseasonal (IS) variations seen in analysis of length of day, atmospheric angular momentum variations, and outgoing long wave radiation, as well as in height or wind fields up to 10 hPa (∼32 km). These are fairly broadband spectral features centered approximately at 17, 23, and 45 days. The 17‐day oscillation we find is more prominent than that found in the data sets reflecting variations mainly in the troposphere and stratosphere. This oscillation is most likely associated with the second symmetric s = 1 free Rossby mode. The data from South Pole Station should not reflect this mode, as such, because the spectral components that produce airglow variations should vanish at the pole unless they are zonally symmetric; however, free oscillations having zonal wave number zero with such long periods should not exist. We suggest that, at least at South Pole Station, the 17‐day oscillation is driven by interactions between the global Rossby mode and stationary wave number one planetary waves. In addition to the low‐frequency oscillations that have been reported in the IS literature, we see a wave in the 12–14 day period range. At South Pole Station that wave may also reflect forcing by a global free mode.

Transformation of elastic wave energy to the energy of motion of bodies

The motion of a body along an elastic guide under the effect of an incident wave is considered. An equation describing the longitudinal motion of a body along an arbitrary guide is derived from the laws governing the energy and momentum variations for the case when the incident wave generates a single reflected wave. The equations that describe the motion of a body along a string and along a beam corresponding to the Bernoulli-Euler model are considered as examples. The process of the body acceleration along a beam of the aforementioned type is investigated. For the subcritical velocities, the law governing the motion of the body and the ratio of the kinetic energy variation to the energy supplied to the body are determined.

Measurements of concentration per size class in a dense polydispersed jet using planar laser-induced fluorescence and phase Doppler techniques

In dense two-phase flows, it is well known that phase Doppler anemometry is not well suited for the measurement of concentration and mass flux. Laser diagnostics based on fluorescence can provide the dispersed phase concentration but without discrimination between size classes. We present a new method of coupling the two techniques, in order to extract the local value of concentration and flux per size class. The method is applied to an axisymmetric turbulent jet, laden with polydispersed droplets 1–90 μm. Droplet concentration profiles are obtained in the development zone (x/d0 < 20) of the dense jet and are used to study droplet dispersion. The results are then introduced into the momentum transport equations to analyze the influence of droplets on the carrier phase. We show that the local decrease of the rate of variation of mean momentum with mass loading is due both to an increase in interfacial transfer rate and to a decrease in turbulent diffusion effects.

“Physical process version” of the first law and the generalized second law for charged and rotating black holes

We investigate both the ``physical process'' version of the first law and the second law of black hole thermodynamics for charged and rotating black holes. We begin by deriving general formulas for the first order variation in ADM mass and angular momentum for linear perturbations off a stationary, electrovac background in terms of the perturbed non-electromagnetic stress-energy, $\delta T_{ab}$, and the perturbed charge current density, $\delta j^a$. Using these formulas, we prove the "physical process version" of the first law for charged, stationary black holes. We then investigate the generalized second law of thermodynamics (GSL) for charged, stationary black holes for processes in which a box containing charged matter is lowered toward the black hole and then released (at which point the box and its contents fall into the black hole and/or thermalize with the ``thermal atmosphere'' surrounding the black hole). Assuming that the thermal atmosphere admits a local, thermodynamic description with respect to observers following orbits of the horizon Killing field, and assuming that the combined black hole/thermal atmosphere system is in a state of maximum entropy at fixed mass, angular momentum, and charge, we show that the total generalized entropy cannot decrease during the lowering process or in the ``release process''. Consequently, the GSL always holds in such processes. No entropy bounds on matter are assumed to hold in any of our arguments.

Diurnal variations in soil temperature, momentum and sensible heat fluxes at Anand during LASPEX-97

Diurnal variations in soil temperature, momentum and sensible heat fluxes at Anand during LASPEX-97