Flows in clumpy planetary nebulae     

T. W. Hartquist  J. E. Dyson 《Astrophysics and Space Science》1996,245(2):263-273

The dynamics of clumps observed in planetary nebulae are considered. The possibility that SiO maser spots in evolved stars and the planetary nebula clumps are formed by the Parker instability behind shocks in pulsating stars' atmospheres is raised. Molecular observations of the clumps are suggested. The effects of the ablation of clumps on the global flow structure of a more tenuous plasma in which they are embedded are reviewed.  相似文献   

The equilibrium configuration of rapid rotating compact stars and some gravitational effects     

Jun Dong  Jing-li Sun  Pei-chen Zhu 《Chinese Astronomy and Astrophysics》1986,10(4):278-286

In this paper, the equilibrium configurations of rapid rotating compact stars and some gravitational effects are studied within the general relativity by use of the Harrison-Wheeler equation of state and by the self-consistent field method. Numerical calculations show that the equilibrium configuration of a rotating star is a spheroid. For large spin velocities, say, ω > 3.0 × 10² sec⁻¹ the eccentricity and mass increase very rapidly as the angular velocity increases, for the critical angular velocity of the rotating star, the eccentricity is about 0.7, the increase in mass is about 10–35%. The difference of the gravitational redshifts at the surface of the star caused by rotation, and the difference of the light bending when the beam moves in the direction of rotation or in the opposite direction are obvious.  相似文献   

黄河三角洲桩西101站潮沟地貌形态及其水动力学研究   总被引：6，自引：1，他引：6  

尹延鸿  成国栋  范顺庭  王玉荣  朱兰部 《海洋科学》1997,21(5):46-49

在潮沟下游，横跨且生趣与潮沟的横断面设了５个观测站，进行了２４ｈ的水动力测量。结果表明，涨落潮流速流向不对称，在潮沟内，涨落潮流流速差别很大，涨潮流流速最大为９ｃｍ／２，而落潮流流速最大可达３８ｃｍ／ｓ。据推测，潮沟内之所以落潮流流速明显大于涨潮流流速，可能是由于潮沟与岸线斜交有关。  相似文献   

Applied hydrodynamic wave-resistance computation by Fourier transform     

Jorge D'Elia  Mario A. Storti  Sergio R. Idelsohn 《Ocean Engineering》2002,29(3):261-278

An applied Fourier transform computation for the hydrodynamic wave-resistance coefficient is shown, oriented to potential flows with a free surface and infinity depth. The presence of a ship-like body is simulated by its equivalent pressure disturbance imposed on the un-perturbed free surface, where a linearized free surface condition is used. The wave-resistance coefficient is obtained from the wave-height downstream. Two examples with closed solutions are considered: a submerged dipole, as a test-case, and a parabolic pressure distribution of compact support. In the three dimensional case, a dispersion relation is included which is a key resource for an inexpensive computation of the wave pattern far downstream like fifteen ship-lengths.  相似文献   

Response-based method for determining the extreme behaviour of floating offshore platforms     

Said Mazaheri  Martine J. Downie 《Ocean Engineering》2005,32(3-4):363-393

The accurate prediction of extreme excursion and mooring force of floating offshore structures due to multi-variete environmental conditions which requires the joint probability analysis of environmental conditions for the worst case situation is still impractical as the processing of large amount of met-ocean data is required. On the other hand, the simplified multiple design criteria (e.g. the N-year wave with associated winds and currents) recommended by API known as traditional method does lead neither to the N-year platform response nor to the N-year mooring force. Therefore, in order to reduce the level of conservatism as well as uncertainties involved in the traditional method the response-based method can be used as a reliable alternative approach. In this paper this method is described. In order to perform the calculations faster using large databases of sea states, Artificial Neural Networks (ANN) is designed and employed. In the paper the response-based method is applied to a 200,000 tdw FPSO and the results are discussed.  相似文献   

Risk assessment of TBT in the Japanese short-neck clam (Ruditapes philippinarum) of Tokyo Bay using a chemical fate model     

Fumio Horiguchi  Kisaburo Nakata  Naganori Ito  Ken Okawa 《Estuarine, Coastal and Shelf Science》2006,70(4):589

A risk assessment of Tributyltin (TBT) in Tokyo Bay was conducted using the Margin of Exposure (MOE) method at the species level using the Japanese short-neck clam, Ruditapes philippinarum. The assessment endpoint was defined to protect R. philippinarum in Tokyo Bay from TBT (growth effects). A No Observed Effect Concentration (NOEC) for this species with respect to growth reduction induced by TBT was estimated from experimental results published in the scientific literature. Sources of TBT in this study were assumed to be commercial vessels in harbors and navigation routes. Concentrations of TBT in Tokyo Bay were estimated using a three-dimensional hydrodynamic model, an ecosystem model and a chemical fate model. MOEs for this species were estimated for the years 1990, 2000, and 2007. Estimated MOEs for R. philippinarum for 1990, 2000, and 2007 were approximately 1–3, 10, and 100, respectively, indicating a declining temporal trend in the probability of adverse growth effects.A simplified software package called RAMTB was developed by incorporating the chemical fate model and the databases of seasonal flow fields and distributions of organic substances (phytoplankton and detritus) in Tokyo Bay, simulated by the hydrodynamic and ecological model, respectively.  相似文献   

Intercomparison of coastal area morphodynamic models     

J. Nicholson  I. Broker  J. A. Roelvink  D. Price  J. M. Tanguy  L. Moreno 《Coastal Engineering》1997,31(1-4)

Five different coastal area morphodynamic models have been set up to run on the same offshore breakwater layout and an intercomparison carried out on the hydrodynamic and morphodynamic output produced by each scheme. In addition, the predicted morphodynamics was checked against available laboratory and field data.It is concluded that the models are capable of producing realistic estimates for the dominant morphodynamic features associated with offshore breakwaters. Coupling of the wave, current and sediment transport components of each scheme is shown to yield bathymetry which attains a state of equilibrium, unlike models which are based on the initial transport field only.  相似文献   

Structure and energy transfer of unstable hot star winds     

A. Feldmeier 《Astrophysics and Space Science》1994,221(1-2):393-408

Due to the instability of the radiation line force, the winds of hot, luminous stars should show a pronounced time-dependence resulting from the nonlinear growth of initially small perturbations. Following the method of Owocki, Castor & Rybicki (1988), we describe the time-dependent wind structure obtained with an independently developed code. Under the central assumption ofisothermality, our results are in very good agreement with the ones by Owocki et al. We find that the response of the wind to periodic base perturbations remains largely periodic, at least up tor 2...3R _* , with no clear evidence of stochastic behaviour.In order to test the foregoing assumption of isothermality and to compute the X-ray emission from models of structured winds, we have also incorporated theenergy equation into our simulations. We encountered the numerical problem that all radiative cooling zones collapse because of the oscillatory thermal instability (cf. Langer et al. 1981). We present a method to hinder this collapse by changing the cooling function at low temperatures. The resulting wind showsresolved cooling zones; but, for a supergiant wind relatively close to the star (r 10R _* ), the macroscopic wind structure is very similar to isothermal calculations. Most of the hot material is caused by shell-shell collisions.  相似文献   

Coalescing neutron stars as gamma ray bursters?     

M. Ruffert  H. -Th. Janka  G. Schäfer 《Astrophysics and Space Science》1995,231(1-2):423-426

We investigate the dynamics and evolution of coalescing neutron stars. The three-dimensional Newtonian equations of hydrodynamics are integrated by the Piecewise Parabolic Method on an equidistant Cartesian grid. The code is purely Newtonian, but does include the emission of gravitational waves and their back-reaction. The properties of neutron star matter are described by the equation of state of Lattimer and Swesty (1991). Energy loss by all types of neutrinos and changes of the electron fraction due to the emission of electron neutrinos and antineutrinos are taken into account by an elaborate neutrino leakage scheme. We simulate the coalescence of two identical, cool neutron stars with a baryonic mass of 1.6M and a radius of 15 km and with an initial center-to-center distance of 42 km. The initial distributions of density and electron concentration are given from a model of a cold neutron star in hydrostatic equilibrium. We investigate three cases which differ by the initial velocity distribution in the neutron stars. The orbit decays due to gravitational-wave emission and after one revolution the stars are so close that dynamical instability sets in. Within 1 ms the neutron stars merge into a rapidly spinning (P 1 ms), high-density body ( 10¹⁴ g/cm³) with a surrounding thick disk of material with densities 10¹⁰ – 10¹² g/cm³ and orbital velocities of 0.3-0.5 c. The peak emission of gravitational waves has a maximum luminosity of a few times 10⁵⁵ erg/s and is reached for about 1 ms. The amplitudes of the gravitational waves are close to 3 10^–23 at a distance of 1 Gpc and the typical frequency is near the dynamical value of the orbital motion of the merging neutron stars of 2 KHz. In a post-processing step, the rate of neutrino-antineutrino annihilation is calculated from the neutrino luminosities generated during the hydrodynamical simulations. We find the integral annihilation rate to be a few 10⁵⁰ erg/s during the phase of strongest neutrino emission, which is too small to generate the observed bursts considering the fact that the merged object of about 3M will most likely collapse to a black hole within milliseconds.  相似文献   

井水温度微动态形成的水动力学机制研究   总被引：27，自引：4，他引：27  

鱼金子  车用太  刘五洲 《地震》1997,(4)

井水温度微动态观测越来越受到有关学者的关注，已成为我国地震地下流体动态观测的主测项之一。观测结果表明，无论是水温的正常动态还是震前的异常动态的形成，用传统的热传导或热对流机制难以给出合理解释。因此笔者根据观测到的事实、异常特征与同震效应等提出了水动力学机制，即含水层变形→含水层内孔隙压力变化→井－含水层系统内水流变化→井水温度的变化。  相似文献