The von Kármán spectra of turbulent temperature andvelocity fluctuations have been widely used in the literature on turbulenceand electromagnetic, seismic, and acoustic wave propagation in random media.In this paper we provide a phenomenological motivation for the vonKármán velocity spectrum in terms of the quasi-wavelet model ofturbulence developed recently. In this model, turbulence is represented as asuperposition of self-similar localized eddies of many different scales. Wefind a functional form for these eddies that yields the von Kármán velocity spectrum exactly. We also show that other eddy functions producevelocity spectra that have the same general form as the von Kármán spectrum, and we consider possible quasi-wavelet representations of the`Kansas' spectrum and the `-1' spectrum. We also present asystematic determination, based on turbulence similarity theories, of theparameters of the von Kármán spectra of temperature and velocityfluctuations in an unstable atmospheric boundary layer. 相似文献
In the near-field of an earthquake the effects of the rotational components of ground motion may not be negligible compared to the effects of translational motions. Analyses of the equations of motion of horizontal and vertical pendulums show that horizontal sensors are sensitive not only to translational motion but also to tilts. Ignoring this tilt sensitivity may produce unreliable results, especially in calculations of permanent displacements and long-period calculations. In contrast to horizontal sensors, vertical sensors do not have these limitations, since they are less sensitive to tilts. In general, only six-component systems measuring rotations and accelerations, or three-component systems similar to systems used in inertial navigation assuring purely translational motion of accelerometers can be used to calculate residual displacements. 相似文献
Meteoric smoke particles (MSPs) form through the vaporization of meteoroids and the subsequent re-condensation of metallic species in the mesosphere. Recently, iridium and platinum enrichments have been identified in Greenland ice layers and attributed to the fallout of MSPs supplying polar latitudes with cosmic matter during the Holocene. However, the MSP fallout to Antarctica during the Earth's climatic history remains essentially unknown.
We have determined iridium and platinum in deep Antarctic ice from Dome C and Vostok dated back to 240 kyrs BP. We find high super-chondritic fluxes during warm periods and low meteoric accretion during glacial times, a pattern that is opposite to any known climatic variation in dust fallout to polar regions. The proposed explanation of this accretion regime is a weaker polar vortex during warm periods, allowing peripheral air masses enriched in volcanic iridium and platinum to penetrate inland to Antarctica. The MSP signal emerges only during cold phases and is four times lower than in the Greenland ice cap where more snow accumulates. This suggests that wet deposition is an important route of cosmic material to the Earth's surface. 相似文献
We present a model of the subducting Cocos slab beneath Central Mexico, that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment. Based on the recently developed thermal models for the Central Mexico subduction zone, the thermal stresses due to non-uniform temperature contrast in the subducting slab are calculated using a finite element approach. The slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep. The calculation results show a 20 km slab core characterized by a tensional state of stress with stresses up to 70 MPa. On the other hand, the top of the slab experiences high compressive thermal stresses up to 110 MPa, depending on the elastic constants used and location along the flat part of the subducting plate. These compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes, and two different sources of stress are proposed.
The trenchward migration of the Mexican volcanic arc for the last 7 Ma indicates an increase of the slab dip through time. This observation suggests that the gravity torque might exceed the suction torque. Considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0.5 × 1016–1.5 × 1016 N m, the upper half would exhibit tensional stresses of 40–110 MPa that can actually balance the compressive thermally induced stresses.
An alternative stress source might come from the slab pull force caused by the slab positive density anomaly. Based on our density anomaly estimations (75 ± 20 kg/m3), a 350 km slab length, dipping at 20° into the asthenosphere, induces a slab pull force of 1.7 × 1012–4.6 × 1012 N m. This force produces a tensional stress of 41–114 MPa, sufficient to balance the compressive thermal stresses at the top of the flat slab.
The linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60–180 MPa inside the flat slab core. Also, our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40–80 MPa.
This study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath Central Mexico, and also it shows that thermal stresses due to non-uniform reheating of subducting slabs play a considerable role in the total stress field. 相似文献
Bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied
experimentally and simulated numerically. Rhyolitic melts were hydrated at 150 MPa, 780–850 °C to uniform water content of
5.5–5.3 wt%. The pressure was then dropped and held constant at 15–145 MPa. Upon the drop bubbles nucleated and were allowed
to grow for various periods of time before final, rapid quenching of the samples. The size and number density of bubbles in
the quenched glasses were recorded. Where number densities were low and run duration short, bubble sizes were in accord with
the growth model of Scriven (1959) for solitary bubbles. However, most results did not fit this simple model because of interaction
between neighboring bubbles. Hence, the growth model of Proussevitch et al. (1993), which accounts for finite separation between
bubbles, was further developed and used to simulate bubble growth.
The good agreement between experimental data, numerical simulation, and analytical solutions enables accurate and reliable
examination of bubble growth from a limited volume of supersaturated melt. At modest supersaturations bubble growth in hydrated
silicic melts (3–6 wt% water, viscosity 104–106 Pa·s) is diffusion controlled. Water diffusion is fast enough to maintain steady-state concentration gradient in the melt.
Viscous resistance is important only at the very early stage of growth (t<1 s). Under the above conditions growth is nearly
parabolic, R2=2Dtρm(C0–Cf)/ρg until the bubble approaches its final size. In melts with low water content, viscosity is higher and maintains pressure gradients
in the melt. Growth may be delayed for longer times, comparable to time scales of melt ascent during eruptions. At high levels
of supersaturation, advection of hydrated melt towards the growing bubble becomes significant.
Our results indicate that equilibrium degassing is a good approximation for modeling vesiculation in melts with high water
concentrations (C0>3 wt%) in the region above the nucleation level. When the melt accelerates and water content decreases, equilibrium can no
longer be maintained between bubbles and melt. Supersaturation develops in melt pockets away from bubbles and new bubbles
may nucleate. Further acceleration and increase in viscosity cause buildup of internal pressure in the bubbles and may eventually
lead to fragmentation of the melt.
Received: 19 June 1995 / Accepted: 27 December 1995 相似文献
The paper discusses minorities and their problems along borders of post-WWII Europe and of nation states formed in the early 1990's. One focuses on the socio-geographic conditions of the territory settled by minorities. These territories are most often peripheral, less developed areas of the state and are, because of the constant ignorance of the local culture and language, conflict regions, where interests of the minority and majority population of the state, as well as of the neighboring countries clash. Finally, an attempt of a cross-cultural comparison is undertaken to distinguish among major elements of the minority (protection) arrangements of the Alpen-Adria region and of the areas along the long-time closed borders of Central Europe. 相似文献
Simple modelling systems and mechanisms of oscillation excitation are presented that may contribute to the revealing of mechanisms of planetary magnetosphere radiosources generation and wave interactions mechanisms in the Earth ionosphere and magnetosphere as well as the excitation of VLF waves in the near-Earth space.An investigation supported by The Bulgarian National Foundation Scientific Research under Contract No. TH-302/92. 相似文献
The paper presents recent achievements in evaluations of site-dependent seismic hazard in Romania and the capital city of Bucharest caused by the Vrancea focal zone (SE-Carpathians). The zone is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume at depths 60–170 km. The database that was used for the hazard evaluation includes parameters of seismicity, ground-motion source scaling and attenuation models (Fourier amplitude spectra), and site-dependent spectral amplification functions. Ground-motion characteristics were evaluated on the basis of several hundred records from more than 120 small magnitude (M 3.5–5) earthquakes occurred in 1996–2001 and a few tens of acceleration records obtained during four large (M 7.4, 7.2, 6.9 and 6.3) earthquakes. The data provide a basis for probabilistic seismic hazard assessment in terms of peak ground acceleration, peak spectral acceleration and MSK intensity using Fourier amplitude spectra for various exceedance probabilities or average return periods. It has been shown that the influence of geological factors plays very important role in distribution of earthquake ground-motion parameters along the territory of Romania. 相似文献
