The time evolution of a turbulent lock release gravity current, formed by a finite volume of homogeneous fluid released instantaneously into another fluid of slightly lower density, was studied by experimental measurements of the density structure via elaborate digital image processing and by a numerical simulation of the flow and mixing using a two-equation turbulence model. The essential fact that the gravity current passes through an initial slumping phase in which the current head advances steadily and a second self-similar phase in which the front velocity decreases like the negative third power of the time after release is satisfactorily presented by the laboratory observation. An overall entrainment ratio proportional to the distance from the release point is found by the numerical simulation. The renormalization group (RNG) K-E model for Reynolds-stress closure is validated to characterize the gravity current with transitional and localized turbulence.

As an emerging approach for unified resource accounting and energy-utilization assessment for the transportation sector, the exergy method to embody the second law fo thermodynamics is investigated and applied to the energy-utilization assessment of the Chinese transport sector from 1978 to 2002. The exergy consumed by the sector in 2000 was 3.3 times that in 1978. The exergy of the oil consumed by transportation accounted for 30.5% of the total oil consumption in the whole country up to 2000, almost equal to the exergy of the net oil import. In terms of the exergy consumption intersity, taken as the exergy consumption per Converted Tumover, proposed to assess the overall effectiveness of the system, and conventional exergy efficiency to reflect the technical performance of involved transport vehicles, both the energy-utilization effectiveness and efficiency of the sector have been declining generally. Waterways transport was proved the most effective one, and railways, highways and clvil aviation were the second, third and fourth, respectively.The historical evolution and structural variation of energy use in terms of the total exergy consumption, exergy effciency and exergy consumption intensity are revealed and explored against drastic backgrud transitions in related policies and socioeconomies.

Prsented in this paper are a systematic study on the global exergy consumption in the earth and a budget and a budget of the exergy consumption with respect to main terrestrial processes. Based on Szargut's definition of exergy for thermal radiation, a global exergy balamce of the thermodynamic system of the earth, driven by cosmic exeryflow originated from the temperature difference between the sun and the cosmic background, is carried out to give the global cosmic exergy consumption as the multiplication of the cosmic background microwave (CBM) radiation temperature and the global entropy generation due to irreversibility in the earth system. Concrete formulae are derived for cosmic exergy, with emphasis on generalization of a simple blackbody relationship between entropy and energy flux densities to the cases of gray body radiation with moderate or large emissivity associated with the earth system. Global entropy generation is evaluated with a result compared very with a widely accepted datum based on satellite observations in earth science. A budget of the global entropy generation is made with respect to the terrestrial radiation processes associated with the atmosphere and the earth's surface and to the molecular transport phenomena in the material earth. A mechanism of multiplication goveming transformation between cosmic exergy and terrestrial exergy is developed. An overall exergy budget of the earth system, based on the entropy budget by means of the Gouy-Stodola law, are presented with essential implication to the problem of global sustainability.

This study deals with the formulation, mathematical property and phydival meaning of the simplified Navier-Stokes (SNS) eguations. The tensorial SNS equations proposed is the simplest in, form and is applicable to flow tields with arbitrary body boundaries. The zones of influence and dependence of the SNS equations, which are of primary importance to numerical solutions, are expounded for the first rime from the viewpoint of subcharacteristics. Besides, a detailed snalysis of the diffusion process in flow fields shows that the diffnsion effect has an influence zone globalty windward and an upwind propagation greatly depressed by, convection. The maximum upwind influential distance of the viscous effect and the relative importance of the viscous effect in the flow direction to that in the direction normal to the flow are represented by the Reynolds number, which illustrates the conversion of the complete Navier-Stokes (MS) equarions to the SNS equations for flows with large Reynolds numher.

A compact fourth-order upwind finite difference scheme, which takes classieal first-order upwind scheme as basic representation and thus has no grid Reynoldsaumber limitarion, is developed for the convection including model equations of fluid flow and a problem of high Rayleigh-number natural convection with boundary-layer effect are given to illustrate the excellent behavior of the present scheme.