集成于方箱防波堤的双气室振荡水柱波能装置转换效率研究

为实现建造、运行和维护等方面的资源共享,将振荡水柱(oscillating water column,简称OWC)波能转换装置与现有海工结构集成耦合,已成为目前海洋波浪能转换利用的热点问题。以集成于方箱防波堤的双气室OWC装置为研究对象,借助开源代码平台OpenFOAM和造/消波工具箱waves2Foam,采用流体体积法(VOF)捕捉自由面和6自由度(6DOF)动网格求解器模拟垂荡运动响应,对在不同规则波作用下,中墙相对宽度、中墙相对吃水对装置波能转换效率及水动力特性的影响进行数值研究。结果表明,较大的中墙相对宽度能够增强装置的波能转换效率(ξ_total(max)=73%)、降低结构物的相对垂荡位移并对装置前后气室内水柱的振荡幅度与压强变化产生影响;增加中墙相对吃水能显著提高气室在中高频波段波能提取效率(ξ_total(max)=78%),并显著拓宽气室的高效频率带宽(0.9≤ω²h/g≤2.2)。     

降雨径流与洪水淹没模型耦合的应用研究

洪水研究包括径流与淹没两种模式。为了探究流域降雨产汇流与淹没情况、提高洪水预报精度,本研究在传统流域水文模型的基础上耦合二维水动力学模型,建立水文-水动力耦合模型。以我国吉林温德河流域为研究实例,模拟了2017年“7·13”洪水在下游口前镇所处子流域洪水淹没过程。首先对基础数据进行预处理,建立HEC-HMS水文模型并进行参数优化后,最终获得流量过程水文结果作为水动力学模型边界条件,之后建立HEC-RAS二维水动力学模型对重要子流域进行淹没模拟。耦合模型计算结果显示,水文模型经多参数优化流量模拟的NSE系数为0.988,水动力计算最大淹没水深达9.3 m相对误差为-5.2%。从泛洪模拟结果来看,子流域上游部分的农田大量被淹,淹没水深范围在0.5~2.0 m,平均流速基本在1 m/s以下。下游口前镇内最大淹没水深接近1 m,水流速度0.2 m/s至1.5 m/s,与实际的淹没情况相吻合。研究表明,所建水文水动力耦合模型模拟计算的结果准确率较高,对具有复杂水文、水力条件的流域的洪水预报具有重要的指导意义。     

Coal seam gas distribution and hydrodynamics of the Sydney Basin,NSW, Australia

This paper reviews various coal seam gas (CSG) models that have been developed for the Sydney Basin, and provides an alternative interpretation for gas composition layering and deep-seated CO₂ origins. Open file CSG wells, supplemented by mine-scale information, were used to examine trends in gas content and composition at locations from the margin to the centre of the basin. Regionally available hydrochemistry data and interpretations of hydrodynamics were incorporated with conventional petroleum well data on porosity and permeability. The synthesised gas and groundwater model presented in this paper suggests that meteoric water flow under hydrostatic pressure transports methanogenic consortia into the subsurface and that water chemistry evolves during migration from calcium-rich freshwaters in inland recharge areas towards sodium-rich brackish water down-gradient and with depth. Groundwater chemistry changes result in the dissolution and precipitation of minerals as well as affecting the behaviour of dissolved gases such as CO₂. Mixing of carbonate-rich waters with waters of significantly different chemistries at depth causes the liberation of CO₂ gas from the solution that is adsorbed into the coal matrix in hydrodynamically closed terrains. In more open systems, excess CO₂ in the groundwater (carried as bicarbonate) may lead to precipitation of calcite in the host strata. As a result, areas in the central and eastern parts of the basin do not host spatially extensive CO₂ gas accumulations but experience more widespread calcite mineralisation, with gas compositions dominated by hydrocarbons, including wet gases. Basin boundary areas (commonly topographic and/or structural highs) in the northern, western and southern parts of the basin commonly contain CO₂-rich gases at depth. This deep-seated CO₂-rich gas is generally thought to derive from local to continental scale magmatic intrusions, but could also be the product of carbonate dissolution or acetate fermentation.     

Constraining the cosmological baryon density with X-ray clusters

We study the properties of X-ray galaxy clusters in four cold dark matter models with different baryon fractions Ω_BM, ranging from 5 to 20 per cent. By using an original three-dimensional hydrodynamic code based on the piecewise parabolic method, we run simulations on a box with a size of 64  h ⁻¹ Mpc and we identify the clusters by selecting the peaks in the X-ray luminosity field. We analyse these mock catalogues by computing the mass function, the luminosity function, the temperature distribution and the luminosity–temperature relation. By comparing the predictions of the different models to a series of recent observational results, we find that only the models with low baryonic content agree with the data, while models with larger baryon fraction are well outside the 1σ error bars. In particular, the analysis of the luminosity functions, both bolometric and in the energy band [0.5–2] keV, requires Ω_BM ≲ 0.05 when we fix the values h  = 0.5 and n  = 0.8 for the Hubble parameter and the primordial spectral index, respectively. Moreover we find that, independently of the cosmological scenario, all the considered quantities have very little redshift evolution, particularly between z  = 0.5 and 0.     

Non-linear fluid dynamics of eccentric discs

A new theory of eccentric accretion discs is presented. Starting from the basic fluid-dynamical equations in three dimensions, I derive the fundamental set of one-dimensional equations that describe how the mass, angular momentum and eccentricity vector of a thin disc evolve as a result of internal stresses and external forcing. The analysis is asymptotically exact in the limit of a thin disc, and allows for slowly varying eccentricities of arbitrary magnitude. The theory is worked out in detail for a Maxwellian viscoelastic model of the turbulent stress in an accretion disc. This generalizes the conventional alpha viscosity model to account for the non-zero relaxation time of the turbulence, and is physically motivated by a consideration of the nature of magnetohydrodynamic turbulence. It is confirmed that circular discs are typically viscously unstable to eccentric perturbations, as found by Lyubarskij, Postnov & Prokhorov, if the conventional alpha viscosity model is adopted. However, the instability can usually be suppressed by introducing a sufficient relaxation time and/or bulk viscosity. It is then shown that an initially uniformly eccentric disc does not retain its eccentricity as had been suggested by previous analyses. The evolutionary equations should be useful in many applications, including understanding the origin of planetary eccentricities and testing theories of quasi-periodic oscillations in X-ray binaries.     

Model dependence of transonic properties of accretion flows around black holes

We analytically study how the behaviour of accretion flows changes when the flow model is varied. We study the transonic properties of the conical flow, a flow of constant height and a flow in vertical equilibrium, and show that all these models are basically identical, provided that the polytropic constant is suitably changed from one model to another. We show that this behaviour is extendible even when standing shocks are produced in the flow. The parameter space where shocks are produced remains roughly identical in all these models when the same transformation among the polytropic indices is used. We present applications of these findings.     

The origin of the initial mass function and its dependence on the mean Jeans mass in molecular clouds

We investigate the dependence of stellar properties on the mean thermal Jeans mass in molecular clouds. We compare the results from the two largest hydrodynamical simulations of star formation to resolve the fragmentation process down to the opacity limit, the first of which was reported by Bate, Bonnell & Bromm. The initial conditions of the two calculations are identical except for the radii of the clouds, which are chosen so that the mean densities and mean thermal Jeans masses of the clouds differ by factors of 9 and 3, respectively.
We find that the denser cloud, with the lower mean thermal Jeans mass, produces a higher proportion of brown dwarfs and has a lower characteristic (median) mass of the stars and brown dwarfs. This dependence of the initial mass function (IMF) on the density of the cloud may explain the observation that the Taurus star-forming region appears to be deficient in brown dwarfs when compared with the Orion Trapezium cluster. The new calculation also produces wide binaries (separations >20 au), one of which is a wide binary brown dwarf system.
Based on the hydrodynamical calculations, we develop a simple accretion/ejection model for the origin of the IMF. In the model, all stars and brown dwarfs begin with the same mass (set by the opacity limit for fragmentation) and grow in mass until their accretion is terminated stochastically by their ejection from the cloud through dynamically interactions. The model predicts that the main variation of the IMF in different star-forming environments should be in the location of the peak (due to variations in the mean thermal Jeans mass of the cloud) and in the substellar regime. However, the slope of the IMF at high masses may depend on the dispersion in the accretion rates of protostars.