Automated watershed subdivision for simulations using multi-objective optimization

A qualitative trial-and-error approach is commonly used to define watershed subdivisions through varying a single topographic threshold value. A methodology has been developed to quantitatively determine spatially variable threshold values using topography and a user-defined landscape reference layer. Optimization and topographic parameterization algorithms were integrated to create solutions that minimize the number of sub-watersheds and maximize the agreement between the discretized watershed and the reference layer. The system was evaluated using different reference datasets such as soil type, land management, and landscape form. Comparison of simulated results indicated that the scenario using land management as the reference layer yielded results closer to the scenario subdivided using a constant topographic threshold but with approximately 10 times more sub-catchments and therefore indicating customization of the watershed subdivision to the user-defined reference layer. The proposed optimization technology could be used in adequately applying watershed modeling technology in developing conservation practice implementation plans.     

大亚湾夏季表层浮游细菌生物量分布与环境变量的关系

根据2008年夏季(7月)在大亚湾进行的现场综合调查资料,分析表层海水浮游细菌生物量的水平分布特征,探讨细菌生物量与环境变量的关系。大亚湾夏季浮游细菌生物量变化范围为14.00～118.80μg·L^-1,海区平均值为55.23±29.43μg·L^-1。大亚湾夏季细菌生物量水平分布大体上呈现南部高于北部的趋势,在北纬22°35′以北和以南的海区均各自表现为东南部高于西北部的趋势。水深、硝酸盐、铵盐、亚硝酸盐和总碱度等5个参数与浮游细菌生物量之间存在着中等水平的相关性(0.15相似文献   

Study on Vortex-Induced Motions of A New Type of Deep Draft Multi-Columns FDPSO

A numerical simulation and an experimental study on vortex-induced motion (VIM) of a new type of deep draft multi-columns floating drilling production, storage and offloading (FDPSO) are presented in this paper. The main dimension, the special variable cross-section column and the cabin arrangement of the octagonal pontoon are introduced based on the result. The numerical simulation is adapted to study the effects of current incidence angles and reduced velocities on this platform''s sway motion response. The 300 m water depth equivalent truncated mooring system is adopted for the model tests. The model tests are carried out to check the reliability of numerical simulation. The results consist of surge, sway and yaw motions, as well as motion trajectories. The maximum sway amplitudes for different types of offshore platform is also studied. The main results show that the peak frequencies of sway motion under different current incidence angles and reduced velocities vary around the natural frequency. The analysis result of flow field indicates that the change of distribution of vortex in vertical presents significant influences on the VIM of platform. The trend of sway amplitude ratio curve of this new type FDPSO differs from the other types of platform. Under 45° current incidence angle, the sway amplitude of this new type of FDPSO is much smaller than those of other types of offshore platform at 4.4 ≤ V_r ≤ 8.9. The typical ‘8’ shape trajectory does not appear in the platform''s motion trajectories.     

Radial Permeability Measurements for Shale Using Variable Pressure Gradients

Shale gas is becoming an important component of the global energy supply, with permeability a critical controlling factor for long-term gas production. Obvious deviation may exist between helium permeability determined using small pressure gradient(SPG) methods and methane permeability obtained under actual field production with variable pressure gradients(VPG). In order to more accurately evaluate the matrix permeability of shale, a VPG method using real gas(rather than He) is established to render permeability measurements that are more representative of reservoir conditions and hence response. Dynamic methane production experiments were performed to measure permeability using the annular space in the shale cores. For each production stage, boundary pressure is maintained at a constant and the gas production with time is measured on the basis of volume change history in the measuring pump. A mathematical model explicitly accommodating gas desorption uses pseudo-pressure and pseudo-time to accommodate the effects of variations in pressuredependent PVT parameters. Analytical and semi-analytical solutions to the model are obtained and discussed. These provide a convenient approach to estimate radial permeability in the core by nonlinear fitting to match the semi-analytical solution with the recorded gas production data. Results indicate that the radial permeability of the shale determined using methane is in the range of 1×10-6– 1×10-5 mD and decreases with a decrease in average pore pressure. This is contrary to the observed change in permeability estimated using helium. Bedding geometry has a significant influence on shale permeability with permeability in parallel bedding orientation larger than that in perpendicular bedding orientation. The superiority of the VPG method is confirmed by comparing permeability test results obtained from both VPG and SPG methods. Although several assumptions are used, the results obtained from the VPG method with reservoir gas are much closer to reality and may be directly used for actual gas production evaluation and prediction, through accommodating realistic pressure dependent impacts.     

冻融循环下砂岩损伤演化及本构模型

冻融交替对岩石宏观强度产生弱化。基于冻融循环条件下的压缩试验结果,利用损伤力学研究材料强度特征及损伤演化,建立岩土材料的损伤模型。结果表明:随着循环次数的增加,砂岩的力学强度参数均逐渐减小,抗压强度与弹性模量的降低幅度可以达到49.70%、83.40%,意味着冻融循环对材料力学性质损伤显著;定义岩石综合损伤变量,得到损伤演化特征曲线,基于高斯函数变化特征,构建冻融循环条件下材料损伤本构模型,对比分析试验与模型曲线,发现该模型能够较好地描述试验结果。基于高斯函数的损伤模型能较好地反映材料变形行为特征,研究成果可以为类似本构模型的建立提供参考。     

象山港内三维动边界潮流的数值模拟

象山港是一具有广阔潮滩和大潮差区域的半封闭型海湾,进行数值模拟时必须引入动边界技术来考虑露漫滩的影响.在二维模式中引入动边界技术相对成熟一些,而将这些二维模式使用的露漫滩处理方法简单地搬入三维模式中,常遇到由于边界不连续变动而产生的数值"噪音"及计算不稳定现象.原始POM(Princeton ocean model)模型采用固定边界,没有使用动边界技术,故无法模拟漫滩流.借鉴BURCHARD et al 2004年的研究工作,在原始POM模型的基础上,突破其为固定边界模式的局限,基于尽量使模式物理上满足流动简化需要的原则,引入了一种较为自然的动边界处理技术,使得在非常浅水处和复杂的地形情况下,应用合理的数学表达式体现该物理过程.当总水深大于设定的临界水深时,体现的数学方程是通常形式的动量方程;而当总水深接近设定的最小允许水深时,模型的动量方程被简化为变化率、摩擦力和正压梯度等项之间的平衡关系,此时由于摩擦力占优,相应的流速也会趋于0,这样可以有效避免计算中出现负水深.除在非常浅水处对动量方程作简化外,对于陡坡地形附近的露漫滩问题,当发生一个网格点上的水位低于相邻格点海床的现象时,还需对正压梯度的求解作一个简单的数值处理,以避免不合理值的出现.此外,通过修改原始POM模型的底摩擦系数表达式,可使模型在模拟露漫滩过程时计算更加稳定.改进后的POM模型被用来模拟象山港海域内的三维潮流场,模型计算结果与2个潮位站、1个锚系站和2个船测连续站资料进行了比较,符合良好.整个模型很好地再现了象山港内大区域三维涨、落潮流特征和变化规律,同时象山港顶区域涨憩、落急、落憩和涨急4个特征时刻的流场水平和垂直分布变化过程反映该模型能很好地反映涨、落潮造成的潮间带的露漫滩过程,由此说明对原始POM模型所作的修改是十分有效和可行的,有利于进一步扩充POM模型的应用范围.     

Period search using the DMRT method: The properties of the test function

The test function for the method proposed by DUMONT, MORGULEFF, RUTILY, and TERZAN (1978) is investigated analytically and numerically for some examples with constant and variable periods. The properties of the test function of the DMRT method are discussed in comparison with that of the leastsquare method.     

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基于局部惩罚型变权的建设用地生态适宜性空间模糊评价——以陕西延安宝塔区为例

生态适宜性评价有助于优化建设用地空间布局,针对生态适宜性的模糊性以及常权评价的不足,采用局部惩罚型变权来改进空间模糊综合评价方法,并合理确定惩罚水平,能有效突出瓶颈因素对评价结果的限制性作用,更好体现生态适宜性评价的渐变性和模糊性,合理反映生态适宜性状况.结果显示:调权水平为0.25,惩罚水平α为0.841 8时,变权...     

1998年夏季长江特大洪水入海的化学水文学特征

根据1998年夏季长江流域特大洪水期间黄海、东海的现场调查资料，讨论了洪水入海对黄海和东海陆架区化学水文学的影响。给出了迄今调查获得的长江冲淡水及其营养物质的最大扩展范围；发现了表层溶解氧和pH分布的一个突出现象，即在南黄海南部和东海北部的远岸海域存在一溶解氧和pH高值封闭区；指出浮游植物的光合作用主要发生在南黄海南端至东海北部海域，而不是在长江口门内。南黄海南部，东海北部和东海近岸海域过量无机氮的存在表明，与其它海洋生态系统不同，上述海域的初级生产可能是磷限制而非氮限制。