湍流动能闭合方法在中尺度模式中的应用

湍流动能闭合方法是近年来发展起来的用以模拟大气边界层的一种方法,本文对其做了简化,仅在边界层内使用这一方法,进一步减少计算量,使之更适用于中尺度数值模拟。对一维旺加拉（Wangara）资料进行的试验表明,这种方法保留了边界层计算的精度,又能节省机时。然后将这种方法应用于三维中尺度钟形山地形的模拟,描述了湍流动能的分布,以及湍流活动对山后回流区面积和强度的影响。     

An Investigation of Higher-Order Closure Models for a Forested Canopy

Simultaneous triaxial sonic anemometer velocity measurements vertically arrayed at six levels within and above a uniform pine forest were used to examine two parameterization schemes for the triple-velocity correlation tensor employed in higher-order closure models. These parameterizations are the gradient-diffusion approximation typically used in second-order closure models, and the full budget for the triple-velocity correlation tensor typically employed in third-order closure models. Both second- and third-order closure models failed to reproduce the measured profiles of the triple-velocity correlation within and above the canopy. However, the Reynolds stress tensor profiles (including velocity variances) deviated greatly from the measurements only within the lower levels of the canopy. It is shown that the Reynolds stresses are most sensitive to the parameterization of the triple-velocity correlation in these lower canopy regions where local turbulent production is negligible and turbulence is mainly sustained by the flux transport term. The failure of the third-order closure model to reproduce the measured third moments in the upper layers of the canopy-top contradicts conclusions from a previous study over shorter vegetation but agrees with another study for a deciduous forest. Whether the third-order closure model failure is due to the zero-fourth-cumulant closure approximation is therefore considered. Comparisons between measured and predicted quadruple velocity correlations suggest that the zero-fourth-cumulant approximation is valid close to the canopy-atmosphere in agreement with recent experiments.     

Non-local mixing of momentum in the convective boundary layer

Recently Frech and Mahrt proposed a closure scheme which includes alarge-scale stress term to represent the effects of non-local momentummixing in the convective boundary layer. Here large-eddy simulation (LES)datasets are used to evaluate the performance of this scheme across a rangeof stabilities between neutral and highly convective conditions, and as afunction of baroclinity. Generally the inclusion of the non-local term inthe closure model leads to results in better agreement with LES, althoughsome modifications to the model formulation are suggested.     

工程测量教学中误差基本概念的应用

主要分析了工程测量教学中误差基本概念的应用,描述了偶然误差和闭合差等概念,举出了教学中的一些例子。     

立堵截流计算机辅助设计软件开发

截流是整个施工导流工程中最关键的工序之一,通过计算机仿真,模拟截流施工的进程,能够为决策机构提供及时、可靠的依据。为此,探讨了截流计算机辅助设计软件的开发技术,介绍了截流计算机辅助设计系统(JLCADS)的主要功能。     

蚀变岩质量平衡计算方法介绍

针对流体与岩石之间在反应（蚀变）过程中普遍存在的闭合问题，指出不能直接比较蚀变前后岩石成分的质量，而需要进行质量平衡计算，在这种计算中，首先要有合理的假设前提，即已知岩石质量变化前后的体积变化，或者岩石在质量变化中存在着不活动元素。介绍了选择不活动元素的多种方法及质量平衡计算的原理和方法。     

基于遥感的漳平市杉松林地表可燃物负荷量估测

 杉木林和马尾松是分布于漳平市的两种易燃的可燃物类型。其可燃物负荷量的多少决定着林火行为。在利用ASTER计算杉木林小班平均植被覆盖度的基础上估算其郁闭度,通过与地面调查结果比较分析,校正已发生明显变化的小班郁闭度。根据已有估测方程,结合小班数据中的年龄、胸径和树高等因子对杉木地表可燃物负荷量进行了估测,据此对各杉木小班的燃烧性、燃烧强度及火险等级等因素进行初步评价。最后对漳平市另一易燃的可燃物马尾松类型开展地表可燃物负荷量估测及分析。     

Eddy covariance measurements of carbon dioxide,latent and sensible energy fluxes above a meadow on a mountain slope

Carbon dioxide, latent and sensible heat fluxes were measured by means of the eddy covariance method above a mountain meadow situated on a steep slope in the Stubai Valley in Austria, based on the hypothesis that, due to the low canopy height, measurements can be made in the shallow equilibrium layer where the wind field exhibits characteristics akin to level terrain. In order to test the validity of this hypothesis and to identify effects of complex terrain in the turbulence measurements, data were subjected to a rigorous testing procedure using a series of quality control measures established for surface-layer flows. The resulting high quality dataset comprised 36% of the original observations, the substantial reduction being mainly due to a change in surface roughness and associated fetch limitations in the wind sector dominating during nighttime and transition periods. The validity of the high quality dataset was further assessed by two independent tests: (i) a comparison with the net ecosystem carbon dioxide exchange measured by means of ecosystem chambers, and (ii) the ability of the eddy covariance measurements to close the energy balance. The net ecosystem CO₂ exchange measured by the eddy covariance method agreed reasonably well with ecosystem chamber measurements. The assessment of the energy balance closure showed that there was no significant difference in the correspondence between the meadow on the slope and another one situated on flat ground at the bottom of the Stubai Valley, available energy being underestimated by 28% and 29%, respectively. We thus conclude that, appropriate quality control provided, the eddy covariance measurements made above a mountain meadow on a steep slope are of similar quality as compared to flat terrain.     

Assessing the potential for leaf-off LiDAR data to model canopy closure in temperate deciduous forests

Estimates of canopy closure have many important uses in forest management and ecological research. Field measurements, however, are typically not practical to acquire over expansive areas or for large numbers of locations. This problem has been addressed, in recent years, through the use of airborne light detection and ranging (LiDAR) technology which has proven effective in modeling canopy closure remotely. The techniques developed to use LiDAR for this purpose have been designed and evaluated for datasets acquired during leaf-on conditions. However, a large number of LiDAR datasets are acquired during leaf-off conditions since their primary purpose is to generate bare-earth Digital Elevation Models. In this paper, we develop and evaluate techniques for leveraging small-footprint leaf-off LiDAR data to model leaf-on canopy closure in temperate deciduous forests.We evaluate three techniques for modeling canopy closure: (1) the canopy-to-total-return-ratio (CTRR), (2) the canopy-to-total-pixel-ratio (CTPR), and (3) the hemispherical-viewshed (HV). The first technique has been used widely, in various forms, and has been shown to be effective with leaf-on LiDAR datasets. The CTRR technique that we tested uses the first-return LiDAR data only. The latter two techniques are new contributions that we develop and present in this paper. These techniques use Canopy Height Models (CHM) to detect significant gaps in the forest canopy which are of primary importance in estimating closure.The techniques we tested each showed good promise for predicting canopy closure using leaf-off LiDAR data with the CTPR and HV models having particularly high correlations with closure estimates from hemispherical photographs. The CTRR model had performance on par with results from previous studies that used leaf-on LiDAR, although, with leaf-off data the model tended to be negatively biased with respect to species having simple and compound leaf types and positively biased for coniferous species. The CTPR and HV models also showed some slight negative biases for compound-leaf species. The biases for the CTPR and HV models were mitigated when the CHM data were smoothed to fill in small gaps. The CHM-based models were robust to changes in the CHM model resolution which suggests that these methods may be applicable to a variety of small-footprint LiDAR datasets. In this research, the new CTPR and HV methods showed a strong ability to predict canopy closure using leaf-off data, however, future work will be needed to test the applicability of the models to variations in LiDAR datasets, forest types, and topography.     

Numerical modeling of hydraulic fracture propagation,closure and reopening using XFEM with application to in‐situ stress estimation

In this paper, a fully coupled model is developed for numerical modeling of hydraulic fracturing in partially saturated weak porous formations using the extended finite element method, which provides an effective means to simulate the coupled hydro‐mechanical processes occurring during hydraulic fracturing. The developed model is for short fractures where plane strain assumptions are valid. The propagation of the hydraulic fracture is governed by the cohesive crack model, which accounts for crack closure and reopening. The developed model allows for fluid flow within the open part of the crack and crack face contact resulting from fracture closure. To prevent the unphysical crack face interpenetration during the closing mode, the crack face contact or self‐contact condition is enforced using the penalty method. Along the open part of the crack, the leakage flux through the crack faces is obtained directly as a part of the solution without introducing any simplifying assumption. If the crack undergoes the closing mode, zero leakage flux condition is imposed along the contact zone. An application of the developed model is shown in numerical modeling of pump‐in/shut‐in test. It is illustrated that the developed model is able to capture the salient features bottomhole pressure/time records exhibit and can extract the confining stress perpendicular to the direction of the hydraulic fracture propagation from the fracture closure pressure. Copyright © 2016 John Wiley & Sons, Ltd.