植被层湍流的大涡模拟

研究植被层湍流的大涡模拟,发展了一个TSF（transientstructurefunction）亚格于模式,尽可能真实地处理植被湍流这种既有强剪切,又有热对流的流动．我们建立了植被湍流数据库,并进行了较为详细的分析研究．湍流统计量如平均风速剖面、雷诺应力、湍流脉动能等等,与有关观测结果作了对比,符合较好．大涡模拟计算同样发现已由现场观测到的、在强对流情况时出现的温度场斜坡型有组织结构．     

建筑群环境风场的特性及模拟--风环境模拟研究之一

建筑风环境是研究建筑物与城市风场相互作用的一门空气动力学与大气科学的交叉学科、将空气动力学惯用的风洞实验和流场显示研究手段，直接用于城市大气边界层和建筑物周边流场变化规律的研究．充分利用数值计算的广度和深度与风洞实验相结合，相互验证、相互补充、弥补了目前实验手段尚无法实现的研究．本以北京商务中心区(CBD)为研究背景，论述了建筑风环境中大气边界层的模拟和具体建筑物风场的显示方法，并与数值计算进行对比，取得了非常一致的结果．为该地区的城市规划和环境保护，提出了科学依据．     

铸造充型流动过程的三维数值模拟

本文以ＳＯＬＡ－ＶＯＦ方法为基础，结合作者提出的三维自由表面边界速度确定方法，实现了铸造充型流动过程的三维数值模拟，模拟结果准确地提供了液体金属在充型过程中的流动形态、充型时间、流量等实用的参数．这表明，通过数值计算的手段，对用实验方法难以观测的铸造充型流动过程进行分析研究是有意义可行的，所得到的参数有助于铸造工艺的设计及优化     

用于模拟颗粒增强复合材料破坏过程的梁—颗粒细观模型的实验验证

本文的主要目的是验证梁－颗粒细观模型在模拟混凝土和砂岩类颗粒增强复合材料连续破坏过程的有效性，文中首先介绍了梁－颗粒细数值模型的基本原理，然后给出了由细钢丝粘结成的正方体试验的单轴抗压实验结果，最后用梁－颗粒细观模型对物理实验进行了数值模拟，研究结果表明，物理实验和数值模拟所得到的试件破坏模式和荷载－位移曲线，两者基本一致，从而初步证明了梁－颗粒细观数值模型及模拟颗粒增强复合材料破坏过程，以及解释     

45钢层裂研究

对４５钢进行了层裂实验，对软回收试件进行了显微金相观测，观察了材料中微损伤的形核、增和聚合机制。将文献［３］中的动态损伤模型编入一维Ｌａｇｒａｎｇｅ有限差分动力学程序，对层裂实验进行了数值模拟，计算结果与实验吻合较好。     

用RNGK─ε模式数值模拟180°弯道内的湍流分离流动

将Ｙａｋｈｏｔ与Ｏｒｓｚａｇ新近提出的ＲＮＧＫ－ε湍流模式推广应用于１８０°强曲率弯道内的湍流分离流动的数值模拟，计算在任意曲线坐标下进行，并采用速度协变分量作为求解变量以保证计算的高度稳定性，控制方程的求解采用通常的控制容积法，文中给出了详细的数值计算结果，并与实验结果进行了比较，结果表明，ＲＮＧＫ－ε湍流模式能有效地模拟有强曲率影响的湍流分离流动，展示了这一模式在工程湍流计算中的前景     

用于模拟颗粒增强复台材料破坏过程的梁-颗粒细观模型的实验验证

本文的主要目的是验证梁－颗粒细观模型在模拟混凝土和砂岩类颗粒增强复合材料连续破坏过程的有效性。文中首先介绍了梁－颗粒细观数值模型的基本原理，然后给出了由细钢丝粘结成的正方体试件的单轴抗压实验结果，最后用梁－颗粒细观模型对物理实验进行了数值模拟。研究结果表明，物理实验和数值模拟所得到的试件破坏模式和荷载－位移曲线，两者基本一致。从而初步证明了梁－颗粒细观数值模型是模拟颗粒增强复合材料破坏过程，以及解释其损伤机理的一个有效工具。     

四种炸药爆轰波绕射数值模拟研究

报导了采用二维Ｌａｇｒａｎｇｅ反应流体动力学程序ＷＳＵ数值模拟计算ＲＨＴ－９０１、ＪＯ－９１５９、ＪＢ－９００１炸药绕直角和液体硝基甲烷炸药绕６０、９０、１２０角的爆轰波绕射过程，对绕爆机理进行了分析。经与实验观测结果比较，数值模拟较准确地再现了爆轰波的绕射过程。     

用RNG K-E模式数值模拟180°弯道内的湍流分离流动

将Ｙａｋｈｏｔ与Ｏｒｓｚａｇ新近提出的ＲＮＧＫ－ε湍流模式推广应用于１８０°强曲率弯道内的湍流分离流动的数值模拟，计算在任意曲线坐标下进行，并采用速度协变分量作为求解变量以保证计算的高度稳定性，控制方程的求解采用通常的控制容积法，文中给出了详细的数值计算结果，并与实验结果进行了比较，结果表明，ＲＮＧＫ－ε湍流模式能有效地模拟有强曲率影响的湍流分离流动，展示了这一模式在工程湍流计算中的前景     

可压缩流向涡与激波轴对称干扰的数值模拟

用ＮＳ方程数值模拟了可压缩流向涡和激波轴对称相互作用现象．数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较．结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征．提出了判断流向涡与运动激波相互作用中旋涡破碎的准则     

植物固沙区土壤水热运移耦合模型研究

根据Ｐｈｉｌｉｐ与Ｖｒｉｅｓ提出的土壤中水热交换的耦合理论，建立了植物固沙区土壤水热运移的耦合模型，考虑了液态水和汽态水运移对温度的变化，分析了植物根系吸水对土壤水热交换的影响，给出了植物蒸腾量、土壤蒸发量、根系汲水率及土壤表面热通量等有关变量的计算公式．利用本模式并采用具有二阶精度的Ｃｒａｎｋ－Ｎｉｃｏｌｓｏｎ格式对非线性扩散方程进行离散，对沙坡头植物固沙区土壤水热交换过程进行了数值模拟，模拟结果和实测值进行了比较，总体上符合较好，证明本模型具有实用价值，可为改造沙漠提供科学依据     

A heat transfer measurement of jet impingement with high injection temperature

Local heat transfer from an impinging high temperature jet is studied using a method based on the heat thin foil technique and on the infrared thermography. Heat thin foil technique is used to impose several heat fluxes. For each flux, the temperature distribution is recorded using infrared imaging. Local heat transfer coefficients and adiabatic wall temperatures are determined by means of a linear regression method. This procedure is validated for a single round jet impinging on a flat plate for a range of injection temperatures. To cite this article: M. Fénot et al., C. R. Mecanique 333 (2005).     

Two-phase flow of refrigerants in 85 μm-wide multi-microchannels: Part II – Heat transfer with 35 local heaters

This article is the second part of a study on flow boiling of R236fa and R245fa. This part presents the heat transfer coefficients obtained in a 12.7 mm silicon evaporator composed of 135 microchannels with 85 μm wide and 560 μm high channels separated by 46 μm wide fins. There were 35 local heaters and temperature measurements arranged in a 5 × 7 array. The heat transfer results were uniform in the lateral direction to the flow (attributable to the inlet restriction) and a function of the heat flux, vapor quality and mass flux. The steady-state standard deviation of the local base temperature was less than 0.2 °C, inferring that the boiling process was very stable. For wall heat fluxes over 45 kW/m², the heat transfer coefficient curves were V-shaped, decreasing for intermittent flow regimes and increasing for annular flow. The three-zone model of Thome et al. (2004) was the best heat transfer prediction method when setting the dryout thickness equal to the channel roughness.     

A simple equilibrium model for shallow-cumulus-topped mixed layers

A new equilibrium model for shallow-cumulus-topped mixed layers is presented. A variant on the w _* closure for the shallow-cumulus mass flux is applied that retains the convective area-fraction in its formulation. As opposed to being constant, the fraction is explicitly modeled using a statistical closure as a function of the saturation deficit and humidity variance at cloud base. As a consequence, important new interactions are introduced between the convective transport, humidity, and depth of the mixed layer. This mechanism, which we call the mass-flux humidity feedback, helps determine the character of the equilibrium state such that the mixed-layer top is maintained close to the cloud-base height. Due to the strong sensitivity of the mass flux to the area fraction, the latter thus acts as a regulator or valve mechanism on moist convective transport. As a consequence, the mixed-layer model is able to explain the robustness of many aspects of the shallow-cumulus boundary layer that is typically found in observations and large-eddy simulations (LESs). The model is evaluated for a single-LES case as well as for global climatology obtained from a 40-year reanalysis of meteorological data by the European Centre for Medium-range Weather Forecasts (ECMWF). LES characteristics of convective mass flux, cloud fraction, humidity variance, cloud-base height, and surface fluxes of heat and humidity are reproduced. The solution on reanalysis fields reproduces the spatial structure of mixed-layer temperature and humidity and their associated surface fluxes in the subtropical Atlantic and Pacific trade wind regions. Furthermore, the spatial structure of the convective area-fraction matches that of synoptic surface observations of frequency of occurrence of shallow cumulus. Particularly striking is the smooth onset of the convective area-fraction and mass flux along the trade-wind trajectory that is reproduced, from zero to typical trade-wind values. The cumulus onset represents the necessity for shallow-cumulus mass flux to occur in order to close the mixed-layer budgets of heat, moisture, and mass, as a response to the changing magnitude of large-scale subsidence and free tropospheric humidity along the trajectory. Finally, the mass flux model is implemented in an intermediate-complexity tropical climate model to study its behavior when fully interactive with the larger-scale flow. A climate run then shows that the model is stable, due to the mass-flux humidity feedback acting to keep the shallow-cumulus boundary layer close to its equilibrium state for long, climatological timescales.     

Stochastic parameterization of shallow cumulus convection estimated from high-resolution model data

In this paper, we report on the development of a methodology for stochastic parameterization of convective transport by shallow cumulus convection in weather and climate models. We construct a parameterization based on Large-Eddy Simulation (LES) data. These simulations resolve the turbulent fluxes of heat and moisture and are based on a typical case of non-precipitating shallow cumulus convection above sea in the trade-wind region. Using clustering, we determine a finite number of turbulent flux pairs for heat and moisture that are representative for the pairs of flux profiles observed in these simulations. In the stochastic parameterization scheme proposed here, the convection scheme jumps randomly between these pre-computed pairs of turbulent flux profiles. The transition probabilities are estimated from the LES data, and they are conditioned on the resolved-scale state in the model column. Hence, the stochastic parameterization is formulated as a data-inferred conditional Markov chain (CMC), where each state of the Markov chain corresponds to a pair of turbulent heat and moisture fluxes. The CMC parameterization is designed to emulate, in a statistical sense, the convective behaviour observed in the LES data. The CMC is tested in single-column model (SCM) experiments. The SCM is able to reproduce the ensemble spread of the temperature and humidity that was observed in the LES data. Furthermore, there is a good similarity between time series of the fractions of the discretized fluxes produced by SCM and observed in LES.     

On the calculation of wall temperatures in the post dryout heat transfer region

A non-equilibrium post dryout heat transfer model for calculating the wall temperature distribution in vertical upflows is presented in this study. The model is based upon the three path heat transfer formulation developed by MIT researchers (Laverty & Rohsenow 1964, Forslund & Rohsenow 1968, Hynek et al. 1969 and Plummer et al. 1974) that involves heat transfer from wall to vapor, from wall to droplets in contact with the wall and from vapor to liquid droplets in the vapor core. Downstream gradients for the bulk vapor temperature, vapor quality, droplet size and vapor velocities are identical to those used by Hynek et al. (1969) and Plummer et al. (1974). Conditions at the dryout location are calculated using a modified version of a technique developed by Hynek et al. (1969).A procedure for determining an average droplet diameter based on a size distribution is introduced. Migration of droplets through the boundary layer and droplet deposition flux are predicted with the model of Gani? & Rohsenow (1979). Heat transfer from the wall to the impinging liquid droplets is calculated with a correlation by Holman & McGinnis (1969). Mechanisms contributing to wall to droplet heat transfer are identified as (a) droplet-wall contact, (b) intensive droplet evaporation inside the boundary layer, and (c) destruction of the boundary layer due to droplet migration to, and rebound from, the hot surface. The significance of the average droplet size and size distribution is demonstrated through its control over the free stream evaporation and droplet deposition rates.Predicted uniform heat flux wall temperature profiles for water, nitrogen and freon 12 are in good agreement with the data of Era et al. (1966), Bennett et al. (1967), Forslund & Rohsenow (1968), Ling et al. (1971), Groeneveld (1972) and Janssen & Kervinen (1975).     

Heat and mass transfer of a water film falling down a tilted plate with radiant heating and evaporation

This paper has concerned the heat and mass transfer of a water film falling down a tilted plate with radiant heating and water evaporation. A cluster of physical models was developed for evaluating the properties of heat and mass transfer. A fully implicit control-volume finite-difference procedure was used to solve the coupling equations. The effects of various parameters on heat and mass transfer were investigated. The results showed that the mass fraction of water vapor in ambient atmosphere and the flow turbulence played key roles in the heat and mass transfer. The ambient atmospheric temperature dramatically affected the sensible heat flux. However its effect on the latent heat flux is negligibly small. The magnitude of solar incident flux had an intense influence on the water film temperature. Received on 29 January 1998     

Turbulent latent and sensible heat flux in the presence of evaporative droplets

Direct numerical simulations of an Eulerian-based carrier phase are performed which are two-way coupled in momentum and energy to Lagrangian droplets within a Boussinesq-type incompressible formulation, where the droplets are allowed to evaporate and condense and are thus coupled to the vapor field of the carrier phase. Turbulent planar Couette flow is simulated under varying boundary forcings to understand the degree to which evaporating droplets modify vertical fluxes of energy in horizontally homogeneous systems. In particular, the separate influences on both sensible and latent heat are substantial but opposite in sign, and the local relative humidity can result in droplet-induced heat or moisture fluxes which counteract the prescribed background gradients. The influence of droplet Stokes number is also considered, where it is shown that both clustering and turbophoresis play important roles in determining the magnitude of the droplet-induced sensible and latent heat fluxes.     

Turbulent mixed convection of heat and water vapor transfers in a two-dimensional vegetation canopy

The present study consists in a numerical investigation of turbulent mixed-convection of heat and water vapor transfers inside two-dimensional (2-D) vegetation canopy, in the surrounding atmosphere and in a wet underground. The time-averaged Navier-Stokes equations are used to characterize the flow field surrounding the canopy and within it. Reynolds shear stresses are calculated using the eddy turbulence model and the Prandtl mixing length. The governing equations are solved numerically using an implicit finite difference method and Thomas algorithm. The present model is used for the determination of the micro climatic profiles such as streamlines, isotherms and iso-concentration. Special emphasis is laid on the systematic analysis of the total evaporation rate (evapotranspiration), the local and average heat fluxes, the Nusselt and Sherwood numbers. The effects of Leaf Area Density distribution, the canopy stomata regulation, as well as the atmospheric forcing conditions on the transfers, are presented and analysed. The results show that buoyancy force caused by properties variation reduces the local heat and mass transfer coefficients, and that this reduction increases at lower wind velocities.     

Forced-convection film condensation on a horizontal cylinder with wavy surface structure

The forced-convection film condensation on a horizontal cylinder with wavy surface structure was performed by boundary-layer-approximation. The local/mean heat fluxes were obtained for the effects of tube temperature, wave number, and wave amplitude. The mean heat flux increases with decreasing wavy amplitude and tube temperatures. Furthermore, when β = 20 and α = 0.005, the mean heat flux slightly increases from 1.1 to 3.6% compared with that of smooth tube, depending on tube temperature.