考虑植被影响的陆气耦合模式

以黄淮海平原大气边界层的观测数据为基础，发展了一个土壤－植被－大气多层模式，对大气和地表进行耦合模拟．模式对植被冠层作多层划分，以助于细致了解植被冠层沿高度分布的各物理量，为非均匀下垫面的参数化提供依据．运用观测结果对数值模拟结果进行验证，表明该模式较成功地模拟了陆气相互作用过程．同时，对模拟进行了敏感性数值试验，以了解植被在陆气相互作用中的影响大小     

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.     

Turbulence structure in a diabatically heated forest canopy composed of fractal Pythagoras trees

We investigate the turbulent flow through a heterogeneous forest canopy by high-resolution numerical modeling. For this purpose, a novel approach to model individual trees is implemented in our large-eddy simulation (LES). A group of sixteen fractal Pythagoras trees is placed in the computational domain and the tree elements are numerically treated as immersed boundaries. Our objective is to resolve the multiscale flow response starting at the diameter of individual tree elements up to the depth of the atmospheric surface layer. A reference run, conducted for the forest flow under neutral thermal stratification, produces physically meaningful turbulence statistics. Our numerical results agree quantitatively with data obtained from former field-scale LESs and wind tunnel experiments. Furthermore, the numerical simulations resolve vortex shedding behind individual branches and trunks as well as the integral response of the turbulent flow through the heterogeneous forest canopy. A focus is the investigation of the turbulence structure of the flow under stable thermal stratification and in response to the heating of the fractal tree crowns. For the stratified flows, statistical quantities, e.g. turbulent kinetic energy and vorticity, are presented and the turbulent exchange processes of momentum and heat are considered in detail. The onset and formation of coherent structures such as elevated shear layers above the diabatically heated forest canopy are analyzed. For the stably stratified flow, temperature ramps above the forest canopy were simulated in agreement with previous observations. Thermally driven vortices with a typical diameter of the canopy height were simulated when the tree crowns were diabatically heated. The impact of the coherent flow structures on the heat flux is investigated.     

Shallow Flows Over a Permeable Medium: The Hydrodynamics of Submerged Aquatic Canopies

Aquatic flow over a submerged vegetation canopy is a ubiquitous example of flow adjacent to a permeable medium. Aquatic canopy flows, however, have two important distinguishing features. Firstly, submerged vegetation typically grows in shallow regions. Consequently, the roughness sublayer, the region where the drag length scale of the canopy is dynamically important, can often encompass the entire flow depth. In such shallow flows, vortices generated by the inflectional velocity profile are the dominant mixing mechanism. Vertical transport across the canopy–water interface occurs over a narrow frequency range centered around f _v (the frequency of vortex passage), with the vortices responsible for more than three-quarters of the interfacial flux. Secondly, submerged canopies are typically flexible, coupling the motion of the fluid and canopy. Importantly, flexible canopies can exhibit a coherent waving (the monami) in response to vortex passage. This waving reduces canopy drag, allowing greater in-canopy velocities and turbulent stresses. As a result, the waving of an experimental canopy reduces the canopy residence time by a factor of four. Finally, the length required for the set-up and full development of mixing-layer-type canopy flow is investigated. This distance, which scales upon the drag length scale, can be of the same order as the length of the canopy. In several flows adjacent to permeable media (such as urban canopies and reef systems), patchiness of the medium is common such that the fully developed condition may not be representative of the flow as a whole.     

Shallow Flows Over a Permeable Medium: The Hydrodynamics of Submerged Aquatic Canopies

Aquatic flow over a submerged vegetation canopy is a ubiquitous example of flow adjacent to a permeable medium. Aquatic canopy flows, however, have two important distinguishing features. Firstly, submerged vegetation typically grows in shallow regions. Consequently, the roughness sublayer, the region where the drag length scale of the canopy is dynamically important, can often encompass the entire flow depth. In such shallow flows, vortices generated by the inflectional velocity profile are the dominant mixing mechanism. Vertical transport across the canopy–water interface occurs over a narrow frequency range centered around f _v (the frequency of vortex passage), with the vortices responsible for more than three-quarters of the interfacial flux. Secondly, submerged canopies are typically flexible, coupling the motion of the fluid and canopy. Importantly, flexible canopies can exhibit a coherent waving (the monami) in response to vortex passage. This waving reduces canopy drag, allowing greater in-canopy velocities and turbulent stresses. As a result, the waving of an experimental canopy reduces the canopy residence time by a factor of four. Finally, the length required for the set-up and full development of mixing-layer-type canopy flow is investigated. This distance, which scales upon the drag length scale, can be of the same order as the length of the canopy. In several flows adjacent to permeable media (such as urban canopies and reef systems), patchiness of the medium is common such that the fully developed condition may not be representative of the flow as a whole. This paper has previously been published in Transp Porous Med (2009) 78: 309–326; DOI .     

Destabilising effects of plant flexibility in air and aquatic vegetation canopy flows

With an analytical model coupling a mixing layer flow with an oscillating vegetation canopy through a drag force, we show that for both the cases of wind over a crop field and a water stream over aquatic plants, the dynamics of the plants is responsible for increasing the growth rate of the coupled instability in the lock-in range through two mechanisms. Because the flow and the vegetation canopy move in phase and thus minimise their interactions, the drag dissipation is decreased. The correlation between the two components of the perturbation flow velocity in the mixing layer is increased and makes the perturbation flow more efficient at extracting energy from the mean flow.     

Study on transient aerodynamic characteristics of parachute opening process

In the research of parachute, canopy inflation process modeling is one of the most complicated tasks. As canopy often experiences the largest deformations and loadings during a very short time, it is of great difficulty for theoretical analysis and experimental measurements. In this paper, aerodynamic equations and structural dynamics equations were developed for describing parachute opening process, and an iterative coupling solving strategy incorporating the above equations was proposed for a small-scale, flexible and flat-circular parachute. Then, analyses were carried out for canopy geometry, time-dependent pressure difference between the inside and outside of the canopy, transient vortex around the canopy and the flow field in the radial plane as a sequence in opening process. The mechanism of the canopy shape development was explained from perspective of transient flow fields during the inflation process. Experiments of the parachute opening process were conducted in a wind tunnel, in which instantaneous shape of the canopy was measured by high velocity camera and the opening loading was measured by dynamometer balance. The theoretical predictions were found in good agreement with the experimental results, validating the proposed approach. This numerical method can improve the situation of strong dependence of parachute research on wind tunnel tests, and is of significance to the understanding of the mechanics of parachute inflation process. The project supported by the National Natural Science Foundation of China (10377006). The English text was polished by Yunming Chen.     

Numerical Simulation of Coherent Structures over Plant Canopy

This paper reports large eddy simulations of the interaction between an atmospheric boundary layer and a canopy (representing a forest cover). The problem is studied for a homogeneous configuration representing the situation encountered above a continuous forest cover, as well as for a heterogeneous configuration representing the situation similar to an edge or a clearing in a forest. The numerical results reproduces correctly all the main characteristics of this flow as reported in the literature: the formation of a first generation of coherent structures aligned transversally with the wind flow direction, the reorganization and the deformation of these vortex tubes into horse-shoe structures. The results obtained when introducing a discontinuity in the canopy (reproducing a clearing or a fuel break in a forest), are compared with the experimental data collected in a wind tunnel; here, the results confirm the existence of a strong turbulence activity inside the canopy at a distance equal to 8 times the height of the canopy, referenced in the literature as the Enhance Gust Zone (EGZ) characterized by a local peak of the skewness factor.     

Simulation of parachute FSI using the front tracking method

We use the front tracking method on a spring system to model the dynamic evolution of parachute canopy and risers. The canopy surface and the riser string chord of a parachute are represented by a triangulated surface mesh with preset equilibrium length on each side of the simplices. The stretching and wrinkling of the canopy and its supporting string chords (risers) are modeled by the spring system. The spring constants of the canopy and the risers are chosen based on the analysis of Young's surface modulus for the canopy fabric and Young's string modulus of the string chord. Damping is added to dissipate the excessive spring internal energy. The current model does not have radial reinforcement cables and has not taken into account the canopy porosity. This mechanical structure is coupled with the incompressible Navier–Stokes solver through the “Impulse Method”. We analyzed the numerical stability of the spring system and used this computational module to simulate the flow pattern around a static parachute canopy and the dynamic evolution during the parachute inflation process. The numerical solutions have been compared with the available experimental data and there are good agreements in the terminal descent velocity and breathing frequency of the parachute.     

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     

城市大气环境的大涡模拟研究进展

本文回顾城市大气污染扩散的研究实践与进展, 介绍城市大气环境流动的特点和用现代计算流体力学手段开展城市大气环境的大涡模拟研究方法, 包括: 数学模型, 控制方程、亚网格湍流模式、定解条件及其数值方法. 其中, 重点介绍中尺度到微尺度的耦合模型与适用于复杂城市下垫面(满足大涡模拟分辨率要求且计算量较小)的组合模型;并给出实际算例和结果分析, 包括准确度的统计估算和湍流特性等. 最后, 讨论城市大气环境数值模拟方法进一步改进的方向和城市大气环境流动与污染物扩散数值研究应重点关注的几个问题.     

Interaction of a line vortex with a round parachute canopy

The interaction of a rectilinear vortex with an inflated round parachute canopy model was studied experimentally in a water tunnel where the vortex core was aligned with the axis of the canopy. Three different canopy diameters were used, and the canopy model was attached to a streamlined forebody. Dye flow visualization indicated that vortex breakdown was present when the core trajectory was within the canopy opening. Vortex breakdown occurred about one to two canopy diameters upstream of the canopy opening. The vortex core completely disintegrated when it interacted with the forebody near the canopy centerline. The vortex breakdown and disintegration caused unsteady, asymmetric deformations on the canopy surface. A reduction in the time-averaged drag and an increase in the fluctuating drag was observed when the vortex core was within the canopy opening. The disintegration of the vortex core near the canopy centerline lessened the drag reduction brought on by the presence of the core.     

Parallel finite element simulation of large ram-air parachutes

In the near future, large ram-air parachutes are expected to provide the capability of delivering 21 ton payloads from altitudes as high as 25,000 ft. In development and test and evaluation of these parachutes the size of the parachute needed and the deployment stages involved make high-performance computing (HPC) simulations a desirable alternative to costly airdrop tests. Although computational simulations based on realistic, 3D, time-dependent models will continue to be a major computational challenge, advanced finite element simulation techniques recently developed for this purpose and the execution of these techniques on HPC platforms are significant steps in the direction to meet this challenge. In this paper, two approaches for analysis of the inflation and gliding of ram-air parachutes are presented. In one of the approaches the point mass flight mechanics equations are solved with the time-varying drag and lift areas obtained from empirical data. This approach is limited to parachutes with similar configurations to those for which data are available. The other approach is 3D finite element computations based on the Navier–Stokes equations governing the airflow around the parachute canopy and Newton’s law of motion governing the 3D dynamics of the canopy, with the forces acting on the canopy calculated from the simulated flow field. At the earlier stages of canopy inflation the parachute is modelled as an expanding box, whereas at the later stages, as it expands, the box transforms to a parafoil and glides. These finite element computations are carried out on the massively parallel supercomputers CRAY T3D and Thinking Machines CM-5, typically with millions of coupled, non-linear finite element equations solved simultaneously at every time step or pseudo-time step of the simulation. © 1997 John Wiley & Sons, Ltd.     

Measurements in an urban-type boundary layer

Wind tunnel measurements of the boundary layer flow over a very rough surface comprising a staggered array of cubes are presented and discussed. Attention is concentrated on the near-wall region, including the canopy region below the tops of the roughness elements. Particle image velocimetry (PIV) and laser Doppler anemometry were used to identify the dominant features of the mean and turbulent flow and these are compared with the better-known features of the flow above the roughness. Spatial correlation data, extracted from the PIV images, are used to provide information about eddy structures and it is shown that these differ in some crucial respects from those typical of more classical boundary layers. The implications of the results are discussed in terms of their relevance to flows within the urban environment.     

Measurement of the flow structures in the wakes of different types of parachute canopies

We measured flow structures with stereoscopic particle image velocimetry (stereo-PIV) in the turbulent wakes of three parachute canopies, which had the same surface area, but different geometries. The tested parachute canopies included ribbon canopy, 8-branches canopy, and cross canopy. The obtained results showed that the geometry of the parachute canopies had significant influences on the flow structures in the wakes of these three canopies. In addition, the variation of Reynolds number did not lead to a dramatic change in the distributions of velocity, vorticity, Reynolds stress, and turbulent kinetic energy.     

Supersonic flow around a large body in an exponential atmosphere

The problem of axisymmetric supersonic flow around a large body is solved in the case when the body is moving at constant velocity in an exponential atmosphere. The nonsteady conditions in the incoming flow are characterized by the Strouhal number. From numerical solutions for different Strouhal numbers, the deviation of the flow from quasisteady conditions is investigated.Translated from Izvestiya Akdemii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 158–161, May–June, 1978.It remains to thnak G. I. Petrov for supervising the work and for discussing the results.     

Problem concerning a plane explosion in a heterogeneous atmosphere with account taken of the post-breakthrough stage

The problem is considered concerning a plane explosion in an exponential and standard atmosphere. The heterogeneity of the medium exerts an extremely marked influence on the gas flow. As shown in [1], under the conditions of an exponential atmosphere the upper part of the shock-wave front recedes to an infinite distance, after a finite time. This phenomenon has received the name breakthrough of the atmosphere. A numerical investigation of a powerful plane explosion in an exponential atmosphere at the stage before breakthrough is contained in [2]. In [3], asymptotic boundary conditions are proposed which permit the gas flow after breakthrough to be calculated. In the present paper, a numerical solution of this problem is obtained at an interval of time which exceeds by a factor of 10–15 the time of break-through. The effect of counterpressure and gravity is studied. Some results are given for a plane explosion in a standard atmosphere.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 124–131, May–June, 1976.The authors thank L. A. Chudov for attention to the project and for useful advice.     

探测器对超音速刚性盘-缝-带型降落伞系统的影响

研究了流体初始马赫数为 2.0 时, 探测器的存在与否对刚性盘-缝-带型降落伞系统气动减速性能以及流场流体结构特性的影响. 对于非定常可压缩流体的数值模拟, 流场采用了三层块结构自适应网格加密技术, 配合混合形式的TCD (tuned center difference)和WENO (weighted essentially non-oscillatory)计算格式以及基于拉伸涡亚格子模型的大涡模拟方法来处理超音速流中的激波以及大尺度湍流旋涡结构等. 结果表明: 无探测器时, 降落伞系统的流场结构稳定, 扰动较小; 有探测器存在时, 探测器后端的湍流尾迹和伞衣内部逆向运动溢出的流体与伞衣前端的弓形激波周期性的相互作用, 使得激波位置发生前移、激波倾角变小, 伞衣内部流场难以达到平衡稳定状态. 这加剧了降落伞系统的气动阻力振荡脉动变化, 降低了降落伞系统气动阻力系数, 同时也使得降落伞系统流场尾迹结构更加复杂.      

Longitudinal oscillations of bodies of revolution and flow at supersonic velocities

The method of aerodynamic derivatives [1–3] can be used for the investigation of the flow around a body executing oscillations with a small amplitude. The characteristics of the flow are expressed in the form of functions which are determined from the solution of the linearized equations of gasdynamics and describe the flow pattern with adequate accuracy. The present article is devoted to the discussion of the results of solution of the general nonstationary problem in nonlinear formulation. Supersonic flows around a hemisphere and a cylindrical front end executing arbitrary harmonic oscillations along the axis of symmetry or experiencing the corresponding oscillations of the flow (turbulent atmosphere) are discussed as examples. The effect of the nonlinearity on the flow pattern is demonstrated for different Strouhal numbers. The results are compared with those of the linear theory and with the results obtained from the solution of the corresponding stationary problems. The solution is obtained by using the characteristic method in form [4].     

B平面上斜压波热力结构特性的实验研究

首次用转环实验模拟方法研究了β效应对斜压波热力结构的影响,发现β效应有抑制流动的水平混合和垂直混合的作用,使流动趋于正压;β平面上急流随高度降低而减弱,在急流的内外两侧各有一个无量纲温度值分布的突跃区,它们的空间结构与大气环流中的极锋锋区和北极锋锋区的结构相似。