建筑密度对建筑物群内风能利用的影响

针对建筑物群内风能应用问题,采用CFD方法,对建筑密度分别为26%、20%、18%、16%、14%的5种建筑物群周围风速和湍流强度特征开展研究,分析建筑密度对建筑物群内风力机合理安装位置的影响方式。结果表明:在低于1.5H高度范围内,建筑物群的建筑密度越大,同一安装高度上适合于安装风力机的区域就越大,即越有利于建筑物群内风能的应用;在高度高于1.5H后,建筑密度对建筑物群内风力机安装位置的影响消失;无论建筑密度大小,在低于1.2H的高度范围内,建筑物群内不适合安装风力机;在高度高于1.45H后,可优先考虑将风力机安装于建筑物群内中间一排建筑物顶面,在建筑物顶面可优先将风力机安装于拐角位置;5种建筑密度的建筑物群内只考虑风速要求即可确定风力机的合理安装位置。

雷诺方程中湍流涡黏性的Richardson势算子本构模型

证明三维湍流Richardson扩散方程中的变系数拉普拉斯算子是基本解为r-7/3的一个势算子,并称其为Richardson势算子,其中,r是空间中两点的欧几里得距离.基于Kolmogorov标度律和湍流快扩散(super-diffusion)理论,运用隐式微积分方程建模方法提出雷诺方程中的Richardson势算子湍流涡黏性本构方程.

Characterization of Rayleigh Convection in Interfacial Mass Transfer by Lattice Boltzmann Simulation and Experimental Verification

Concentration gradient induced Rayleigh convection can influence effectively interracial mass transfer processes, but the convection phenomena are known as mesoscopic and complex. In order to investigate this phenomenon, a two-equation Lattice Boltzmann Method （LBM） is proposed to simulate the velocity and the concentra-tion distributions of Rayleigh convection generated in the CO2 absorptlon into ethanol liquid.The simulated results on velocity distributions are experimentally verified by PIV （particle image velocimetry technique） measurements. In order to simplify the analysis, the convection in the simulation as well as in the experiment, the Rayleigh convection was manipulated into a single down flow pattern, The simulated results show that the concentration contours agree qualitatively with the schlieren images in the literature. The experimental and simulated results show that theRayleigh convection under investigation is dominated by the flow in the downward direction and impels exchange of the liquid between the interfacial vicinity and the liquid bulk promoting the renewal of interfacial liquid, and hence enhances mass transfer. The comparison between the simulated and experimental results demonstrated that the proposed LBM is a promising alternative for simulating mass transfer induced Rayleigh convection.

Channel Response to Gravel Mining Activities in Mountain Rivers

s: The removal of bed material from active river channels usually affects the bed profile of the streambed, causing progressive degradation upstream and downstream of the extraction site. These effects can extend for kilometers affecting hydraulic structures located in the vicinity of the river reach. In this paper, the geomorphic effects of gravel mining are reviewed and summarized. Some cases in Venezuelan streams are presented to illustrate the problem. To describe the processes of erosion and sedimentation in a gravel extraction pit, a recent developed mathematical model for the simulation of flow and sediment transport in gravel-cobble bed streams is applied to a hypothetical case of gravel mining in a river channel. A simple rectangular dredge pit is imposed as initial condition in the channel bed, and changes in bed elevations and grain size distribution of bed material are calculated by using the numerical model. The process of deposition within the pit, and the downstream and upstream migration of the erosion wave are well simulated by the model and closely resemble the phenomena observed in laboratory experiments. The response of the friction coefficient to the changes in flow and bed elevations shows the importance in modeling adequately flow resistance and sediment transport in gravel-cobble bed streams.

An epidemiological study of collagenous colitis in southern Sweden from 2001-2010

AIM： To estimate the incidence of collagenous colitis （CC） in southern Sweden during 2001-2010. METHODS： Cases were identified by searching for CC in the diagnostic registers at the Pathology Departments in the county of Skane. The catchment area comprised the south-west part of the county （394 307 inhabitants in 2010） and is a mixed urban and rural type with limited migration. CC patients that had under- gone colonoscopy during the defined period and were living in this area were included in the study regardless of where in Skane they had been diagnosed. Medical records were scrutinized and uncertain cases were re- assessed to ensure that only newly diagnosed CC cases were included. The diagnosis of CC was based on both clinical and histopathological criteria. The clinical crite-rion was non-bloody watery diarrhoea. The histopatho- logical criteria were a chronic inflammatory infiltrate in the lamina propria, a thickened subepithelial collagen layer ≥10 micrometers （um） and epithelial damage such as flattening and detachment. RESULTS： During the ten year period from 2001-2010, 198 CC patients in the south-west part of the county of Skane in southern Sweden were newly diagnosed. Of these, 146 were women and 52 were men, i.e., a female： male ratio of 2.8：1. The median age at diag- nosis was 71 years （range 28-95/inter-quartile range 59-81）, for women median age was 71 （range 28-95） years and was 73 （range 48-92） years for men. The mean annual incidence was 5.4/105 inhabitants. During the time periods 2001-2005 and 2006-2010, the mean annual incidence rates were 5.4/105 for both periods [95% confidence interval （CI）： 4.3-6.5 in 2001-2005 and 4.4-6.4 in 2006-2010, respectively, and 4.7-6.2 for the whole period]. Although the incidence varied over the years （minimum 3.7 to maximum 6.7/105） no increase or decrease in the incidence could be identi- fied. The odds ratio （OR） for CC in women compared to men was estimated to be 2.8 （95% CI： 2.0-3.7）. The OR for women 65 years of age or above compared to below 65 years of age was 6.9 （95% CI：5.0-9.7）, and for women 65 years of age or above compared to the whole group the OR was 4.7 （95% CI： 3.6-6.0）. The OR for age in general, i.e., above or 65 years of age compared to those younger than 65 was 8.3 （95% CI： 6.2-11.1）. During the last decade incidence figures for CC have also been reported from Calgary, Canada dur- ing 2002-2004 （4.6/105） and from Terrassa, Spain dur- ing 2004-2008 （2.6/105）. Our incidence figures from southern Sweden during 2001-2010 （5.4/10s） as well as the incidence figures presented in the studies during the 1990s （Terrassa, Spain during 1993-1997 （2.3/10s）, OI- msted, United States during 1985-2001 （3.1/10s）, Orebro, Sweden during 1993-1998 （4.9/10s）, and Iceland during 1995-1999 （5.2/10s） are all in line with a north- south gradient, something that has been suggested be- fore both for CC and inflammatory bowel disease.CONCLUSION： The observed incidence of CC is com- parable with previous reports from northern Europe and America. The incidence is stable but the female： male ratio seems to be decreasing.

Lattice Boltzmann simulation of double diffusive natural convection in a square cavity with a hot square obstacle

Double diffusion convection in a cavity with a hot square obstacle inside is simulated using the lattice Boltzmann method. The results are presented for the Rayleigh numbers 104,105 and 106, the Lewis numbers 0.1, 2 and 10 and aspect ratio A(obstacle height/cavity height) of 0.2, 0.4 and 0.6 for a range of buoyancy number N = 0 to- 4 with the effect of opposing flow. The results indicate that for |N| b 1, the Nusselt and Sherwood numbers decrease as buoyancy ratio increases, while for |N| N 1, they increase with |N|. As the Lewis number increases, higher buoyancy ratio is required to overcome the thermal effects and the minimum value of the Nusselt and Sherwood numbers occur at higher buoyancy ratios. The increase in the Rayleigh or Lewis number results in the formation of the multi-cell flow in the enclosure and the vortices will vanish as |N| increases.

Convection in the Rayleigh-Bénard Flow with all Fluid Properties Variable

In the present paper, the effect of variable fluid properties （density, viscosity, thermal conductivity and specific heat） on the convection in the classical Rayleigh-Benard problem is investigated. The investigation concerns water, air, and engine oil by taking into account the variation of fluid properties with temperature. The results are obtained by numerically solving the governing equations, using the SIMPLE algorithm and covering large temperature differences. It is found that the critical Rayleigh number increases as the temperature difference increases considering all fluid properties variable. However, when the fluid properties are kept constant, calculated at the mean temperature, and only density is considered variable, the critical Rayleigh number either decreases or remains constant.

On the Rayleigh-Taylor Instability for Two Uniform Viscous Incompressible Flows

The authors study the Rayleigh-Taylor instability for two incompressible immiscible fluids with or without surface tension, evolving with a free interface in the presence of a uniform gravitational field in Eulerian coordinates. To deal with the free surface, instead of using the transformation to Lagrangian coordinates, the perturbed equations in Eulerian coordinates are transformed to an integral form and the two-fluid flow is formulated as a single-fluid flow in a fixed domain, thus offering an alternative approach to deal with the jump conditions at the free interface. First, the linearized problem around the steady state which describes a denser immiscible fluid lying above a light one with a free interface separating the two fluids, both fluids being in(unstable) equilibrium is analyzed. By a general method of studying a family of modes, the smooth(when restricted to each fluid domain) solutions to the linearized problem that grow exponentially fast in time in Sobolev spaces are constructed, thus leading to a global instability result for the linearized problem.Then, by using these pathological solutions, the global instability for the corresponding nonlinear problem in an appropriate sense is demonstrated.

A Rayleigh-Plesset based transport model for cryogenic fluid cavitating flow computations

The present article focuses on modeling issues to simulate cryogenic fluid cavitating flows.A revised cavitation model,in which the thermal effect is considered,is derivated and established based on Kubota model.Cavitating flow computations are conducted around an axisymmetric ogive and a 2D quarter caliber hydrofoil in liquid nitrogen implementing the revised model and Kubota model coupled with energy equation and dynamically updating the fluid physical properties,respecitively.The results show that the revised cavitation model can better describe the mass transport process in the cavitation process in cryogenic fluids.Compared with Kubota model,the revised model can reflect the observed"frosty"appearance within the cavity.The cavity length becomes shorter and it can capture the temperature and pressure depressions more consistently in the cavitating region,particularly at the rear of the cavity.The evaporation rate decreases,and while the magnitude of the condensation rate becomes larger because of the thermal effect terms in the revised model compared with the results obtained by the Kubota model.

Measurements of heat transport by turbulent Rayleigh-Bénard convection in rectangular cells of widely varying aspect ratios

High-precision measurements of the Nusselt number Nu for Rayleigh-B6nard （RB） convection have been made in rectangular cells of water （Prandtl number Pr ≈ 5 and 7） with aspect ratios （F~, Fy） varying between （1, 0.3） and （20.8, 6.3）. For each cell the data cover a range of a little over a decade of Rayleigh number Ra and for all cells they jointly span the range 6x105 〈 Ra 〈1011. The two implicit equations of the Grossmann-Lohse （GL） model together with the empirical finite conductivity cor- rection factorf（X） were fitted to obtain estimates of Nu∞ in the presence of perfectly conducting plates, and the obtained Nu∞ is independent of the cells＇ aspect ratios. A combination of two-power-law, Nu∞= O.025Ra0.357＋O.525Ra0.168, can be used to de- scribe Nu∞（Ra）. The fitted exponents 0.357 and 0.168 are respectively close to the predictions 1/3 and 1/5 of the 11μ. and 1Vμ re- gimes of the GL model. Furthermore, a clear transition from the II. regime to the IVμ regime with increasing Ra is revealed.