Scale, boundary and inlet condition effects on impinging jets

The sensitivity of the orthonormal impinging jets with respect to scale (Reynolds number), boundary conditions (geometry and surface roughness) as well as inlet conditions is investigated. Due to the unsteady separation in the near-wall region the flow field is Reynolds number dependent. The depth of the boundary layer formed on the impinging surface decreases, while the maximum radial velocity increases with Reynolds number below a critical, Re_cr, value. Above one order of Re_cr the flow becomes asymptotically independent of Reynolds number. When Reynolds number reaches a fully roughness region the depth of the surface layer increases with roughness height only. The flow is found to be only weakly dependent on the distance between the jet and the surface for distances larger than the ring-vortex formation length. Radial confinements of diameters less than approximately 10 jet diameters and axial confinements placed at less than 1 jet diameter above the surface affect the pressure distribution on the impinging plate. The inlet turbulence affects mostly the free-jet flow region.     

Theoretical and experimental investigation of impinging jet ventilation and comparison with wall displacement ventilation

This paper focuses on evaluating the performance of a new impinging jet ventilation system and compares its performance with a wall displacement ventilation system. Experimental data for an impinging jet in a room are presented and non-dimensional expressions for the decay of maximum velocity over the floor are derived. In addition, the ventilation efficiency, local mean age of air and other characteristic parameters were experimentally and numerically obtained for a mock-up classroom ventilated with the two systems. The internal heat loads from 25 person-simulators and lighting were used in the measurements and simulations to provide a severe test for the two types of ventilation systems. In addition to a large number of experimental data CFD simulations were used to study certain parameters in more detail. The results presented here are part of a larger research programme to develop alternative and efficient systems for room ventilation.     

Experimental measurement and numerical simulation of an impinging jet with application to thunderstorm microburst winds

Wind profiles and characteristics in a thunderstorm downburst are significantly different from that of regular boundary layer winds. This paper deals with the experimental and numerical simulation of a type of thunderstorm wind, namely the microburst, to study the outflow velocity characteristics. The microburst is simulated as a round jet, impinging onto a flat plate. A generic empirical equation for radial velocity profile is developed based on the experimental data, using hotwire, pressure rakes and particle image velocimetry (PIV). The experimental results are used to validate CFD simulations and to find the applicability of different turbulence models for this kind of flow. Favorable agreement between numerical and experimental studies indicates that CFD can be used for this kind of complex flow.     

Parametric study on the along-wind response of the CAARC building to downbursts in the time domain

A primary investigation on the along-wind responses of tall buildings to thunderstorm downbursts is presented in this paper. Related characteristics of downbursts are first reviewed. By assuming that downbursts are fully correlated over the building height, a simple model is applied for downburst wind speeds. As a representative, the CAARC building is modelled as a linear 2-D cantilever beam and a base-pivoted single degree of freedom rigid body as well, and subjected to non-stationary conceptual, actual, and wind tunnel-simulated downburst velocity time histories. For comparison, a typical stationary wind speed record is also applied to the building. The maximum dynamic magnification factor (MDMF) is introduced as a non-dimensional variable to measure and compare the effect of non-stationary downbursts on the building. The definition of MDMF is the ratio of the maximum dynamic response to the corresponding maximum static response. Relationships between MDMF of the first normal coordinate and MDMFs of responses are derived. For the parametric study, MDMFs of the first normal coordinate for all the time histories are obtained as functions of the building first circular frequency and the first modal damping ratio. Effects of gust profiles and building characteristics on MDMFs and structural along-wind responses are observed theoretically and by the parametric study. The characterizing period for downbursts could be . Differences between MDMFs from the downbursts and those from the typical wind are noted. In practice, the procedure and the results presented here may be used to estimate the maximum dynamic response of a tall building subjected to severe downburst winds.     

Numerical simulations of triaxial shearing-infiltration tests

The failure of steep slopes during rainfall is commonly associated with a decrease in matric suction in the unsaturated soil zone above the water table. The shear strength characteristics of residual soil under water infiltration have been studied in the laboratory using unsaturated triaxial tests. This paper presents a development of a numerical model for simulating a triaxial shearing-infiltration test to investigate the shear strength characteristics of a compacted kaolin under infiltration condition. Both the hydraulic and mechanical responses of the compacted kaolin are modeled using the commercial software SIGMA/W and in-house software YS-Slope. The numerical analyses result and their validation against laboratory test results are presented and discussed in this paper. The results from the numerical analyses show good agreements with those from the laboratory tests, indicating that the proposed numerical model can be used to simulate the triaxial shearing-infiltration tests in laboratory.     

Numerical simulations of kinetic formation mechanism of Tangjiashan landslide

Tangjiashan landslide is a typical high-speed landslide hosted on consequent bedding rock.The landslide was induced by Wenchuan earthquake at a medium-steep hill slope.The occurrence of Tangjiashan landslide was basically controlled by the tectonic structure,topography,stratum lithology,slope structure,seismic waves,and strike of river.Among various factors,the seismic loading with great intensity and long duration was dominant.The landslide initiation exhibited the local amplification effect of seismic waves at the rear of the slope,the dislocation effect on the fault,and the shear failure differentiating effect on the regions between the soft and the hard layers.Based on field investigations and with the employment of the distinct element numerical simulation program UDEC(universal distinct element code),the whole kinetic sliding process of Tangjiashan landslide was represented and the formation mechanism of the consequent rock landslide under seismic loading was studied.The results are helpful for understanding seismic dynamic responses of consequent bedding rock slopes,where the slope stability could be governed by earthquakes.     

A comparison of three turbulence models for the prediction of parallel lobed jets in perforated panel optimization

The general context of the present study is the design of high induction HVAC air diffusers by means of passive jet control. When the diffuser is a perforated panel with lobed orifices (Meslem et al. 2010), the optimization of jet induction consists in improving the orifice’s geometry, the spacing between orifices and their arrangement on the panel. In this study, the flow field of a turbulent twin cross-shaped jet is investigated numerically using the standard k-ε model, the Shear Stress Transport (SST) k-ω model and the Reynolds Stress Model (RSM). The results are compared with PIV measurements. The objective is to assess their capability and limitations to predict the significant features of twin jet flow when the flow is numerically resolved through a lobed diffuser. It is shown that the k-ε and RSM models are more appropriate for predicting potential jet core length, the change in jet centreline streamwise velocity, and flow expansion in the symmetry plane of the twin jet flow. However, these models overestimate the overall flow expansion and the jet volumetric flow rate. The SST k-ω model seems more appropriate for the prediction of such dynamic integral quantities. A high level of turbulent kinetic energy predicted by the k-ε and RSM models in the near field of jets is probably the reason for this overestimation of jet induction. The SST k-ω model would appear to be the most appropriate tool for optimizing orifice design, orifice to orifice spacing and relative orifice orientation on a perforated panel diffuser.     

Study of FDS simulations of buoyant fire-induced smoke movement in a high-rise building stairwell

Numerical simulations of fire-induced smoke movement in the stairwell of a high-rise building are conducted using FDS, version 6.0.1, with default settings. Twelve scenarios are considered. The required fineness of the grid has been determined in earlier work by considering both the fire source and the vent flow, and by assessing the velocity profile at the bottom opening and the vertical distribution of temperature in the stairwell. In the present study, the results including the airflow velocity at the bottom opening, vertical distribution of temperature, the temperature at the middle opening, pressure distribution, and neutral plane height in the stairwell, are compared to experimental data. For the average velocity through the bottom opening, a maximum deviation of 16.23% is obtained. Good agreement is achieved for the vertical temperature inside the stairwell (maximum relative deviation of 12.3%). By analyzing the temperature at the middle opening, it is found that the smoke moves faster than in the experiment. The influence of the staircase on the pressure distribution is demonstrated by comparing two cases: one with and one without staircase. The difference between the pressure inside the stairwell and the pressure outside increases with height, due to fire-induced buoyancy. However, the pressure difference evolution is non-monotonic when there are staircases inside the stairwell. The neutral plane height value, as obtained by post-processing the simulation results, is too high in the simulations, compared to experimental data and the corresponding analytical expression. Finally, the influence of the turbulence model is shown to be negligible.     

Numerical simulations of wind measurements at Amsterdam Airport Schiphol

In this paper two typical results of numerical simulations of wind measurements at an airport are discussed. The first case deals with the long reach of some isolated individual ‘roughness elements’. The positioning of the farms and haystacks result in a weak vortex which travels over a very long distance. The second case is about the influence of an industrial complex just outside the airport premises. In this case the actual wake behind the buildings is the dominant flow feature. The numerical simulations are able to explain the measured disturbances.     

基于ANSYS的深基坑土钉墙支护数值模拟

采用有限元软件ANSYS程序对基坑开挖进行数值模拟,以土钉墙为例得出每步开挖的土体位移变形图,得出该方法在深基坑支护设计应用中有很强的实用性、可靠性和经济性,对基坑设计及工程施工起到了很好的指导作用。     

Surface reconstruction with spherical harmonics and its application for single particle crushing simulations

Particle morphology has great influence on mechanical behaviour and hydro/thermal/electrical conductivities of granular materials.Surface reconstruction and mesh generation are critical to consider realistic particle shapes in various computational simulations.This study adopts the combined finitediscrete element method(FDEM)to investigate single particle crushing behaviour.Particle shapes were reconstructed with spherical harmonic(SH)in both spherical and Cartesian coordinate systems.Furthermore,the reconstructed surface mesh qualities in two coordinate systems are investigated and compared.Although the efficiency of the two SH systems in reconstructing star-like shapes is nearly identical,SH in Cartesian coordinate system can reconstruct non-star-like shapes with the help of surface parameterisation.Meanwhile,a higher triangular mesh quality is generated with spherical coordinate.In single particle crushing tests,the low mesh quality produces more fluctuations on load-displacement curves.The particles with more surficial mesh elements tend to have a lower contact stiffness due to more contact stress concentrations induced by complexity of morphology features and more volumetric tetrahedral elements.The fracture patterns are also influenced by mesh quality and density,e.g.a particle with fewer mesh elements has a simpler fragmentation pattern.This study serves as an essential step towards modelling particle breakage using FDEM with surface mesh directly from SH reconstruction.     

Numerical study on static and dynamic fracture evolution around rock cavities

In this paper, a numerical code, RFPA^2D (rock failure process analysis), was used to simulate the initiation and propagation of fractures around a pre-existing single cavity and multiple cavities in brittle rocks. Both static and dynamic loads were applied to the rock specimens to investigate the mechanism of fracture evolution around the cavities for different lateral pressure coefficients. In addition, characteristics of acoustic emission (AE) associated with fracture evolution were simulated. Finally, the evolution and interaction of fractures between multiple cavities were investigated with consideration of stress redistribution and transference in compressive and tensile stress fields. The numerically simulated results reproduced primary tensile, remote, and shear crack fractures, which are in agreement with the experimental results. Moreover, numerical results suggested that both compressive and tensile waves could influence the propagation of tensile cracks; in particular, the reflected tensile wave accelerated the propagation of tensile cracks.     

雷暴冲击风作用下球壳型屋面模型风压特性试验研究

以球壳型大跨度屋面为研究对象,进行雷暴冲击风作用下的刚性模型风洞试验。按照相似准则制作了不同类型球壳型屋面模型,利用射流装置、粗糙元等模拟了雷暴冲击风流场,研究了失跨比、高跨比及同一矢跨比下的跨度等结构参数以及屋面所处流场位置的变化对冲击风作用下屋面的风压极值位置及其数值大小、气流分离点位置等的影响。试验结果与CFD数...     

特殊单立管排水系统消除支管水舌探讨

排水支管水舌直接关系到排水与通气集于一根管道的特殊单立管排水系统的排水能力和气压稳定,小文就特殊单立管排水系统如何尽量消除支管水舌影响展开探讨,可为特殊单立管排水系统特殊管件的研发、改进和工程设计提供一些思路。     

Effect of fibres on early age cracking of concrete tunnel lining. Part II: Numerical simulations

The early-age cracking of concrete structures increases permeability and diffusivity and moreover accelerates the penetration of liquid, gas and aggressive agents. Consequently, the serviceability of these structures could be reduced drastically. Early-age cracking might be due to external loading, but also to the internal or external restraint resulting from autogenous, drying and thermal shrinkage. This study focuses more specifically on these latter phenomena.In the first part of this study (see effect of fibres on early-age cracking of concrete tunnel lining - Part I: Laboratory testing), ring tests were performed to investigate the sensitivity of concrete to cracking due to both shrinkage strain and type of fibre (two organic fibres and one steel fibre were studied).Ring test results were then used to validate the capacity of a chemo-thermo-viscoelastic damage model aimed at reproducing the complex behaviour of fibre-reinforced concrete subjected to restrained shrinkage through identifying the material parameters with standardised tests. The numerical simulations conducted on a real tunnel lining show that for the studied geometries and concrete mixtures, thermal shrinkage constitutes the major phenomenon capable of causing early-age transverse cracks and moreover crack opening is highly dependent on the type of reinforcement. Modifications to both fibre type and lining thickness may serve to avoid the onset of transverse cracks.     

Numerical simulations on fire spread and smoke movement in an underground car park

The Fire Dynamic Simulator code is used to investigate fire spread and smoke movement in a large underground car park under different fire scenarios. Initially, by comparing with experimental results of heat release rate of a single car fire, the development of car fire is designed by letting surface densities of the fuel over the car. Fire spread and movement of smoke are then investigated under different ventilation conditions. Simulated results show that the development of car fire in the underground car park can be classified into four stages; namely an initial stage, a developed stage, an extinction and re-burning stage and another fast-developed stage. Affected by ventilation systems, fire develops rapidly resulting in consuming most oxygen quickly followed by early extinction of the fire. After extinction of the fire, with more ambient air drawn into the car park due to ventilation, re-ignition takes place with accelerated development. In addition, detailed field distributions of temperature and velocity vectors are given. It is found that the smoke layer decent to the top of the car after 15 min and the hot smoke flows in a disorderly manner resulting in the spread of fire more rapidly.     

Numerical simulations of wind-driven rain on building envelopes based on Eulerian multiphase model

A numerical simulation approach for evaluation of wind-driven rain (WDR) on building envelopes is presented based on Eulerian multiphase model. Unlike existing methods, which are generally on the basis of Lagrange frame to deal with raindrop motions by trajectory-tracking techniques, the present approach considers both wind and rain motions and their interactions under Euler frame. By virtue of the Eulerian multiphase model, the present method could significantly reduce the complexity in evaluations of WDR parameters, simplify the boundary condition treatments and is more efficient to predict transient states of WDR, spatial distributions of rain intensity, impacting rain loads on building surfaces, etc. A numerical example shows that the simulation results by the present method agree well with available experimental and numerical data, verifying the accuracy and reliability of the WDR simulation approach based on the Eulerian multiphase model. It is also demonstrated through the validation example that the present method is an effective tool for numerical evaluations of WDR on building envelopes.     

圆弧焊接薄壁圆筒残余应力场的分析研究

由于圆弧焊接的薄壁圆筒在高科技工程,如航空机器、压力容器和核工程中的广泛应用,对焊接导致的如焊接变形和残余应力等缺陷的研究就显得非常重要。介绍了一个计算程序,用以分析低碳钢薄壁圆筒在圆弧焊接中的温度分布以及随后将产生的残余应力场。基于数值模拟的参数研究用于分析焊接所产生的残余应力中的关键参数的影响。考虑了低碳钢、焊丝和双椭圆热源模型的基于温度的热机械性能。通过全尺寸焊接试验中的瞬时温度分布和残余应力场证实了有限元模拟方法的精确性。其目的在于提供数据以证实薄壁圆筒现有焊接工艺的有效性,从而将预加应力导致的结构在使用期间失效的可能性降至最低。     

Low-level atmospheric jets as main mechanism of long-range transport of power plant plumes in the Lake Baikal Region

This paper considers some peculiarities of anthropogenic admixtures propagation from large regional Coal Power Plants towards the South Baikal using experimental data and model estimates. Results of observations show that the main mechanism for transfer of atmospheric admixtures towards the South Baikal is low-level atmospheric jets having high velocity and a weak turbulent mixing. A weak mixing of emitted plume with the surrounding air (within a jet flow) results in deficiency of oxidants in it and in slowing of chemical transformation of some admixtures during the transfer.