Gas dispersion near a cubical model building. Part I. Mean concentration measurements

The dispersion of effluent plumes emitted on or in the near-wake region ($\text{x/H ? 5.0}$) of a cubical model building has been examined. The model study was performed in a wind tunnel with a simulated neutrally stratified shear layer. Mean concentration measurements were made on the model building for three different roof vent locations and three different building orientations. A full-scale measurement was conducted in the near-wake region for central roof vent release.The concentration level on the lee face of a model building is greatly reduced by the presence of a sharp edge on the model. The optimum location for the intake vent on the building, for equal vent exhaust to vent intake distance, is a position away from the downwind direction and where it cannot “see” the exhaust vent. Orientation of the building at an angle of 45° results in a secondary peak concentration on the building and in the near-wake region.     

Scalar dispersion in model building wakes

This work is a study of the dispersion processes operating within the near aerodynamic wake of surface-mounted building models in deep turbulent boundary layers. A light-scattering technique was used to measure the residence times of a particulate tracer, both locally and globally, within the wakes of a number of models. The model geometry and orientation were chosen to provide several quite different “types” of wake, from momentum wakes to those dominated by coherent axial vortex structures. The major conclusion is that turbulent diffusion is the dominant mechanism of tracer dispersal, even for vortex-dominated wakes. The effects of the vortex structures are manifest in a reduction in size of the wake “bubble”, which then has a unique relationship to the residence time.     

Transport and deposition of particles near a building model

Particle transport, dispersion and deposition near a building model were studied. The stress transport model of FLUENT™ code was used for simulating the mean airflow. The instantaneous turbulence fluctuating velocity was simulated by a Gaussian-filtered white-noise model. A Lagrangian particle tracking computational procedure was developed and was used for simulating particle transport and deposition in the vicinity of the building model. The computational model accounted for the drag and lift forces acting on the particle, as well as the effect of Brownian force, in addition to the gravitational sedimentation effects. For particles in the size range of 0.01–10 μm, the corresponding deposition rates on various surfaces of the building model were evaluated.     

Dynamic sub-grid scale turbulent Schmidt number approach in large eddy simulation of dispersion around an isolated cubical building

In this paper, the effects of the sub-grid scale (SGS) turbulent Schmidt number, Sc _SGS, on the large eddy simulation of dispersion on and around an isolated cubical model building with a flush vent located on its roof are examined. Constant and dynamic Sc _SGS approaches for SGS turbulent mass flux modeling are employed. Simulation results are compared with the available wind tunnel measurements. Furthermore, the influence of the grid resolution on the accuracy of results predicted by the dynamic Sc _SGS approach is investigated. Detailed statistical analysis of Sc _SGS demonstrates that the dynamically computed Sc _SGS at different locations varies by a factor of almost 5 and a considerable deviation of Sc _SGS from its common values of 0.5 and 0.7 occurs. Particularly, in the vicinity of the building where the concentration gradients are noticeable, Sc _SGS has a larger variation. Also, the probability of occurrence of 0.2 < Sc _SGS <1.5 is more than 90 percent and the Sc _SGS mean values are nearly around 0.8 to 1 with a maximum variance of 0.2. In addition, by refining the grid, the differences between the predictions of constant and dynamic Sc _SGS approaches decrease. This is due to the reduction of sub-grid scales contribution to turbulent dispersion. It is confirmed that dynamic Sc _SGS approach is a practical alternative to the constant Sc _SGS approach, effectively eliminating a user-defined model coefficient.     

Prediction of the pressure distribution on a cubical building with implicit LES

This paper presents a numerical study of the flow around a cubical building in an atmospheric boundary layer. The Reynolds number of the flow is Re=4×10⁶. Different types of turbulence models, steady state RANS, hybrid RANS/LES and LES, are used and the simulation results are compared to field measurement data of the mean pressure distribution. The objective is to build an approach to perform simulations on coarse grids with low computational cost. The outcome is that the Implicit LES (ILES) method is the most accurate for coarse grid simulations. In order to verify the sensitivity of the results to the Reynolds number, also simulations of a wind tunnel experiment at Reynolds number 4×10⁴ are performed. We demonstrate that also for lower Reynolds numbers, although not optimal, the ILES approach leads to quite good results.     

Smoke measurements in large-and small-scale fire testing — Part II

A new extinction photometer design has been developed that will provide the stability and accuracy necessary for taking smoke measurements in fire testing.     

Scale-adaptive simulation of unsteady flow and dispersion around a model building: spectral and POD analyses

The present paper evaluates the relative performance of scale-adaptive simulation (SAS) in modelling unsteady concentration and flow fields around a model building relative to other transient simulations such as large eddy simulation (LES) and unsteady Reynolds-averaged Navier–Stokes (URANS) models. A novel application of proper orthogonal decomposition (POD) and time–frequency analysis is carried out in order to evaluate the transient behaviour and dominant structures of the flow fields predicted by SAS and LES. Results represent the outstanding performance of SAS in comparison with the URANS computation based on the SST k–ω model. This better performance is related to the accurate reproduction of unsteady fluctuations around the model building by SAS. In addition, the quantitative and qualitative agreements for the shapes and magnitudes of POD modes between SAS and LES confirm the LES-like behaviour of SAS in the wake region. However, in terms of computational performance, SAS imposes an extra CPU cost as compared with LES for the same grid resolution.     

Numerical simulation of pollutant dispersion around a building complex

The dispersion of exhausted pollutants from a building roof stack situated in the wake of a neighbouring tower has been studied using computational fluid dynamics (CFD) with the realizable k–? turbulence model for closure. Two scales are considered in this work, full-scale (1:1) and wind tunnel scale (1:200).Of primary interest are the distributions of the plume and of the pollutant concentrations on the building roof as well as on the leeward wall of the tower. Two stack heights and pollutant exhaust velocities have been considered for the distribution of pollutant concentrations in the neighbourhood of the building from which the pollutant is emitted. Results are compared with measurements from field and wind tunnel experiments to estimate the accuracy of simulations.     

A two-layer turbulence model for simulating indoor airflow: Part II. Applications

Energy efficient buildings should provide a thermally comfortable and healthy indoor environment. Most indoor airflows involve combined forced and natural convection (mixed convection). This paper used a two-layer model to predict such flows. This model used a one-equation model for near-wall region and the standard k–ε model for the outer-wall region. Its validation in six cases shows good agreements between the computed results and measured data. In addition, the computing cost required by the model has been reduced significantly.     

Across-wind vibrations of structure of circular cross-section. Part II. Development of a mathematical model for full-scale application

A model is presented for predicting the across-wind response of slender structures of circular cross-section to turbulent shear flow. Fundamental aspects of the model have been described in a companion paper [1], the key feature of the model being the representation of all motion-dependent phenomena by a nonlinear damping force. The development described in this paper is concerned with extending the earlier work, which was confined to two-dimensional laboratory configuration, such that the model is able to provide estimates of response for full-scale structures.Based on the work outlined above, a computer program was developed to obtain estimates of response in full-scale situations. The program was used to compute the response of a 330 m TV tower and the results were compared with the measured response. In general, the computed and measured responses are in fair agreement.The uncertainties associated with some of the input parameters are noted, and are considered a greater hindrance to further development than any theoretical difficulties.     

Pollutant dispersion in a large indoor space. Part 2: Computational fluid dynamics predictions and comparison with a scale model experiment for isothermal flow

This paper reports on an investigation of the adequacy of computational fluid dynamics (CFD), using a standard Reynolds Averaged Navier-Stokes (RANS) model, for predicting dispersion of neutrally buoyant gas in a large indoor space. We used CFD to predict pollutant (dye) concentration distribution in a water-filled scale model of an atrium with a continuous pollutant source in the absence of furniture and occupants. Predictions from the RANS formulation are comparable with an ensemble average of independent identical experiments. Model results were compared with pollutant concentration data in a horizontal plane from experiments in a scale model atrium. Predictions were made for steady-state (fully developed) and transient (developing) pollutant concentrations. Agreement between CFD predictions and ensemble averaged experimental measurements is quantified using the ratios of CFD-predicted and experimentally measured dye concentration at a large number of points in the measurement plane. Agreement is considered good if these ratios fall between 0.5 and 2.0 at all points in the plane. The standard k-epsilon two-equation turbulence model obtains this level of agreement and predicts pollutant arrival time to the measurement plane within a few seconds. These results suggest that this modeling approach is adequate for predicting isothermal pollutant transport in a large room with simple geometry. PRACTICAL IMPLICATIONS: CFD modeling of pollutant transport is becoming increasingly common but high quality comparisons between CFD and experiment remain rare. Our results provide such a comparison. We demonstrate that the standard k-epsilon model provides good predictions for both transient and fully developed pollutant concentrations for an isothermal large space where furnishings are unimportant. This model is less computationally intensive than a large eddy simulation or low Reynolds number k-epsilon model.     

Pollutant dispersion in a large indoor space: Part 1 -- Scaled experiments using a water-filled model with occupants and furniture

Scale modeling is a useful tool for analyzing complex indoor spaces. Scale model experiments can reduce experimental costs, improve control of flow and temperature conditions, and provide a practical method for pretesting full-scale system modifications. However, changes in physical scale and working fluid (air or water) can complicate interpretation of the equivalent effects in the full-scale structure. This paper presents a detailed scaling analysis of a water tank experiment designed to model a large indoor space, and experimental results obtained with this model to assess the influence of furniture and people in the pollutant concentration field at breathing height. Theoretical calculations are derived for predicting the effects from losses of molecular diffusion, small scale eddies, turbulent kinetic energy, and turbulent mass diffusivity in a scale model, even without Reynolds number matching. Pollutant dispersion experiments were performed in a water-filled 30:1 scale model of a large room, using uranine dye injected continuously from a small point source. Pollutant concentrations were measured in a plane, using laser-induced fluorescence techniques, for three interior configurations: unobstructed, table-like obstructions, and table-like and figure-like obstructions. Concentrations within the measurement plane varied by more than an order of magnitude, even after the concentration field was fully developed. Objects in the model interior had a significant effect on both the concentration field and fluctuation intensity in the measurement plane. PRACTICAL IMPLICATION: This scale model study demonstrates both the utility of scale models for investigating dispersion in indoor environments and the significant impact of turbulence created by furnishings and people on pollutant transport from floor level sources. In a room with no furniture or occupants, the average concentration can vary by about a factor of 3 across the room. Adding furniture and occupants can increase this spatial variation by another factor of 3.     

Developing a model for fibrous building materials

Ventilation systems are now used to ensure optimal indoor temperature and humidity in contemporary buildings such as showrooms, museums, offices, and homes. Winter indoor humidity can be very low due to the low humidity contained within fresh outdoor air supply. Humidification becomes necessary to raise indoor humidity, which also raises primary energy demand.Three approaches have been examined in a research project to reduce humidification/dehumidification energy consumption:

• (1) 

  Moisture storage (absorption and emission of moisture-peaks)

• (2) 

  Air flow control optimization

• (3) 

  Moisture recovery by the ventilation system

This paper focuses on the first approach, air humidification and dehumidification using moisture storage. A model was developed to illustrate both the microscopic and macroscopic hysteresis of moisture storage, and transport capacities of fibrous materials. The necessary parameters for the model have been obtained using measurements from a number of different materials that were used as humidity buffers. Precise equipment to measure humidity was constructed, tested, and used over a very long measurement period, during which detailed measurements of moisture absorption and emission were measured from different types of fibreboard sheets. The simulations generated by the model showed very good agreement with the measured results.     

达州环城路加油站顺层岩质边坡稳定性分析

对达州环城路加油站顺层岩质边坡稳定性进行了现状和预测性分析,确定该边坡层面和裂隙面多为不稳定结构面;开挖后,在外部营力作用下,边坡可能沿岩层面发生整体或局部的滑动破坏;以坡脚位置层面为潜在滑动面进行分析,暴雨工况下边坡不稳定;提出了防治方案建议。研究结论能够对类似的工程有一定的借鉴作用。     

Analysis of full-scale wind and pressure measurements on a low-rise building

Significance of full-scale experiments, analyzing wind and pressure fields in the proximity or on low-rise buildings, is evident from the attention that has been dedicated by researchers to these programs in the recent past. In the south and southeastern regions of US this problem is of particular relevance due to the presence of hurricanes and high-speed winds. This paper presents some recent results derived from a three-year monitoring of a structure located near the coast of North Carolina.In the first part of this study, attention is devoted to the characterization of the wind field around an instrumented house; a comprehensive investigation on wind velocity and turbulence characteristics during the passage of three tropical storms and other significant events is summarized. In the second part, results associated with the meteorological studies are used to assist the interpretation of pressure time histories related to such extreme events.Analyses associated with the derivation of normalized pressure coefficients were concentrated on the identification of direction-dependent pressure characteristics, correlation among consecutive taps and potential effects of the wind unsteadiness on the maximum and minimum values. Building geometry and local topography effects had an important and direct influence on these analyses.     

央视新址火灾对幕墙的反思（二）

本文通过对央视新址附楼火灾和近年幕墙、门窗、采光顶一些火灾的剖析反思,阐明了积极防范幕墙、门窗、采光顶建筑物火灾危险的重要性。结合新的防火规范及经验,从专业角度提出建筑幕墙、门窗、采光顶防火在设计和施工方面的技术处理要点。     

A wind-tunnel study on exhaust-gas dispersion from road vehicles—Part II: Effect of vehicle queues

By a reduced-scale wind-tunnel experiment, we investigate the dispersion behavior of exhaust gas from automobiles. Based on the results of single-vehicle cases in Part I of our work, we consider vehicle queues consisting of passenger cars (P) and small-size trucks (T). The roles of the vehicles before and after the gas-emitting vehicle are as follows. By their turbulent wake, the preceding vehicles make the concentration field approximately symmetric in the spanwise direction despite the significant lateral offset of the exhaust pipe. The vehicles behind the gas-emitter expand the exhaust plume considerably in the spanwise direction; T expands the plume also in the vertical direction in its roll-up wake, while P scoops up the oncoming plume only around the centerline. For vehicle queues of mixing ratio P:T=2:1, the concentration fields when one of the component vehicles emit the gas are measured. It turns out that the plume shape is determined mostly by the type of the gas-emitting vehicle whereas the type of the following vehicles has minor effect. We also present an analytical procedure to approximate the overall contribution from all the queue-forming vehicles by a Gaussian line-source plume formula.     

A unified model for the simulation of oil,gas and biomass space heating boilers for energy estimating purposes. Part II: Parameterization and comparison with measurements

A semi-physical model for the simulation of oil, gas and biomass space heating boilers has been parameterized based on measurements on nine different boiler units and simulation results have been compared to results obtained from measurements in steady state and transient operation. Although the agreement between simulated and measured boiler efficiencies was within the range of measurement uncertainties in most cases, model improvements are expected to be possible concerning the heat capacitance modelling in cycling on/off operation as well as influences of start and stop behaviour on the overall efficiency. It is found that electricity consumption during cycling on/off operation of small pellets or oil space heating boilers may have a significant influence on the overall energy balance of these units. This influence increases strongly with decreasing heat load and increasing number of on/off cycles.     

Integration of chaos theory and mathematical models in building simulation: Part II: Conceptual frameworks

Current mathematical models in building research have been limited in most studies to linear dynamics systems. A literature review of past studies investigating chaos theory approaches in building simulation models suggests that as a basis chaos model is valid and can handle the increasing complexity of building systems that have dynamic interactions among all the distributed and hierarchical systems on the one hand, and the environment and occupants on the other. The review also identifies the paucity of literature and the need for a suitable methodology of linking chaos theory to mathematical models in building design and management studies. This study is broadly divided into two parts and presented in two companion papers. Part (I), published in the previous issue, reviews the current state of the chaos theory models as a starting point for establishing theories that can be effectively applied to building simulation models. Part (II) develop conceptual frameworks that approach current model methodologies from the theoretical perspective provided by chaos theory, with a focus on the key concepts and their potential to help to better understand the nonlinear dynamic nature of built environment systems. Case studies are also presented which demonstrate the potential usefulness of chaos theory driven models in a wide variety of leading areas of building research. This study distills the fundamental properties and the most relevant characteristics of chaos theory essential to (1) building simulation scientists and designers (2) initiating a dialogue between scientists and engineers, and (3) stimulating future research on a wide range of issues involved in designing and managing building environmental systems.