A venturi-shaped roof for wind-induced natural ventilation of buildings: Wind tunnel and CFD evaluation of different design configurations

Wind tunnel experiments and Computational Fluid Dynamics (CFD) are used to analyse the flow conditions in a venturi-shaped roof, with focus on the underpressure in the narrowest roof section (contraction). This underpressure can be used to partly or completely drive the natural ventilation of the building zones. The wind tunnel experiments are performed in an atmospheric boundary layer wind tunnel at scale 1:100. The 3D CFD simulations are performed with steady RANS and the RNG k-? model. The purpose of this study is twofold: (1) to evaluate the accuracy of steady RANS and the RNG k-? model for this application and (2) to assess the magnitude of the underpressures generated with different design configurations of the venturi-shaped roof. The CFD simulations of mean wind speed and surface pressures inside the roof are generally in good agreement (10–20%) with the wind tunnel measurements. The study shows that for the configuration without guiding vanes, large negative pressure coefficients are obtained, down to −1.35, with reference to the free-stream wind speed at roof height. The comparison of design configurations with and without guiding vanes shows an – at least at first sight – counter-intuitive result: adding guiding vanes strongly decreases the absolute value of the underpressure. The reason is that the presence of the guiding vanes increases the flow resistance inside the roof and causes more wind to flow over and around the roof, and less wind through it (wind-blocking). As a result, the optimum configuration is the one without guiding vanes.     

Experimental investigations of the indoor natural ventilation for different building configurations and incidences

This paper presents an experimental investigation of the indoor natural ventilation in terms of wind pressures on the surfaces of cubic buildings of a street located within a high density urban area. Wind tunnel tests over 1:100 scale models for four typical building patterns of a highly populated urban area have been carried out. The variables of the experiments were the building configurations and the incident wind direction. The experimental data are presented in terms of wind pressure coefficient measured on the surfaces of the buildings. The study results gave the evidence that buildings configuration and wind direction are very important factors in determining the induced natural ventilation within urban domains since they characteristically influence the flow yielding differences in wind pressures.     

Evaluating emergency ventilation strategies under different contaminant source locations and evacuation modes by efficiency factor of contaminant source (EFCS)

Emergency ventilation plays an important role in protecting occupants when a hazardous contaminant is released indoors. A number of studies have been conducted to better understand how to protect indoor occupants with effective ventilation strategies. However, little attention has been paid to the impact of the non-uniform and time-dependent distribution of occupants during evacuation. A new concept, Efficiency Factor of Contaminant Source (EFCS), has recently been proposed to evaluate the performance of emergency ventilation by comprehensively considering the spatial and temporal distributions of both the contaminant and occupants. This paper aims to: (1) propose and demonstrate a procedure for determining an optimal ventilation strategy by using EFCS; (2) examine the effects of source locations, ventilation modes, and evacuation modes on the performance of emergency ventilation. One hundred cases with ten ventilation modes, two evacuation modes, and five source locations were investigated numerically. The results show that the EFCS concept can provide a reasonable way to evaluate the performance of emergency ventilation. The threats of different source locations may vary over a large range, and certain measures should be taken to monitor and prevent the releases at high threat locations. A system equipped with multiple ventilation modes is necessary since no universal ventilation mode can successfully mitigate all hazardous situations. The effects of an evacuation mode may be more significant than that of a ventilation mode under certain situations.     

An analytical and numerical study of solar chimney use for room natural ventilation

The solar chimney concept used for improving room natural ventilation was analytically and numerically studied. The study considered some geometrical parameters such as chimney inlet size and width, which are believed to have a significant effect on space ventilation. The numerical analysis was intended to predict the flow pattern in the room as well as in the chimney. This would help optimizing design parameters. The results were compared with available published experimental and theoretical data. There was an acceptable trend match between the present analytical results and the published data for the room air change per hour, ACH. Further, it was noticed that the chimney width has a more significant effect on ACH compared to the chimney inlet size. The results showed that the absorber average temperature could be correlated to the intensity as: (T_w = 3.51I^0.461) with an accepted range of approximation error. In addition the average air exit velocity was found to vary with the intensity as (ν_ex = 0.013I^0.4).     

A comparison between CFD and Network models for predicting wind-driven ventilation in buildings

The aim of this paper is to compare the implementation of computational fluid dynamics (CFD) and Network models for airflow rate estimation in buildings. The CFD software used is Fluent 5.5. Comparison between the predicted and simulated airflow rate is suggested as a validation method of the implemented CFD code, while the common practice is to compare CFD outputs to wind tunnel or full-scale measurements. This could be useful for studies that have no access to laboratory or full-scale testing facilities. Results obtained from testing a number of cases have been compared and analysed, considering normal and oblique wind directions. The comparison held between mathematical and CFD results generally showed a good agreement, which seems to justify the use of CFD code for predicting natural ventilation in buildings.     

Reduced-scale building model and numerical investigations to buoyancy-driven natural ventilation

Similarities of a reduced-scale building model using air as the working fluid for buoyancy-driven natural ventilation have been analyzed and experiments were carried out using the scaled model for a common natural ventilation building, which has open office floor plans connected to a central atrium. Both open and closed cases have been investigated for the stack vents, located at the top of the atrium. Inputs for the scaled building model were taken from results measured in the prototype building by the authors. The parameters of the scaled building model's experiments thus were used as inputs into a computational fluid dynamics (CFD) simulation model to compare predicted and measured airflow patterns, temperatures and velocity distributions in the scaled building model.     

Characterization and prediction of the volume flow rate aerating a cross ventilated building by means of experimental techniques and numerical approaches

The paper presents an extensive experimental and numerical study on a cross-ventilated building providing important features of the induced flow patterns at the two openings as a function of the free stream wind velocity's magnitude and its incidence angle. The experimental data are measured via anemometers across the openings, whilst the numerical methodology is based on the time-dependant solution of the governing Navier-Stokes equations. The experimental data are compared to the corresponding numerical results, revealing the unsteady character of the flow field especially at large incidence angles. Furthermore, additional information regarding the flow field near the opening edges, not easily extracted by experimental methods, provide an in depth sight in the main characteristics of the flow field both at the openings but also inside the building. Finally, a new methodology for the approximation of the volume flow rate aerating the building based on experimental measurements of the velocity field at the openings is presented.     

Model development and experimental validation of a floriculture greenhouse under natural ventilation

Greenhouse technology is an effective method of cultivation of flowers, crops, etc. under controlled environment. For any greenhouse, ventilation performance is a major factor in production, influencing the yield and quality of the products. Natural ventilation can be effectively used to maintain greenhouse microclimate, conducive to plant growth, when the ambient conditions are not extreme. The present paper discusses the modeling aspects of a floriculture greenhouse suitable for operation in typical Indian climate under natural ventilation. Combined ridge and sidewall ventilation is considered in the model. The model is validated against the test results of an experimental greenhouse. Parametric analysis is also done to understand the effects of variations in parameters such as wind speed, solar radiation intensity, effective greenhouse height etc. The study reveals that the performance of a greenhouse under natural ventilation is influenced considerably by parameters such as intensity of solar radiation, effective distance between the side and the roof vents, free wind speed etc.     

体育馆形态的非对称性与自然通风

该文针对三种形态体育馆形态,利用CFD模拟出各自的风环境,探讨对比在相同的边界条件下,不同的体育馆形态对于室内风环境的耦合影响,进而为建筑师选择空间形态提供重要的科学依据。     

A parametric analysis of a tunnel climatic prediction and planning model

This paper presents results of a series of detailed parametric studies analysis conducted on a computer based tunnel climatic prediction model developed at the University of Nottingham.An extensive number of computer simulations have been undertaken using the drivage model. Sensitivity exercises have been undertaken to analyze the effect that variations to the various input and computational parameters have on the predicted climatic conditions. The results of these model sensitivity exercises have been compared against both manually measured and continuously recorded digital climatic survey data collected from within a rapid development drivage of a representative UK coal colliery. The conclusions drawn from these comparative analyses are reported.The influence of the main climate model variables studied are the rock thermal parameters, the total efficiency of the auxiliary forcing fan, and the electrical equipment utilization (i.e., percentage of power loading given off of sensible heat into airstream). The results of these analyses have identified the range of values that may be taken by these parameters in order that the tunnel climate prediction model satisfactorily reproduces the measured psychrometric conditions.     

Natural cross-ventilation in buildings: Building-scale experiments,numerical simulation and thermal comfort evaluation

The constantly increasing energy consumption due to the use of mechanical ventilation contributes to atmospheric pollution and global warming. An alternative method to overcome this problem is natural ventilation. The proper design of natural ventilation must be based on detailed understanding of airflow within enclosed spaces, governed by pressure differences due to wind and buoyancy forces. In the present study, natural cross-ventilation with openings at non-symmetrical locations is examined experimentally in a test chamber and numerically using advanced computational fluid dynamics techniques. The experimental part consisted of temperature and velocity measurements at strategically selected locations in the chamber, during noon and afternoon hours of typical summer days. External weather conditions were recorded by a weather station at the chamber's site. The computational part of the study consisted of the steady-state application of three Reynolds-Averaged Navier-Stokes (RANS) models modified to account for both wind and buoyancy effects: the standard k–?, the RNG k–? and the so-called “realizable” k–? models. Two computational domains were used, corresponding to each recorded wind incidence angle. It is concluded that all turbulence models applied agree relatively well with the experimental measurements. The indoor thermal environment was also studied using two thermal comfort models found in literature for the estimation of thermal comfort under high-temperature experimental conditions.     

Investigation of air quality,comfort parameters and effectiveness for two floor-level air supply systems in classrooms

The method of distributing the outdoor air in classrooms has a major impact on indoor air quality and thermal comfort of pupils. In a previous study, ([11] Karimipanah T, Sandberg M, Awbi HB. A comparative study of different air distribution systems in a classroom. In: Proceedings of Roomvent 2000, vol. II, Reading, UK, 2000. p. 1013–18; [13] Karimipanah T, Sandberg M, Awbi HB, Blomqvist C. Effectiveness of confluent jets ventilation system for classrooms. In: Idoor Air 2005, Beijing, China, 2005 (to be presented).) presented results for four and two types of air distribution systems tested in a purpose built classroom with simulated occupancy as well as computational fluid dynamics (CFD) modelling.     

Verification of the accuracy of CFD simulations in small-scale tunnel and atrium fire configurations

In preparation for the use of computational fluid dynamics (CFD) simulation results as ‘numerical experiments’ in fire research, the agreement with experimental data for two different small-scale set-ups is discussed. The first configuration concerns the position of smoke-free height in case of fire with vertical ventilation in an atrium. The second set-up deals with the critical velocity for smoke backlayering in case of fire in a horizontally ventilated tunnel. An N-percent rule is introduced for the determination of the presence of smoke in the simulation results, based on the local temperature rise. The CFD package FDS is used for the numerical simulations. The paper does not scrutinize the detailed accuracy of the results, as this is hardly possible with any state-of-the-art experimental data at hand. Rather, the global accuracy is discussed with current numerical implementation and models in FDS, considering continuous evolution over different version releases with time. The agreement between the experiments and numerical simulations is very promising. Even when quantitative agreement with experimental data is not perfect, the trends are very well reproduced in the simulations. While much additional work is required, both in CFD as in ‘real’ experiments, the results are encouraging for the potential of state-of-the-art CFD to be used as numerical experiments.     

Experimental and numerical study of room airflow under stratum ventilation

This paper investigates the air movement, air temperature profile and gaseous contaminant transportation in an individual office with stratum ventilation. The room temperature is elevated compared with conventional standards. The experimental investigation is carried out in an environmental chamber with the presence of heat generating rectangles used to simulate an occupant and a computer. Measurements of temperature, velocity, and CO₂ concentration are carried out for nine plumb lines in the chamber. Up to sixteen points are measured along each plumb line. The experimental data of the aforesaid three parameters of the individual office in warm condition under stratum ventilation are presented. The experimental data collected are used to validate a re-normalization group (RNG) k–? turbulence model used for the warm condition. The agreements between the predicted values and experimental results are acceptable, which demonstrates the feasibility of simulating indoor airflows at elevated room temperature under stratum ventilation by the RNG k–? turbulence model.     

Modelling of cowl performance in building simulation tools using experimental data and computational fluid dynamics

Exhaust cowls are used in conjunction with hybrid ventilation systems to efficiently convert wind energy into negative pressure and thus minimize the electrical energy required by the extract fan. Yet the fact that cowl performance is largely dictated by operating conditions imposes particularly stringent demands on modelling. This paper demonstrates, by way of a concrete example, the need for and potential benefits of a new methodological approach to the modelling of cowls. The study focuses on a specific modelling strategy, applied within a building simulation program, for a cowl used in a hybrid ventilation system. The method is progressively simplified to produce four variants, which chiefly vary according to their level of detail and, hence, the associated modelling effort. Wind pressure coefficients at facade, above roof and in the cowl are needed for all model variants. Some of the investigated variants rely on CFD computations of airflow around the building to determine these values. This study uses the example of a single-family house (SFH) to identify those criteria requiring particular attention in the performance of CFD numerical flow analyses. All four variants are examined on the basis of this example to determine which simplifications to the model are appropriate and permissible without unduly compromising the accuracy of the results.     

CFD simulations of natural ventilation behaviour in high-rise buildings in regular and staggered arrangements at various spacings

Natural ventilation, which is in line with the concepts of sustainability and green energy, is widely acknowledged nowadays. Prevailing winds in urban areas are unavoidably modified by the increasing number of closely placed high-rise buildings that significantly modify the natural ventilation behaviour. This paper explores the effects of building interference on natural ventilation using computational fluid dynamics (CFD) techniques. The cross-ventilation rate (temporal-average volumetric airflow rate) of hypothetical apartments in a building cluster under isothermal conditions was examined using the standard two-equation k − ? turbulence model. The sensitivity of ventilation rate to wind direction, building separation and building disposition (building shift) was studied. Placing buildings farther away from one another substantially promoted the ventilation rate, cancelling the unfavourable interference eventually when the building separation was about five times the building width (the optimum separation). The characteristic flow pattern leading to this behaviour was revealed. With the adoption of building disposition, the optimum separation could be reduced to three times the building width. In addition, the airflow rates could be doubled with suitable shifts. Building disposition is therefore one of the feasible solutions to improve the natural ventilation performance in our crowded environment.     

Numerical modelling of industrial indoor environments: A comparison between different turbulence models and supply systems supported by field measurements

Ventilation, heating and cooling systems in industrial premises are important issues as they are related to both energy cost and indoor climate management and the health of the premises’ occupants. The present paper has two aims: (1) to evaluate the performance of Computational Fluid Dynamics (CFD) for planning new or renovating existing industrial ventilation systems, and (2) to evaluate the performance of two different supply principles in a contaminant-intensive process with temperature and density stratification.     

The ventilation and climate modelling of rapid development tunnel drivages

The extraction of minerals and coal at greater depth, employing higher-powered machinery to increase production levels has imposed an increased burden on ventilation systems to maintain an acceptable working environment. A deterioration in the climate experienced within these workings may also adversely affect the health and safety of the workforce. In the UK, mineral extraction is now being practiced at depths of over 1000 m. In addition, the adoption of continuous miner and tunnel bolting support methods has permitted improved development rates to be achieved at the cost of increased emissions of dust, gas and heat and humidity. There is a recognized need to improve the efficiency in the design and operation of auxiliary ventilation systems to maintain an adequate underground environment and climate. Any improvement achieved in the quality, quantity and control of the delivered ventilation will assist in the provision of improved gas and dust dilution and climatic control. Due to the constraints imposed by the mining method, there may be an economic or practical limit to the climatic improvement that may be obtained by the sole use of ventilation air. Where this limit is identified, there may be the need to consider the selective application of air-cooling systems. The paper details the construction of a computer based climatic prediction tool developed at the University of Nottingham. This work builds upon earlier research (Ross et al., 1997, Proceedings of 6th International Mine Ventilation Congress, SME, Littleton, CO, pp. 283–288) that developed a prototype model for short tunnel developments. The current model predicts the psychrometric and thermodynamic conditions within long rapid development single entry tunnel drivages. The model takes into account the mass and heat transfer between the strata, water, machinery and the ventilation air. The results produced by the model have been correlated against ventilation, climatic and operational data, obtained from a number of rapid tunnel developments within UK deep coalmines. The paper details the results of a series of correlation and validation studies conducted against the ventilation and climate survey data measured within 105s district Tail Gate tunnel development at Maltby Colliery, UK. The paper concludes by presenting the results of a case study that illustrate the application of the validated model to the design and operation of an integrated mine ventilation and cooling system. The case study illustrates the effect that an increased depth and hence increased virgin strata temperature has on the climate experienced within rapid tunnel developments. Further investigations were performed to identify the optimum cooling strategy that should be adopted to maintain a satisfactory climate at the head of the drivage.     

Climate change,thermal comfort and energy: Meeting the design challenges of the 21st century

This paper addresses the dual challenge of designing sustainable low-energy buildings while still providing thermal comfort under warmer summer conditions produced by anthropogenic climate change—a key challenge for building designers in the 21st century. The main focus is towards buildings that are ‘free running’ for some part of the summer, either being entirely naturally ventilated or mixed-mode (where mechanical cooling is only used when thought to be essential). Because the conditions in these buildings will vary from day to day it is important to understand how people react and adapt to their environment. A summary is made of recent developments in this area and of the climate data required to assess building performance. Temperatures in free running buildings are necessarily closely linked to those outside. Because the climate is changing and outside summer temperatures are expected to increase, the future will offer greater challenges to the designers of sustainable buildings aiming to provide either entirely passive or low-energy comfort cooling. These issues are demonstrated by predictions of the performance of some case study buildings under a climate change scenario. The examples also demonstrate some of the important principles associated with climate-sensitive low-energy design.     

Experimental and CFD evidence of multiple solutions in a naturally ventilated building

This paper considers the existence of multiple solutions to natural ventilation of a simple one-zone building, driven by combined thermal and opposing wind forces. The present analysis is an extension of an earlier analytical study of natural ventilation in a fully mixed building, and includes the effect of thermal stratification. Both computational and experimental investigations were carried out in parallel with an analytical investigation. When flow is dominated by thermal buoyancy, it was found experimentally that there is thermal stratification. When the flow is wind-dominated, the room is fully mixed. Results from all three methods have shown that the hysteresis phenomena exist. Under certain conditions, two different stable steady-state solutions are found to exist by all three methods for the same set of parameters. As shown by both the computational fluid dynamics (CFD) and experimental results, one of the solutions can shift to another when there is a sufficient perturbation. These results have probably provided the strongest evidence so far for the conclusion that multiple states exist in natural ventilation of simple buildings. Different initial conditions in the CFD simulations led to different solutions, suggesting that caution must be taken when adopting the commonly used 'zero initialization'.