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Full-scale validation of CFD simulations of buoyancy-driven ventilation in a three-story office building
Abstract
Computational fluid dynamics (CFD) is frequently used to support the design of naturally ventilated buildings; however, the model accuracy should be thoroughly assessed, ideally through validation with full-scale measurements. The present study aims to (1) validate transient CFD simulations with uncertainty quantification (UQ) for buoyancy-driven natural ventilation against full-scale experiments in an operational atrium building, and (2) quantify the sensitivity of the CFD results to the thermal boundary conditions. The UQ and sensitivity analysis consider uncertainties in the initial and boundary conditions for the temperatures. Considering the volume-averaged air temperature on each floor, the predictions and measurements agree well with discrepancies less than 0.3 °C. When considering the temperature averaged over smaller zones on each floor, two trends can be observed. First, in zones not adjacent to windows, the discrepancies between the CFD and measurements can be explained by uncertainty in the boundary conditions and the measurements. Second, in zones adjacent to windows, higher discrepancies are observed due to oscillations in the inflow jets just downstream of the windows, and due to geometrical simplifications in the CFD model. The sensitivity analysis demonstrates that the boundary conditions for the thermal mass surface temperature and the outdoor temperature have a dominant effect on the indoor air temperature predictions, with their relative importance varying as a function of proximity to the windows.
... Refs. [8,9]). A converged CFD simulation does not necessarily mean a correct solution. ...
... Many suggestions and guidelines regarding this topic are available in technical documentation of CFD codes. A sensitivity analysis to identify the parameters at the boundaries that have a dominant effect on CFD predictions seems to be a useful strategy [9]. Furthermore, Djunaedy [78] proposed a guideline to select the appropriate simulation approach for certain flow problems. ...
... There are CFD studies which provide accurate predictions of natural ventilation flows when the interior of the building is considered as the computational domain, and the ventilation flow rates are prescribed as boundary conditions at the window openings. However, from a design point of view, it is important to know if CFD can predict ventilation flow rates given initial and boundary conditions for temperature and far-field wind conditions, so that both indoor and outdoor environments must be modelled [9]. In these cases, the two environments are linked by an interface at the openings of the building, forming the wind pressure coefficient [79]. ...
This narrative review describes the capabilities of computational fluid dynamics (CFD) to support the scientific analysis of fluid flows inside buildings, focusing on natural ventilation. The challenges posed by CFD, such as mesh generation, boundary conditions specification, choice of turbulence or radiation models and the ability to estimate the accuracy of results are explored. For the first time, this work provides a summary of verification and validation studies relating to CFD models of different built environments, and detailed validation studies of naturally ventilated spaces. This review summarises the most common guidelines and conclusions drawn from literature relating to CFD modelling of indoor environments that are naturally ventilated. The work demonstrates current practices in CFD simulation of naturally ventilated indoor environments, highlighting the importance of quality assured validation data to support the credibility of models. The review shows that, despite the presence of best practice guidelines for verification and validation of computational models, the grid verification was infrequently reported in the literature when presenting CFD results of indoor environmental conditions. Moreover, a third of reviewed validation studies were only qualitative and lacked specific validation criteria. Credible CFD analysis of natural ventilation strategies in buildings requires the ability to interpret strongly variable field measurements when specifying boundary conditions, other computational parameters and validating model results. This research provides a background and general guidelines for researchers who are commencing work in the field of CFD simulation of indoor environments for flow problems relating to natural ventilation.
... We placed HOBO® Temp-RH 2.5% Data Logger (UX100-011) sensors in two adjacent rooms in a multi-family residential building, and measured indoor temperatures, ( ) and relative humidity, ( ). In hopes of capturing the average room temperature, sensors were intentionally positioned away from drafts that could come in through windows [2]. We also collected data on the ambient temperature, ( ) and relative humidity ( ) from a local weather station . ...
... In order to more confidently identify where the changes in window state occur, we examine the first and second derivatives of ′ ( ), ′ ( ) and 2 ′ ( ) 2 . The second derivative is particularly effective for identifying change points. ...
Natural cooling, utilizing non-mechanical cooling, presents a low-carbon and low-cost way to provide thermal comfort in residential buildings. However, designing naturally cooled buildings requires a clear understanding of how opening and closing windows affect occupants' comfort. Predicting when and why occupants open windows is a challenging task, often relying on specialized sensors and building-specific training data. This limits the scalability of natural cooling solutions. Here, we, propose a novel unsupervised method that utilizes easily deployable off-the-shelf temperature and humidity sensors to detect window operations. The effectiveness of our approach is evaluated using an empirical dataset and compared with a state-of-the-art support vector machine (SVM) model. The results demonstrate that our proposed method outperforms the SVM on key indicators, except when indoor and outdoor temperatures have small differences. Unlike the SVM's sensitivity to time series characteristics, our proposed method relies solely on indoor temperature and exhibits robust performance in pilot studies, making it a promising candidate for developing a highly scalable and generalizable window operation detection model This work demonstrates the potential of unsupervised data-driven methods for understanding window operations in residential buildings. By enabling more accurate modeling of naturally cooled buildings, our work aims to facilitate the widespread adoption of this low-cost and low-carbon technology.
... CFD (computational fluid dynamics) is a commonly used method for evaluating the natural ventilation performance of buildings since it can provide more detailed and accurate information than physical experiments, especially in complex flow systems [26,27]. In this study, a series of simulations with CFD methods were carried out by using FLUENT software to reveal the conditions of airflow fields or temperature distributions in outdoor and indoor environments. ...
... CFD (computational fluid dynamics) is a commonly used method for evaluating the natural ventilation performance of buildings since it can provide more detailed and accurate information than physical experiments, especially in complex flow systems [26,27]. In this study, a series of simulations with CFD methods were carried out by using FLU-ENT software to reveal the conditions of airflow fields or temperature distributions in outdoor and indoor environments. ...
Recently, climate governance has entered a new phase of accelerating decarbonization. In order to achieve low-carbon buildings, natural ventilation has been widely used as it requires no fan power. However, there are great challenges for achieving effective natural ventilation in large-space public buildings especially in areas characterized by hot-summer and cold-winter climatic regions, due to empirically unsuitable ambient temperatures and theoretically complex joint effect of wind pressure and thermal buoyancy. Therefore, this numerical study was conducted on the performance of a natural ventilation strategy in a large-space public building in a hot-summer and cold-winter region by using computational fluid dynamics (CFD) methods. Simulations were performed by applying FLUENT software for obtaining airflow distributions within and around a typical low-carbon public building. The temperature distribution in the atrium of the building was simulated particularly for analyzing the natural ventilation performance in a large-space area. Results demonstrated that thermal pressure was dominant for the large-space building in the case study. The average indoor airflow velocities on different floors ranged from 0.43 m/s to 0.47 m/s on the windward side which met indoor ventilation requirements. Most areas of wind velocities could meet ventilation requirements. The natural ventilation performance could be improved by increasing the relative height difference between the air inlets and air outlets. These findings could help provide references and solutions for realizing natural ventilation in low-carbon large-space public buildings in hot-summer and cold-winter regions.
... In Figures 9 and 10, it can be noticed that the presence of buoyancy induces a vertical stratification of air temperature and water mass fraction. A similar effect was seen by Chen et al. [58] in an office building. The same analysis can be repeated with the open-roof configuration. ...
Computational fluid dynamics (CFD) may aid the design of barn ventilation systems by simulating indoor cattle thermal welfare. In the literature, CFD models of mechanically and naturally ventilated barns are proposed separately. Hybrid ventilation relies on cross effects between air change mechanisms that cannot be studied using existing models. The objective of this study was to develop a CFD methodology for modelling animal thermal comfort in hybrid ventilated barns. To check the capability of CFD as a design evaluation tool, a real case study (with exhaust blowers) and an alternative roof layout (with ridge gaps) were simulated in summer and winter weather. Typical phenomena of natural and mechanical ventilation were considered: buoyancy, solar radiation, and wind together with high-speed fans and exhaust blowers. Cattle thermal load was determined from a daily animal energy balance, and the assessment of thermal welfare was performed using thermohygrometric indexes. Results highlight that the current ventilation layout ensures adequate thermal welfare on average, despite large nonuniformity between stalls. The predicted intensity of heat stress was successfully compared with experimental measurements of heavy breathing duration. Results show strong interactions between natural and mechanical ventilation, underlining the need for an integrated simulation methodology.
... MAE and RMSE are statistical quantities to evaluate the similarity or difference between two data sets and have been used in similar previous studies [70][71][72][73]. The MAE values for points a, b, and c are 0.047, 0.064, and 0.108, respectively, and all were within a range of ±11%. ...
Indoor Air Quality (IAQ) is a significant health determinant in the building environment. Ventilation plays a significant role in providing comfortable working conditions and securing contaminant removal from indoor spaces. The heat and effluents generated by cooking activity need to be exhausted from the kitchen space to control the IAQ and provide thermal comfort. The study investigates the ventilation performance concerning IAQ of five institutional kitchens in an educational campus. This study evaluates the influence of building parameters on ventilation performance and IAQ using experimental research and Computational Fluid Dynamics (CFD). The empirical part includes field measurement of indoor airflow and IAQ indicators (particulate matter, CO2, temperature, and RH). The Integrated Environmental Solutions Virtual Environment (IESVE) software performs the CFD analysis. The 'Local Mean Age of air' (LMA) is used as a parameter to analyze IAQ. The findings reveal that the LMA reduces with increased cross-ventilation at the operational level and the presence of local exhaust ventilation, such as chimneys. Moreover, a higher slenderness ratio, lower aspect ratio, and lower building volume reduce the age of air.
... Traditional observational approaches such as wind tunnel experiments are limited in their ability to compute such information. CFD is becoming more popular, especially for creating natural ventilation systems in buildings with opened-roof structures such as atria, because it can provide a detailed analysis of the flow and temperature, avoiding the need for empirical correlations regarding flow and heat transfer rates [7]. This research centers on developing a mathematical model of the wind tunnel procedure in compliance with Section 207F of the National Structural Code of the Philippines 2015 through FE-based CFD analysis. ...
This research paper evaluates the effects of wind tunnel testing on low-rise buildings using a Finite Element-based computational fluid dynamics (CFD) model in order to compare the results of the traditional Directional Procedure (Building of All Heights Method) to the results of the CFD analysis. This study utilized all necessary parameters for the Wind Tunnel Procedure in accordance with the National Structural Code of the Philippines section 207F together with the guidelines prescribed in ASCE 7-16. The mathematical prototype of 3 different buildings with the same dimensions were created using midas NFX, and the necessary boundary conditions for CFD analysis were set up. The pressure caused by wind load on these buildings were evaluated and compared with the Directional procedure in Section 207B of the NSCP (Buildings of all heights method). Atmospheric turbulence was also modelled using Kinetic Energy and Length Scale method utilizing 2-equation k-ε turbulence model. The results show significant differences in pressure effects on walls with considerable number of openings and unique façade features. CFD results also show accurate internal pressure and velocity profile in areas near the wall openings and building edges. This research is particularly relevant for the design and construction of buildings in areas prone to strong winds, such as the Philippines, where accurate wind load calculations are crucial to ensuring structural safety and integrity. While numerous studies in other countries have explored the impact of wind on low-rise structures through mathematical simulations, there remains a substantial gap in the Philippines regarding research concerning wind tunnel effects on structures using mathematical and numerical approaches. This study aims to fill this gap by connecting the existing National Structural Code of the Philippines with contemporary trends in mathematical simulations.
... Energy simulation (ES) is widely used in the field of architecture. Computational fluid dynamics (CFD), a type of computer-aided engineering tool, is also widely used for analyzing houses and offices [9,10], for studying energy-efficient designs for heat pumps [11], as well as to perform blood flow analyses in medical fields [12] and aerodynamic analyses of aircraft [13]. ES is an excellent tool for performing transient calculations for building construction, making it suitable for power consumption estimation. ...
The use of radiant panels in homes has increased recently because they do not cause a drafty feeling, unlike air conditioners. However, air conditioners are more power-efficient than radiant panels and have a higher coefficient of performance (COP). Therefore, combining radiant panels and air conditioning can provide an optimal solution for thermal control in residences. Energy simulation (ES) and computational fluid dynamics (CFD) can be used to simulate such environments. ES is suitable for non-steady state calculations, and combined with appropriate modeling, enables an accurate estimation of power consumption. Effective dehumidification becomes necessary, during summer as the relative humidity in a room increases. Both air conditioners and radiant panels can achieve this. This study developed a simulation tool that incorporates the effects of dehumidification. Based on a relative evaluation, a case was proposed where both energy efficiency and comfort were satisfied by jointly using air conditioners and radiant panels. The study found that a small number of panels could achieve the most balanced operation. The results of this study can serve as a reference for general housing, and the developed simulation tool can be applied to product development and building material design.
The recent events impacting public health highlight the need for investigating airflow patterns in confined spaces like elevator cabins. It is essential to ensure proper ventilation, prevent the accumulation of contaminants, and ultimately promote a healthy indoor environment for occupants. In this study, an evaluation of the airflow distribution, temperature, and mean age of air control within an occupied elevator cabin is presented. For that, a CFD model that simulated the airflow patterns in an elevator cabin was developed, validated, and used to conduct the study under six air ventilation scenarios, involving mechanical ventilation with air curtains or displacement flows. The proposed ventilation configurations in Cases 2–6 enhanced the airflow circulation within the elevator. Among these configurations, Case 4, a case of displacement flow, exhibited the most favourable conditions, providing an ACH of 27.05, a mean air age of 84.45 s in the breathable plane, an air change effectiveness of 1.478, and a temperature of 25 °C near the doors and around the occupied zone. It is important to highlight Case 3, which had a lower ACH of 21.2 compared to Case 4. Despite this, Case 3 presented a mean average air age of approximately 122.84 s and an air change effectiveness of 1.309. Based on these findings, displacement ventilation (Case 4) is recommended as the most effective configuration, followed by Case 3, which also showed improved air circulation compared to the other scenarios. The results evidence that the ventilation configuration is particularly influential when aiming to promote air ventilation and improve air age conditions in elevator cabins.
Night-time passive cooling is an energy-efficient cooling strategy, but the design of passive cooling systems relies on imperfect computational models, which require validation. This paper assesses the importance of spatial variability in the temperature field when performing model validation. Full-scale temperature measurements in a three story atrium building reveal spatial variability of up to 3∘C on each floor during the natural ventilation process. Validation of a dynamic thermal model with uncertainty quantification reveals accurate volume-averaged air temperature predictions. Discrepancies are on the order of the sensor accuracy (0.3∘C), and are primarily due to slightly under-predicted cooling rates in the model. Importantly, this trend would be identified incorrectly when validating the model against the building's built-in sensors, which consistently record 0.05–1.63 ∘C higher temperatures than the volume-averaged air temperature. These findings highlight the importance of spatial variability and careful temperature sensor placement in naturally ventilated buildings.
Natural ventilation can provide building occupants with thermal comfort and a healthy indoor environment. Among all the design related parameters that affect ventilation performance, ventilation mode (i.e. single-sided and cross ventilation) is perhaps the main one. The current study uses full-scale in-situ measurements to investigate the effect of ventilation mode on thermal comfort and ventilation performance of a high-rise case study unit. Two modes of natural ventilation, single-sided and cross ventilation, are investigated. Air velocity, temperature and relative humidity were measured for both ventilation modes on two consecutive days in summer. Indoor thermal conditions were evaluated using the extended Predicted Mean Vote (PMV) and Standard Effective Temperature (SET*) comfort models. In addition, the relationship between reference wind speed and internal airflow, airflow distribution, and the effect of wind direction on internal airflow were investigated for both single-sided and cross ventilation. Finally, the implications of the research outcomes on natural ventilation design are discussed. Indoor thermal conditions were found to be within the comfort zone for more than 70% of the time under cross ventilation operation while single-sided ventilation provided adequate thermal conditions for only 1% of the time. Results from this study highlight a significantly better performance of cross ventilation over single-sided ventilation.
Natural ventilation (NV) is an important and efficient passive technique to reduce building cooling energy need and improve indoor air quality. NV design requires profound knowledge and accurate prediction of air flow and heat transfer in and around buildings. This paper reviews the important NV models and simulation tools and the comparisons of their prediction capabilities. In one hand, a review of the analytical models reveals that these models are generally only applicable to specific geometries and driving forces. In other hand, results of comparison and assessment between airflow models have shown that the current one can be used to model most NV mechanisms, with an exception of wind-driven single-sided ventilation. For the predictable cases, the most accuracy is achieved for cases with small and simple openings. For larger openings and especially complicated openings, the model's predictions are less accurate. Furthermore, the model is heavily dependent on several somewhat ambiguous coefficients including: wind profile exponent, pressure coefficient, and discharge coefficient.
Despite significant impacts of natural ventilation on atria, indoor thermal conditions and decreasing energy usage load, there is lack of knowledge about how various design parameters influence atria thermal conditions. The complexity of natural ventilation design of atria and inadequate simulation design tools may lead to inaccurate atria thermal prediction. In the past 25 years, researchers have developed and suggested various methods such as experimental, theoretical and numerical modeling to identify thermal and ventilation performances of atria. However, the diversity of the modeling conditions makes it difficult to achieve proper conclusions which link all contributing parameters to atrium thermal conditions. This paper provides a comprehensive review of previous studies on the role of natural ventilation in atrium buildings in different climates, efficient design parameters and their application to improvement of thermal performance and decrease of energy consumption. These parameters include various atrium configurations and components such as atrium geometry, opening characteristics, roof properties, materials and fenestration characteristics. The review further highlights different ventilation techniques which can be applied in atria as assisted ventilation methods. The cited parameters can be categorized into those affecting thermal performance and ventilation performance. The outlet opening size is the most influential parameter that affects both indoor thermal condition and ventilation behaviors of atria and consequently decreases energy usage load.
The relative freshness of indoor air in breathing zone can be measured by ventilation effectiveness. Numerous research articles in literature have investigated ventilation effectiveness under different ventilation schemes, different inlet/outlet positions, and different diffusor types. These researches seem to have a goal to find a solution to optimize ventilation effectiveness through manipulating ventilation system. In reality, however, the occupants of a rented office room have no right to manipulate the ventilation system; instead, they have to accept whatever rented to them. An important issue thus arises: how to improve ventilation effectiveness without changing ventilation system? This paper has built a CFD model about a typical office room, validated it by published experimental data in literature, and then applied it to twelve typical office situations/cases of different furniture layouts under different ventilation schemes. The simulation results of twelve cases show that furniture layout is an important factor in indoor airflow and temperature fields, and the quality of air in breathing zone can be significantly improved by adjusting furniture layout without making any change in ventilation system.
An atrium is a central feature of many modern naturally ventilated building designs. The atrium fills with warm air from the adjoining storeys: this air may be further warmed by direct solar heating in the atrium, and the deep warm layer enhances the flow. In this paper we focus on the degree of flow enhancement achieved by an atrium which is itself ‘ventilated’ directly, by a low-level connection to the exterior. A theoretical model is developed to predict the steady stack-driven displacement flow and thermal stratification in the building, due to heat gains in the storey and solar gains in the atrium, and compared with the results of laboratory experiments.Direct ventilation of the atrium is detrimental to the ventilation of the storey and the best design is identified as a compromise that provides adequate ventilation of both spaces. We identify extremes of design for which an atrium provides no significant enhancement of the flow, and show that an atrium only enhances the flow in the storey if its upper opening is of an intermediate size, and its lower opening is sufficiently small.
The paper develops proposals for a model of turbulence in which the Reynolds stresses are determined from the solution of transport equations for these variables and for the turbulence energy dissipation rate ε. Particular attention is given to the approximation of the pressure-strain correlations; the forms adopted appear to give reasonably satisfactory partitioning of the stresses both near walls and in free shear flows.
Numerical solutions of the model equations are presented for a selection of strained homogeneous shear flows and for two-dimensional inhomogeneous shear flows including the jet, the wake, the mixing layer and plane channel flow. In addition, it is shown that the closure does predict a very strong influence of secondary strain terms for flow over curved surfaces.
Natural ventilation and cooling can significantly reduce building energy consumption, but inherent variability in the boundary and operating conditions makes robust design and operation a challenging task. Computational models can be leveraged to evaluate the effects of these uncertainties and support robust design; however, the model accuracy should be thoroughly assessed, ideally through validation with carefully designed full-scale measurements. This study presents a novel approach for using computational fluid dynamics (CFD) simulations with uncertainty quantification to optimize temperature sensor placement in buildings with buoyancy-driven natural ventilation, such that the measurements are representative of the volume-averaged temperature, while also supporting analysis of the spatial variability in the temperature field. The approach uses a polynomial chaos expansion method to quantify the effect of the variability in the boundary and initial conditions on the predicted buoyancy-driven flow and indoor temperature field. Results for an operational building that employs night-time cooling are analyzed to identify sensor locations that will support robust observation of the time series of the volume-averaged temperature and the temperature range. The approach is shown to significantly reduce the potential error between the temperature recorded at a random location and the true volume-average temperature, while also providing a representative measure of the temperature range in the building. The results indicate that this approach can also support identifying optimal locations for temperature sensors used for operational control.
Windexchangers are relatively small structures located on the building rooftop to promote natural ventilation. This paper presents a Computational Fluid Dynamics (CFD) validation study, sensitivity analysis and performance comparison of three windexchanger (WE) configurations applied to a generic isolated building with a windward window. The study is limited to wind-driven (isothermal) ventilation, for wind perpendicular to the windward facade. The CFD simulations are based on the 3D-steady Reynolds-Averaged Navier–Stokes (RANS) equations. The validation study is performed with experimental results from a previously published water channel test. The sensitivity analysis focuses on the domain size, grid resolution and turbulence model. The performance evaluation of the three WE configurations is based on the mean velocity and mean static pressure coefficients in the vertical centerplane, the ventilation volume flow rate and the volume percentage of the living zone with air speed ratio equal to or above 0.10. The WE configuration with four openings and one duct shows the highest ventilation flow rate (0.232 m³/s) and the highest volume percentage (21%). This study shows that the assessment and selection of WE configurations should not only be based on volume flow rate or ACH but should consider the living zone air speed ratio as well, specifically concerning the flow distribution in the living zone.
Energy demand in cooling-dominant climates can be reduced by implementation of natural ventilation as a passive cooling strategy. Accordingly, suitable evaluation and prediction tools are requirements for effectively introducing natural ventilation in building design. In addition, the rapid emergence of multi-storey buildings can accelerate energy consumption, especially in cases of inappropriate building design. This study, therefore, proposes a process model for the better integration and evaluation of natural ventilation design into the overall building design process for multi-storey buildings. To achieve this, available methods of natural ventilation evaluation were identified through a literature review and classified into three main categories: analytical and empirical methods, computational simulations, and experimental methods. Strengths and limitations of each method are then evaluated with regards to accuracy of the results, cost, applicability to complex geometries, resolution of results, and time cost. Finally, a process model was proposed based on the methods' advantages and limitations, as well as needs of each design stage and recommends the most suitable integration of natural ventilation evaluation methods into the overall design process.
The potential for prediction error when using computational fluid dynamics (CFD) for investigating internal building airflows is assessed in the current paper. The ability of a proprietary CFD code, CFX, to simulate buoyant and forced airflow regimes, typical of a naturally ventilated building, are investigated using two experimental case studies from the literature. Comparisons are then made between simulated and measured airflows for a naturally ventilated building. Results from the two case studies indicate that structured meshes are less dependent on mesh density and yield consistent convergence and accuracy when coupled with the k-ε or k-ω turbulence models. Comparison of CFD predicted airflows with the full-scale building airflows was challenging due to scatter in the measured data. A structured mesh in conjunction with either the k-ε or k-ω turbulence models, showed reasonable correlation with the measured airflows, with both models performing equally.
Knowledge of temperature stratification in indoor environments is important for occupant thermal comfort and
indoor air quality and for the design and evaluation of displacement ventilation systems. This paper presents a
detailed and systematic evaluation of the performance of 3D steady Reynolds-Averaged Navier-Stokes (RANS) CFD simulations to predict the temperature stratification within a room with a heat source and two ventilation openings. The indoor air quality within the room is also investigated by assessing the distribution of the age of air. The evaluation is based on validation with full-scale measurements of air temperature. A sensitivity analysis is performed to investigate the impact of computational grid resolution, turbulence model, discretization schemes and iterative convergence on the predicted temperatures and age of air. The results show that steady RANS with the SST k-ω model can accurately predict the temperature stratification in this particular indoor environment. However, of the five commonly used turbulence models, only the SST k-ω model and the standard k-ω model succeed in reproducing the thermal plume structure and the associated thermal stratification, while the three k-ε models clearly fail in doing so. In addition, the iterative convergence criteria have a major impact on the predicted age of air, where it is shown that very stringent criteria in terms of scaled residuals are required to obtain accurate results. These required criteria are much more stringent than typical default settings. This paper is intended to contribute to improved accuracy and reliability of CFD simulations for displacement ventilation assessment.
A brief history is given of research into natural ventilation over the first fifty years of the Journal, from the personal perspective of the author. One of my aims is to give an indication of the contribution that the Journal has made to reporting research on this subject. Another aim is to provide a background for researchers who are starting out in research. There are nine areas of research that I have identified, namely:- steady envelope flow models; flow characteristics of openings; unsteady envelope flow models; internal air motion, zonal models and stratification; contaminant transport; age of air; CFD and its applications; scale modelling; full-scale measurements. For reasons given in the text, some of these topics are only very briefly mentioned. For example, the sections on contaminant transport and CFD are relatively brief, partly because they are the subject of other papers in this issue. (Similarly, the feasibility of natural ventilation relies on the adaptability of the occupants to be comfortable, but thermal comfort is not covered at all here, because it too is the subject of other papers.) The paper concludes with a few personal comments on the design process, since the underlying aim of research is to minimise the risks associated with naturally ventilated buildings.
In the past 50 years, Computational Wind Engineering (CWE) has undergone a successful transition from an emerging field into an increasingly established field in wind engineering research, practice and education. This paper provides a perspective on the past, present and future of CWE. It addresses three key illustrations of the success of CWE: (1) the establishment of CWE as an individual research and application area in wind engineering with its own successful conference series under the umbrella of the International Association of Wind Engineering (IAWE); (2) the increasing range of topics covered in CWE; and (3) the history of overview and review papers in CWE. The paper also outlines some of the earliest achievements in CWE and the resulting development of best practice guidelines. It provides some views on the complementary relationship between reduced-scale wind-tunnel testing and CFD. It re-iterates some important quotes made by CWE and/or CFD researchers in the past, many of which are still equally valid today and which are provided without additional comments, to let the quotes speak for themselves. Next, as application examples to the foregoing sections, the paper provides a more detailed view on CFD simulation of pedestrian-level wind conditions around buildings, CFD simulation of natural ventilation of buildings and CFD simulation of wind-driven rain on building facades. Finally, a nonexhaustive perspective on the future of CWE is provided.
The suitability of night ventilation to reduce the cooling demand in buildings can be evaluated by coupling Airflow Network Models to Building Energy Simulation tools. To estimate wind-induced ventilation, pressure coefficients (CO on the building envelope are key inputs, as well as local wind speed and direction. C-P data obtained by primary sources such as measurements or CFD simulations are considered the most reliable but can be difficult to obtain. An easy alternative are C-P secondary sources, such as databases providing literature data correlations. Therefore an issue arises regarding the choice of the source of pressure coefficients. This paper investigates the effects of C-P from primary and secondary sources on the predicted energy saving potential of night ventilation of an isolated office building for several European climates and some relevant design conditions and simulation parameters. Different C9 sources produce a dispersion of C9 data and differences in the calculated night ventilation rates up to 15%. Contrary to what might be expected, these differences influence only marginally the resulting passive cooling effects. Overall a stronger impact is observed for the colder climates, where higher temperature differences occur between desired indoor temperature and night-averaged outdoor temperature. Finally, for the building under study, the choice of the Cp source appears less crucial than the choice of other building simulation parameters, such as the internal Convective Heat Transfer Coefficient. This study can support building designers towards accurate energy simulations of naturally ventilated buildings.
In this paper the performance of various turbulence models potentially suitable for the prediction of indoor air flow and temperature distributions in an atrium space was evaluated in a systematic way. The investigation tested these models for various thermal conditions in atria of different geometrical configuration in two existing buildings using the Reynolds Averaged Navier–Stokes (RANS) modeling approach. The RANS turbulence models that were tested include the one-equation model (the Spallart–Allamaras) and two-equation models (the standard k-ɛ, RNG k-ɛ, realizable k-ɛ, standard k-ω and SST k-ω models). The radiation exchange between the surfaces of the atrium space was considered using the Discrete Transfer Radiation Model (DTRM). The resultant steady state governing equations were solved using a commercial CFD solver FLUENT. The numerical results obtained for a particular time of the day were compared with the experimental data available. Relatively good agreement between the experimental and CFD predictions was obtained for each model. However, from the results obtained, it was found that the performance of two-equation turbulence models is better than one-equation model and among the two-equation models, the SST k-ω model showed relatively better prediction capability of the indoor environment in an atrium space than k-ɛ-models.
A glazed atrium is a common feature in shopping centres and in the lobbies of residential and commercial buildings. This research aimed to discover the vertical temperature distribution and cooling load reduction relating to displacement ventilation in atrium spaces. The air temperatures at different levels (0.1, 0.7, 1.2, 1.7, 3, 5 and 7 m), within a 25 m high exhibition atrium, were monitored. Using the measured surface temperatures as boundary conditions, three-dimensional computational fluid dynamic (CFD) simulations were performed to examine the vertical temperature distribution. Further, by using parametric studies, the influences of occupant load, equipment load, lighting load, floor and wall surface temperatures, and supply air conditions were investigated with 12 CFD simulation cases. Based on the simulation results, load reduction factors - with three grouping methods of heat sources - were then developed. Temperature stratification was present even under the condition of a low cooling load. Load reductions were lower for occupant and equipment loads than for a lighting load. The best grouping method was to consider the heat released from the floor as an individual group of heat sources, which is an indication that secondary heat from the floor should not be neglected during the design of displacement ventilation.
Predicting the performance of natural ventilation is difficult, especially for large scale naturally ventilated buildings, due to the lack of accurate and efficient prediction tools. This paper presents a strategy, integrating a multi-zone model and computational fluid dynamics (CFD), to improve natural ventilation prediction and design methods. Large openings and atrium configurations are broadly used in naturally ventilated buildings to promote buoyancy force and optimize air movement. How to properly deal with this typical configuration for a multi-zone model and integrated simulation is discussed and compared in this paper. In order to validate a newly developed multi-zone model program, MMPN, this paper investigated both buoyancy ventilation and wind-buoyancy combined ventilation. Integration strategies, transferring data (velocity or pressure) from a multi-zone model program to CFD as boundary conditions, are also studied.
Numerous turbulence models have been developed in the past two decades, and many of them can be used in predicting airflows and turbulence in enclosed environments. It is important to evaluate the generality and robustness of the turbulence models for various indoor airflow scenarios. This study evaluated the performance of eight turbulence models, potentially suitable for indoor airflow, in terms of accuracy and computing cost. These models cover a wide range of computational fluid dynamics (CFD) approaches, including Reynolds averaged Navier-Stokes (RANS) modeling, hybrid RANS and large-eddy simulation (or detached-eddy simulation [DES]), and large-eddy simulation (LES). The RANS turbulence models tested include the indoor zero-equation model, three two-equation models (the RNG k-∊, low Reynolds number k-∊, and SST k-ω models), a three-equation model ( model), and a Reynolds-stress model (RSM). The investigation tested these models for representative airflows in enclosed environments, such as forced convection and mixed convection in ventilated spaces, natural convection with medium temperature gradient in a tall cavity, and natural convection with large temperature gradient in a model fire room. The air velocity, air temperature, Reynolds stresses, and turbulent heat fluxes predicted by the models were compared against the experimental data from the literature. The study also compared the computing time used by each model for all cases. The results reveal that LES provides the most detailed flow features, while the computing time is much higher than for RANS models, and the accuracy may not always be the highest. Among the RANS models studied, the RNG k-ω and a modified model perform the best overall in four cases studied. Meanwhile, the other models have superior performance only in particular cases. While each turbulence model has good accuracy in certain flow categories, each flow type favors different turbulence models. Therefore, we summarize in the conclusions and recommendations both the performance of each particular model in different flows and the best suited turbulence models for each flow category.
Atria are becoming an increasingly common feature of new buildings. They are often included for their aesthetic appeal; however, their effect on building indoor environment can be significant. Building simulation tools have the potential to assist designers in enhancing energy efficiency by providing information on the temperature and velocity fields inside the atrium for specified geometries and ambient conditions. The unique nature of the physical phenomena that govern the complex flows in atria, however, are not usually considered in traditional building energy simulation programs. These physical phenomena include turbulent natural convection, radiative heat transfer and conjugate heat transfer. Computational fluid dynamics (CFD) has the potential for modeling fluid flow and heat transfer resulting from the phenomena; however, careful validation is required in order to establish the accuracy of predictions. This paper provides a systematic validation of a commercial CFD code against experimental measurements of the underlying physical phenomena. The validation culminates in the simulation of an existing atrium. This work indicates that CFD can be used to successfully simulate the heat transfer and fluid flow in atria geometries and provides recommendations regarding turbulence and radiative heat transfer modeling.
In recent years, highly glazed atria are popular because of their architectural aesthetics and advantage of introducing daylight into inside. However, cooling load estimation of such atrium buildings is difficult due to complex thermal phenomena that occur in the atrium space. The study aims to find out a simplified method of estimating cooling loads through simulations for various types of atria in hot and humid regions. Atrium buildings are divided into different types. For every type of atrium buildings, both CFD and energy models are developed. A standard method versus the simplified one is proposed to simulate cooling load of atria in EnergyPlus based on different room air temperature patterns as a result from CFD simulation. It incorporates CFD results as input into non-dimensional height room air models in EnergyPlus, and the simulation results are defined as a baseline model in order to compare with the results from the simplified method for every category of atrium buildings. In order to further validate the simplified method an actual atrium office building is tested on site in a typical summer day and measured results are compared with simulation results using the simplified methods. Finally, appropriate methods of simulating different types of atrium buildings are proposed.
Single-sided natural ventilation is a simple and energy-efficient method to passively cool a building, thus reducing or avoiding air-conditioning use. CFD has the potential to give detailed information about the complex flow generated by the interaction of buoyancy and wind in single-sided ventilation. In this paper, three experiments on single-sided ventilation are reproduced by CFD, using two turbulence models (RANS and LES). A detailed description of the experimental set-up, the numerical methods and the comparison method between experiments and simulations are provided. Results from RANS and LES are compared to experiments in terms of average and turbulent flow field, local airspeed, turbulence and temperature at the opening and airflow rates. The comparison shows that LES has the potential to provide more accurate results than RANS in most of the cases, capturing better the turbulent characteristics of the flow. However, the computational cost of LES is at least an order of magnitude higher than that of RANS.
Quite extensive measurements undertaken in a three-story atrium space (floors 14th–16th) with a hybrid solar-assisted natural ventilation system in the Engineering building of the Concordia University, Montreal, Canada have recently become available. The thermal conditions of the atrium space have been studied numerically using the Reynolds Averaged Navier-Stokes (RANS) modeling approach. The RANS turbulence models that were tested include the standard k-ε, RNG k-ε, ‘realizable’ k-ε, and SST k-ω turbulence models. The radiation exchange between the surfaces of the atrium space was considered using the Discrete Transfer Radiation Model (DTRM). The resultant steady state governing equations were solved using a commercial CFD solver FLUENT. Numerical results were obtained for the conditions existing when measurements were taken in the Concordia atrium on typical clear days with the blinds fully open or fully closed and with the natural ventilation system (NV) ON or OFF. The CFD model predictions were validated by the comparison against the experimental measurements available and it was found that the numerical predictions implying CFD model agree well with the measurements.
Computational fluid dynamics (CFD) is used to investigate buoyancy-driven natural ventilation flows in a single-storey space connected to an atrium. The atrium is taller than the ventilated space and is warmed by heat gains inside the single-storey space which produce a column of warm air in the atrium and drive a ventilation flow. CFD simulations were carried out with and without ventilation openings at the bottom of the atrium, and results were compared with predictions of analytical models and small-scale experiments. The influence of key CFD modelling issues, such as boundary conditions, solution controls, and mesh dependency were investigated. The airflow patterns, temperature distribution and ventilation flow rates predicted by the CFD model agreed favourably with the analytical models and the experiments. The work demonstrates the capability of CFD for predicting buoyancy-driven displacement natural ventilation flows in simple connected spaces.
Design guidelines for natural ventilation (NV) in buildings focus on the potential hourly air change (ACH) rates based on the building space parameters. Critically, external airflow data is often assumed on the basis of a single mean wind speed and an associated prevailing wind direction. This can result in significant variation in ventilation rates and comfort conditions when non-design external wind conditions prevail. This paper describes a CFD study aimed at examining the influence of variations in external wind speed and direction on the air change rate for the atrium space of a two-storey naturally ventilated building. The building atrium is ventilated by a series of entry vents on one wall of the building in conjunction with roof vents. External wind speeds from 25 to 250% of the mean site wind speed (5.7 m/s) were examined and found to result in an almost linear increase in the ACH rate. For a single wind speed, the relationship between wind direction and the ACH rate was also found to be approximately linear for wind directions between 0° and 90° (orthogonal and parallel) to the wall vent openings, but non-linear for other wind directions (90–135°). More generally, the significant variation in the atrium ACH rate with changes in external wind conditions, evident in this particular building model, illustrates the importance of considering non-design wind conditions when designing NV buildings.
The thermal performance of an atrium integrated with photovoltaic (PV) modules has been evaluated. Computational fluid dynamics (CFD) was applied to the prediction of air flow and temperature distribution in the atrium. CFD was then used to investigate the effect of ventilation strategies on the performance of PV arrays. CFD modelling indicated that for effective cooling of roof PV arrays, cool outdoor air should be introduced through an opening positioned close to the roof or an air channel underneath the roof.
Global sensitivity indices for rather complex mathematical models can be efficiently computed by Monte Carlo (or quasi-Monte Carlo) methods. These indices are used for estimating the influence of individual variables or groups of variables on the model output.
This paper presented an overview of the tools used to predict ventilation performance in buildings. The tools reviewed were analytical models, empirical models, small-scale experimental models, full-scale experimental models, multizone network models, zonal models, and Computational Fluid Dynamics (CFD) models. This review found that the analytical and empirical models had made minimal contributions to the research literature in the past year. The small- and full-scale experimental models were mainly used to generate data to validate numerical models. The multizone models were improving, and they were the main tool for predicting ventilation performance in an entire building. The zonal models had limited applications and could be replaced by the coarse-grid fluid dynamics models. The CFD models were most popular and contributed to 70 percent of the literature found in this review. Considerable efforts were still made to seek more reliable and accurate models. It has been a trend to improve their performance by coupling CFD with other building simulation models. The applications of CFD models were mainly for studying indoor air quality, natural ventilation, and stratified ventilation as they were difficult to be predicted by other models.
Full-scale measurements of indoor environmental conditions and natural ventilation in a large semi-enclosed stadium: Possibilities and limitations for CFD validation
- van Hooff