On the numerical study of contaminant particle concentration in indoor airflow

The application of three turbulence models—standard k–ε, re-normalization group (RNG) k–ε and RNG-based large eddy simulation (LES) model—to simulate indoor contaminant particle dispersion and concentration distribution in a model room has been investigated. The measured air phase velocity data obtained by Posner et al. [Energy and Buildings 2003;35:515–26], are used to validate the simulation results. All the three turbulence model predictions have shown to be in good agreement with the experimental data. The RNG-based LES model has shown to yield the best agreement. The flow of contaminant particles (with diameters of 1 and 10 μm) is simulated within the indoor airflow environment of the model room. Comparing the three turbulence models for particle flow predictions, the RNG-based LES model through better accommodating unsteady low-Reynolds-number (LRN) turbulent flow structure has shown to provide more realistic particle dispersion and concentration distribution than the other two conventional turbulence models. As the experimental approach to access indoor contaminant particle concentration can be rather expensive and unable to provide the required detailed information, the LES prediction can be effectively employed to validate the widely used k–ε models that are commonly applied in many building simulation investigations.     

Evaluating the influence of the width of inlet slot on the prediction of indoor airflow: Comparison with experimental data

The aim of this work is to investigate the influence of two values of inlet slot width on the velocity characteristics and turbulent intensity of the airflow inside a rectangular room. The experimental data used to check the numerical results concerns a rectangular room where the air is supplied horizontally on the upper left and is exhausted through an opening on the lower right on the opposite side. The performance of three turbulence models, standard k-?, RNG k-?, and k-ω, in predicting the three-dimensional airflow in that room has also been investigated. The results for Reynolds number of 5000 are presented for dimensionless horizontal velocities and turbulent kinetic energy for two planes of the room and two inlet arrangements, one opening as large as the room and another with half of the width of the room. The results have indicated that the main features of the flow were captured by the three turbulence models investigated. On the whole, the performance of the standard k-? model was better than those of the other two turbulence models. In particular, the k-ω model performed better in the configuration with the largest air opening than in that with the smallest one, while the RNG k-? model presented the opposite behavior. The comparative study between both geometries demonstrated that for slots much smaller than the width of the room, three-dimensional effects become important.     

The effect of ceiling configurations on indoor air motion and ventilation flow rates

The purpose of this paper is to evaluate the effects of a building parameter, namely ceiling configuration, on indoor natural ventilation. The computational fluid dynamics (CFD) code Phoenics was used with the RNG k–? turbulence model to study wind motion and ventilation flow rates inside the building. All the CFD boundary conditions were described. The simulation results were first validated by wind tunnel experiment results in detail, and then used to compare rooms with various ceiling configurations in different cases. The simulation results generated matched the experimental results confirming the accuracy of the RNG k–? turbulence model to successfully predict indoor wind motion for this study. Our main results reveal that ceiling configurations have certain effects on indoor airflow and ventilation flow rates although these effects are fairly minor.     

Nonlinear eddy viscosity modeling and experimental study of jet spreading rates

Indoor airflow pattern is strongly influenced by turbulent shear and turbulent normal stresses that are responsible for entrainment effects and turbulence‐driven secondary motion. Therefore, an accurate prediction of room airflows requires reliable modeling of these turbulent quantities. The most widely used turbulence models include RANS‐based models that provide quick solutions but are known to fail in turbulent free shear and wall‐affected flows. In order to cope with this deficiency, this study presents a nonlinear k‐ε turbulence model and evaluates it along with linear k‐ε models for an indoor isothermal linear diffuser jet flow measured in two model rooms using PIV. The results show that the flow contains a free jet near the inlet region and a wall‐affected region downstream where the jet is pushed toward the ceiling by entrainment through the well‐known Coanda effect. The CFD results show that an accurate prediction of the entrainment process is very important and that the nonlinear eddy viscosity model is able to predict the turbulence‐driven secondary motions. Furthermore, turbulence models that are calibrated for high Reynolds free shear layer flows were not able to reproduce the measured velocity distributions, and it is suggested that the model constants of turbulence models should be adjusted before they are used for room airflow simulations.     

Particle dispersion and deposition in ventilated rooms: Testing and evaluation of different Eulerian and Lagrangian models

Indoor particle dispersion in a three-dimensional ventilated room is simulated by a Lagrangian discrete random walk (DRW) model and two Eulerian models: drift flux model and mixture model. The simulated results are compared with the published measured data to check the performance of the three models for indoor particle dispersion simulation. The deposition velocity of the particles is also computed and compared with published data. The turbulent airflow is modeled with the renormalization group (RNG) k−ε and a zero equation turbulence model. Comparison of the calculated air velocities with measurement shows that both the two turbulence models can simulate the airflow well for the presented case. For the Lagrangian DRW model, a post-process program is used to state the particle trajectories and transfer the results to particle concentration distribution. For Eulerian models, the effect of particle deposition towards wall surfaces is incorporated with a semi-empirical particle deposition model. The comparison shows that both the Lagrangian DRW model and drift flux model yield satisfactory predictions, while the predicted results by the mixture model are not satisfied. The deposition velocity obtained by the three models match the experimental data well.     

Sensitivity of the predicted convective heat transfer in a cooled room to the computational fluid dynamics simulation approach

Computational fluid dynamics (CFD) plays an increasingly important role in the design, analysis and optimization of engineering systems. However, CFD does not necessarily provide reliable results. The most crucial numerical solution error is caused by inadequate grid resolution, and the key modelling error sources in CFD in ventilated indoor environments are turbulence modelling and diffuser modelling. Many researchers already proposed guidelines, but they based their analyses on local variables. In response, underlying study intended to verify the impact of the CFD simulation approach on the convective heat flux, an integral quantity. The authors tested several grids, Reynolds averaged Navier–Stokes turbulence models and diffuser models for three convection regimes in a cooled room. The diffuser modelling had a much larger impact than the grid and the turbulence modelling, as long as the jet dominated the airflow. So, CFD users, who want to model forced/mixed convection airflow indoors, certainly need to pay attention to the diffuser modelling.     

Unsteady thermal performance analysis of a room with serial and parallel duct radiant floor heating system using hot airflow

In this study, the unsteady thermal performance of a test room heated by circulating hot airflow under the floor was analyzed with a developed mathematical model based on heat transfer equilibrium among the air flow, the floor and the indoor air. The time variations in the indoor air temperature for the serial duct floor heating system were investigated theoretically and experimentally. The time variations in the floor surface and the indoor air temperatures were predicted theoretically for the parallel duct floor heating system. Experiments on the time variations of the dimensionless numbers such as Nu for the airflow in duct and the indoor air, Gr for the indoor air and the heat ratios of convection and radiation to total heat for the serial duct floor heating system were performed. The theoretical and experimental results showed a good agreement.     

Improving predictions of heat transfer in indoor environments with eddy viscosity turbulence models

Heat transfer modelling in indoor environments requires an accurate prediction of the convective heat transfer phenomenon. Because of the lower computational cost and numerical stability, eddy viscosity turbulence models are often used. These models allow modification to turbulent Prandtl number, and near wall correction which influences stagnation points, entrainment, and velocity and time scales. A modified v ²–f model was made to correct the entrainment behaviour in the near wall and at the stagnation point. This new model was evaluated on six cases involving free and forced convection and room airflow scenarios and compared with the standard k–ε, and k–ω–SST models. The results showed that the modification to the v ²–f model provided better predictions of the buoyant heat transfer flows while the standard k–ε failed to reproduce and underestimate the convective heat transfer. The k–ω–SST model was able to predict the flow field well only for a 2D square cavity room, and 3D partitioned room case, while it was poor for the other four cases.     

A study of the dispersion of expiratory aerosols in unidirectional downward and ceiling-return type airflows using a multiphase approach

Dispersion characteristics of expiratory aerosols were investigated in an enclosure with two different idealized airflow patterns: the ceiling-return and the unidirectional downward. A multiphase numerical model, which was able to capture the polydispersity and evaporation features of the aerosols, was adopted. Experiments employing optical techniques were conducted in a chamber with downward airflow pattern to measure the dispersion of aerosols. Some of the numerical results were compared with the chamber measurement results. Reasonable agreement was found. Small aerosols (initial size 相似文献   

Indoor air flow analysis based on lattice Boltzmann methods

The modeling of convective flows based on a 3D lattice Boltzmann approach for low Mach number flows with variable density combined with a large eddy turbulence model is presented. The ability to handle non-Boussinesq density variation problems is depicted for two-dimensional Rayleigh–Bénard convection at a Rayleigh number Ra=800,000.A complex three-dimensional example shows the status of our work with respect to turbulent flow in and around a building, so far without consideration of the energy equation in the full scale 3D case. Integrated within a CAD environment, the spatial geometric model, based on an IFC building product data model, is discretized using a hierarchic data structure. Results are presented for a Reynolds number Re=75,000 computed on a high-performance parallel vector computer.State-of-the-art visualization techniques integrate the simulation results and the CAD model into a virtual reality environment. The VR environment allows also for an interactive analysis of thermal comfort criteria, being demonstrated for an indoor air flow simulation of an open-plan office.     

Feasibility of utilizing numerical viscosity from coarse grid CFD for fast turbulence modeling of indoor environments

Computational fluid dynamics (CFD) is a useful tool in building indoor environment study. However, the notorious computational effort of CFD is a significant drawback that restricts its applications in many areas and stages. Factors such as grid resolution and turbulence modeling are the main reasons that lead to large computing cost of this method. This study investigates the feasibility of utilizing inherent numerical viscosity induced by coarse CFD grid, coupled with simplest turbulence model, to greatly reduce the computational cost while maintaining reasonable modeling accuracy of CFD. Numerical viscosity introduced from space discretization in a carefully specified coarse grid resolution may have similar magnitude as turbulence viscosity for typical indoor airflows. This presents potentials of substituting sophisticated turbulence models with inherent numerical viscosity models from coarse grid CFD that are often used in fast CFD analysis. Case studies were conducted to validate the analytical findings, by comparing the coarse grid CFD predictions with the grid-independent CFD solutions as well as experimental data obtained from literature. The study shows that a uniform coarse grid can be applied, along with a constant turbulence viscosity model, to reasonably predict general airflow patterns in typical indoor environments. Although such predictions may not be as precise as fine-grid CFDs with well validated complex turbulence models, the accuracy is acceptable for indoor environment study, especially at an early stage of a project. The computing speed is about 100 times faster than a fine-grid CFD, which makes it possible to simulate a complicated 3-dimensional building in real-time (or near real-time) with personal computer.     

Numerical investigation of the unsteady flow characteristics of human body thermal plume

Human thermal plume is quite important to the study of airflow organization in the indoor environment, especially in the micro-environment research such as personalized ventilation, infectious disease transmission through air, etc. In order to investigate the unsteady fluctuation of the thermal plume around human body, a series of transient numerical simulations are conducted in this study. Numerical simulation based on 9.7 million grids and 0.02 s time step is performed to obtain the detail quantitative data of flow field. The obvious fluctuation and separation are captured in the upper flow region of human body based on the high resolution grids. The maximum time-averaged velocity of the thermal plume is found to be 0.25 m/s while the maximum fluctuate velocity is about 0.07 m/s. The further analysis of frequency spectrum shows that the thermal plume around the body is mainly dominated by the low frequency fluctuation which is lower than 1 Hz and the principal frequency is around 0.1 Hz. In order to overcome the drawback of the high computation cost for application of the engineering simulation, a new numerical simulation method combining a modified k–ε turbulence model and coarse grids is presented. This modified k–ε model can reduce the calculation error of Reynolds stress in the flow region of natural convection through redefining the turbulence viscosity coefficient segmentally and avoid a high numerical viscosity appeared due to the central difference scheme. It can reasonably predict the general fluctuation velocity and the frequency distribution during simulation process in coarse grids and show a huge potential to be applied to the engineering application.     

Applicability of LES to the turbulent wake of a rectangular cylinder—Comparison with PIV data

The applicability of the large eddy simulation (LES) technique to wake flows past a bluff body should be clarified in order to improve numerical accuracy for the estimation of aerodynamic forces and pressures acting on a bluff body. Here, we conduct both LES and particle image velocimetry (PIV) measurement for turbulent flows past rectangular cylinders with a depth/breadth ratio ranging from D/B=2.00 to 3.00 at Re=2000 and 20,000, which sensitively change turbulence structures due to flow separation and reattachment. For LES, we use the dynamic Smagorinsky-type SGS (sub-grid scale) model. First, at Re=2000 where pure turbulent viscosity without numerical viscosity is realized, the accuracy of the SGS modeling for the prediction of not only aerodynamic characteristics but also turbulence statistics in the wake of rectangular cylinders is examined in comparison with PIV data. Furthermore, at Re=20,000 where the numerical dissipation must be incorporated, we clarify the unfavorable effects of the numerical dissipation on the turbulence structures in the wake.     

Interzonal air and moisture transport through large horizontal openings in a full-scale two-story test-hut: Part 2 – CFD study

The aim of this paper is to study the air and moisture transport through a large horizontal opening in a full-scale two-story test-hut with mixed ventilation by means of computational fluid dynamics (CFD) simulations. CFD allows extending the experimental study presented in the companion paper [1] and overcoming some limitations of experimental data. More than 80 cases were simulated for conditions similar to those tested experimentally and for additional ventilation rates and temperature difference between the two rooms. CFD simulations were performed in Airpak and the indoor zero-equation turbulence model was used. The CFD model was extensively validated with the distributions of air speed, temperature and humidity ratio measured across the two rooms, as well as with the measured interzonal mass airflows through the horizontal opening. CFD simulation results show that temperature difference between the two rooms and ventilation rate strongly influence the interzonal mass airflows through the opening when the upper room is colder than the lower room, while warm convective air currents from the baseboard heater and from the moisture source placed in the lower room cause upward mass airflows when the upper room is warmer than the lower room. Finally, empirical relationships between the upward mass airflow and the temperature difference between the two rooms are developed.     

室内空气流动的简捷数值模拟方法

概要介绍了N点风口动量模型、新零方程湍流模型及误差预处理数值算法，利用这些模型和方法对某采用百叶风口的办公室内温度场和速度场进行模拟计算，并与实验数据进行比较，表明计算值与实测值吻合得较好；在普通微机上能快速合理地模拟室内温度场和速度场，可用于实际工程指导通风空调气流组织设计和分析。     

深圳市建筑室内外脉动风特性实测研究

利用现场实测的方法对深圳市某办公建筑室内外的自然风脉动特性进行研究,经实测数据的统计分析,得到建筑室内外的平均风速、湍流强度、湍流积分尺度及湍流的谱特性等脉动风特性.分析结果表明：处在建筑群冠层的平均风速、湍流积分尺度、湍流能量要高于城市密集层的值;建筑室内出风口的湍流强度比进风口处的略大,说明出风口处的脉动特征较为明显;建筑室内的进、出风口振幅能量不同,而频率波动的近似一致性,说明风的振动频率随着空间尺度稳定传递.     

A two-layer turbulence model for simulating indoor airflow: Part I. Model development

Energy efficient buildings should provide a thermally comfortable and healthy indoor environment. Most indoor airflows involve combined forced and natural convection (mixed convection). In order to simulate the flows accurately and efficiently, this paper proposes a two-layer turbulence model for predicting forced, natural and mixed convection. The model combines a near-wall one-equation model [J. Fluid Eng. 115 (1993) 196] and a near-wall natural convection model [Int. J. Heat Mass Transfer 41 (1998) 3161] with the aid of direct numerical simulation (DNS) data [Int. J. Heat Fluid Flow 18 (1997) 88].     

Teeter excursions of a two-bladed horizontal-axis wind-turbine rotor in a turbulent velocity field

A theoretical model is developed for predicting the teeter excursions of the rotor of a horizontal-axis wind turbine due to turbulence. A single-degree-of-freedom model is used and it is assumed that the atmospheric turbulence is homogeneous and isotropic and that Taylor's frozen-turbulence hypothesis is obeyed.The effects of rotor size, Lock number, turbulence intensity and the introduction of a δ₃ hinge are examined. An approximate but rapid means of assessing the response of a given rotor to particular site conditions is described.     

Measurement of the Reynolds stress structure and turbulence characteristics of the wind above a two-dimensional trapezoidal shape of hill

Wind tunnel experiments were carried out to measure the mean velocity and turbulence structure of the wind flow over a two-dimensional trapezoidal shape of hill. The quadrant analysis technique was employed to analyze the structure of the Reynolds stress. Analysis of the turbulent velocity spectrum of the wind above the hill under different wind attack angles is conducted. The fractional speed-up ratios of the present measured results are found in agreement with the wind tunnel data of Lemelin et al. (J. Wind Eng. Ind. Aerodyn. 28 (1988) 117) for the case of the wind attack angle of 30°. Measurements of the mean velocity profiles disclose that the speed-up phenomenon is mostly manifest at Z/H=0.6 for the case of wind attack angle of 10°. Turbulence intensity profiles measured at different locations show that the turbulence intensity decreases as shifting from far upstream location of the hill (X/H=−20) to the downstream location at the center of hill (X/H=0). The decrease of the turbulence intensity is obviously at the distance close to the surface of the hill. Results of the quadrant analysis indicate that the sweep and ejection events are the major contributors to the Reynolds stress. Others like inward and outward interaction events make negative contributions. The values of the stress fractions of ejection and sweep events become the lowest as the wind attack angle is 20°. Analysis of the turbulent velocity power spectrum density shows that the spectrum density is increasing in the lower-frequency region as the wind attack angle increases. The power spectrum density is found to decrease for increase in the wind attack angle at the higher-frequency region.