Steady and unsteady RANS simulations of pollutant dispersion around isolated cubical buildings: Effect of large-scale fluctuations on the concentration field

The performance of unsteady Reynolds-Averaged Navier–Stokes equations (URANS) for simulations of flow and dispersion fields around isolated cubical buildings has been examined in this study. URANS results were compared with those obtained from steady-RANS (SRANS) computations and experiments. The comparison determines not only the applicability of URANS simulations, but also the contribution of unsteady large-scale fluctuations to pollutant dispersion around buildings. Three different source locations, i.e. upwind, rooftop and downwind releases, were considered for pollutant dispersion around the building. It was found that the improvement of the predicted concentration field achieved by URANS largely depends on the source location. Although this improvement was not as significant in the upwind and rooftop release cases, the prediction accuracy achieved by URANS was substantially improved for the downwind release case, for which, the unsteady-RANS simulations yielded larger estimates of the momentum and concentration diffusions behind the building than SRANS did, improving the accuracy of the estimation of the mean concentration

CFD prediction of flowfield and snowdrift around building complex in snowy region

A CFD ( Computational Fluid Dynamics) technique is applied to the prediction of snowdrift around a 9-storey apartment building. A modified version of the Launder-Kato-type k-εmodel (LK model[5,9,10]) is used. In the first part of the paper, results of a preliminary study for a flowfield around a cube placed in a channel flow are presented. Here, results obtained by using the modified LK model are compared with those of measurements and the standard k-ε model. The latter part describes the numerical prediction of snowdrift around a 9-storey apartment building which is presently under construction in Nagaoka City, Niigata Prefecture. Special emphasis is given to snowdrift into the elevator hall

Experimental and Steady-RANS CFD Modelling of Cross-ventilation in Moderately-dense Urban Areas

Computational fluid dynamics (CFD) models based on the steady Reynolds-averaged Navier Stokes (SRANS) equations are vastly used for calculation of airflow field inside and around cross-ventilated buildings. However, most of the developed CFD guidelines ignore CFD challenges related to cross-ventilation modeling in terms of flow unsteadiness, high level of gradients of airflow parameters, and complex interactions between the indoor and outdoor flows.Hence, a systematic parametric study was performed in this study for a generic cross-ventilated building model with a planar area ratio of 0.25 against different wind angles while effects of different CFD parameters, including advection and diffusion terms discretization methods, mesh generation techniques, and turbulence models on prediction accuracy and convergence behavior of CFD solver were comprehensively studied.Results show that a particularly generated unstructured tetrahedral mesh configuration with significantly lower mesh numbers can provide comparable results with structured hexahedral mesh configuration. Furthermore, second-order discretization scheme for advection terms encounters convergence issues against the normal wind angle, but generally presents more accurate results against oblique wind angles. Moreover, two-equation turbulence models showed very low accuracy in the case of normal wind angle, but acceptable results were found for oblique wind angles

CFD simulations of near-field pollutant dispersion with different plume buoyancies

This study performs computational fluid dynamics simulations for flow and dispersion fields around an isolated cubic building model with tracer gases being exhausted from an exit behind the building. The tracer gases have three different buoyancies according to the difference in density with ambient air and, therefore, behave as neutral, light, and heavy gases. The performance of steady Reynolds-averaged Navier–Stokes (RANS) simulations with the Boussinesq approximation is examined herein by comparing the simulation results with the experimental results for different plume buoyancies. The steady RANS computations can generally reproduce the impact of plume buoyancy on the mean concentration in the experimental results even if the model performance for heavy gases is better than that for light gases and worse than that for neutral gases. This tendency is closely related to the prediction accuracy of the mean velocity and turbulent kinetic energy behind the building, which is restricted by the steady RANS simulations. The study also confirmed that the buoyancy modeling in the ε equation shows a negligible influence on the results

Ten Questions Concerning Modeling of Near-Field Pollutant Dispersion in the Built Environment.

Outdoor air pollution is a major current environmental problem. The precise prediction of pollutant concentration distributions in the built environment is necessary for building design and urban environmental assessment. Near-field pollutant dispersion, involving the interaction of a plume and the flow field perturbed by building obstacles, is an element of outdoor air pollution that is particularly complex to predict. Modeling methodologies have been discussed in a wide range of research fields for many years. The modeling approaches are categorized into field measurements, laboratory (wind and water tunnel) experiments, (semi-) empirical models, and computational fluid dynamics (CFD) models. Each of these approaches has advantages and disadvantages. It is therefore important to use due consideration for the underlying theory and limitations when applying these modeling approaches. This paper considers some of the most common questions confronting researchers and practitioners in the modeling of near-field pollutant dispersion in the built environment

Modelling enhancement of cross-ventilation in sheltered buildings using stochastic optimization

Accurate representation of turbulence phenomenon in Computational Fluid Dynamics (CFD) modeling of cross-ventilation around and inside buildings is a challenging and complex problem, especially under the sheltering effect of surrounding buildings. Steady Reynolds Averaged Navier-Stokes (RANS) models are broadly used in many practical applications. However, these models mainly fail to predict accurate distribution of flow characteristics in the cavity formed between the buildings, and hence miscalculate the attributed cross flow and airflow rate through buildings. In this study, a novel and systematic methodology is proposed to enhance the accuracy of the model for the urban study applications such as cross-ventilation in the sheltered buildings.A microclimate CFD model for a case study of a cross-ventilation experimental work by Tominaga and Blocken (2015) was firstly constructed and validated. In the next step, the closure coefficients of themodel were modified using a stochastic optimization and Monte Carlo Sampling techniques. The probability density function (PDF) of all closure coefficients were given to the optimizer and proper objective function defined in terms of different validation metrics. The modified coefficients obtained from the developed systematic method could successfully simulates the cross-ventilation phenomena inside the building with an airflow rate prediction error less than 8% compared to the experiment while other RANS models predicted the airflow rate with up to 70% error. The effectiveness of the optimization technique was also discussed in terms of validation metrics and pressure coefficients

CFD analysis of cross-ventilation flow in a group of generic buildings: Comparison between steady RANS, LES and wind tunnel experiments

Computational fluid dynamics (CFD) results generated by the steady Reynolds-averaged Navier-Stokes equations (SRANS) model and large eddy simulation (LES) are compared with wind tunnel experiments for investigating a cross-ventilation flow in a group of generic buildings. The mean flow structure and turbulence statistics are compared for SRANS based on different two-equation turbulence models with LES based on the Smagorinsky subgrid-scale turbulence model. The LES results show very close agreement with the experimental results in the prediction of the time-averaged velocity, wind surface pressure around and inside the building, and crossing flow through the openings. In contrast, SRANS fails to predict the most important features of cross-ventilation. LES reproduces well the anisotropic turbulence property around and inside the cross-ventilated building, which is closely related to the transient momentum transfer caused in street canyon flows and has a significant influence on the mean flow structure. In contrast, SRANS could not inherently reproduce such transient fluctuations and anisotropic turbulence property, which results in low accurate predictions for the time-averaged velocity components, wind surface pressure distribution and crossing airflow rate up to 100% error

Experimental Study on Cross-ventilation of a Generic Building in Highly-dense Urban Areas: Impact of Planar Area Density and Wind Direction

This study presents the experimental results on cross-ventilation in a generic low-rise building placed in highly-dense urban configurations. Flow visualization studies were conducted by utilization of a smoke generator in order to investigate the nature of the flow pattern inside and around the cross-ventilated building. Moreover, distribution of the wind surface pressure coefficients over windward and leeward façades and internal walls of the target building were measured using a pressure tap system. Furthermore, the airflow rate crossing through the openings was measured using a tracer gas method. Different building configurations, representing highly-dense urban areas, as well as different wind angles were investigated in this study Surprisingly, the experimental results reveal a noticeable difference between the mechanism of cross-ventilation in moderately-dense and highly-dense buildings arrangements. A clear leeward jet with a highly-transient nature can be observed, which is generated due to a leeward vortex formed by the target and downstream buildings. As another novel finding of this study, the cross-ventilation is understood to be highly transient in highly-dense urban areas with a strong periodic fresh air pulsation through the windward and leeward openings. This behavior is fundamentally far from the steady state models considered for such cross-ventilation scenarios in literature

RANS model calibration using stochastic optimization for accuracy improvement of urban airflow CFD modeling

In this study, a systematic calibration methodology is proposed for enhancing the accuracy of urban airflow simulations using computational fluid dynamics (CFD) models based on the Reynolds-averaged Navier-Stokes (RANS) equations. In the calibration process, high-quality data from different sources are used to define the validation metrics, which are then utilized as the objective function in a stochastic optimization solver to find optimal values for closure coefficients of the RANS turbulence model. The proposed calibration method is applied to three different urban case studies, including an unstable atmospheric boundary layer (ABL) around a high-rise building, a sheltered cross-ventilated low-rise building, and a group of low-rise buildings located in a highly packed urban area.The significant advantage of using the obtained calibrated coefficients is observed over the existing coefficients embedded in CFD tools as well as the ones recommended by other calibration methods in literature. Thus, this study proves the necessity of finding a group of customized optimum closure coefficients for RANS turbulence models suitable for a wide range of urban flow problems

PIV Measurement of Flow around Buildings in Boundary-Layer Wind Tunnel

Particle Image Velocimetry (PIV) measurements were performed around two kinds of three dimensional building models those were placed in a wind tunnel. Special attention was paid to uniformity of emitting tracers and clearness of images. For a cubic model, flow separation around frontal corner and vortexes in front of and behind the model is observed in measured results. For a street canyon model, a vortex in the canyon is clearly shown. It was confirmed that PIV technique is very effective to measure the complicated velocity field around buildings in detail in a wind tunnel