Niigata Institute of Technology

Motamedi H., Shirzadi M., Tominaga Y., Mirzaei P.A.

Airborne transmission is an important route of spread of viral diseases (e.g., COVID-19) inside the confined spaces. In this respect, computational fluid dynamics (CFD) emerged as a reliable and fast tool to understand the complex flow patterns in such spaces. Most of the recent studies, nonetheless, focused on the spatial distribution of airborne pathogens to identify the infection probability without considering the exposure time. This research proposes a framework to evaluate the infection probability related to both spatial and temporal parameters. A validated Eulerian-Lagrangian CFD model of exhaled droplets is first developed and then evaluated with an office case study impacted by different ventilation strategies (i.e., cross- (CV), single- (SV), mechanical- (MV) and no-ventilation (NV)). CFD results were analyzed in a bespoke code to calculate the tempo-spatial distribution of accumulated airborne pathogens. Furthermore, two indices of local and general infection risks were used to evaluate the infection probability of the ventilation scenarios. The results suggest that SV has the highest infection probability while SV and NO result in higher dispersions of airborne pathogens inside the room. Eventually, the time history of indices reveals that the efficiency of CV and MV can be poor in certain regions of the room.

Okaze T., Kikumoto H., Ono H., Imano M., Ikegaya N., Hasama T., Nakao K., Kishida T., Tabata Y., Nakajima K., Yoshie R., Tominaga Y.

We designed and exclusively performed a wind tunnel experiment to obtain high-quality and high-resolution measurement results for three components of mean and fluctuating velocities around a 1: 1: 2 isolated building to establish a benchmark case for large-eddy simulation (LES) as a part of the activities of the Architectural Institute of Japan. Subsequently, we conducted LES to investigate the effect of computational grid arrangement on turbulent statistics by changing three grid arrangements. Finally, we performed a comparison by changing spatial discretization schemes, sub-grid scale (SGS) turbulence models, and convergence criteria. A grid in which the width of a building was discretized into 20 grids yielded sufficient resolution to reproduce the mean flow and second-order turbulent statistics. Under this appropriate grid arrangement, in the condition that the blending ratio of the first-order upwind interpolation scheme with the second-order linear interpolation scheme from 5 to 10%, the appropriate second-order statistics of wind velocity around the isolated building was obtained and numerical oscillation was effectively removed. Then, with this grid, any SGS model yielded appropriate results. In addition, the convergence criteria had little effects on the LES results if stable solution was obtained. Because the convergence criteria greatly affect the total calculation cost, weaker appropriate criteria should be set. • We conducted a comparison study of LES of flow field around a building. • A grid size dividing the building width into 20 grids provided sufficient resolution. • 5–10% upwind blended linear interpolation scheme yielded appropriate results. • The difference in results using different SGS turbulence models was small. • The convergence criteria of the PISO algorithm had minimal effect on the results.

Tominaga Y., Shirzadi M.

The wind velocity and surface pressure of the model of a group of urban blocks made of low-rise buildings with a high-rise building in the center were measured using a wind tunnel. Not only the time-averaged velocity field, but also the turbulent statistics, i.e., the turbulent kinetic energy, normal component of the Reynolds stresses, gust factor (GF), and probability density function (PDF), and power spectrum density (PSD) of instantaneous velocity components were investigated. Based on the results, the impact of high-rise buildings on the general flow structure and the surrounding pedestrian wind environment was discussed. The presence of high-rise buildings drastically changes the pedestrian wind environment around buildings in terms of time-averaged and instantaneous flow properties due to the complex interaction between the flows around the building and in and above the surrounding street canyons. High-rise buildings create a large GF region where the downwash flow and the street flow collide. A clear correlation between the GF and the time-averaged horizontal velocity magnitude was observed regardless of the height of the central building. The PDF of instantaneous velocity components around the high-rise building was characterized by a large skew not only near the building corners of the high-rise building but also on the surrounding streets. The high-rise building produces large PSDs of the velocity fluctuation components, particularly near the upwind corner at a high frequency and on the surrounding street at an intermediate frequency. Furthermore, the results of the time-averaged and instantaneous flow properties provide a useful database for computational fluid dynamics (CFD) validation and model development studies. • Wind tunnel measurement of turbulent flow around a building group were performed. • Impact of high-rise buildings on the surrounding pedestrian wind is discussed. • Various turbulent statistics of velocity at gust wind environment are investigated. • Complex interaction between downwash and street flows is essential.

Shirzadi M., Tominaga Y., Mirzaei P.A.

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 neglect 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 cell numbers can provide comparable results with structured hexahedral mesh configurations. Furthermore, the second-order discretization scheme for the 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 show a very low accuracy in the case of the normal wind angle, but acceptable results can be found for oblique wind angles.

Shirzadi M., Mirzaei P.A., Tominaga Y.

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.

Potsis T., Tominaga Y., Stathopoulos T.

The paper reviews the evolution of computational wind engineering from environmental and structural perspectives, since the inaugural conference of computational wind engineering held in Tokyo 30 years ago (CWE 92). The progress in computational methodologies and important aspects for accurate analysis are discussed. As a groundwork for the application of computational fluid dynamics (CFD) to various environmental issues, the importance of accurate modeling of atmospheric boundary layer, urban boundary layer, and urban canopy layer is pronounced. Environmental applications refer to urban micro-climate, pedestrian level wind, near-field pollutant dispersion, natural and urban ventilation, urban wind energy and snow/sand erosion and accumulation. Structural applications refer to wind loading on low- and high-rise buildings, including wind directionality. The most seminal contributions are examined, and their results are presented. It becomes clear that the engineering community has gained more benefits from environmental than structural computational wind engineering applications, mainly due to the usually less demanding computational needs for the former. Future challenges in CFD applications are thoroughly discussed and the need to bridge the gap between environmental and structural applications is highlighted.

Shirzadi M., Tominaga Y., Mirzaei P.A.

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 facades 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.

Shirzadi M., Tominaga Y.

In this study, a multi-fidelity shape optimization framework is proposed for the pedestrian-level wind environment (PLWE). In the proposed framework, low-fidelity computational fluid dynamics (CFD) models based on steady Reynolds-averaged Navier–Stokes equations (RANS) models and high-fidelity CFD models based on large-eddy simulation (LES) are efficiently integrated into the optimization process to improve the optimization reliability while maintaining its computational speed in an affordable range for practical engineering applications. The optimization solver is coupled with an approximation model generated by low-fidelity CFD samples obtained using a design of experiments (DOE) technique. The optimal candidates are then evaluated according to the degree of improvement of the objective function compared to the reference case. If the degree of improvement shows significant deviations between the low-fidelity and high-fidelity models, suitable corrections and modifications are applied to improve the reliability of the optimization process. The applicability of the proposed method was investigated in terms of minimizing the high-wind-speed area, as the optimization objective, around a high-rise building considering (a) uniform urban blocks and (b) real urban blocks with different frequency distributions of wind directions associated with two different local wind climates. In summary, a significant reduction in the critical strong-wind area around the target building was realized using the proposed optimization framework. Furthermore, the application of the proposed multi-fidelity optimization framework highlighted the importance of considering the local wind climate in architectural design. • A novel multi-fidelity shape optimization for pedestrian-level wind environment is proposed. • Low-fidelity and high-fidelity CFD models based on SRANS and LES are efficiently integrated into the optimization process. • This approach increases the reliability of the optimization process while maintaining its computational speed. • The frequency distributions of wind directions associated with different local wind climates were considered for a real urban block. • A significant reduction in the critical strong-wind area around the target building was realized using the proposed method.

Ma H., Zhou X., Tominaga Y., Gu M.

Plume buoyancy is one of the most crucial driving forces of gas pollutant dispersion in weak wind regions such as the recirculation region. In this study, steady Reynolds-averaged Navier-Stokes (SRANS) simulation and large eddy simulation (LES) are conducted simultaneously to investigate the flow and concentration fields around a cubic building, in which the positive and negative buoyancies produced by the density difference between pollutants and the ambient air are considered at the same time. The performances of SRANS and LES are quantitatively evaluated by comparing the simulated results with the data from wind tunnel tests. In general, LES provides more accurate predictions for the time-averaged concentration fields, especially for the cases of neutral gas and heavy gas. Also, by studying the effect of different plume buoyant forces, the research finds that the increased vertical turbulent flux of light gas in the recirculation region behind the building enhances the dilution of pollutant. However, the heavy gas has an adverse effect. It is also found that the positive plume buoyancy raises the concentration fluctuation above the pollutant source considerably, while the negative plume buoyancy has a certain inhibitory effect on it. ∙ The effect of positive and negative plume buoyancy is investigated simultaneously. ∙The prediction accuracy of RANS and LES for buoyant gas dispersion is evaluated. ∙Negative buoyancy restrains the dilution of pollutants in wake region. ∙Positive buoyancy raises the concentration fluctuation behind the building.

Blachier F., Blais A., Elango R., Saito K., Shimomura Y., Kadowaki M., Matsumoto H.

Amino acid supplementation may be indicated to correct for insufficient amino acid intake in healthy individuals, and in specific physiological or pathophysiological situations. However, there is a concern to not supplement beyond the tolerable upper intake level (UL) by determining parameters of no-observed-adverse-effect level (NOAEL) or lowest-observed-adverse-effect level (LOAEL) for each amino acid. Since the NOAEL and LOAEL values are at least one order of magnitude different when comparing the values obtained in rats and humans, the aim of this review is to evaluate to what extent the amino acid UL measured in the rat model, when referenced to the dietary usual consumption (UC) and dietary requirement (RQ) for indispensable amino acids, may be used as an approximation of the UL in humans. This review then compares the ratios of the NOAEL or LOAEL over UC and RQ in the rat model with the same ratios calculated in humans for the nine amino acids (arginine, serine, glycine, histidine, leucine, lysine, methionine, phenylalanine, and tryptophan) for which this comparison can be done. From the calculations made, it appears that for these 9 amino acids, the calculated ratios for rats and humans, although rather different for several amino acids, remains for all of them in the same order of magnitude. For tryptophan, tyrosine, and valine, the ratios calculated in rats are markedly different according to the sex of animals, raising the view that it may be also the case in humans.