Search

Global urbanization has underscored the significance of urban microclimates for human comfort, health, and building/urban energy efficiency. However, analyzing urban microclimates requires considering a complex array of outdoor parameters within computational domains at the city scale over a longer period than indoors. As a result, numerical methods like Computational Fluid Dynamics (CFD) become computationally expensive when evaluating the impact of urban microclimates. The rise of deep learning techniques has opened new opportunities for accelerating the modeling of complex nonlinear interactions and system dynamics. Recently, the Fourier Neural Operator (FNO) has been shown to be very promising in accelerating solving the Partial Differential Equations (PDEs) and modeling fluid dynamic systems. In this work, we apply the FNO network for real-time three-dimensional (3D) urban microclimate simulation. For modeling large-scale urban microclimate problems, CityFFD simulates urban microclimate features based on the semi-Lagrangian approach and fractional stepping method with the Smagorinsky large eddy simulation model. In our simulation, the 1200 sequential time steps are used as training data. We retain and analyze the data from all stages, including the spin-up period, because we wish to understand how the flow develops transiently from initial conditions, and both one-step and sequential timestep predictions are analyzed. When applied to unseen data with different wind directions, the FNO model has a 0.3% one-step prediction error and a maximum error of 5%. A real-time simulation of urban microclimates in 3D is possible with the FNO approach, which is 25 times faster than the traditional numerical solver. • Fourier neural operator (FNO) was applied to simulate 3D urban wind field. • FNO is demonstrated to learn 3-D N–S equations for reconstruction of urban microclimate. • Different instantaneous wind direction results obtained when trained only for one wind direction. • Real-time predictions achieved within 0.08 s: 25 times faster than physics-based GPU solver. • Limitations identified and FNO codes and training data available for public sharing. [ABSTRACT FROM AUTHOR]

Air curtains have been widely used to reduce infiltration through door openings and save heating/cooling energy in different types of buildings. Previous studies have found that there exist three aerodynamics conditions: optimum condition (OC), inflow break-through (IB), and outflow break-through (OB) conditions, which are important for categorizing air curtain performance subject to such key parameters including supply speed and angle, and presence of a person during an actual operation. However, few studies have focused on the effects of these parameters on air curtain performance in terms of resisting infiltration and reducing exfiltration. This research presents a parametric study of air curtain performance based on reduced-scale experiments and full-scale numerical simulations. It was found that increasing air curtain supply angle improves air curtain performance when it is operated under the OC and IB conditions but creates excessive exfiltration under the OB condition. Increasing supply speed of air curtain generally improves the air curtain performance whereas this improvement deteriorates with the increase of supply angle under the OB condition. The presence of person, either directly under or below the air curtain, almost has no effect on the infiltration/exfiltration during the OC condition. Moreover, the person in the doorway can block airflow from both directions, contributing to less infiltration under the IB condition and less exfiltration under the OB condition than without the person. This study provides valuable insights into air curtain aerodynamics performance under different operational conditions and key contributing parameters. [ABSTRACT FROM AUTHOR]

Thermal stratification in large warehouses is a common phenomenon in winter, especially for cold climates. Buoyancy-driven warm air tends to move upwards towards ceilings creating vertical temperature gradients with upper higher temperatures, and consequently leading to excessive heat losses through roofs. These warehouses therefore require thermal destratification through certain air mixing strategies, the selection of which often relies on engineering rules of thumb and experiences but can be better determined through combined experimental and numerical approaches. This paper investigated different thermal destratification methods in two industrial warehouses based on field measurements and CFD simulations with the focus on the quantification of thermal stratification, and the suitable methods modeling air mixing devices, e.g. ceiling rotating fans and bucket fans. The numerical models were first validated by field measurements and then used to simulate different destratification strategies and to compare associated energy impacts. The CFD methods for modeling air mixing devices and their performances can be applied to other similar cases. [ABSTRACT FROM AUTHOR]

In sparsely occupied large industrial and commercial buildings, large-diameter ceiling fans1 (LDCFs) are commonly utilized for comfort cooling and destratification; however, a limited number of studies were conducted to guide the operation of these devices during the COVID-19 pandemic. This study conducted 223 parametrical computational-fluid-dynamics (CFD) simulations of LDCFs in the U.S. Department of Energy warehouse reference building to compare the impacts of fan operations, index-person, and worker-packing-line locations on airborne exposures to infectious aerosols under both summer and winter conditions. The steady-state airflow fields were modeled while transient exposures to particles of varying sizes (0.5–10 μm) were evaluated over an 8-h period. Both the airflow and aerosol models were validated by measurement data from the literature. It was found that it is preferable to create a breeze from LDCFs for increased airborne dilution into a sparsely occupied large warehouse, which is more similar to an outdoor scenario than a typical indoor scenario. Operation of fans at the highest feasible speed while maintaining thermal-comfort requirements consistently outperformed the other options in terms of airborne exposures. There is no substantial evidence that fan reversal is beneficial in the current large space of interest. Reversal flow direction to create upward flows at higher fan speeds generally reduced performance compared with downward flows, as there was less airflow through the fan blades at the same rotational speed. Reversing flow at lower fan speeds decreased airflow speeds and dilution in the space and, thus, increased whole-warehouse concentrations. • 223 parametrical CFD simulations of large-diameter ceiling fans (LDCFs) in a 3220 m2 warehouse. • 8-h transient airborne particle transmissions were modeled by a validated drift-flux model. • The breeze from LDCFs and higher fan speeds benefits airborne dilutions and depositions. • Reversing LDCFs lowers air speeds and dilutions and thus results in higher airborne concentrations. • New LDCF operation guidelines were recommended for both summer and winter operations. [ABSTRACT FROM AUTHOR]

Trees can effectively regulate the urban microclimate, while the change of microclimate conditions in turn affect the physiological state of trees. In this paper, CFD simulations are performed by coupling canopy energy balance (CEB) model to study the effects of trees on outdoor microclimate. The results show that the tree has different effects on outdoor microclimate under different wind speeds, air conditions, solar radiation and stomatal resistance. Increasing the wind speed will weaken the cooling effect of the tree. This weakening effect can extend to a distance of about 3.2 times of the tree canopy width behind the tree when the inflow air temperature is 30 °C. However, the influence range at high wind speeds is greater than that at low wind speeds. Air temperature and relative humidity have opposite effects on the sensible and latent heat fluxes of the tree, while they both have negligible effects on the net radiation flux. The humidification effect of the tree will be weakened as the relative humidity increases. Solar radiation has a greater effect on leaf surface temperature (LST) than on air temperature. The net radiation flux is high at the top and bottom of the tree and comparatively low at the central section, which is related to the vertical distribution of leaf area density (LAD). Trees can mitigate the effects of environmental changes on the LST by regulating stomatal resistance, and the reduction of stomatal resistance leads to a greater cooling effect. • CFD study trees effect on urban microclimate under different influencing factors. • Trees have different effects on urban microclimate in different environments. • Increasing wind speed weakens cooling effect of trees under the same conditions. • Air parameters have little effect on the net radiation flux absorbed by trees. • The distribution of net radiation flux is highly correlated with leaf area density. [ABSTRACT FROM AUTHOR]

Highlights: [•] Air curtain was characterized by optimum condition, inflow/outflow break-through. [•] Infiltration models were developed for air curtains based on door usage frequencies. [•] A new parameter, flow modifier, is added to characterize air curtain infiltration. [•] Air curtain reduces more infiltration than single door, and door with vestibule. [•] Air curtain could cause more exfiltration than door without air curtain. [Copyright &y& Elsevier]

Abstract: The hot smoke test is often used for commissioning fire smoke management system in atrium buildings, in which liquid fuel is burnt to generate a buoyant plume mixed with artificial tracer smoke to model a fire smoke. The method is usually costly and often causes safety concerns. This paper studied an alternative method of using a cold smoke test, in which pure helium is used to create the buoyant plume. A method was developed to determine the supply rate of pure helium necessary to achieve the same buoyancy effect as that of the corresponding hot smoke test. Computational fluid dynamics (CFD) simulations of the helium smoke tests were conducted and compared to the measured hot smoke tests in a full-scale naturally ventilated atrium and a sub-scale atrium with mechanical ventilation. A new method was added in the CFD model to track the smoke layer height for the simulations of helium smoke based on the concentrations of smoke and helium. It is found that the predicted smoke layer heights based on the mass fractions of the tracer smoke are generally close to the measured ones in the hot smoke tests of different heat release rates. A non-dimensional temperature in the hot smoke test is also found closely related to the dimensionless helium concentrations in the helium smoke test for the atria modeled. Although the consumption of pure helium for a full-scale helium smoke test can be very high, it is promising to use the pure helium smoke test in the lab-scale experiments as the preliminary tests of full-scale and/or lab-scale testing of real fires. [Copyright &y& Elsevier]

Abstract: Designing ventilation systems for buildings requires a suitable tool to assess the system performance. This investigation assessed seven types of models (analytical, empirical, small-scale experimental, full-scale experimental, multizone network, zonal, and CFD) for predicting ventilation performance in buildings, which can be different in details according to the model type. The analytical model can give an overall assessment of a ventilation system if the flow could be approximated to obtain an analytical solution. The empirical model is similar to the analytical model in terms of its capacities but is developed with a database. The small-scale model can be useful to examine complex ventilation problems if flow similarity can be maintained between the scaled model and reality. The full-scale model is the most reliable in predicting ventilation performance, but is expensive and time consuming. The multizone model is a useful tool for ventilation design in a whole building, but cannot provide detailed flow information in a room. The zonal model can be useful when a user has prior knowledge of the flow in a room. The CFD model provides the most detailed information about ventilation performance and is the most sophisticated. However, the model needs to be validated by corresponding experimental data and the user should have solid knowledge of fluid mechanics and numerical technique. Thus, the choice for an appropriate model is problem-dependent. [Copyright &y& Elsevier]

The Reynolds independence has been important for scaled experiments of urban aerodynamics in wind environment engineering. It helps to circumvent the difficulty in conserving the Re number for large scaling ratios. However, the criteria for Re-independence and its working mechanism have not been well investigated quantitatively. In this study, a dimensionless numerical analysis is conducted to provide a microscopic view of the problem both theoretically and numerically. The conditions of the similarity and Re-independence are identified theoretically. A series of simulations of the typical urban canopy layer model with three typical building-street aspect ratios were performed to quantify the similarity levels by some new evaluation metrics, local adapted deviation rate (ADR), the turbulence viscosity ratio (TVR), and the fraction of laminar viscosity (μ ' / μ e f f '). It is found that one explanation for the similarity of Re-independence is that either the convection of the flow dominates the diffusion, and/or the turbulence viscosity dominates the laminar viscosity inside the diffusion term. A regional critical Re number (R e c r − r ) concept is introduced to describe the case-specific and region-specific Re-independence: it is around 10,000–30,000 for the canyon and building height regions, and 60,000–140,000 for the near-wall regions. • Numerical dimensional analysis is conducted for urban aerodynamics. • Conditions of similarity and Re-independence are identified theoretically. • Re-independence and critical Re numbers are problem-specific and region-specific. • Similarity-prone and Re-independence-prone regions can be found quantitatively. • A general numerical dimensional analysis process with key parameters is proposed. [ABSTRACT FROM AUTHOR]

• An urban building energy model was integrated with an urban fast fluid dynamics model for resilience study. • City fast fluid dynamics is the new urban fast fluid dynamics model with graphics processing unit computing. • City building energy model is the new urban energy model with extensive building archetypes. • 1971 snowstorm of the century was modeled for vulnerability and survivability study. • Retrofitting measure was analyzed to evaluate improved urban resilience. In the past decade, urban building energy models have been developed to address the increasing concerns over energy consumption and greenhouse gas emission due to rapid urbanization and building resilience as a result of climate change. These models can estimate energy consumption, GHG emission and resilience response of buildings in an urban area, and evaluate retrofit strategies for architects, engineers, researchers and policy makers. It has been recognized that local microclimate and neighborhood effects play an important role in urban building energy modeling. Creating an urban building energy model also requires the collection of extensive building data, which is a time-consuming process. In this study, we developed an integrated platform by combining CityFFD (City Fast Fluid Dynamics), an urban-scale fast fluid dynamics model for microclimate modeling, and CityBEM (City Building Energy Model), a new urban building energy model with a library of 1700 building archetypes for facilitating urban model creation. Local aerodynamics and heat transfer information are exchanged between both models at each time step. Graphics processing unit computing is also applied to CityFFD for simulation speedup. The simulation of the 1971 Montreal snowstorm of the century was conducted as a case study of more than 1500 buildings of an island near Montreal, Canada for the investigation of their resilience against the three-day power outage due to the storm. Building retrofit analysis was also conducted to evaluate the added level of resilience. The results show that the proposed platform can produce high-resolution results of building thermal load, microclimate condition, and building behavior during weather extremes. [ABSTRACT FROM AUTHOR]

The ability to predict the concentration of hydrogen in a partially confined space is significant to the safe use of hydrogen-related products such as fuel cell vehicles. Hydrogen release and subsequent dispersion are frequently investigated using commercially-available computational fluid dynamic (CFD) models once those are calibrated with available experimental data. Due to the explosion safety concerns of using hydrogen, accidental scenarios are often replicated with helium as a hydrogen simulant in experiments. Currently, there is no validated, theoretical analogy to correlate the helium data in order to predict the spatial and temporal distribution of hydrogen in the enclosure. The aim of this paper is to assess different theoretical relationships for the similarity between hydrogen and helium leakage in an enclosure. Experiments were first carried out to obtain measured data with helium leakage in a set of scenarios and to validate the present CFD model. Three methods, namely equal volumetric flow rate, equal buoyancy and a newly proposed correlation derived from equal concentration, were employed to determine the equivalent hydrogen release rate, using which the validated CFD model was then used, with the physical property values for hydrogen, to stimulate hydrogen dispersion as compared to that of helium. The accuracy of these different methods at different stage of release and location is discussed. The present result thus provides a guide when using helium experiment to validate hydrogen simulation in different scenarios, which is of importance to the investigation of hydrogen safety. [ABSTRACT FROM AUTHOR]

An air curtain is often installed at a building entrance as an air barrier to reduce infiltration and resultant energy and air quality concerns. Due to its direct contact with the ambient environment, wind could affect the air curtain's performance significantly, whereas its interaction with the wind has not been well understood. This study carried out a series of experiments on a sub-scaled building model with an air curtain in an atmospheric boundary layer wind tunnel. A CFD model was also built to further investigate the air curtain jet interactions with the wind. The CFD model was verified and validated through Particle Image Velocimetry measurements for different air curtain and wind conditions and in terms of the function of the dimensionless air infiltration rate versus the dimensionless pressure difference across the air curtain. This study defined the wind pressure coefficient for air curtains based on their performance under wind compared to that without wind. The wind pressure coefficient was found to be constant for different air curtain supply speed, supply angle, and wind speed, but varies with wind direction, so its profile was provided as a function of wind direction. This study provides insights into air curtain performance under various wind conditions. • Air curtain performance was studied under different wind conditions. • Wind effect on air curtain was quantified by a new wind pressure coefficient. • New wind pressure coefficient depends on wind direction but not on wind speed. • Wind pressure coefficient was constant for various air curtain speeds and angles. [ABSTRACT FROM AUTHOR]