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Modeling the urban climate using computational fluid dynamics (CFD) is essential for assessing urban thermal comfort and developing heat wave mitigation solutions. One mitigation strategy may rely on the enhancement of urban ventilation and the use of porous urban environments, as the lift-up, or on pilotis, buildings. Lift-up buildings have their ground floor in whole or in part supported by columns and shear walls, allowing wind flow through the building at pedestrian level. CFD modeling of these intricate geometries is computationally challenging for large urban neighborhoods. Simplifications, such as removing columns to obtain an acceptable computational cost, lead to erroneous results in the global flow pattern. To balance accuracy and computational cost, the impact of complex ground floor geometry on wind flow is modeled using a porosity sink term in the Navier-Stokes equations based on the Darcy-Forchheimer law. The improved CFD modeling of a simple lift-up building is validated with wind tunnel measurements. Simulations of wind flow around a realistic lift-up building for different wind directions determine the Forchheimer coefficients required for the porosity approach. Comparison of reference and numerical porosity results demonstrates a very good agreement in mean velocity patterns. The study demonstrates that oversimplifications, such as removing all columns, leads to an unacceptable overestimation of the wind velocity at pedestrian level. The paper highlights the benefits of the numerical porosity approach and its necessity for accurate urban-scale simulations. • The complexity of lift-up buildings' open ground floor can be modeled using artificial porosity. • The porous approach has been validated using wind tunnel measurements. • The Darcy-Forchheimer law can be used to model artificial porosity based on CFD simulations. • The porosity approach predicts pedestrian-level wind at a lower computational cost. • Oversimplified ground floor porosity leads to an unacceptable overestimation of the pedestrian-level wind speed. [ABSTRACT FROM AUTHOR]

An urban microclimate model is used to design a smart wetting protocol for multilayer street pavements in order to maximize the evaporative cooling effect as a mitigation measure for thermal discomfort during heat waves. The microclimate model is built upon a computational fluid dynamics (CFD) model for solving the turbulent air, heat and moisture flow in the air domain of a street canyon. The CFD model is coupled to a model for heat and moisture transport in porous urban materials and to a radiative exchange model, determining the net solar and thermal radiation on each urban surface. A two-layer pavement system, previously optimized for maximal evaporative cooling applying the principles of capillary pumping and capillary break, is considered to design a smart wetting protocol answering the questions "when," "how much," and "how long" a pavement should be artificially wetted. It was found for the current optimized pavement solutions that a daily amount of 6 mm wetting over 10 min in the morning, preferentially between 8:00 and 10:00, guarantees a maximal evaporative cooling for 24 h during a heat wave. [ABSTRACT FROM AUTHOR]

Wind-driven rain (WDR) is one of the largest moisture sources affecting the hygrothermal performance and durability of building envelopes. Accurate estimation of WDR loads on building facades as boundary condition is important for the hygrothermal analysis of building envelopes. In this work, computational fluid dynamics (CFD) simulations and semi-empirical models are used to compare WDR load on building facades in the urban environment and open field. The research focuses on two historical buildings in the city center of Zurich, Switzerland. There is a very large difference in terms of the local wind-flow fields around buildings in the urban environment and open field. Raindrop trajectories in the open field are more horizontal due to higher wind speeds around the building. In the urban environment, the wind speed is lower yielding droplets following trajectories that are more parallel to the facades. The building overhang is more effective at rain protection in the urban environment compared to the building in the open field due to a larger rain sheltering effect, since the rain trajectories are more parallel to the facades. There is a very large difference in WDR load and distribution on the facade of buildings in the urban environment and open field. WDR loads on buildings in the urban environment are much smaller than in the open field. When semi-empirical models are used for this case study, there is large uncertainty in the prediction and the predicted WDR load can be many times larger compared to the load predicted by CFD. • WDR simulation in the urban environment and open field is performed. • WDR load on buildings in the urban environment is smaller than in open field. • There is a large uncertainty in the prediction of WDR by semi-empirical models. [ABSTRACT FROM AUTHOR]

The maximum spreading of an impinging droplet on a rigid surface is studied for low to high impact velocity, until the droplet starts splashing. We investigate experimentally and numerically the role of liquid properties, such as surface tension and viscosity, on drop impact using three liquids. It is found that the use of the experimental dynamic contact angle at maximum spreading in the Kistler model, which is used as a boundary condition for the CFD-VOF calculation, gives good agreement between experimental and numerical results. Analytical models commonly used to predict the boundary layer thickness and time at maximum spreading are found to be less correct, meaning that energy balance models relying on these relations have to be considered with care. The time of maximum spreading is found to depend on both the impact velocity and surface tension, and neither dependency is predicted correctly in common analytical models. The relative proportion of the viscous dissipation in the total energy budget increases with impact velocity with respect to surface energy. At high impact velocity, the contribution of surface energy, even before splashing, is still substantial, meaning that both surface energy and viscous dissipation have to be taken into account, and scaling laws depending only on viscous dissipation do not apply. At low impact velocity, viscous dissipation seems to play an important role in low-surface-tension liquids such as ethanol. [ABSTRACT FROM AUTHOR]

Background and Aims Leaf transpiration is a key parameter for understanding land surface–climate interactions, plant stress and plant structure–function relationships. Transpiration takes place at the microscale level, namely via stomata that are distributed discretely over the leaf surface with a very low surface coverage (approx. 0·2–5 %). The present study aims to shed more light on the dependency of the leaf boundary-layer conductance (BLC) on stomatal surface coverage and air speed. Methods An innovative three-dimensional cross-scale modelling approach was applied to investigate convective mass transport from leaves, using computational fluid dynamics. The gap between stomatal and leaf scale was bridged by including all these scales in the same computational model (10−5–10−1 m), which implies explicitly modelling individual stomata. Key Results BLC was strongly dependent on stomatal surface coverage and air speed. Leaf BLC at low surface coverage ratios (CR), typical for stomata, was still relatively high, compared with BLC of a fully wet leaf (hypothetical CR of 100 %). Nevertheless, these conventional BLCs (CR of 100 %), as obtained from experiments or simulations on leaf models, were found to overpredict the convective exchange. In addition, small variations in stomatal CR were found to result in large variations in BLCs. Furthermore, stomata of a certain size exhibited a higher mass transfer rate at lower CRs. Conclusions The proposed cross-scale modelling approach allows us to increase our understanding of transpiration at the sub-leaf level as well as the boundary-layer microclimate in a way currently not feasible experimentally. The influence of stomatal size, aperture and surface density, and also flow-field parameters can be studied using the model, and prospects for further improvement of the model are presented. An important conclusion of the study is that existing measures of conductances (e.g. from artificial leaves) can be significantly erroneous because they do not account for microscopic stomata, but instead assume a uniform distribution of evaporation such as found for a fully-wet leaf. The model output can be used to correct or upgrade existing BLCs or to feed into higher-scale models, for example within a multiscale framework. [ABSTRACT FROM AUTHOR]

As cities and their population are subjected to climate change and urban heat islands, it is paramount to have the means to understand the local urban climate and propose mitigation measures, especially at neighbourhood, local and building scales. A framework is presented, where the urban climate is studied by coupling a meteorological model to a building-resolved local urban climate model, and where an urban climate model is coupled to a building energy simulation model. The urban climate model allows for studies at local scale, combining modelling of wind and buoyancy with computational fluid dynamics, radiative exchange and heat and mass transport in porous materials including evaporative cooling at street canyon and neighbourhood scale. This coupled model takes into account the hygrothermal behaviour of porous materials and vegetation subjected to variations of wetting, sun, wind, humidity and temperature. The model is driven by climate predictions from a mesoscale meteorological model including urban parametrisation. Building energy demand, such as cooling demand during heat waves, can be evaluated. This integrated approach not only allows for the design of adapted buildings, but also urban environments that can mitigate the negative effects of future climate change and increased urban heat islands. Mitigation solutions for urban heat island effect and heat waves, including vegetation, evaporative cooling pavements and neighbourhood morphology, are assessed in terms of pedestrian comfort and building (cooling) energy consumption. [ABSTRACT FROM AUTHOR]

Abstract: Knowledge on convective water vapour exchange at leaf-air interfaces is required to assess transpiration of leaves via stomata and evaporation of droplets, which can both be considered as microscopic moisture sources, heterogeneously distributed across the leaf. An innovative modelling approach was proposed to investigate such convective mass transport from leaf surfaces, using computational fluid dynamics (shear-stress transport k–ω turbulence model with low-Reynolds number modelling). The main novelty lies in the fact that a large range of spatial scales (10−5–10−1 m) was included and that the individual microscopic sources were modelled discretely. The convective exchange from the leaf model was strongly dependent on three parameters: surface coverage, air speed and source size. The relation between the convective flow rate and both the coverage ratio (CR) and the microscopic Sherwood number, i.e., the ratio of the source size to the viscous sublayer thickness, was quantified. It was shown that well-established convective transfer coefficients, obtained from plates or leaf models for a CR of 100%, can result in a significant overprediction of the convective exchange, compared to more realistic, lower CR. Furthermore, small variations in stomatal density (CR), e.g., due to biological variability, were shown to have a large impact on the convective exchange and droplets were found to evaporate more rapidly at low CR. The decrease in mass transfer rate due to stomatal closure was quantified as well. The proposed numerical modelling approach can be applied to increase our understanding of leaf transpiration and droplet evaporation, but also of the leaf boundary-layer microclimate and the transport processes therein. A critical discussion of the modelling assumptions allowed to identify focus points for future model refinement as well as future research. [Copyright &y& Elsevier]

Abstract: Accurate predictions of convective heat and mass transfer at air–porous material interfaces are essential in many engineering applications, one example being optimisation of industrial drying processes with respect to energy consumption and product quality. For porous-material modelling purposes, simplified convective transfer coefficients (CTCs) are often used to avoid explicit air-flow modelling. Alternatively, conjugate models have been introduced recently and are being more widely used. Conjugate modelling has the advantage that it does not require the use of CTCs or of the heat and mass transfer analogy. Instead, these CTCs can be identified a-posteriori. In this study, an overview of the existing methods to predict convective heat and mass transfer at air–porous material interfaces is given, with a specific focus on conjugate modelling. The improved accuracy of this approach is indicated based on two case studies, namely hygroscopic loading and convective drying. A large spatial and temporal variability of the CTCs is found by means of conjugate modelling. This approach provides increased accuracy, which is especially relevant for complex flow problems, such as in industrial drier systems. However, the sensitivity to the convective boundary conditions can be limited in some cases, e.g. for hygroscopic loading. Instead of improving accuracy significantly here, conjugate modelling will rather impose an additional modelling effort, which often requires conjugate model code development as these models are not readily available. Before embarking on a conjugate modelling study, it is advised to perform a sensitivity analysis with respect to the convective boundary conditions: in some cases, sufficient accuracy can be obtained using empirical CTCs from literature. [Copyright &y& Elsevier]

Vegetation can provide transpirative cooling in cities and is therefore being increasingly integrated as an essential part of Urban Heat Island (UHI) mitigation strategies. However, the behaviour of vegetation must be accurately understood to determine the effectiveness of vegetation based solutions. In this study, vegetation is modelled as a porous medium in a computational fluid dynamics model for flow of moist air, where a leaf energy balance model is used to determine the heat fluxes. We study the cooling effect of a single row of trees at noon with solar altitude at 90° for various environmental factors (wind speed, air temperature, relative humidity and solar radiation intensity) and tree properties (leaf size, stomatal resistance and leaf area density). Furthermore, the influence of tree height and number of tree rows on the cooling effect are studied. The Universal Thermal Climate Index (UTCI) around the trees is estimated to determine the impact of transpirative cooling on pedestrian thermal comfort. The study shows that, at low wind speeds, pedestrians would only perceive a local benefit of transpirative cooling. However, vegetation extracts overall more heat from the flow at higher wind speeds. A study on the influence of environmental conditions quantifies to which extent a single row of trees provide maximum cooling during hot and dry conditions. The shading provided by trees improves thermal comfort more that transpirative cooling of a single row of trees. Furthermore, taller trees are more beneficial as the vegetation canopy with high leaf temperatures is further away from the pedestrian level. [ABSTRACT FROM AUTHOR]

Convective drying is one of the most important forms of moisture removal in macroporous materials exposed to environmental loads. In this study, the influence of airflow on the convective removal of water vapor (passive scalar) from porous asphalt (PA), a macroporous material, is investigated for different wind speeds and levels of liquid water height within the material, using computational fluid dynamics (CFD). The PA domain for the CFD simulations is reconstructed from three-dimensional X-ray micro-computed tomography images. Steady Reynolds-averaged Navier-Stokes (RANS) k- ε model with low Reynolds number modeling for the near-wall region is used for the turbulence modeling. Simulations are carried out for three wind speeds, 0.1, 1.5 and 10 m/s, and three depths of the scalar (vapor) source, 0.13, 15 and 29 mm, within the specimen. Only the highest wind speed has a clear influence on the total vapor flux measured at the interface. Additionally, at the highest wind speed, air entrainment in pores near the air-material interface plays an important role in vapor removal. At lower wind speeds, diffusion within the material is the dominant vapor transport mechanism. At a wind speed of 10 m/s, strong vortices are observed which inhibit vapor removal in cavity-type pores by recirculating vapor within the pore. However, at the same wind speed, vapor transport in a non-cavity-type pore system is found to be very strong due to strong air convection. [ABSTRACT FROM AUTHOR]

Abstract: Urban heat islands affect the energy use for cooling in an urban environment, as well as human comfort and health. Water evaporation from moist surfaces could potentially reduce the local temperature in urban areas, a process known as evaporative cooling. This paper introduces a coupled model to study the effect of evaporative cooling on the temperature conditions in an urban street canyon. A computational model for determining convective heat and mass exchanges between the canyon walls and the air is proposed. The model couples three sub-models: (i) a Computational Fluid Dynamics (CFD) model, which solves heat and vapor transfer in the air, (ii) a Building Envelope Heat and Moisture (BE-HAM) transport model which solves heat and moisture transfer within the porous building walls and (iii) a radiation model (RAD) which determines the radiative heat exchange between the surfaces. An efficient coupling strategy has been developed and applied to investigate the drying of a wet windward wall of a street canyon. The effect of evaporation on the reduction of the surface and air temperatures in a street canyon is analyzed and the influence of these temperature reductions on the Physiological Equivalent Temperature (PET) is shown to be important. [Copyright &y& Elsevier]