Search

We introduce a novel methodological advancement by clustering paired near-surface air temperature with the planetary boundary layer height (PBLH) to characterize intra-city clusters for analytics. To illustrate this approach, we analyze three heatwaves (HW): the 2019 HW in Paris, the 2018 HW in Montreal, and the 2017 HW in Zurich. We assess cluster-based characteristics before, during, and after heatwave events. Using the objective hysteresis model, we determine the overall strength coefficient of the hysteresis loop between ground storage flux and all-wave downward radiative flux, ranging from 0.414 to 0.457 for urban clusters and from 0.126 to 0.157 for rural clusters during the heatwave periods. Across all cities, we observe a consistent refueling-restoration mode in the cumulative ground heat flux as the heatwaves progress. Future developments of this proposed two-component clustering approach, with the integration of more influential physics, will offer a more comprehensive characterization of cities for urban climate analytics.

Rising demand for space cooling has been placing enormous strain on various technological, environmental, and societal dimensions, resulting in issues related to energy consumption, environmental sustainability, health and well-being, affordability, and equity. Holistic approaches that combine energy efficiency optimization, policy-making, and societal adaptation must be rapidly promoted as viable, timely solutions. We interpret the 30-year upward trend and spikes in urban cooling demand from the perspective of climate change, urbanization, and background climates, focusing on five representative cities: Hong Kong, Sydney, Montreal, Zurich, and London. An unequivocal, worrying upward trend in cooling demand is observed in meteorological data from 1990 to 2021, using cooling degree hours (CDH) as a city-scale metric. The surge in cooling energy demand can be largely attributed to global warming, urban heat islands, and extreme heat events. Further, our quantification of the impact of the base temperature, in relation to the historical CDH, reveals that a 20% energy saving could be achieved instantly within a rather broad range of temperature and humidity by increasing the setpoint temperature by one degree, while characteristic sensational and physiological levels can be maintained at 'acceptable' and 'physiological thermal neutrality' respectively. However, the potential of reducing cooling demand can be nonlinearly and significantly lowered due to the presence of compound high relative humidity and high air temperature. To reduce cooling energy demand rapidly in a warming climate, we highlight the necessity of promoting hard and soft behavioral adaptation along with regulatory intervention for the operation of space cooling systems., Comment: 9 pages, 2 figures

We present a comprehensive study of nanoparticle deposition from drying of colloidal droplets. By means of lattice Boltzmann modeling and theoretical analysis, various deposition patterns, including mountain-like, uniform and coffee ring, as well as un-/symmetrical multiring/mountain-like patterns are achieved. The ratio of nanoparticles deposited at droplet peripheries and center is proposed to quantify different patterns. Its value is controlled by the competition between the capillary flow and nanoparticle diffusion, leading to a linear dependence on an effective P\'eclet number, across over three orders of magnitude. Remarkably, the final deposition pattern can be predicted based on material properties only.

Nowadays, cities are frequently exposed to heatwaves, worsening the outdoor thermal comfort and increasing cooling energy demand in summer. Urban forestry is seen as one of the viable and preferable solutions to combating extreme heat events and urban heat island (UHI) in times of climate change. While many cities have initiated tree-planting programmes in recent years, the evolving impact of trees on street microclimate, in a time span of up to several decades, remains unclear. We investigate the cooling effects of linden trees in five groups, i.e., 10-20, 20-30, 30-40, 40-60, and 60-100 years old. The leaf area index (LAI) and leaf area density (LAD) vary nonlinearly as the trees grow, peaking at different ages. Computational fluid dynamics (CFD) simulations solving microclimate are performed for an idealized street canyon with trees of varied age groups. Turbulent airflow, heat and moisture transport, shortwave and longwave radiation, shading and transpiration are fully coupled and solved in OpenFOAM. The meteorological data, including air temperature, wind speed, moisture, and shortwave radiation of the heatwave in Zurich (June 2019), are applied as boundary conditions. The results show that young trees in the age group of 10-20 years old provide little heat mitigation at the pedestrian level in an extreme heat event. Optimal heat mitigation by trees is observed for the group of 30-60 years old trees. Finally, the potential impact of growing trees as a heat mitigation measure on air ventilation is evaluated., Comment: 4 figures, 8 pages

Plant response is not only dependent on the atmospheric evaporative demand due to the combined effects of wind speed, air temperature, humidity, and solar radiation, but is also dependent on the water transport within the leaf-xylem-root system. Therefore, a detailed understanding of such dynamics is key to the development of appropriate mitigation strategies and numerical models. In this study, we unveil the diurnal dynamics of the microclimate of a Buxus sempervirens plant using multiple high-resolution non-intrusive imaging techniques. The wake flow field is measured using stereoscopic particle image velocimetry, the spatiotemporal leaf temperature history is obtained using infrared thermography, and additionally, the plant porosity is obtained using X-ray tomography. We find that the wake velocity statistics is not directly linked with the distribution of the porosity but depends mainly on the geometry of the plant foliage which generates the shear flow. The interaction between the shear regions and the upstream boundary layer profile is seen to have a dominant effect on the wake turbulent kinetic energy distribution. Furthermore, the leaf area density distribution has a direct impact on the short-wave radiative heat flux absorption inside the foliage where 50% of the radiation is absorbed in the top 20% of the foliage. This localized radiation absorption results in a high local leaf and air temperature. Furthermore, a comparison of the diurnal variation of leaf temperature and the net plant transpiration rate enabled us to quantify the diurnal hysteresis resulting from the stomatal response lag. The day of this plant is seen to comprise of four stages of climatic conditions: no-cooling, high-cooling, equilibrium, and decaying-cooling stages., Comment: 44 pages, 19 figures, preprint

In liquid spray applications, the sprays are often created by the formation and destabilization of a liquid sheet or jet. The disadvantage of such atomization processes is that the breakup is often highly irregular, causing a broad distribution of droplet sizes. As these sizes are controlled by the ligament corrugation and size, a monodisperse spray should consist of ligaments that are both smooth and of equal size. A straightforward way of creating smooth and equally sized ligaments is by droplet impact on a mesh. In this work we show that this approach does however not produce monodisperse droplets, but instead the droplet size distribution is very broad, with a large number of small satellite drops. We demonstrate that the fragmentation is controlled by a jet instability, where initial perturbations caused by the injection process result in long-wavelength disturbances that determine the final ligament breakup. During destabilization the crests of these disturbances are connected by thin ligaments which are the leading cause of the large number of small droplets. A secondary coalescence process, due to small relative velocities between droplets, partly masks this effect by reducing the amount of small droplets. Of the many parameters in this system, we describe the effect of varying the mesh size, mesh rigidity, impact velocity, wetting properties, keeping the liquid properties the same by focusing on water droplets only. We further perform Lattice Boltzmann modeling of the impact process that reproduces key features seen in the experimental data., Comment: 11 pages, 12 figures, 6 supplemental movies (not included)

Separating petroleum hydrocarbons from water is an important problem to address in order to mitigate the disastrous effects of hydrocarbons on aquatic ecosystems. A rational approach to address the problem of marine oil water separation is to disperse the oil with the aid of surfactants in order to minimize the formation of large slicks at the water surface and to maximize the oil-water interfacial area. Here we investigate the fundamental wetting and transport behavior of such surfactant-stabilized droplets and the flow conditions necessary to perform sieving and separation of these stabilized emulsions. We show that, for water soluble surfactants, such droplets are completely repelled by a range of materials (intrinsically underwater superoleophobic) due to the detergency effect; therefore, there is no need for surface micro/nanotexturing or chemical treatment to repel the oil and prevent fouling of the filter. We then simulate and experimentally investigate the effect of emulsion flow rate on the transport and impact behavior of such droplets on rigid meshes to identify the minimum pore opening (w) necessary to filter a droplet with a given diameter (d) in order to minimize the pressure drop across the mesh and therefore maximize the filtering efficiency, which is strongly dependent on w. We define a range of flow conditions and droplet sizes where minimum droplet deformation is to be expected and therefore find that the condition of is sufficient for efficient separation. With this new understanding, we demonstrate the use of a commercially available filter--without any additional surface engineering or functionalization--to separate oil droplets from a surfactant stabilized emulsion with a flux of 11,000 L m$^{-2}$ hr$^{-1}$ bar$^{-1}$. We believe these findings can inform the design of future oil separation materials.

Wood, due to its biological origin, has the capacity to interact with water. Sorption/desorption of moisture is accompanied with swelling/shrinkage and softening/hardening of its stiffness. The correct prediction of the behavior of wood components undergoing environmental loading requires that the moisture behavior and mechanical behavior of wood are considered in a coupled manner. We propose a comprehensive framework using a fully coupled poromechanical approach, where its multiscale implementation provides the capacity to take into account, directly, the exact geometry of the wood cellular structure, using computational homogenization. A hierarchical model is used to take into account the subcellular composite-like organization of the material. Such advanced modeling requires high resolution experimental data for the appropriate determination of inputs and for its validation. High-resolution x-ray tomography, digital image correlation, and neutron imaging are presented as valuable methods to provide the required information.

The hygro-mechanical behavior of a hierarchical cellular material, i.e. growth rings of softwood is investigated using a two-scale micro-mechanics model based on a computational homogenization technique. The lower scale considers the individual wood cells of varying geometry and dimensions. Honeycomb unit cells with periodic boundary conditions are utilized to calculate the mechanical properties and swelling coefficients of wood cells. Using the cellular scale results, the anisotropy in mechanical and swelling behavior of a growth ring in transverse directions is investigated. Predicted results are found to be comparable to experimental data. It is found that the orthotropic swelling properties of the cell wall in thin-walled earlywood cells produce anisotropic swelling behavior while, in thick latewood cells, this anisotropy vanishes. The proposed approach provides the ability to consider the complex microstructure when predicting the effective mechanical and swelling properties of softwood.