The Physics in Natural Ventilation of Cities and Buildings

Asian cities are mostly taller, denser, deeper and larger than those in the West, and the magnitudes of building drag or urban heat island circulation and their effects on city ventilation are also stronger. The physics of urban climate in these large cities is complex, as a combined result of local circulation and synoptic winds modified by the mountainous topography and land/sea breeze, if any. Natural ventilation of a city refers to the penetration and distribution of rural air into an urban canopy layer. The weakened city ventilation has become one major reason for worsening urban warming and air pollution in cities. Two distinct situations need to be considered, i.e. when the synoptic wind is strong; and when the synoptic wind is weak respectively. For the former, designers are interested to manage city ventilation for removal of the urban heat, moisture and pollutant, or retain of urban heat and moisture. The latter become mostly the conditions for the worst urban extreme heat or haze scenarios to occur. Natural ventilation of a building refers to the introduction of outdoor air into a building by natural forces such as wind and buoyancy. High-rise buildings present an interesting challenge as the top of the building may be in the urban roughness layer or even beyond the atmosphere boundary layer. Many excellent review papers exist on relevant urban airflows (e.g. Roth, 2000, Britter and Hanna 2003, Arnfield, 2003, Belcher 2005), but not specifically on city ventilation. City ventilation is mainly driven by winds and buoyancy forces such as slope flows, sea-land breezes, etc. The importance of city ventilation may be seen by long recognition that the restricted air flows were the causes of the all major pollution disasters (Brimblecombe and Sturges, 2009). Rigby et al (2006) presented a rose analysis showing the influence of boundary layer ventilation. The wind speed in London is often found to be lower than in a rural area, whilst occasionally accelerated due to urban heat island effects (Lee, 1979). The purpose here is to review the status of our understanding of the physics in city ventilation under both strong and weak wind conditions. It is known that understanding the urban air flows in calm wind conditions is crucial, as most urban heat wave and severe air pollution episodes occur when wind calmness and inversion coexist, leading to formation of a heat dome or urban heat island circulation. Heat dome comprises a convergent inflow at the lower atmospheric level, divergent outflow at the upper, and a dome-shaped flow field resulting from entrainment and overshoot at the top. Numerous field studies worldwide have confirmed the existence of UHIC during the day and night in many cities. It is interesting that though a strong wind would destroy the heat dome and breakup the inversion, a weak wind may only elongate the dome to become a plume or dome shadow, transport the pollutant downstream to other cities. How such a weak wind impact on the dome formation has not been well studied. Examples given here including wind weakening phenomenon in a dense high-rise city (Peng et al. 2018), the roles of heat dome formation on urban extreme high temperature events, spread of SARS CoV virus when there is inversion, and the urban heat domes (Fan et al 2017) and their merging (Fan et al 2018). Different methods are available for investigation, i.e. simple theoretical estimates (Fan et al 2017), water tank models (Fan et al 2016), city scale CFD (Wang and Li, 2016), and meso-scale WRF (Wang et al 2017). It is concluded that there is a need to establish the need and an approach for designing city climate and environment as for buildings, for example, designing building density and height in a city for better urban climate, and between-city distance needed to avoid regional haze formation

The Impact of City Scale Morphological and Anthropogenic Heat Parameters on Daily Temperature Cycles

AbstractUrban heat island (UHI) is generally considered to be one of the major problems to human beings due to accelerated urban growth and anthropogenic heat release. To further investigate the cause of UHI, an improved Zero-dimensional City Air Temperature (zCAT) model was proposed for analyzing urbanization effect on urban thermal environment and applied to the city of Hong Kong. Comparison of model result with measured meteorology data revealed that the improved model was able to predict daily varying urban air temperature with good accuracy, with insignificant effect on the model performance based on different weather condition. We conclude that building height and plan area ratio play an important role on daily cycle of urban air temperature

Polychlorinated Biphenyls (PCBs) Enhance Metastatic Properties of Breast Cancer Cells by Activating Rho-Associated Kinase (ROCK)

Background: Polychlorinated biphenyls (PCBs) are a family of structurally related chlorinated aromatic hydrocarbons. Numerous studies have documented a wide spectrum of biological effects of PCBs on human health, such as immunotoxicity, neurotoxocity, estrogenic or antiestrogenic activity, and carcinogensis. The role of PCBs as etiologic agents for breast cancer has been intensively explored in a variety of in vivo, animal and epidemiologic studies. A number of investigations indicated that higher levels of PCBs in mammary tissues or sera correlated to breast cancer risk, and PCBs might be implicated in advancing breast cancer progression. Methodology/Principal Findings: In the current study, we for the first time report that PCBs greatly promote the ROCK activity and therefore increase cell motility for both non-metastatic and metastatic human breast cancer cells in vitro. In the in vivo study, PCBs significantly advance disease progression, leading to enhanced capability of metastatic breast cancer cells to metastasize to bone, lung and liver. Additionally, PCBs robustly induce the production of intracellular reactive oxygen species (ROS) in breast cancer cells; ROS mechanistically elevate ROCK activity. Conclusions/Significance: PCBs enhance the metastatic propensity of breast cancer cells by activating the ROCK signaling, which is dependent on ROS induced by PCBs. Inhibition of ROCK may stand for a unique way to restrain metastases in breast cancer upon PCB exposure

A scalable and physiologically relevant system for human induced pluripotent stem cell expansion and differentiation

Human induced pluripotent stem cells (iPSCs) and their derivatives are needed in large numbers for various biomedical applications. However, scalable and cost-effective manufacturing of high quality iPSCs and their derivatives remains a challenge. In vivo, cells reside in a 3D microenvironment that has plenty of cell-cell and cell-ECM (extracellular matrix) interactions, sufficient supply of nutrients and oxygen, and minimal hydrodynamic stresses. The current iPSC culturing methods, however, provide highly-stressed culturing microenvironments, leading to low culture efficiency. For instance, we and others showed iPSCs typically expanded 4-fold/4 days to yield ~2.0x10^6 cells/mL with current 3D suspension culturing. These cells occupy ~0.4% of the bioreactor volume. To our best knowledge, the largest culture volume demonstrated to date for iPSCs is less than 10 liters. There is a critical need to develop new culture technologies to achieve the iPSCs’ potential. Please click Additional Files below to see the full abstract

The impact of building operations on urban heat/cool islands under urban densification: a comparison between naturally-ventilated and air-conditioned buildings

Many cities are suffering the effects of urban heat islands (UHI) or urban cool islands (UCI) due to rapid urban expansion and numerous infrastructure developments. This paper presents a lumped urban-building thermal coupling model which captures the fundamental physical mechanism for thermal interactions between buildings and their urban environment. The benefits of the model are its simplicity and high computational efficiency for practical use in investigating the diurnal urban air temperature change and its asymmetry in a city with both naturally-ventilated (NV) and air-conditioned (AC) buildings. Our model predicts a lower urban heat island and higher urban cool island intensity in a city with naturally-ventilated buildings than for a city with air-conditioned buildings. During the urban densification (from a low-rise, low-density city to a high-rise, high-density one), the increases in the time constant and internal heat gain give rise to asymmetric warming phenomena, which become more obvious in a city with air-conditioned buildings rather than naturally-ventilated ones. Unlike previous studies, we found that a low-rise, low-density city experiences a stronger urban cool island effect than a high-rise, high-density city due to less heat being emitted into the urban atmosphere. The urban cool/heat island effect will firstly increase/decrease, and then rapidly decrease/increase and ultimately disappear/dominate with increasing time constant in the process of urbanization/urban densification

Stationary distribution of stochastic SIRS epidemic model with standard incidence

We study stochastic versions of a deterministic SIRS (Susceptible, Infective, Recovered, Susceptible) epidemic model with standard incidence. We study the existence of a stationary distribution of stochastic system by the theory of integral Markov semigroup. We prove the distribution densities of the solutions can converge to an invariant density in L1. This shows the system is ergodic. The presented results are demonstrated by numerical simulations