Search

© 2020 Elsevier B.V. Heat waves are unusually high temperature events over consecutive days and may cause adverse impacts such as morbidity and mortality. The interaction between heat waves and urban heat island (UHI) effects has remained a subject of debate, as some studies prove heat wave-UHI synergy while others do not. Furthermore, heat waves affect tropical cities more severely than mid-latitude cities, but there is a disproportionate lack of heat wave studies focusing on tropical cities. We attempt to narrow this gap by studying the heat wave in Singapore in April 2016 using ground observations and the Weather Research and Forecasting (WRF) model. Compared to non-heat wave days, the ground observations show that daytime temperatures can be 3 °C higher during the heat wave. Despite the temperature spike, the UHI intensity is not amplified during the heat wave, maintaining its peak near 2.5 °C during both heat wave and non-heat wave periods. WRF simulation results also agree well with measurements and predict UHI peaks near 2.5 °C during both periods, showing no heat wave-UHI synergy. The spatially averaged UHI intensity also shows no such synergy. There is no significant change of wind speed, soil moisture availability or heat storage flux during the heat wave. Therefore, the lack of heat wave-UHI synergy in our study is consistent with current understanding of factors contributing to UHI. This study shows that the heat wave-UHI interaction in a tropical city can be different from that in cities in the temperate climate zone and more studies should be conducted in tropical cities, which are projected to suffer larger impacts of increasing heat stress.

Low wind speeds can inhibit passive ventilation and reduce thermal comfort in tropical cities. We use both experimental and numerical approaches to investigate pedestrian-level wind speed enhancement with void decks (empty spaces at the ground floors) in urban street canyons. Water channel experiments show that void decks allow wind to pass through them, thereby enhancing pedestrian-level wind speeds up to twofold. Computational fluid dynamics models validated with the experiments were used to conduct two parametric studies. The first study varied the void deck height and showed that the wind enhancement effects increase with taller void decks, as expected. The second study varied the canyon aspect ratio and showed that variations in the canyon aspect ratio had less influence on wind enhancement compared to the variations in the void deck height. This is because a void deck induces a secondary vortex, which counter-rotates with the vortex driven by the freestream. Increasing the canyon aspect ratio stretches the vortices vertically but does not alter the (normalized) flow fields. Our findings reveal the potential of void decks to channel wind into street canyons, including narrow (high height-to-width aspect ratio) canyons, which suffer from poor ventilation. The enhanced wind speeds are beneficial for pollutant dispersion and outdoor thermal comfort in tropical cities. Keywords: Void ground floor; Building porosity; Urban street canyon; Water channel experiment; CFD simulation

Cities suffer from low wind speeds due to the blockage effects of urban structures. Low wind speeds inhibit a city's ability to self-ventilate and reduce thermal comfort in tropical cities. Building porosity has been shown to be an effective architectural feature to channel winds into the pedestrian level. However, previous studies have focused on building porosity in single buildings or two-dimensional urban street canyons. We extended the investigations of building porosity to three-dimensional urban street canyons with computational fluid dynamics simulations. Validated numerical models were used to conduct parametric studies on void decks (empty spaces at the ground floors). By allowing winds to flow through them, void decks can enhance pedestrian-level wind speeds by more than twofold compared to the reference case without void decks. The wind enhancement effect is the strongest at the first canyon and decreases downstream. The effectiveness of a void deck is greatly improved by increasing the void deck height. The average wind speed in the first canyon increases from 13 percent to 59 percent (of the freestream wind speed) by increasing the void deck height from 2 m to 6 m. On the other hand, the building height and canyon (height-to-width) aspect ratio have a much smaller influence on void-deck effectiveness. Non-uniform building height in an array of buildings also has a minor influence on the effectiveness. Therefore, void decks are equally effective to enhance pedestrian-level wind speeds in urban areas with tall buildings and buildings with non-uniform height. Keywords: Void ground floor; Building porosity; CFD simulation; Wind field modification; Non-uniform building height

Reduced-scale experiments and full-scale field measurements show contradictory buoyancy effects from heated windward walls. Reduced-scale experiments exhibit significant thermal effects, but full-scale field measurements show a negligible thermal effect on the overall flow fields. This paper investigates this discrepancy by using at both scales Computational Fluid Dynamics simulations with Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES). Compared to experimental and field measurements, RANS models perform well at reduced scale but over-predict the thermal effects of heated windward walls at full scale. On the other hand, LES results agree well with measurements at both scales. Therefore, LES should be used for full-scale simulations of street canyon flows with heated windward walls. To date, there has been no explanation for the discrepancy of opposing thermal effects between reduced-scale experiments and full-scale field measurements although Richardson number similarity is satisfied. We provide an explanation by showing that in reduced-scale experiments with heated windward walls, the assumption of Reynolds number independence is invalid. In canyon flows with thermally induced buoyancy, unless the flow is proven independent of both Reynolds number and Grashof number, we should not generalize results from reduced-scale experiments to full-scale street canyons. Keywords: Urban street canyon; Large eddy simulation; Reynolds number independence; Dimensionless group similarity; Full-scale simulation; Buoyant flow

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017., Cataloged from PDF version of thesis., Includes bibliographical references (pages 77-79)., A densely built environment has low wind speed at the pedestrian level due to flow obstruction induced by buildings. Urban street canyons, the outdoor spaces formed between buildings, often have much lower wind speed than the atmospheric wind above the roof level. In tropical regions, wind plays an important role to improve outdoor thermal comfort of urban inhabitants by increasing the convective heat transfer from body surfaces. This thesis explores four types of passive architectural interventions to boost pedestrian-level wind speed in urban street canyons, namely void decks (open ground level), the wind catcher, the reversed wind catcher, and step-up/ step-down canyons. The proposed interventions were first studied experimentally in a recirculating water channel, where an atmospheric flow across an array of two-dimensional canyons was simulated with reduced-scale models of buildings. The velocity profiles in the third to sixth canyons were measured with Acoustic Doppler Velocimetry. Compared with the reference case, void decks enhance near-ground flows in all measured canyons by up to a factor of two, but the enhancement effect weakens in downstream canyons. The wind catcher enhances the flow in the target canyon by 2.5 times with no significant effect in other canyons. The reversed wind catcher and the step-up/ step-down canyons reduce flows in the downstream canyons. The experimental data was used to validate computational fluid dynamics (CFD) models. CFD simulation results agree well with the experimental results for all cases. The validated CFD models were then used to study the void decks and the wind catcher in three-dimensional canyons. Void decks double near-ground flows in all canyons. The wind catcher increases near-ground flow in the target canyon by only 50% due to leakage at the sides. An improved wind catcher with sidewalls (to prevent leakage) triples near-ground flow in the target canyon. These findings prove the potential of void decks and the w, by Lup Wai Chew., S.M.

Dense urban areas restrict air movement, causing airflow in urban street canyons to be much lower than the flow above buildings. Boosting near-ground wind speed can enhance thermal comfort in warm climates by increasing skin convective heat transfer. We explored the potential of a wind catcher to direct atmospheric wind into urban street canyons. We arranged scaled-down models of buildings with a wind catcher prototype in a water channel to simulate flow across two-dimensional urban street canyons. Velocity profiles were measured with Acoustic Doppler Velocimeters. Experiments showed that a wind catcher enhances pedestrian-level wind speed in the target canyon by 2.5 times. The flow enhancement is local to the target canyon with little effect in other canyons. With reversed flow direction, a “reversed wind catcher” has no effect in the target canyon but reduces the flow in the immediate downstream canyon. The reversed wind catcher exhibits a similar blockage effect of a tall building amid an array of lower buildings. Next, we validated Computational Fluid Dynamics (CFD) simulations of all cases with experiments and extended the study to reveal impacts on three-dimensional ensembles of buildings. A wind catcher with closed sidewalls enhances maximum pedestrian-level wind speed in three-dimensional canyons by four times. Our results encourage better designs of wind catchers to increase wind speed in targeted areas.

Most real life bubble dynamics applications involve multiple bubbles, for example, in cavitation erosion prevention, ultrasonic baths, underwater warfare, and medical applications involving microbubble contrast agents. Most scientific dealings with bubble-bubble interaction focus on two similarly sized bubbles. In this study, the interaction between two oscillating differently sized bubbles (generated in tap water) is studied using high speed photography. Four types of bubble behavior were observed, namely, jetting toward each other, jetting away from each other, bubble coalescence, and a behavior termed the "catapult" effect. In-phase bubbles jet toward each other, while out-of-phase bubbles jet away from each other. There exists a critical phase difference that separates the two regimes. The behavior of the bubbles is fully characterized by their dimensionless separation distance, their phase difference, and their size ratio. It is also found that for bubbles with large size difference, the smaller bubble behaves similarly to a single bubble oscillating near a free surface.