In urban areas, successful technological advancements such as taller buildings, energy-efficient ventilation systems, drones and potentially air taxis, remain at the mercy of wind, a significant natural physical phenomenon.

Researchers at the University of Surrey studied building wakes, the disturbed airflow around buildings, in an effort to improve the air quality in urban environments. They examined the differences between a single tall building and a cluster of tall buildings.

"Understanding how to model the wake of tall buildings is the first step to enable city planners to reduce the heat-island effect as well as improve urban air quality," said Joshua Anthony Minien, a researcher in mechanical engineering"

A team from Oklahoma State University attached sensors to robotic aircraft to take more cohesive measurements of building wakes, or the disturbed airflow around buildings. Source: Jamey JacobA team from Oklahoma State University attached sensors to robotic aircraft to take more cohesive measurements of building wakes, or the disturbed airflow around buildings. Source: Jamey Jacob

By conducting experiments in a wind tunnel, varying the grouping, aspect ratio and spacing of tall buildings, the team discovered that a cluster of buildings and a single, isolated building have similar wake characteristics when measured far enough downstream. The researchers also found that the wakes of building clusters are significantly affected by changes in wind direction.

Airflow also affects building ventilation. Researchers at the University of Cambridge developed a model that will help designers of natural ventilation systems.

"The ability to predict ventilation flow rates, purging times and flow patterns is important for human comfort and health, as highlighted by the need to prevent the airborne spread of coronavirus," said researcher Nicholas Wise. He, along with engineering professor Gary Hunt, found a flaw in current models of passive natural ventilation systems that use displacement flow, where cooler night air enters the building through one opening and warmer air that accumulates during the day exits through a different opening.

The researchers discovered, through mathematical modeling, that the purge of warm air slows. "Every displacement flow transitions to unbalanced exchange flow," said Wise. They found that rooms with an additional small low-level opening significantly speeds up room cooling compared to rooms with only a high-level opening.

Looking ahead to a future in which a variety of autonomous aircraft could fill urban skies, researchers from Oklahoma State University have developed a way to model the environmental hazards such vehicles might encounter, with the goal of providing a way for them to navigate cities more safely. They presented their findings on modeling and predicting urban airflow at the 73rd Annual Meeting of the American Physical Society’s Division of Fluid Dynamics.

“Urban environments present enormous challenges for drone and urban air mobility platforms," said researcher Jamey Jacob, who led the team. "In addition to the challenges of traffic congestion and obstacles, critical technology gaps exist in modeling, detecting, and accommodating the dynamic urban local wind field as well as in precision navigation through uncertain weather conditions."

Using sensors attached to robotic aircraft, the researchers were able to take better measurements of building wakes and combine the data with numerical predictions for a clearer picture of these complex urban wind patterns.

Equipping all drones and other unmanned aircraft with sensors would be a game-changing opportunity for monitoring, predicting and reporting hazardous weather events, said Jacob. This could benefit conventional aircraft as well.

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