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Scientific Method / Science & Exploration

Urban wind flows deposit pollutants in repetitive patterns

A formula modeling grimy gusts will be valuable for city-planners.


Mathematicians have published a paper suggesting that pollutant-carrying urban flows—winds that navigate city obstacles—deposit particles in repetitive patterns leading to build-ups in specific city locations.

As wind weaves through the everyday fumes generated by a city, it gathers particles from car exhausts, dust and other minute pollutants. Using a mathematical dynamical systems formula dictated by Lagrangian mechanics, the Arizona State University and University of Notre Dame team plotted the random motion of these particles for the first time. The model, built to simulate the wind motion over long periods of time, found that the process was not random at all—a coherent pattern rapidly emerged, demonstrating that the particles were repeatedly deposited in the same area.

"In previous studies, the existence of these patterns in fluid flows was only verified with idealised 'theoretical' flows," said lead author Wenbo Tang. "It was not known if such structures were robust enough to manifest in the environment."

The model used realistic urban geometry and tested the theory at different times of the day to reach its conclusion that "major coherent structures survive to organise local flow patterns, and weaker structures are smoothed out due to random motion."

The system has plenty of value for city-planners, who could use it to model a roadmap of pollutant free city-centres. The data would reveal areas where the particles would be most concentrated, highlighting city quarters that need extra attention to avoid a substantial build-up.

The study was published in Volume 24 of the journal Physics of Fluids.

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