Roberto Putzu
Professor
Fluid mechanics laboratory CMEFE, Hepia University of Applied Sciences Western Switzerland
Switzerland

Abstract


Air quality is a growing concern for the health of the global population living in large urban centers. Gas dispersion predictions in complex built environments can be relevant for those public and private authorities involved in urban development projects. Several applied science universities of Western Swiss (HES-SO) initiated a long-term research project called Clean City in order to develop tools and methods to assess the impact of architectural and transportation solutions from the pollutant dispersion standpoint. The work here presented focuses on the work carried out by the collaboration of hepia, he-VS and eia-fr in the framework of the Clean City project.

The Clean City project, aims at identifying and qualifying a methodology to perform aerodynamic numerical investigations on complex urban areas. Aiming at this final scope, several intermediate steps were undertaken:

1) A series of numerical investigations were first conducted on a simple cube shaped building emitting a pollutant from its top roof, the used codes were both commercial and opens ource codes. The results were compared with experimental information available in litterature.
2) The 3D cadastral surface of the "p?quis" urban area was used to print a city mock-up to be used in the wind tunnel investigations. In parallel, this 3D model was adapted for the numerical aerodynamic investigation.
3)Test campaigns were conducted in the hepia Geneva wind tunnel to provide experimental results to be compared with the simulations.

The Wind tunnel investigations were carried out on a 1:500 scale model of the P?quis neighbourhood manufactured at hepia from the 3D cadastre data provided by SITG. The hepia wind tunnel is a large circulating facility with a 2 m large and 1.5 m high closed test-section. The model was oriented in the wind tunnel in order to replicate a typical intense and gusty wind condition of Geneva called ?bise?. Pollutant dispersion phenomena were reproduced by the emission of an air-SF6 mixture at constant volume ratio of the species from 150 continuous point sources on the model building rooftops. The mixture was supplied at three different flow rates which reproduce three different fuel consumption levels of the boilers installed in each apartment of the neighbourhood. Measurements of the 3D aerodynamic field and pollutant concentrations of the neighborhood were performed in order to validate the CFD code. The implementation of an hybrid experimental/numerical approach together with drone field measurements represent an innovative tool to move ahead in the comprehension of the dispertion phenomena in a complex urban area.