**Particles and whirlpools**

With their expertise in fluid mechanics in general and in the study of turbulence in particular, the researchers studied the effects of turbulence on airborne particles. Turbulence is naturally present in complex fluid flows, for example in an airflow when it meets the blades of a wind turbine, or in a water flow as it passes through a hydro turbine, in the atmosphere. And therefore, in the air we breathe!

Particularly interesting in these times of health crisis, their work, carried out in collaboration with their colleagues from CORIA, IMAG, the University of Vermont and the SAFRAN Group, has benefited from 10 million hours of computing time on the Joliot-Curie supercomputer, located on the CEA site in Paris-Saclay. Indeed, the behaviour of fluids are governed by the Navier-Stokes equation, in which turbulence is intrinsically contained. Turbulence is a phenomenon that extends over a wide range of spatial scales.

To reduce (astronomical) computational times, turbulence at the smallest spatial scales can be universally described by a simple model: vortices or turbulences are always simulated over the larger scales accessible by computation, and the smaller ones are modelled. This approach known as Large Scale Simulation (LSS) greatly reduces the computational time while maintaining a very satisfactory accuracy.

Here, the researchers employed this method, integrated into the YALES 2 calculation code, to study the trajectory of droplets as a function of their size, external flow (ventilation), etc. The method is based on the YALES 2 calculation code. "The results, which are still being acquired and interpreted, could be used to assess the value of protective equipment such as visors. "But before that, the results will have to be interpreted by professionals in biology and virology to draw practical conclusions.