Air flow through net covered arched structures

Permeable coverings facilitate indoor ventilation due to their porosity, the characteristics of the air flow passing through a net depends on the Reynolds number of the system. So far, only the permeability of nets used in application involving low Reynolds numbers have been studied, while the behaviour of nets at high Reynolds numbers, has been poorly studied. The ventilation of a net covered tunnel was numerically and experimentally studied analysing the airflow around and through the structure. The airflow through a two-dimensional tunnel structure (8.0 m wide and 2.5 m high) covered with various nets was numerically calculated by Computational Fluid Dynamics (CFD) simulations. A full scale tunnel structure, of the same geometry, covered with four different nets, characterised by different porosity, was built in order to validate the numerical CFD results. Wind velocity was measured outside and inside the structure by means of two handheld rotary cup anemometers. Data were gathered in the middle of each sector in order to avoid edge effects. The estimation of the indoor/outdoor air velocity reduction ratio shows a good agreement between measured and calculated values, especially for nets with a medium-low solidity ratio. For low solidity ratios the numerical results overestimate the indoor air velocity. Moreover the full scale measurements confirm the dependence of the “a” factor in the Forchheimer equation with the characteristics of the net.