Wind tunnel measurements of scale effects on 3D turbulent flow in an urban street canyon configuration

This study examines scale effects in a wind tunnel study of an urban street canyon. A simplified modelling configuration was employed, comprising five consecutive canyons of height-to-width and length-to-width aspect ratios equal to 1 and 8, respectively, for a free-stream turbulence intensity of ~5%. Both 3D-3C Robotic Particle Tracking velocimetry and Constant Temperature Anemometry measurements were performed for Reynolds numbers exceeding the commonly accepted critical value for mean flow (~10000–20000) by a factor of ~4, i.e. ranging from 23000 to 57000. Focusing on time-averaged velocity components and turbulence kinetic energy, a novel framework is introduced to facilitate the local separation of actual Reynolds number dependence from measurement uncertainty while global indices, applied to the canyon-scale measurement volume, are implemented. Results indicate that the critical values of the Reynolds number (e.g. in the range 10000–20000) that are widely adopted in the wind tunnel testing literature may be insufficient to ensure Reynolds number independence for all examined variables and for the whole volume of urban street canyon configurations, especially near wall boundaries, as Reynolds number values up to 47000 may be necessary.