Turbulence measurements in a 90-deg square duct using laser-Doppler velocimetry

Measurements of mean velocity and turbulence intensity in various cross-stream planes in a 90-deg square duct of 1.5 radius ratio for the Reynolds number of 47,000 are presented. The bend was preceded by a short upstream tangent with air as flowing fluid. The technique used was laser-Doppler velocimetry. The boundary layer at the bend inlet was found to be less than 0.1 of the hydraulic diameter, and resulted in a secondary velocity maximum of 0.35 of the bulk flow velocity. The secondary motion was characterized by four-vortex and six-vortex structures near the bend exit. The results also showed that the turbulence intensity was anisotropic in the bend and in the short downstream distance from the bend exit. High levels of turbulence intensity were found near the duct walls and in the regions were Dean vortices appear.