Flow distortion in a circular-to-rectangular transition duct

Experiments were made for three circular-to-rectangular transition ducts with different transition lengths at Reynolds numbers which ranged from 4 × 10³ to 2 × 10⁴. The Reynolds number is based on the inlet boundarylayer thickness and a reference freestream velocity measured upstream of the transition duct. The results indicate that the major driving force for flow in the duct to behave three dimensionally is the cross-streamwise pressure gradient, which results from the contoured wall. A three-dimensional separation bubble was found to occur on the diverging wall at Re = 4 × 10³, but disappeared at higher Reynolds numbers when the boundary layer upstream of the geometrical deformation section was turbulent. The secondary flow pattern developed at the exit cross-sectional plane was mapped out in detail by a three-dimensional velocity measurement technique. The results further indicated that the development of the secondary flow in the corner region was mainly caused by the mean-flow distortion effect.