Wake influence on flow in a circular-to-rectangular transition duct. A water-tunnel study using visualization techniques

A water tunnel facility was employed to study the flow in a circular-to-rectangular transition duct with a ring-type bluff plate placed upstream. Experiments were performed by using visualization techniques with colored dye, fluorescent dye and particle mixture, at Reynolds numbers of Re_D = 7.5 × 10³ to 6.7 × 10⁴. Four bluff-plate models were selected for experiments allowing a comparison of the results obtained. Experimental observations, in general, indicate that the unsteady effects of the wake behind the bluff plate influence the characteristics of flow near the wall of the transition duct significantly. For the smallest bluff plate studied, even though the wake eddies do not contact the boundary-layer flow throughout the transition duct, the unsteady effect induced by the wake is reflected from the behaviors of the boundary layer.