In this paper separated and reattaching laminar flows produced by a sudden inward expansion within confined annular geometries are experimentally studied. Using Particle Image Velocimetry (PIV), combined with refractive index matching, the structure of fluid flow at the expansion region was investigated. Detailed measurement of the velocities, the reattachment length (Lr) and the relative eddy intensity (ξ) are obtained for two different expansion ratios of ε = 1.4 and 1.6. The measured variation of the reattachment length as a function of Reynolds number (50 < Re < 600) was found to be non-linear for these expansion ratios. Eddy intensity was found to depend strongly on both Reynolds number and expansion ratio, with the relationship between ξ and Re being non-linear. Overall corresponding numerical predictions obtained by Nag & Datta [1] are in good agreement with the measurement results presented.

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