Flow through a sudden contraction occurs in a variety of engineering applications including injection moulding, extrusion, thermoforming and many polymer melt processing applications. Instabilities observed during polymer melts processing can have an important limiting factor affecting production rates in many industrial area. Analysis of contraction flow is a useful process to identify these defects. In this study, the flow of a Newtonian fluid in an axisymmetrical 4:1 sudden contraction geometry is studied using quantitative flow visualization and Particle Image Velocimetry (PIV) technique. The flow is driven by the effect of gravity under atmospheric conditions in room temperature. Study of the Newtonian entrance problem is useful precursor to the study of the entrance behaviour of viscoelastic fluids. Time averaged and instantaneous velocity vector maps of the flow field, time-averaged velocity and vorticity contours as well as axial and radial velocity profiles right above the contraction for different Reynolds numbers are presented. (c) 2007 Elsevier Ltd. All rights reserved.