Velocity field along vertical production tubing. Model height is fixed at 3.2 m but inner diameters D are: (a) 0.04 m, (b) 0.06 m, (c) 0.075 m and (d) 0.09 m respectively. In each of (a–d) the referenced positions along the pipe correspond to the location of the actual pressure sensors (S1, S2, S3, S4) placed on the physical experimental rig. The length-to-diameter L/D ratios along the pipes are as indicated on the geometric models.