Time histories of sample tracers and stretch rates in a straight tube. (a) Deformation of tracers at 4 normalized times t/T = (0, 0.25, 0.60, 0.75) marked by (1, 2, 3, 4). (b) Time histories of orientation vector mi released at y/a = 0.48 (the middle of the red line at t/T = 0). Subscript i (1, 2, 3) correspond to (x, y, z) in (red, green, blue). The black line with circle is the enlarged view of the sT curve in (c). (c) Time histories of of the stretch rates of a massless particle released at y/a = 0.48. Line, sI (instantaneous stretch rate); line with circle, sT (time-averaged stretch rate).