Bubble formation frequency is measured by averaging the time intervals between two consecutive pulses in each record. A typical result is shown here. Images at the left hand side are experimental pictures taken by CCD camera. And images at the right hand side are the numerical simulations of the experiments. A hypodermic needle is inserted on the side wall of an acrylic tube and its orifice is exposed into a liquid flow inside the tube.
Water flows from left to right.
Time unit is millisecond. One may see that it is a very fast process, and the comparison between the experimental data and the numerical simulations are favorable overall.