Abstract
We have already developed a Single-Tip Optical fiber Probe (S-TOP): an efficient measurement technique for a velocity and diameter of a bubble or droplet. The objective of the present study is to improve the measurement accuracy of the S-TOP. We use a pre-signal to rationally correct the S-TOP measurement error owing to a random position penetrated by the S-TOP on the bubble surface. Hidden potentials of the pre-signal were revealed through a 3D numerical simulation based on a ray tracing method. The simulator can evaluate the complicated optical signals of the S-TOP by computing the polarization and energy of every incident ray. Based on the numerically simulated result corresponding to the single-bubble measurement, we clarified that the pre-signal was caused by light reflection at the bubble frontal surface. Furthermore, the characteristics of the pre-signal were quantified; the pre-signal intensively appears just before the S-TOP touches the bubble’s central region. Using these characteristic phenomena, we can easily detect the touched position (pre-signal threshold method). This method enables us to achieve an accurate measurement in a bubbly flow. The measured chord lengths via the S-TOP were compared with the lengths of the bubble minor axis obtained from visualization. In consequence, a difference of the results between the pierced chord length and the bubble minor axis was considerably decreased.
