## 1 Introduction  
Cavitation is an omnipresent phenomenon observed during flows in valves, pipes, pressure vessels, and so on. Its occurrence favors considerable material losses in complex situations, which requires costly replacements to be made thereby causing significant performance drawbacks to industries. Studies on fluid dynamics about solving issues related to cavitation due to bubbles collapse are being done by researchers from both universities and companies. To be more precise, understanding bubble dynamics in valves requires in-depth investigations as the presence of void fractions in optically dense multiphase flows have been hindering the observation of bubbles [1]. To the best of our knowledge, fewer studies about bubble morphology in throttle valves have been done, albeit with both the invasive techniques (impedance probe and optical fiber probe) and the non-invasive techniques (PIV, PTV, PT) being prevalent. The non-invasive approach has widely been promoted through high speed photography in bubble measurement studies, inclusive of the 2D bubble columns [2; 3; 4; 5], the channels [6; 7; 8; 9], the flat plates [10], the hydrofoils [11; 12], the mixing tanks [13], the liquid-solid interface [14], the dynamically loaded journal bearings [15], the ultrasonic devices [16; 17; 14; 18], the axisymmetric geometry [10], the throttle orifice, and so on. To start with, the first ever experiment on capturing the motion of Helium-filled bubbles in an engine using a single camera and multiple mirrors, was done by [PERSON] and [PERSON] [19] in 1982. Next, [PERSON] and [PERSON] [20] implemented a measuring method by using a single high speed cinc camera through a split field mirror to track small resin beads (tracers) and [PERSON] et al. proposed a \"3 Dsynthetic aperture imaging (SA imaging)\" by using nine (9) high speed photon cameras to capture the bubbly flow induced by a turbulent circular plunging jet. Similarly, to study the fluid flow, virtual stereo vision was implemented in this research to observe and reconstruct the 3D bubbles formed in the area between the valve seat, the valve core and the outlet port. [PERSON] et al. [21] applied virtual binocular stereo vision in a glass-made water tank to match and reconstruct the bubble trajectory motion through a \"3D polar coordinate homonymy correlation algorithm\", thereby determining the analogous relation of alike bubbles from two-half images. Additional studies by [PERSON] et al. [22; 23; 24] dealt with the bubble behaviour characteristics in the gas-liquid two-phase flow, modality factors of bubbles, intrinsic and extrinsic parameters of the virtual cameras, and the segmentation of multi-bubbles. Likewise, by implementing robust algorithms to estimate the velocities and reconstruct the trajectories of bubbles, broad investigations were made by [PERSON] et al. [25], [PERSON] et al. [26], [PERSON] et al. [20], [PERSON] et al. [16], [PERSON] et al. [27], [PERSON] [28], and [PERSON] et al. [29], respectively. For a clearer approach of the stereo vision concept, significant contributions were brought by the following researchers: [PERSON] et al. [30] established a 3D mathematical model to measure a 3D point through a combination of the single camera stereo vision sensor with planar mirror imaging. Upon comparison with the binocular stereo vision, the output in terms of calibration, measurement speed and errors resulted in being more accurate. [PERSON] et al. [31] studied a nearly non-dispersed 2D bubbly flow in a thin channel by means of a high speed camera- image processing routine to validate bubble clusters trajectory and sealing arguments to estimate their lifespan. Moreover, bubbly flows were continuously studied by [PERSON] et al. [8; 32], [PERSON] [33], [PERSON] et al. [3], and [PERSON] et al. [5] from the last decade. Prolonged observations in dense bubbly flows containing overlapped ellipse-like bubbles through image analysis using algorithms were achieved by [PERSON] et al. [34], [PERSON] et al. [35; 36], and [PERSON] et al. [37], respectively.