An experimental study was carried out on basic phenomena

pinch-off characteristics and penetration depth of underwater gas jet ejecting out of a vertical annular nozzle. The process of gas jet development was recorded by a high-speed camera

and jet interfaces were tracked by processing pictures with an edge detection algorithm. The pinch-off spatial distribution was determined by summation of downstream position across all recorded times. The gas jet penetration distance was calculated by counting transient pixel values divided by measured time duration of gas observed for all pixel locations. The color contour was used to indicate time percentage for gas to occupy a certain location in the field of view. The results show that underwater annular-nozzled gas jets exhibit two stages of continuous jet and centralized pinch-off. More pinch-off occurs near nozzle and less pinch-off occurs further away from nozzle in the study conditions. Pinch-off frequency at momentum jet section for all nozzles is reduced with the increase of gas flow rate. Back-attack phenomenon upon pinch-off is the result of axial obstruction and transverse expansion of subsequent gas instead of gas flowing backwards and impinging on nozzle end. The jet penetration distance is proportional to gas flow rate for ring seams of same sizes.