Influencing Factors and Scaling Law of Bubble Detachment at a Submerged Orifice

Bubble formation and detachment are important in the drag reduction of underwater vehicle by using gas injection, which are involved in the integrated scaling law considering bubbles coalescence. The parameters of influencing factors are theoretically derived by dimensionless analysis on this issue. Then the effects are shown by analyzing results with various parameters. Results indicate that, the viscous force can be neglected near the default condition, and the bubble is difficult to detach if the viscosity is very large. Surface tension is the major constraint force in the default condition, which is also sensitive. The bubble easily crushes if the surface tension is small, while the neck of bubble can hardly break up if the surface tension is large. For the contact angle, the bubble profile significantly exceeds the orifice boundary if the wall surface is hydrophobic, which makes detachment period becomes larger. INTRODUCTION Bubble formation and detachment are involved in many engineering applications. Owning to its practical importance, the phenomenon of bubble formation and pitch-off has been widely studied theoretically and experimentally for decades [113]. As new application directions, bubble drag reduction (BDR) and air-layer drag reduction (ALDR) are important methods to reduce the Skin-friction drag of high speed underwater vehicles [14-17]. For the external multiphase flow field around underwater vehicles, the major similarity parameters include Froude number, cavitation number and Euler number. However, on the issue of bubble and air layer formation, firstly the gas ejects from the orifice as the form of discrete bubbles, and then the bubbles coalesce into continuous air layer. Coalescence is the key process of flow pattern transition of drag reduction, because the effect of ALDR is more significant than BDR [14]. However besides the influencing factors of the external flow, the flow pattern may be closely related to the flow field and bubble state near the orifice. Therefore this requires indepth study on the microscopic flow filed of bubble formation and detachment, especially the dynamic characteristics. We need to obtain the influencing factors and the scaling laws, which can be combined with the factors of the macroscopic external flow to derive the integrated scaling law of bubbles coalescence and ALDR. In the present paper, numerical and experimental methods on bubble formation and detachment are established. The parameters of influencing factors are theoretically derived by dimensionless analysis. Then the effects are shown by analyzing results with various parameters 1. EXPERIMENTAL AND NUMERICAL METHODS The experiment of bubble formation on submerged orifice was conducted at atmospheric pressure and temperature. Experiment setup is sketched in Fig.1. A glass tank with stainless steel plate at the bottom was used. The cross section of the tank was 500mm×500mm with an orifice in the middle of the plate. The diameter of the orifice was 2mm. Compressed air was forced to the orifice through a long plastic tube and retaining valve. A flow rate controller was used to control the gas flow rate. In experiment of present work, the depth of water in the glass tank was 100mm and the gas flow rate was 100ml/min. The high-speed camera is placed on the side of the tank, above and parallel to the bottom plate. Proceedings of the ASME 2016 International Mechanical Engineering Congress and Exposition IMECE2016 November 11-17, 2016, Phoenix, Arizona, USA