XU CHANG

Ph.D. student

National University of Singapore

Xu Chang is presently a PhD student in the department of Mechanical Engineering, National University of Singapore. She received her B.Eng. at the University of Strathclyde and her M.Sc. at the University of Southampton. She has also worked as a research intern at the Institute of Mechanics, Chinese Academy of Science. Her research interests include Computational Fluid Dynamics (CFD), multiphase flow, cloud cavitating and supercavitating flow.

Natural And Ventilated Cavitating Flow Over High-Speed Underwater Vehicle

Cavitation occurs when a vehicle runs fast underwater. Liquid water will phase change into vapour in low pressure region. The unsteady phenomenon of cavitating flow can cause erosion, noise, and vibration in underwater vehicles, which could be one of the most important speed barriers for high-speed vehicles such as torpedo, hydrofoil craft, and wing-in-ground craft. 
The mechanism involved in the cavitating flow needs to be studied to determine how the instability can be controlled. We presented the experimental and numerical investigations on the unsteady cloud cavitating flow generated by neighbouring boundaries. The unsteady phenomenon, such as the asymmetry cavity, cavity shedding and collapsing, could induce large lateral force and cloud be harmful to the vehicle structure. 
In order to avoid the evolution and collapse of cavitation bubbles, flow control methods are of great significance in theory and engineering applications. Supercavitators can help produce continuous and stable supercavitating flow over the underwater vehicle to maintain stable cruising and/ or reduce viscous drag. We presented a new passive flow control method using small protrusions as cavitators. The cavitators can be placed along the axis of the underwater vehicle to produce supercavitating flow around the vehicle. Compared with the case without the cavitator, the supercavitating hydrofoil controlled by the cavitator shows a significant difference in the lift, while the drag changes little. The change of lift is closely related to the position of the cavitator. Such a passive flow control method can only be applied when the cavitation number is low. 
At moderate cavitation number, air ventilation is needed. Concerning the active flow control by ventilating non-condensable air into the cavity, it is important to understand the flow dynamics of the mixture of air and vapour inside the cavity. The ventilated cavitating flow over an axis-symmetric projectile is investigated at a moderate cavitation number, which is much more stable than the natural cloud cavitating flow. The findings can provide good guidance on improving the hydrodynamic performance and stability condition of high-speed underwater vehicles.