Online Measurement of Water Entry Cavity Using Outward Electrical Capacitance Tomography

Online measurement of water entry cavities is important for the stable manipulation of high-speed underwater vehicles. However, there are almost no feasible methods to track and measure the cavity online when the vehicles are moving. For this purpose, an online measurement method using outward electrical capacitance tomography (Outward-ECT) is proposed to obtain the circumferential cavity profile on the outer surface of an axisymmetric blunt body during water entry. An Outward-ECT physical model is developed to incorporate the effect of finite boundaries on the cavity measurement in laboratory tests. Based on this physical model, simulation is conducted and a cavitator equipped with Outward-ECT electrodes is designed and fabricated, enabling the reconstruction of the cavity profile during water entry. Experiments were carried out to verify the proposed method, where images taken by a high-speed camera are used as references. It is found that the cavity profiles reconstructed by the Outward-ECT are consistent with their references when the cavitation enters the water in different attitudes, where the relative measurement errors of the cavity diameter are less than 5.9%. Analyzing the boundary measurements by the Outward-ECT sensor shows that the global capacitance is related to the water entry stages and attitudes of the cavitator as well as the initial entry velocity. Overall, the success of this approach would provide a feasible method for the online measurement of the water entry cavity when high-speed underwater vehicles are moving, which is essential for their online control to maintain a stable cavity.