Reducing Hull Drag in Conventional Watercraft Through an Induced Cavitation Field: A Numerical/Predictive Approach

Despite the relatively low viscosity of water, viscous friction drag on a conventional watercraft’s hull produces a significant parasitic power load. At high speeds (speed to length ratio of 0.4 with speed expressed in knots), viscous drag, which is directly proportional to the craft’s submerged area, can account for up to 50% of total hull resistance [1]. The overall goal of this project is to create a system that induces a cavitation field to reduce the parasitic power load due to drag on watercraft with  partially submerged hulls . A prominent example of cavitation-assisted drag reduction is the fully submerged rocket-powered cavitating torpedo [2].  A nozzle at the front of the torpedo, coupled to a gas ejection system, induces a cavitation field (air bubble capsule) around the torpedo, thereby reducing drag.  However, challenges arise when using induced cavitation drag reduction on fully submerged crafts. Only a few small regions of the craft’s surface are in contact with the surrounding water. Control options are limited to those surfaces, making thrust and direction control challenging [3]. Furthermore, if any portion of the watercraft were to pierce the cavitation field, an imbalance in forces is generated, causing both violent oscillatory motion and instability [4]. Tiny disturbances in the cavitation field may also trigger these violent reactions [5]. These challenges provided motivation to develop a different method of applying cavitation-assisted drag reduction to watercraft. This project was a numerical/predictive study of utilizing cavitation fields induced by actively controlled mechanical protrusions, called fins, to reduce viscous hull drag on conventional watercraft with partially submerged hulls (Eg: naval vessels, recreational vessels, cargo vessels). The phrase ‘cavitation field’, as used in this project, refers to the air-ventilated cavity (air bubble) induced by the actively controlled fins. A “system engineering” approach was used to study three prominent, interacting subsystems, viz., a cavitation field subsystem, a fluid power subsystem, and a signal processing/machine learning subsystem. The study yielded two tools that can be utilized in further numerical and experimental study of cavitation-assisted viscous hull drag reduction to make maritime transport more efficient. The first tool created was a one-dimensional system-level simulation of the proposed drag reduction system and its impact on the required propulsion power of a watercraft with a partially submerged  hull . The numerical results of the simulation indicated that the drag reduction system significantly reduced the required propulsion power of the modeled watercraft. The second tool developed in this study is a multilayer, feedforward neural network that successfully emulated a PID controller’s operation to actuate the drag reduction appendage used to induce cavitation fields. The successful emulation of this PID controller indicates that the neural network can replace the PID controller as the control scheme in the system-level simulation, thus providing the system with a predictive, rather than reactive, control scheme.