A Study on the Hydrodynamic Characteristics of a Synthetic Jet Propelled Underwater Vehicle

Abstract

In this article, the drag characteristics of an underwater vehicle propelled by a synthetic jet (SJ) is numerically studied. The underwater vehicle studied is REMUS Autonomous underwater vehicle (AUV). The cruising speed studied is up to 10 knots. The actuation parameters of the SJ actuator are the same under different cruising speeds. When the SJ actuator works, the drag of the vehicle changes periodically. The SJ-induced additional pressure drag increases with increasing cruising speed. The additional friction force is negligible. The thrust of the actuator decreases with increasing cruising speed. The thrust outputted by the SJ actuator when used for underwater vehicle propulsion is smaller than the thrust of an isolated actuator. The mathematical model of the additional drag, the model of the thrust, and the velocity distribution is given to assist the analysis in this article. 1. Introduction The advance in propulsion technique is the key to the evolution of underwater robots. The most common method for underwater robot propulsion is using screw propellers. Thrust is generated through the high speed rotation of the blades. The fluid is accelerated not only in the axial direction but also in the radial direction; however, only fluid acceleration in the axial direction can generate thrust. A significant part of the energy is wasted in the radial direction without thrust generation. As a result, the efficiency of screw propellers is low. Furthermore, the protruded screw propellers induce significant added resistance. This further reduces the total hydrodynamic efficiency of the vehicle. Nature always inspires the technical revolution in engineering. In nature there exist many marine creatures such as squids adopting jet propulsion. The jet created by squids is called pulsed jet. On the other hand, the jet of screw propellers is called steady jet. Studies about pulsed jet showed that compared with the steady jet, pulsed jet possess higher propulsion efficiency (Krueger 2001; Krueger & Gharib 2003, 2005; Moslemi & Krueger 2010; Lydia et al. 2011; Robert & John 2013). Seikman (1963), Weihs (1977), and recently Krueger and Gharib (2003) have shown that pulsed jets can give rise to a greater average thrust than steady jets of an equivalent mass flow rate. Lydia A. Ruiz, Robert W. Whittlesey, and John O. Dabiri found through experiment that pulsed jet has a propulsion efficiency that is 40% higher than that of steady jet, and a drag-based hydrodynamic efficiency increase more than 70% (Lydia et al. 2011). The mechanism behind this high efficiency is the formation of the vortex. The vortex will induce a radial velocity gradient at the nozzle exit. The velocity distribution becomes more ununiform compared with steady jet. As a result, the pressure at the nozzle exit will increase. This is observed by Krueger through experiment.