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Propelling Underwater Vehicles Using Vortex Ring Generation
Ann Marie Polsenberg, Joel Burdick

Abstract. As robots designed to operate underwater become more common, it is useful to look at ways to make them more efficient. Autonomous Underwater Vehicles (AUVs) carry their power source with them, so improving the efficiency of the vehicle will also increase the maximum duration time for missions that the vehicle can perform. One area in which efficiency is very important is the propulsion system. We propose that vortex ring generators may be a viable way to propel these vehicles. This idea stems from looking at aquatic animals, such as squid, which use this mechanism. Our work involves the modeling, design, construction and analysis of synthetic jets. The next step will be to design a small vehicle that uses these thrusters and to begin an investigation into the control of such a vehicle.

Summary. Autonomous Underwater Vehicles are routinely used for oceanographic, industrial and explorative purposes. As their uses increase, so does the desire to expand their capabilities. We are currently looking at methods which would allow efficient slow propulsion of these vehicles. Currently, most AUVs are propelled by a single stern propeller. This configuration, while excellent for cruising situations, does not allow stationkeeping (hovering in one place). The single aft propeller is optimized for cruising, not for stability at slow speeds. Thus, we would like to design a propulsion system specifically for these scenarios. It is our belief that at low Reynolds numbers, we may be able to show pulsed jet propulsion to be desirable over propellers.

Our inspiration is the pulsed jet propulsion of cephalopods and salps. (Weihs, 1977) These creatures propel themselves by pulsing water out of a cavity in their body. The fluid rolls up into vortex rings as it exits the body. To create our own vortex ring generator, we are using a synthetic jet. Synthetic Jets have been used in the aerospace industry for flow control. (Glezer 2002, Rathnasingham 1997) These jets consist of a chamber with a vibrating membrane at one end, and a lid with an orifice in it at the other. On the down-stroke, the vibrating membrane sucks water in from around the orifice. (Fig 1a) On the up-stroke, water is ejected through the orifice, creating rings. (Fig 1b) As these rotating rings pull water through them a jet is created. Benefits of these jets include the small volume of space that they take up, as well as their having zero net mass flux. As the apparatus is sucking water in and pushing it out through the same orifice, there is no need to have separate intake and discharge equipment.

Figure 1 (a and b)

Our initial synthetic jet design consists of a mechanically actuated vibrating rubber diaphragm at the bottom of an acrylic cylinder. As it is a modular design, we can easily switch orifice plates. This allows us to test various plate thicknesses, as well as orifice diameters. (Figure 3) Preliminary testing shows that our system does create a jet.
(Figure 4) We are currently in the process of quantifying our results.

Figure 3

Figure 4

Future work will include developing a fluid model for the system, and using this model to create a method for optimizing the thrust produced from the jet. Once we have achieved this, it is our intention to begin designing small vehicles which will use these jets for propulsion and maneuvering.

References
A. Glezer, and M Amitay. Synthetic Jets. Annual Review of Fluid Mechanics, 34:503-29, pages 503-529, 2002.

R. Rathnasingham, and K. Breuer. Couples Fluid-Structural Characteristics of Actuators for Flow Control. AIAA Journal, 35:5, pages 832-837, 1997.

D. Weihs. Periodic jet propulsion of aquatic creatures. In W. Nachtigal, editor, Bewegungsphysiologie-Biomechanik, pages 171-175, 1977.