Numerical investigation of the propulsion efficiency of real squids using computational fluid dynamics

Abstract

The use of ships and vessels could be traced back to the old Egyptians (B.C. 3000) while first submarine has been built four centuries ago by England. Although several centuries past already, there has not been any significant improvements developed for the motion mechanisms of submarine vehicles. The only change in the submarine vehicles could be seen in the engines which provide power to the propellers. These type of changes while making underwater travel better, the efficiency of the motion mechanism stayed limited with the technology of the propeller. Nowadays, in addition to land and air travels sea travel has been becoming very important. Especially, country like Turkey surrounded by sea in all three directions, it would be very important to have high maneuver ships and underwater vehicles having state of the art technology. Based on this goal nowadays Turkey has started to design and manufacture her own ships. In addition to having ships over the sea, development of modern submarine vehicles would enhance the Turkey’s defense power as well as make Turkey leader in the surrounding sea. Today, the performance (i.e., high maneuver and acceleration skills) of the vehicles which provide under water travel has been limited by their size and engines which power the propeller they have. When looked at the under the water, sea creature like squid can be observed as an creature can speed up to 40 – 50 km/h in a short duration of time. Although length of a squid can vary from 1 cm to 10 m and weight can change from 1 kg to 500 kg, regardless of the size and weight a squid can exhibit incredible swimming speed. This high speed gives squid a priceless opportunity to escape from her enemies and caught her food. In this project getting motivated by the perfect swimmers of the underwater namely a squid, it is aimed to develop an underwater vehicle which propels itself using an unsteady jet. In the example of swimming technique of a squid, certain amount of water will be taken to the inside the robot-squid. Then this water will be ejected with high acceleration that will in turn provide propulsion. Based on the Newton’s third law (action-reaction) robot-squid will gain propulsion on the opposite side that water has been pushed. Squid uses her nozzle to eject pressurized water in her mantle cavity. It was noticed that the squid's nozzle is made from a soft tissue and the nozzle diameter can be adjusted by the squid during water ejection. Having a different nozzle diameter could directly affect the squid's swimming speed as well as magnitude of the thrust she would gain. The magnitude of the propulsion will directly be related to the both amount and velocity of the pushed fluid (change of momentum). While traveling in a fluid, vehicles must produce certain amount of thrust to be able to move. Especially, underwater vehicles need to produce a thrust that is equal to the drag force acting on them so that their traveling fluid becomes possible. This thrust could be calculated from momentum equation. In this project the solid model of the robot-squid will be obtained from a squid’s CT-scan. Therefore, robotsquid will have a solid model that will resemble very much to the real squid. During the ejection of the fluid from squid there will be interaction between fluid and the solid part of the squid. To provide information transfer between solid and fluid, fluid structure interaction has been defined on the computational fluid Dynamics (CFD) model. When CFD model is run, velocity vector and pressure values are obtained. These values are used to compare the efficiencies of unsteady jet and steady jet, to calculate friction and pressure drag, to obtain the relation between resistance coefficient of robot-squid and squid’s Reynolds number. Since this work uses real squid geometry for calculating drag force, lift force, request jet velocity at different jet nozzle diameters, thrust force and jet efficiency when squid moves at various velocities underwater, it will be first in the literature.