PROPULSION is the mechanism to move a ship or boat by means of thrustproduced by the propeller.Non Propulsion is the mechanism used to move a ship or boat by means ofthrust produced without propeller.What is Biomimetic Propulsion? Mechanical Propulsion derived in some way from biology are said to be “biomimetic” or “bioinspired” depending on the fidelity to the original biological system.
Oscillating Fins The use of oscillating fins by aquatic animals for propulsion has inspired researchers to design corresponding propulsors for ships and underwater vehicles. Oscillating fins are being considered for the propulsion of submersible vehicles and Micro air vehicles(MAVs), at low Reynolds number where conventional propulsion techniques becomes inefficient. The method of propulsion evolved by many fast swimming animals is centered on the generation of the thrust from movements of the crescent -shape fin or wing type surface. This includes propulsion from the tail fins of certain fish and marine mammals, carangiform and thunniform propulsion. The maximum propulsive efficiency of a fin whale has been estimated to about 85%
Fish Locomotions and EfficiencyCarangiform Locomotion: Fish in this group are stiffer and faster-moving. The vast majority of movement is concentrated in the very rear of the body and tail. Carangiform swimmers generally have rapidly oscillating tails.Thunniform Locomotion: These are high-speed long-distance swimmers, like tuna (new research shows that the thunniform locomotion is an autapomorphy of the tunas) Here, virtually all the lateral movement is in the tail and the region connecting the main body to the tail (the peduncle). The tail itself tends to be large and crescent shaped. Tunas
Swimming Mode Species Propulsive (Froude) EfficiencyAnguilliform Eel 0.43 – 0.54Carangiform Rainbow Trout 0.74Thunniform False Killer Whale 0.85 – 0.91
Strouhal Number: In dimension analysis, the Strouhal number is a dimensionless number describing oscillating flow mechanisms. The Strouhal number is an integral part of the fundamentals of fluid mechanics. The Strouhal number is often given as St dimensionless Strouhal number, f frequency of vortex shedding, L characteristic length (for example hydraulic diameter) V velocity of the fluid. A Strouhal number is the principle parameter in an oscillating foil.
Von Karman Vortex Street A Von Karman Vortex Street is a term in fluid dynamics for a repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid over bluff bodies. It is named after the engineer and fluid dynamicist, Theodore von Karman.
Wake Structure: One of the major hydrodynamic benefits of flapping foil propulsion is thatthe wake structure is very simple and natural. It is the reverse of a common drag wake known as a Von Karman VortexStreet Here the vortices are shed in an alternating pattern that results in the meanvelocity deficit behind the body. This is a very simple and elegant means of transforming drag into thrust.
Wake Mechanics for Thrust generations in Oscillating Foil Foils oscillating transversely to an oncoming uniform flow produce certain conditions, thrust. It is shown through experimental data from flapping foils and data from fish observation that thrust develops through the formation of reverse Von Karman Street whose preferred strouhal number is between 0.25 and 0.35 and that optimal foil efficiency is achieved within this Strouhal range. When thrust is generated by an oscillating foil, the wake behind it has an average velocity profile with the form of a jet. This Jet flow is associated with a staggered array of vortices moving downstream from the foil, closely resembling the Von Karman vortex street behind bluff objects, but with reverse rotational direction.
Forces acting on Swimming AquaticVertebrates In the horizontal direction, propulsive thrust is the force that drives the aquatic vertebrate forward and is a function of the work output from the fish or marine mammal. Opposing this force is drag that contributes to swimming inefficiencies. In the vertical direction, the weight of the fish is acting downward counteracted by buoyancy and lift forces acting upward. Finally the lift force, which too may act in the downward direction, is typically related to the lift generated during swimming due to the pelvic or pectoral fins.
Comparing Propulsion and Fish Tail A fishs tail and a boats propeller do exactly the same thing in exactly the same way, by producing a longitudinal force from a lateral motion, just like a screw thread. In fact ships propellers are properly called screws. The propeller uses a continuous rotary motion, the tail an oscillating one. Experience and theory have both established that continuous motion is more efficient than oscillation. Although a fish tail is remarkably efficient, it has taken millions of years to get that way. The propeller was optimized in far less time. Many fish depend primarily on their tail beat for propulsion. Such a tail is commonly modelled as a two-dimensional flapping foil.
Flapping Wing Propulsion Numerous biological studies confirm that the best hydrodynamic properties in nature belong to dolphins, whereas maximum relative speeds are achieved in air by birds and insects. The motion of flapping foils is modelled after classical fish caudal fins (tails) to move in heave and pitch and also after turtle or penguin pectoral fins to move in roll and pitch. Flapping-wing propulsion is investigated experimentally and numerically with direct comparisons between experimental and numerical thrust measurements for several geometrically simple configurations. Numerical simulations are performed using linear theory, and a previously developed, unsteady panel method that models one or two independently moving airfoils with three-degrees of freedom and non-linear deforming wakes.
The experimental flapping mechanism utilizes variable aspect ratio wings and optional tip plates to investigate the effect of three- dimensionality. Caudal Fins TypesFlapping Wing Propulsion
Flapping Foil Maneuvering Manoeuvring tests with the oscillating foil are very promising. By adding pitch bias to the harmonic motion, large lift coefficients can be achieved; Fish morphology suggests that control fins for manoeuvrability have unique scalar relationships irrespective of their speed type. Manoeuvring experiments are carried out with fish that are fast, yet manoeuvrable. A related problem in manoeuvring involves, forces created when the foil moves through a single sweep, in still water. Analysis of measurements indicates that the Strouhal number of dolphins is a constant - 0.25 – 0.35.
Angle Of Attack Angle of attack is a term used in fluid dynamics which describes the angle between the lifting body’s reference line and the oncoming flow. Lift Angle of attackDrag Along the Direction of Flow – Drag Perpendicular to the Direction of Flow - Lift
Pitch Bias: During straight-line stability, symmetric propulsion, an oscillating foil device produces instantaneous force vectors with relatively small thrust components. Most of the force produced by the foil is in the form of lift; symmetry causes the mean lift to be zero. By adding a bias, or static offset, to the angle of attack, one can take advantage of these large lift forces for manoeuvring. The simplest method is to add bias to the pitch angle itself, which is linearly related to the angle of attack.• Similar method observed is Impulsive Starting Method.
Flapping Foil Propeller Advantages AdvantagesActive Noise control which helps us in High Speed.performing the Stealth.High propulsive efficiency. Fuel Efficiency good.No Cavitations.Easily adopted from Nature. Disadvantages DisadvantagesLess Speed. More noise creation. Cavitations will be observed.
Conclusion• Biological mechanisms are generally way too complex to make practical in a man made device. It can be done, it is just not easy to duplicate. Propulsion and manoeuvring underwater by flapping foil motion, optimized through years of evolution, is Found everywhere in nature, yet marine propulsors inspired by examples of highly manoeuvrable marine life or aquatic birds are not widely implemented. Further characterization of the hydrodynamic performance of these motions is necessary to improve the control and performance of such propulsors on underwater vehicles. The Main intention of this presentation is to create Spark for Innovations.