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22.10.2012 20:43:55
23.08.2011
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In 1920, the Austrian scientist Franz Wels, who conducted the research into the mechanism of fish tail movements, has concluded that the local movement of the fish produces by curvature of the spine. The curvature of the spine is provoked by the additional reductions of related muscle fibers, which cause the bends of the fish body, always starting with the head, moving on to the tail in the form of wave-like fluctuations and again starting with the head. Moreover, these wave-like fluctuations occur in the plane perpendicular (or close to perpendicular) to the direction of the fish movement. The movements of birds and amphibians have the similar cause. On the basis of these studies Franz Wels constructed and in 1921 patented the device [1], created not only to reproduce the fish movement, but also to carry out useful work by displacing a flow of water. In his work, he suggested that this device can convert mechanical energy of muscular strength or chemical energy of fuel combustion engine into the useful work liquid transferring, and perform the reverse conversion of the kinetic energy of the water flow into the mechanical energy of the motor shaft rotation as well. However, the idea of transformation of energy of the liquid flow into the useful work has not got the confirmation and the efficiency of work of the device in the form of a pump for pumping the liquid was so small, that there was no further development. The author of the idea tried to reproduce mechanically the kinematics of the fish movements with the help of usual crank gear and crank-slide mechanism without deep understanding of the physical nature of these movements.
The author hoped to transfer the force of the flow impact to the element placed in the flow which has the ability to simulate the fish movements, and then, through the slider to the piston engine power. But at the same time the forces of mechanical dampening of the device, were so great, that there were no movements of the piston.
In 1936, the British zoologist professor James Gray made the calculation of power, developed by the dolphin muscles at a speed of V=10 m/s (36 km/h) and found out a fantastic paradox: dolphin can not swim with such speed, so as there is the lack of the developing capacity, it is 7 times less than it is needed [2]. This nonsense became called in science as "the Gray's paradox". But the author of the calculation has based on the fact that the boundary layer of liquid, flowing a body of a swimming dolphin, takes turbulent (screw) character, and this increases the friction between the body and the water. Such an assumption would be quite legitimate if we were talking about the flowing the rigid body the same size and shape like a dolphin. But in this case, the body is alive, flexible, and J. Gray suggested that the explanation of the paradox is possible if the flowing has a laminar (inkjet, calm) character, in this case the friction reduces essentially. In other words, a swimming dolphin has a kind of mechanism for laminarization boundary layer of the liquid.

There was created a whole direction in hydrobionics when scientists were trying to find this mechanism or to deny J. Gray's calculations. Because of the rapid growth of interest to bionics in the early 60's the scientists were began to pay a special attention to the swimming of dolphins. According to the publications the studies in this area were conducted mainly in Great Britain, USA and USSR [3].
Many years the searches of the dolphin's speed secret were conducted mainly in the field of studying of the structure and various properties of dolphin's integument: there was a view of creation of an artificial covering for sea ships which would have an ability of an integument to damp the fluctuations of the boundary layer. However till now there are no convincing proofs which were experimentally confirmed that it's only in the skin. Here it is appropriate to notice that the most high-speed fishes of the world ocean, such as tuna, marlins, swordfish, bodies of these fishes are covered mostly with a big, tough scales, and to talk about some of damping properties of such coverage doesn't make any sense, and the speed of them under the water reaches 100 km/hour.
Meanwhile J. Gray in the same work suggested that the boundary layer laminirization mechanism can be connected with the presence of so-called negative gradient of dynamic fluid pressure along the body of the animal, due to the active motion of the body like the dolphins, as well as the fish. Simply speaking, the nature of the movements of a floating body reduces the pressure of the fluid from head to tail, and this reduces the degree of turbulence. However, so far no one has described theoretically the mechanism of the dolphin's movement, or, moreover, has obtained experimentally proofs (effects) connected with the reproduction of the kinematics of dolphin's.
The Scheme of the wave-like motion of the fish's body, presented in the work [1], resembles the scheme of the single cylinder movement in a longitudinal flow of the liquid at high flow rates, what leads to loss of cylinder steadiness and the emergence of bending-twisting flutter. The dynamics of single-cylinder in a longitudinal flow of liquid is studied in sufficient detail [4-10]. It is well established [6,10]that for small flow rates cylinders make a small random oscillations raised by random resistance of averaged flow; the influence of the averaged flow has an impact on the appearance of a hydrodynamic damping and reduction of the cylinder own frequency. However, at relatively high flow velocities, first the system loses its steadiness as a result of static buckling (the phenomenon of "divergence"), and then there is a "flutter" in it [4,5,10]. And with it in the survey [9, 10] it was shown, that the majority of industrial systems has the cylinders flexible not enough, and flow rate are too high for these phenomena of unsteadiness and flutter emergence could be realized in practice.
The term "self-oscillations" is usually applied to the oscillating object that can take energy from the flow of a fluid medium (air, water), while maintaining the unabated fluctuations. If the energy is taken from a uniform flow, this process of self-oscillations is called the "flutter". The concept of "flutter" is usually applied to the wings of the aircraft and the blade of the turbomachines. Later this name was applied in the study of pipe arrays of heat exchangers. In aviation flutter, among fluctuations and in general among the numerous types of vibrations , which experiences the aircrafts, poses a special danger, because dynamic tension in the aircraft construction what arises as a result of the intense oscillations and this can quickly (sometimes within a few seconds) destroy the aircraft while in flight[11]. That is why the appearance of the flutter in any form is unacceptable.

Always while studying the interaction of oscillating objects with the flow of fluid, the flutter phenomenon was considered as harmful (catastrophic) phenomenon, which can occur only in the technique and leads to the destruction of the object and it cannot be allowed. That is why, all of the research, tests and calculations of the flutter phenomenon were to define the boundary conditions of the flutter and recommendations for the prevention of these conditions [11, 12].
Information concerning the flutter phenomenon in the device for the conversion of the kinetic energy of the fluid medium flow into the useful work is available in the patent [13], which is an improvement of the same patents [14, 15, 16].In these works the author confuses the phenomena of flutter and resonance (conventional forced vibrations) oscillating in the fluid medium flow load of cylindrical objects, when the laws of cylindrical objects oscillations are unpredictable, and oscillations of the objects are failing and it needs to be maintained constantly by the external forces.The main types of the flutter of oscillating objects in the flow of a fluid medium with subsonic flow are stall flutter, lattice flutter and bending-torsion flutter [12].
Theoretically the conditions of formation and analysis of bending-torsion flutter of a single wing (profile) are described in the paper [17]. If there were only bending or only torsional oscillations, it would abate because of aero damping (as well as mechanical damping). The emergence of joint bending-torsional oscillations radically change the picture. The matter of the fact is that bending as well as torsional oscillations of the profile will cause the appearance of unsteady aerodynamic forces and moments (unsteady aerodynamic influence - UAI), work of which can be both positive and negative depending on the angle of phase shift between bending and torsion oscillations. If driven and dedicated energy are balancing, there are persistent harmonic oscillations. So as the aerodynamic forces and moments depend on the speed of the main flow, the balance of energies will come at a certain speed, what is called critical. If the speed exceeds a critical, it should be observed oscillations with increasing amplitude.The most famous and used in technique is the lattice flutter of turbomachine's blades, when the blade's array makes flutter oscillations in the composition of a vibrating grate. In all countries, what are doing aircraft construction or turbine construction, there exist scientific-research centers and institutes on study of the stability and reliability of the oscillations of turbomachine's blades in the interaction with the flow of the fluid medium, as well as to prevent the conditions of flutter occurrence at the work of these mechanisms.In the work [12] is said that the most important, complex and diverse of all of the factors that affect the nature and intensity of oscillations of turbomachine's blades are non-stationary aerodynamic influence (UAI), arising from the oscillations of the blades in a flow. UAI on a blade of vibrating grate of a turomachine is determined by the total aerodynamic influence on this blade at its oscillations in the flow and the additional impact caused by the oscillations of the other blades of the grate, and transmitted through the flow. The emergence of this impact is explaining by the fact that when the blades are oscillating in the flow takes place a periodic change of instantaneous values of angles of attack and velocities of the flow-on relative to indicators of non forced flow by flow-on, as well as the aerodynamic interaction of vibrating blade of a grating, contributing additional disturbances in the flow and changing the regime of flow-on conditions. Thereby, UAI on one or another blade of the grating is determined not only by its own oscillations, but also by oscillations of other blades, which are performed with different phase shift.
In other words, to a variable impact, caused by the forced oscillations of the most visible blade and what is under the opposite phase with its speed, there are added a changing be the time impacts generated by oscillations of the other blades of the grating, as a result of its variable speed and changing of its situation in the space. Under certain conditions (what are depending not only from the shift of phases of the blade's oscillations) , it may be that the additional impact will change its direction for one reason or another, peculiar turbomashines. Unsteady aerodynamic force (moment), forced by oscillations of the blade, will change its sign, which means it will work in phase with the speed of the blade's movement. Depending on the parameters defining the oscillatory process, there are cases, when the direction of the examined UAI coincides with the direction of oscillation of the blade. Then, these Influences make the positive work. If by this the positive work of UAI in absolute value exceeds always negative work of forces of mechanical damping, it would be self-oscillations of blades type flutter. In fact when flutter aerodynamic damping changes sign, and the combined damping take negative values. The appearance of the flutter of the blades is characterized by a loss of dynamic stability of its forced oscillations.

 Usually to start any study of any phenomena is reasonable from a simple, moving to a more complex. In practice, however, there is no information about getting the flutter of a single profile (wing or blades), oscillating in the flow of a fluid medium, regardless from a flow speed.
That is why, to obtain practical results in the study of the mechanism of getting an additional energy during the movement of a dolphin was decided to start the research of bending-torsion flutter of a single working element, with the possibility to perform bending-torsion oscillations in the composition of the array of such working elements,
when unsteady aerodynamic influence (the forces and moments - UAI) arise not only from the interaction of oscillating studying working element with the flow of a fluid medium, but also caused by the influence of one working element to the next working element in the composition of one studied array of working element. In this case, each analyzed working element, included in the composition of the studied array, must have a possibility to perform bending-torsion oscillations, which is characterize the flutter of a single profile.

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