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22.10.2012 20:43:55
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2.5. Unsteady Aerodynamic Influences arising at self-oscillations of the "flutter" type.

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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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