POSSIBILITY OF ACTIVE CASCADE FLUTTER CONTROL WITH SMART STRUCTURE IN TRANSONIC FLOW CONDITION

Junichi Kazawa Toshinori Watanabe

Department of Aeronautics and Astronautics, University of Tokyo 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8656, Japan Phone : +81 3 5841-6624 Fax : +81 3 5841-6622 kazawa@aero. t.u-tokyo. ac. jp

Abstract To study the possibility of active cascade flitter control by application of smart structure, numerical analyses were performed under transonic flaw conditions with passage shock waves by a developed flaw-structure coupled method. In the flow condition of the present study, the unsteady aerodynamic force induced by the shock oscillation was dominant for instability of blade vibration. The direction of blade movement during oscillation was first adopted as the control parameter, because it was known to be a quite influential factor for vibration in­stability of blades in the transonic flows. The method could decrease the passage shock movement near the blade surface and effectively suppress the blade vibra­tion, though it was not effective when the blade stiffness was small. For more effective control, the method in which the trailing edge of blade was actively vibrated was sought to control the passage shock oscillation. The trailing edge oscillation might be realized by, for instance, application of piezo-electric de­vice. The method was revealed to change the unsteady aerodynamic force acted on the blade from exciting to damping force if the phase of trailing edge oscil­lation was properly selected. The suppressing effect of the control method came from its effect on passage shock movement, which was confirmed by developed flow-structure coupled method.

1. Introduction

Active control of surge and rotating stall has been successfully studied in the last decade [1] including phenomenological description of the onset of insta­bility and control mechanism. Concerning the cascade flitter problem, some active control techniques have also been reported, in which acoustic waves are introduced to suppress the flitter [2], or the acoustic impedance on casing wall is actively controlled to reduce the exciting energy of the oscillating blade [3].

65

K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 65-76. © 2006 Springer. Printed in the Netherlands.

On the other hand, the study of "smart structure" has been advanced in the research field of structures and materials. The smart materials can deform themselves reacting to electric signals. Active control techniques, in which the smart materials are applied to the fhp of the aircraft wing or fuselage panel, have been proposed so far [4] to prevent flitter instability of them. If the char­acteristics of smart materials are properly utilized for the active control of cas­cade flitter, an effective and reliable control method can be realized since such materials directly change the structural characteristics of the vibrating materi­als, or can give fexible deformation of the blades.

The authors have studied some methods of cascade flutter control by use of the developed fl>w-structure coupled method. For example, flitter suppres­sion was found to be possible by changing natural frequency of blade in the subsonic fliw condition [5]. The control can be realized by the application of Shape Memory Alloy.

In the present study, two methods of the cascade flutter control were ana­lyzed for suppression of vibration instability in transonic flow conditions with passage shock waves. In the studied fl>w condition, it is well known that the unsteady aerodynamic force induced by the movement of passage shock wave is dominant for blade vibration instability. It is also known that the direction of blade movement during oscillation is a quite influential factor for flutter instability.

Based on the knowledge above, the possibility of flitter suppression by changing the oscillation direction of blades was investigated first. The change in the direction can be carried out with, for instance, some kind of shape mem­ory alloy. The second method was to give forced oscillation on the trailing edge section of blades. It was thought that the occurrence of cascade flitter could be suppressed through the change in the oscillatory motion of shock waves im­pinged to the blade surfaces. The forced vibration of the trailing edge section can be realized with piezo-electric devices.

The results of these numerical studies are reported in the present paper.