Abstract

*The consideration of aeromechanics plays a vital role in the design of machines that operate under aerodynamic forces, such as turbomachinery, and aircraft. The structure of those machines is subject to aeromechanical dynamics, including forced response and flutter. The strength of aeromechanical interaction depends on the level of coupling between flow and structure. One effect that can lead to strong coupling is the interaction between eigenmodes of the structure and eigenmodes of the flow near coincidence. This paper examines the impact of modal coincidence on the linear dynamic stability of aeromechanical systems for two illustrative canonical examples, one dominated by inviscid acoustics, and one by the eigenmode of a wake. Three commonly used analysis techniques are applied and ranked for various levels of coupling: The 1-way coupled work-per-cycle method, a 2-way coupled non-linear modal FSI analysis in time, and an eigenanalysis of the 2-way coupled linear system, based on a state space representation. It is demonstrated that all three methods agree for low to moderate levels of 2-way coupling. At higher levels, the work-per-cycle assessment is insufficient, whereas the FSI and eigenvalue analysis agree well.*

Keywords: Aeromechanics, Aeroelasticity, Flutter, Mode coupling

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1.     INTRODUCTION

 

The risk of flutter and severity of forced response increases with the level of coupling between structure and aerodynamics. Common methods for design and analysis of aeromechanical systems consist of analyzing in-vacuo structural eigenmodes, predicting aerodynamic forces, and then analyzing interactions between both parts of the dynamic system. This works well as long as in-vacuo structural modes are fairly representative of the coupled aeromechanical eigenmodes. However, even in structurally dominant systems coupling can be strong under coincidence of structural and aerodynamic eigenmodes.

Literature on aeromechanical instabilities addresses a variety of problems that relate to the coupling of structural and aerodynamic modes. Abbott \[1\] reports on a labyrinth seal flutter problem that was dominated by the effects of coincidence between a structural mode of the seal, and a circumferential acoustic mode in the seal cavities. Vahdati et al. \[2\] explain the speed sensitivity of fan blade dynamic stability by an interaction between the structural mode and inlet acoustics. Zhang et al. \[3\] study the aeromechanics of an elastically suspended cylinder near coincidence between the structural and vortex-shedding modes.

In this paper we examine the phenomenon of aeromechanical modal coupling and its effect on dynamic stability by studying two canonical cases. The first case is a duct terminated by a nozzle whose throat area is subject to structural motion. The motion controls mass flow and thus interacts with acoustic duct modes. While easy to analyze, this case is relevant for understanding the aeromechanics of labyrinth seals and similar flow control devices. The second example is the much studied circular cylinder in cross flow. Here, we examine the structurally flexible cylinder at sub-critical Reynolds numbers, where aerodynamic modes are stable per se, but fluid-structure mode coupling can lead to aeromechanical instability. In both cases the focus will be on understanding the physics of mode coupling on one hand, but also on understanding the range of validity of various analysis methods (coupled vs. uncoupled).

 

  
REFERENCES

\[1\] Abbott, D. R. *Advances in Labyrinth Seal Aeroelastic Instability Prediction and Prevention*, J. Eng. Power, Apr. 1981, 103(2):308-312

\[2\] Vahdati, M., Smith, N., Zhao, F. *Influence of Intake on Fan Blade Flutter*, J. Turbomach, Aug. 2015, 137(8)

\[3\] Zhang, W., Li, W., Ye, Z., Jiang, Y. *Mechanism of frequency lock-in in vortex-induced vibrations at low Reynolds numbers*, J. Fluid Mech., 2015, 783:72-102


On the Effect and Analysis of Fluid-Structural Mode Coupling

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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