Temporal analysis of transonic flow field characteristics associated with limit cycle oscillations

Limit Cycle Oscillation (LCO) is a sustained, non-divergent, periodic motion experienced by aircraft with certain external store configurations. Flutter, an instability caused by the aerodynamic forces coupling with the structural dynamics, and LCO are related as evidenced by the accuracy with which linear flutter models predict LCO frequencies and modal mechanisms. However, since the characteristics of LCO motion are a result of nonlinear effects, flutter models do not accurately predict LCO onset speed and amplitude. Current engineering knowledge and theories are not sufficient to provide an analytical means for direct prediction of LCO; instead engineers rely heavily on historical experience and interpretation of traditional flutter analyses and flight tests as they may correlate to the expected LCO characteristics for the configuration of concern.;There exists a significant need for a detailed understanding of the physical mechanisms involved in LCO that can lead to a unified theory and analysis methodology. This dissertation aims for a more thorough comprehension of the nature of the nonlinear aerodynamic effects for transonic LCO mechanisms, providing a significant building block in the understanding of the overall aeroelastic effects in the LCO mechanism. Examination of a true fluid-structure interaction (FSI) LCO case (flexible structure coupled with CFD) is considered quasi-incrementally since this capability does not yet exist in the flutter community. The first step in this process is to perform fluid-structure reaction (FSR) simulations, examining the flow-field during rigid body pitch and roll oscillations, simulating the torsional and bending nature of an LCO mechanism. More complicated configurations and motions will be examined as the state of technology progresses. Through this build-up FSR approach, valuable insight is gained into the characteristics of the flow-field during transonic LCO conditions in order to assess any possible influences on the LCO mechanism. This will be accomplished temporally via traditional flow visualization techniques combined with Lissajous and wavelet analyses. By examining the effects that the flow features have on the structure, these analysis techniques will lead to the ability to predict whether LCO is expected to occur for particular configurations once true FSI analysis can be conducted.