Oddvar O. Bendiksen
DOI Number: N/A
Conference number: IFASD-2017-015
Multiple or nested limit cycle oscillations (LCOs) were first observed in wind tunnel tests of an unswept supercritical wing in transonic flow, restricted by the supports to two degrees of freedom (pitch and plunge). In this case the flow was essentially two-dimensional, and the coexistence of two LCOs of different amplitudes can be explained in terms of the transition from Tijdeman Type A to Type B shock motion, which limits the energy flow from the fluid to the structure. For a highly nonsymmetrical wing, the transitions on the upper and lower surfaces will occur at different amplitudes, resulting in nested limit cycles. Recent computational results suggest that nested limit cycles should occur for most transonic wings, if the wings exhibit LCO-type flutter near Mach 1. In this paper, results are presented for flutter cases where 3 nested limit cycles are present in the transonic region. Possible physical mechanisms responsible for multiple LCOs are discussed, as are the computational and experimental implications. Calculations indicate that at some Mach numbers one or more of the nested LCOs may become unstable and eventually disappear by “morphing” into neighboring LCOs. The outer LCO is also susceptible to amplitude instabilities and may gradually transition into strong flutter. In a wind tunnel or flight test this could be dangerous, because a small change in Mach number or dynamic pressure could transform a benign LCO into hard flutter. These observations also have important computational implications in code verification and validation.