Centrifugal Instabilities

It has been shown that counter-rotating structures (Görtler vortices) form in the vortex/surface interaction region over a delta wing at angle of attack. These structures convect around the primary vortex in a helical fashion, causing nearly periodic velocity fluctuations in the flowfield. A centrifugal instability is hypothesized as the driving mechanism for the generation of the structures which form with a preferred frequency as a function of velocity and angle of attack.  The figure shows a summary of the results obtained by Hubner. Through the past research by Hubner and this present research, many questions have been answered regarding the origin of the fluctuations, but there are also many unanswered questions and new questions that need further research to answer. Specifically, what is the mechanism that selects the dominant peak frequency and generates the frequency characteristics seen in experiments? Can experimentally investigating the centrifugal instability mechanism offer insight into this, such that a frequency prediction tool can be developed? Can the results of this investigation be applied to an actual fighter aircraft to reduce tail buffeting? These questions are to be answered by the future research.

 F-15 Tail Buffet Alleviation Results
Introduction:    Spectral Amplification:    Centrifugal Instabilities:
Reduced Frequency Scaling:    Linear Peak Frequency/Velocity Slopes:    Models Tested:
Empirical Peak Frequency Data:    Surface Fence Tests:    Reduction in Spectral Intensity:
Reductions Over all Angles:    Comparison with Flight Tests:    Summary:
Future F-15 Wind Tunnel Tests: