Dynamics of vibration mode competition between flutter and forced vibration in thin-walled scramjet isolators under fluctuating backpressure

Backpressure fluctuations in thin-walled scramjet isolators can induce vibration mode competition between the intrinsic flutter of elastic panels and forced vibration, yet the underlying dynamics of this interaction remain insufficiently understood. This study employed a two-way coupled CFD/CSD framework to investigate the interplay and competition between the panel’s intrinsic flutter and the forced vibration induced by periodic backpressure fluctuations over a broad range of fluctuation frequencies (f_b) and amplitudes (A_b). Under steady backpressure, the elastic panel exhibits flutter-dominated limit cycle oscillations. When fluctuating backpressure is introduced, periodic excitation superimposes on the intrinsic flutter, giving rise to four distinct dynamic regimes: (i) flutter-dominated regime at low f_b; (ii) forced-vibration regime at mid f_b with resonance near panel’s natural frequency; (iii) a transitional regime with a coupled frequency; and (iv) flutter re-emergence at high f_b. At higher A_b, only the flutter-dominated and forced-vibration regimes occur, separated by clear f_b boundaries. With increasing f_b, the panel starts in flutter dominance with near constant oscillation amplitudes, transitions to forced vibration near resonance with rapid amplitude growth, and returns to flutter dominance beyond resonance. Reducing A_b lowers the direct energy input from backpressure fluctuations and promotes the emergence of the transitional regime, which occurs more frequently and persist longer, blurring the regime boundaries between flutter and forced vibration. As a result, vibration mode competition fundamentally governs the structural response, which in turn controls shock-train motion, separation behavior, and overall isolator performance. Regime transitions cause abrupt changes in mean values and oscillation amplitudes of these flow features, with resonance exerting the strongest influence by driving extrema in both metrics. Away from resonance, flutter- or transitional-dominated regimes exhibit weaker sensitivity to f_b, although large A_b can still generate significant unsteadiness.