Direct Numerical Simulations (DNS) are carried out for flat-plate boundary layers at M_a = 2.0 and 6.8 to investigate subharmonic combination-mode phenomena caused by the interaction of two three-dimensional disturbance waves with fundamental and subharmonic frequency. An unexpected long growth of the nonlinearly generated subharmonic combination mode is observed which is independent of the disturbance amplitudes. The relevance of the subharmonic modes for transition is examined by comparison with a pure oblique-type breakdown scenario.
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