Shock wave and boundary layer interactions exist widely in the internal and external flow of high speed vehicles. The complicated unsteady flow field has significant effect on the aerodynamic performance of aircraft. Dynamic modal analyses of unsteady motions are helpful to deeply understand the flow structures and dynamical properties of characteristic frequencies in the interactions, providing information to reveal the complex mechanism of the flow. A modal analysis of the unsteady flow field in shock wave and boundary layer interaction for a 24° compression ramp at a Mach number 2.9 is performed by using Dynamic Mode Decomposition (DMD). The applicability of sparsity-promoting DMD in the compression ramp is systematically evaluated. The differences of and reasons for the spatial structures of the dynamic mode corresponding to characteristic frequencies between turbulent and transitional interactions are studied. The influence of spanwise non-uniformity of the transitional boundary layer on the dynamics mechanism of the high and low frequency modes is analyzed and compared. It is found that low frequency modes are characterized by the separation shock and the foot of separated shear layer, exhibiting the breathing motion of the separation bubble. The spatial structures of high frequency modes are dominated by the alternating structures around the mean sonic line, corresponding to the propagation of instable waves past the shear layer above the separation bubble. Additionally, the spanwise non-uniformity has significant effect on the dynamic properties of low frequency modes, while a little effect on those of high frequency modes.
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