Transition to turbulence of a Mach 6 flat plate boundary layer over a porous surface is investigated by direct numerical simulation considering two Reynolds numbers based on the laminar boundary layer displacement thickness, namely, Re?? = 6000 and Re?? = 20?000. The transition was initiated by perturbing the laminar boundary layer with small random disturbances and was followed all the way to the turbulent state. The porous geometry was modeled by directly resolving the flow within the pores and the damping of the primary Mack mode of instability was verified. The presence of a porous surface was found to reduce the secondary instability growth rate by reducing the amplitude of the second mode saturation. In particular, the pores suppress the growth of the secondary wave in the near wall region, so that the secondary instability mainly happens near the critical layer. Besides the secondary instabilities Fourier analysis shows additional modes growing at the same rate as the primary instability, consistent with a model for sound waves scattering from the porous surface. The transient growth of u?, ??, and T? fluctuations, in the form of streamwise streaks, appears to favor the fundamental type of secondary instability. Additional calculations revealed that an oblique first mode wave is the most amplified mode in this porous surface configuration. This wave is slightly destabilized by the pores. With the oblique first mode excited, the flow becomes turbulent due to the nonlinear interactions without the need for secondary instabilitie
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