A pair of oblique waves at low amplitudes is introduced in a supersonic flat-plate boundary layer at Mach 3. Its downstream development and the concomitant process of laminar to turbulent transition is then investigated numerically using linear-stability theory, parabolized stability equations and direct numerical simulations (DNS). In the present paper, the linear regime is studied first in great detail. The focus of the second part is the early and late nonlinear regimes. It is shown how the disturbance wave spectrum is filled up by nonlinear interactions and which flow structures arise and how these structures locally break down to small scales. Finally, the study answers the question whether a fully developed turbulent boundary layer can be reached by oblique breakdown. It is shown that the skin friction develops such as is typical of transitional and turbulent boundary layers. Initially, the skin friction coefficient increases in the streamwise direction in the transitional region and finally decays when the early turbulent state is reached. Downstream of the maximum in the skin friction, the flow loses its periodicity in time and possesses characteristic mean-flow and spectral properties of a turbulent boundary layer. The DNS data clearly demonstrate that oblique breakdown can lead to a fully developed turbulent boundary layer and therefore it is a relevant mechanism for transition in two-dimensional supersonic boundary layers.

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Oblique breakdown to turbulence was initiated by low amplitude forcing in a laminar flat-plate boundary layer at Mach three. The growth of the instability waves was investigated using spatial Direct Numerical Simulations (DNS). Excellent agreement with theory was obtained in the linear stage corroborating that the entire transition process from the linear regime to the final breakdown was captured. A fully turbulent flow was reached demonstrating that this transition scenario is a viable path to turbulence. Key events in the late nonlinear stage of breakdown are studied in detail.

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The final stage of transition to turbulence in a supersonic flat-plate boundary layer at Mach 3 is investigated using Direct Numerical Simulations (DNS). A set of six simulations was performed to study the development of a pair of oblique waves from their initiation at low amplitudes to final breakdown to turbulence. In a previous paper (part I of this study), the linear and early nonlinear regimes of this so-called "oblique breakdown scenario" were discussed. The focus of the present paper (part II) is on the early turbulent regime, i.e. the region of the flow downstream of the maximum of skin friction. In this region, the flow is shown to have lost periodicity in time and to exhibit the typical mean flow and spectral properties of a turbulent boundary layer as known from the literature and theory. The DNS data clearly demonstrate that oblique breakdown can lead to a fully developed turbulent boundary layer and therefore may be a relevant mechanism for practical applications. authors.@en

A pair of oblique waves at low amplitudes is introduced in a supersonic at-plate boundary layer. Its downstream development and the concomitant process of laminar to turbulent transition is then investigated numerically using Direct Numerical Simulations (DNS). In the present paper, first the linear regime is studied in great detail. Comparisons to linear stability theory clearly determine that the so-called "oblique breakdown" mechanism is initiated. The focus of the second part is the nonlinear regime. It is shown how the spectrum is filled up by nonlinear interactions, what flow structures arise and how these structures locally break down to small scales. Finally, a logarithmic region of the mean streamwise velocity profile is formed indicating the beginning of a fully turbulent flow region. The simulations demonstrate that oblique breakdown is a viable path to turbulence.

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