摘要：

This work focuses on instability mechanisms of high-speed boundary layers over flat plates and cones with a circular cross section. Supersonic transition investigations at Mach 2 and hypersonic transition investigations at Mach 8 are performed using Direct Numerical Simulations (DNS). At wind-tunnel conditions, these simulations allow for comparison with experimental measurements to verify fundamental stability characteristics. For the DNS of boundary-layer transition at Mach 2, the experimental studies by Kosinov et al. (1994) and Ermolaev et al. (1996) for a flat plate serve as reference and provide the physical conditions for the numerical setup. In these experiments, the weakly nonlinear regime of transition was studied resulting in the discovery of asymmetric subharmonic resonance triads. Scrutinizing the experimental data, reveals however the presence of another, possibly competing breakdown mechanism, in the experiments. Both mechanisms were addressed in detail in this work. To better understand geometrical influences, flat-plate and cylindrical geometries are studied under after-shock conditions of the conical investigations (experiments). This allows for a direct comparison with the results of the sharp cone to evaluate the influence of spanwise curvature and cone opening angle. The ratio of the boundary layer thickness to the spanwise radius is used to determine the importance of spanwise curvature effects. For a cone, in downstream direction the radius increases linearly while the boundary layer thickness stays almost constant. Hence, spanwise curvature effects are strongest close to the nose tip and decrease in downstream direction. Their influences on the secondary instability mechanisms provide some preliminary guidance in the design for future high-speed air vehicles.

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