| Summary: | Direct numerical simulations have been performed to study receptivity and breakdown mechanisms for hypersonic flow over blunt leading-edge configurations with imposed freestream acoustic disturbances. Both two-dimensional (2D) and three-dimensional (3D) fast and slow acoustic wave models have been used. The former has been adopted for 2D simulations over a blunt-nose wedge probe designed to measure freestream noise levels in hypersonic wind tunnels; the latter has been used to perform 3D simulations for a span-periodic blunt wedge in unswept and swept configurations, and for a three-dimensional generic forebody model. In the 2D wedge simulations, an analysis of the post-shock wave structure shows that fast acoustic waves are efficiently transmitted across the shock as refracted waves, while slow acoustic waves generate convected waves. The wall response to the fast mode highlights a resonance-modulation behaviour in the nose region. An estimation of the freestream noise levels in the DLR high-enthalpy (HEG) and low-enthalpy (RWG) hypersonic wind tunnels has been performed, showing higher noise levels for the HEG wind tunnel at high Mach numbers. The 3D wedge simulations have been used to study the characteristics of the receptivity and breakdown mechanisms associated with different wave types (fast/slow), disturbance amplitudes, and sweep angles. The fast-mode induced transition has been observed to be a much more rapid and powerful process than the slow-wave related transition, because of the role played by the fast-mode resonance mechanism at the leading edge. Finally, the numerical simulations performed for a generic forebody geometry have enabled comparison with a recent transition experiment carried out in the Mach 6 Purdue hypersonic wind tunnel in noisy conditions. In this case, slow acoustic waves show the most similar transition patterns to the experimental case, and, in particular, are more efficient than fast waves in triggering nonlinear growth of streamwise streaks, related to crossflow inflectional instabilities located in the off-centerline leading-edge region.
|
|---|
