Influence of Nose Bluntness on Hypersonic Boundary-Layer Transition Onset in a Mach 8 Digital Wind Tunnel

Direct numerical simulations (DNS) are used to investigate laminar-turbulent transition over a 7-degree half-angle blunt cone with 5.2 mm nose radius at Mach 8, extending previous studies on the effects of free-stream acoustic disturbances on hypersonic boundary-layer receptivity and transition. Similar to the earlier work, the tunnel-like freestream acoustic
disturbances are modeled from the acoustic radiation from the nozzle-wall turbulent boundary layer from a precursor DNS of the full-scale axisymmetric nozzle of the Sandia HWT-8. The onset of transition is clearly identified through surface quantities, including wall-shear stress, pressure, and heat flux. The transition region is also characterized by a rapid increase in spanwise-averaged, root-mean-square value of fluctuations in these quantities, accompanied by a broadening of both frequency and azimuthal wavenumber spectra. The onset of transition is considerably downstream in comparison with a nominally sharp cone at the same freestream conditions, highlighting the effect of nose bluntness. The peak N-factor for Mack’s second mode disturbances at the transition onset location is very small, 𝑁𝑇 = 1.2, similar to previous observations based on blunt cone experiments at hypersonic conditions. The paper also explores the physical mechanisms underlying the onset of transition despite such low modal amplification.