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Numerical Investigation of Nonlinear Wave Packets in a Hypersonic High-Enthalpy Boundary Layer on a 5°Sharp Cone
Stability and transition of a boundary-layer is numerically investigated for a 5° sharp cone in high-enthalpy flow using weakly nonlinear and strongly nonlinear wave packet simulations. Towards this end, a short-duration pulse through a small hole in the surface of the cone is utilized to excite a broad spectrum of frequencies and streamwise/azimuthal wave numbers. The flow conditions chosen for the simulations are those from the high-enthalpy experiments by Jewell et al. in the T5 tunnel at Caltech. The experimental measurements indicate the presence of turbulent spots. In these experiments, the free-stream temperature is much higher than the wall temperature due to the high-enthalpy conditions. The wall temperature of the cone remains at room temperature (isothermal wall) due to the short-duration of the experiments. Salemi and Fasel have investigated the linear regime for this flow utilizing low-amplitude wave packets. However, to date no nonlinear wave packet numerical simulation data are available for comparison with the hot flow/cold wall experimental data of Jewell et al. The stability investigations discussed in the present paper represent another step towards understanding the physical mechanisms that are maybe responsible for generating the turbulent spots. Our present results indicate that a nonlinear resonance mechanism is present in such experiments and may be a fundamental resonance of the second mode waves. For the strongly nonlinear regime, a significant 2-D higher harmonic arises, which may be Mack's third mode, and becomes increasingly important downstream. According to our previous DNS results, the wave packet in the linear regime changes its character as amplified wave components synchronize their phase speeds with the vorticity/entropy, and slow acoustic modes. The current weakly nonlinear and strongly nonlinear results also exhibit a similar phase speed synchronization. However, the packet envelope spatial modulation changed considerably compared to the linear packet. The packet envelope change during synchronization is not observed in axisymmetric pulse simulations.
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