In this study, the disturbance energy budget is analyzed on the derived disturbance energy norm in hypersonic and high-enthalpy boundary layers with thermal-chemical nonequilibrium (TCNE) effects. The disturbance growth rate is decomposed to quantitatively evaluate the contribution from various classified terms. Hypersonic flat-plate flows are investigated with various free-stream Mach numbers, free-stream temperatures, and wall temperatures. The linear and nonlinear evolutions of disturbances are predicted using linear stability theory and parabolized stability equations. The results show that in the first-mode region, the disturbance growth rates are determined by the production term (destabilizing) and the viscous term (stabilizing), while the former nearly offset the latter. In the second-mode region, the viscous term decreases to the minimum, resulting in the dominance of the second mode. The disturbance of the TCNE source term has a stabilizing effect on the second mode, but at most it reduces the growth rate by $6\%$ in a Mach 10 adiabatic case with the highest free-stream temperature of $900\mathrm{K}$. The production term is mainly responsible for the second-mode growth rate difference between the TCNE flow and calorically perfect gas flow. TCNE changes the disturbance characteristics mainly through the mean flow modification. In the oblique-mode breakdown case, the intensive energy transfer between the selected modes and their harmonic waves is found to occur where they interact strongly with the mean flow.

• Received 11 August 2021
• Accepted 10 February 2022

DOI:https://doi.org/10.1103/PhysRevFluids.7.033901