High-Enthalpy Boundary Layer Research Articles

Interaction between Mack second mode and radiation mode in high-enthalpy boundary layers

Instabilities of high-enthalpy flat-plate boundary layers in thermochemical equilibrium conditions are studied using a linear stability theory (LST) and parabolized stability equations. A supersonic radiation mode is found to be unstable at the downstream location of the second mode. This mode synchronizes with the second mode as its phase velocity approaches cr=1−1/Ma. The amplification rate is expected to be comparable to the second mode when Ma>14 and Tw<14Te. The results of disturbance evolution indicate that the radiation mode is generated by the second mode through a synchronization process. The coefficient for the amplitude response of the radiation mode to the forcing second mode is around 1. After the radiation mode is excited, the second mode itself still exists in the boundary layer, leading to a co-existence of dual unstable modes. Owing to the competition between the two modes, the disturbance amplitudes exhibit significant oscillations. A two-mode amplitude prediction model based on the LST is proposed by properly superposing the two modes' amplitudes. The accurate prediction of the N-factor demonstrates that the proposed model can be used to predict a high-enthalpy boundary layer transition involving these two modes.

Energy transfer of hypersonic and high-enthalpy boundary layer instabilities and transition

Flow instability of hypersonic and high-enthalpy boundary layers with thermal-chemical nonequilibrium (TCNE) effects is studied for Mach numbers of up to 15. It is quantitatively illustrated that the TCNE effects change the disturbance characteristics predominantly through 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. (L${}_{\mathrm{\ensuremath{\Gamma}}}$ and N${}_{\mathrm{\ensuremath{\Gamma}}}$ in the figure are separately the energy transfer through the linear and nonlinear terms in the disturbance energy-norm equation.)

Roughness-Induced Boundary-Layer Transition on a Hypersonic Capsule-Like Forebody Including Nonequilibrium

The present work investigates the influence of high-temperature gas effects on the laminar–turbulent transition induced by a patch of distributed roughness on a hemispherical capsule-like geometry. The freestream conditions correspond to a realistic reentry scenario at Mach 20. At these conditions, chemical reactions and nonequilibrium effects are present in the high-enthalpy boundary layer of the hemisphere. Parallel Direct Numerical Simulations are undertaken to analyze the instability mechanisms in the crossflow-type vortex developing in the wake of a skewed protuberance of the roughness patch. Both linear and nonlinear growth of unsteady disturbances forced in the roughness wake, including laminar–turbulent breakdown, are considered in the analysis. The primary focus of the study is how chemical and thermal nonequilibrium affect the location of the laminar–turbulent transition as well as the level of wall heating in the transitional boundary layer for the considered capsule configuration. The results highlight the necessity to include nonequilibrium effects in this problem of roughness-induced transition at high-altitude reentry conditions because the gas modeling turns out to have a notable influence on the development of instabilities, both in the linear and in the nonlinear ranges.

Roughness-Induced Crossflow-Type Instabilities in a Hypersonic Capsule Boundary Layer Including Nonequilibrium

The current study investigates the unsteady disturbance development in a three-dimensional, high-enthalpy boundary layer on a capsule-like hemispherical geometry with pseudo-random distributed roughness. Direct Numerical Simulations are conducted for a typical reentry scenario at where chemical dissociation takes place. Unsteady disturbances at various frequencies are introduced into the flow to analyze the instabilities developing in the wake of the roughness patch. The amplification of small disturbances is shown for various chemical models (i.e., chemical equilibrium, chemical nonequilibrium, and thermochemical nonequilibrium). The largest disturbance amplification is observed for a crossflow-type vortex developing in the wake of the highest skewed protuberance of the roughness patch. The modes of instability of this crossflow-type vortex are analyzed and compared for different frequencies. The influence of the different nonequilibrium effects on the steady base flow as well as on the disturbance development is quantified and compared. The study highlights the necessity to include nonequilibrium effects in transitional scenarios as the unsteady development of the instabilities are affected strongly by the chemical modeling.

Radiation-Convection Heat Transfer in the Problems of Screening Electomagnetic Fields by a High-Enthalpy Boundary Layer

Radiation-Convection Heat Transfer in the Problems of Screening Electomagnetic Fields by a High-Enthalpy Boundary Layer

Diagnostics and Modeling of Heat Transfer in High-Enthalpy Gas Flows with Local Heat Sources and Sinks

The use of the laser Raman scattering method is considered for the diagnostics of a near-wall high-enthalpy boundary layer on the surface of ablating materials with a view of detecting volumetric-spatial zones of fast gas-phase chemical reactions of exo- and endothermic types between the ablation vapor and gas components of the main strean. Based on the experimental data, a rational mathematical model is suggested which takes into account the influence of the zones on the character of combined heat transfer, with simultaneous presence of heat conduction and radiation.