Computational Analysis of Energy Exchange Mechanisms in Turbulent Flows with Thermal Nonequilibrium

Abstract

The design of hypersonic vehicles is significantly affected by the state of boundary layer. Hypersonic boundary layers can be laminar or turbulent, and in chemical and vibrational nonequilibrium, each with different length and time scales. In turbulent boundary layers, heating augmentation can be an order of magnitude or higher above laminar heating rates. The scales of the internal energy relaxation processes can be of the same order or greater than the turbulent flow scales, and can interact with turbulent motion. Understanding how turbulent motion and internal energy relaxation interact is relevant to flow control. Fundamental flows are studied to understand how energy is exchanged between turbulent motion and internal energy relaxation. Specifically, high-fidelity DNS of vibrational energy relaxation effects in compressible isotropic and temporally evolving shear layers are presented. The energy exchange mechanisms are analyzed by decomposing the flow into incompressible and compressible energy modes. By varying the vibrational relaxation rate, the tuning of the relaxation rate to the turbulent flow is studied. Vibrational energy relaxation is demonstrated to be coupled to the turbulent flow through the compressible modes of the gas. Compressions and expansions generate fluctuations in the thermal state, and the vibrational energy lags behind these fluctuations. Energy is then transferred to or from the vibrational energy mode at a rate proportional to the relaxation time, and the fluctuations are damped. Damping of turbulent quantities are strongest when the vibrational relaxation rate is on the order of the turbulent large structure acoustic rate. Wavenumber specific damping is also observed in isotropic flows when the relaxation time is on the order of the acoustic frequency of the wave. The effects of vibrational relaxation are shown to increase with compressibility. However, the overall effect on turbulent kinetic energy is weak due to the incompressible mode containing significantly more energy than the compressible modes.