Ionization and dissociation effects on boundary-layer stability. (23rd November 2020)

• Record Type: Journal Article
• Authors: Miró Miró, Fernando; Beyak, Ethan S.; Pinna, Fabio; Reed, Helen L.
• Is Part Of: Journal of fluid mechanics. Volume 907(2021)
• Publication Date: 2020-11-23
• Subjects: boundary layer stability, -- compressible boundary layers, -- transition to turbulence; Fluid mechanics -- Periodicals; 532.005
• DOI: 10.1017/jfm.2020.786
• Languages: English
• ISSNs: 0022-1120

Abstract

Abstract: Abstract : The ever-increasing need for optimized atmospheric-entry and hypersonic-cruise vehicles requires an understanding of the coexisting high-enthalpy phenomena. These phenomena strongly condition the development of instabilities leading to the boundary layer's transition to turbulence. The present article explores how shock waves, internal-energy-mode excitation, species interdiffusion, dissociation and ionization condition boundary-layer perturbation growth related to second-mode instabilities. Linear stability theory and the e $^{N}$ method are applied to laminar base flows over a $10^{\circ }$ wedge with an isothermal wall and free-stream conditions similar to three flight-envelope points in an extreme planetary return. The authors explore a wide range of boundary conditions and flow assumptions, on both the laminar base flow and the perturbation quantities, in order to decouple the various phenomena of interest. Under the assumptions of this study, the cooling of the laminar base flow due to internal-energy-mode excitation, dissociation and ionization is seen to be strongly destabilizing. However, species interdiffusion, dissociation and ionization acting on the perturbation terms are seen to have the opposite effect. The net result of these competing effects ultimately amounts to internal-energy-mode excitation and dissociation being destabilizing, and ionization being stabilizing. The appearance of unstable supersonic modes due to high-enthalpy effects is seen to be linked to the diffusion-flux perturbations, rather than the cooling of the laminar base flow (as is commonly believed). The use of the linearized shock boundary condition was seen to have a minor impact in the $N$ -factor envelopes, despite the extremely low relative shock angle.