Rarefied airfoil aerodynamics based on the generalized hydrodynamic model

Numerical studies on rarefied gas flows around an airfoil are conducted by an implicit non-Navier-Stokes-Fourier (NSF) framework, namely, the generalized hydrodynamic (GH) model. A detailed study of the rarefied gas flow around an airfoil is performed following validations. Investigations show that all the characteristics of the rarefied effect on an airfoil observed by the particle method in a previous study are also observed in the present GH study. In addition, the constitutive relationships of the GH model provide a clear explanation regarding the distinctive features of the rarefied effect on the aerodynamic characteristics of an airfoil. Also, a recompression region is found at the trailing edge in a state involving subsonic flow and a high Knudsen (Kn) number. With increasing Kn number, the drag increases rapidly compared to the lift, and this effect results in a sharp decrease in the lift-drag ratio. Moreover, the NSF framework overpredicts the lift-drag ratio for subsonic cases and underpredicts the ratio for supersonic cases in rarefied gas flows with different angles of attack considered. Credible explanations are provided for the limitations of the NSF framework in a numerical study of the rarefied effects, including the near equilibrium state, considering the present constitutive relationships of the GH model. We show that the present GH model provides a new numerical tool for the investigation of the rarefied effect on aerodynamic characteristics. (C) 2019 Elsevier Masson SAS. All rights reserved.