An Improved Version of One-Equation RAS Turbulence Model

Abstract

An improved version of a recently developed one–equation turbulence model called RAS (Rahman–Agarwal–Siikonen) is proposed to account for the distinct effects of low–Reynolds number (LRN) and wall proximity. The turbulent kinetic energy k and the dissipation rate ǫ are evaluated using the R = (k/ǫ)–transport equation together with the Bradshaw and other empirical relations. The associated coefficients are constructed such as to preserve the anisotropic characteristics of turbulence encountered in non–equilibrium flows. In the current version, several improvements to the original RAS model are made which include the introduction of a near– wall eddy–viscosity damping function. An anisotropic destruction coefficient is used to obtain a faster decaying behavior of turbulence destruction in the outer region of the boundary/shear layer, thereby precluding the free–stream dependency. The source term in the transport equation is independent of the Reynolds stress tensor. A comparative assessment of the improved RAS model with the Spalart–Allmaras (SA) one–equation model and the shear stress transport (SST) k–ω model is provided for well– documented non–equilibrium turbulent flows.