Evaluation of Turbulence Models for External Flows

Abstract

An industry-motivated investigation of the application of turbulence models to external fluid flows has been carried out. The motivating application is the melt-blowing fiber production process used by 3M in which friction forces exerted by high-speed air jets stretch pliable polymeric fibers. The complex fluid flow is turbulent and unique. The closest traditional fluid flow is the wall jet. In order to achieve results of sufficient accuracy to be used in the design of the production equipment, a necessary prerequisite is the identification of an appropriate turbulence model. To this end, five turbulence models, commonly offered in commercially available CFD software (FLUENT® Software, ANSYS® CFX® Software, and Star-CD™ Software) were carefully implemented in appropriate numerical simulations of the available experimental data. The investigated turbulence representations included several two-equation models and a direct solution of the RANS equation. The experimental data consisted of measured velocity profiles for the plane wall jet and the cylindrical wall jet. A comparison of the velocity profiles extracted from the simulations with those of experiment enabled a clear conclusion to be drawn about the most suitable turbulence model, which was found to be the standard k − ε model with enhanced near-wall treatment. For all the simulations, the accuracy of the numerical simulations was carefully investigated with respect to three critical issues: number of control volumes, deployment of the control volumes, and the geometry and the extent of the solution domain. The deployment issue is of great significance in the neighborhood of a bounding wall and directly influences the near-wall treatment. The choice of the solution domain controls the boundary conditions and the surfaces at which they are applied. A general conclusion that follows from this study is that it is essential to verify the suitability of a turbulence model before applying it to real-world applications.