Drag prediction in transitional flow over airfoils

When simulating the flow over airfoils at relatively low Reynolds numbers, transition from laminar to turbulent flow plays an important role in determining the flow features and in quantifying the airfoil performance such as lift and drag. Hence a proper modeling of transition, including both the onset and extent, will lead to a more accurate drag prediction. The current paper presents a transition model that combines existing methods for predicting the onset and extent of transition, which are compatible with the Spalart-Allmaras turbulence model (Spalart, P. R., and Allmaras, S. R., "One Equation Turbulence Model for Aerodynamic Flows," AIAA Paper 92-0439, Jan. 1992), in -which the flow is simulated using Fluent, a commercial computational fluid dynamics package. The transition onset prediction uses the Cebeci and Smith correlation (Cebeci, T., and Smith, A. M. O., "CS Method for Turbulent Boundary Layers," Analysis of Turbulent Boundary Layers, 1st ed., Academic Press, New York, 1974, pp. 329-384), which is based on the Michel method, for steady incompressible two-dimensional flow, whereas the extent of transition is quantified by developing a model for the intermittency function. The proposed transition model is implemented into Fluent; it is then used in simulating transitional flow in different well-documented experimental cases that cover single-element and two-element airfoils under different freestream conditions. Given the experimental data, the results obtained with the developed transition model reflect a consistent and significant improvement in drag prediction, when compared with the drag predicted using a fully turbulent flow simulation.