Abstract

When analyzing the flow over airfoils at relatively low Reynolds numbers, transition from laminar to turbulent flow plays an important role in shaping the flow features and in quantifying the airfoil performance such as lift and drag. In most cases, laminar separation bubbles and transition zones that extend over a relatively long portion of the airfoils surface are present. Hence the proper modeling of transition, including both the onset and extent of the transition region, will lead to a more accurate drag prediction. The onset of transition is related to the disturbances that propagate in the laminar flow region. Analyzing such instabilities is carried out either by a full study of the Tollmien-Schlichting waves or by the use of empirical correlations most of which are based on results for two-dimensional incompressible flows. As for the transition extent modeling, different intermittency functions of the linear, algebraic or differential type have been developed. The current work presents a transition model that combines existing methods for predicting the onset and extent of the transition region. The transition onset is predicted using Cebeci and Smith's correlation which is based on Michel's method for incompressible two-dimensional flow while the extent of transition is quantified by developing a linear model for the intermittency function. The proposed transition model is implemented into the Spalart-Allmaras turbulence model available in the commercial software, Fluent, using user defined functions (UDF). It is then used in simulating transitional flow in different well documented experimental cases including single-element and two-element airfoils under different free-stream 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.