Numerical Investigation of Low-Reynolds Number Airfoil Flows Using Transition-Sensitive and Fully Turbulent RANS Models

Abstract

A numerical analysis is performed to study the pre-stall and post-stall aerodynamic characteristics over a group of six airfoils using commercially available transition-sensitive and fully turbulent eddy-viscosity models. The study is focused on a range of Reynolds numbers from 6 × 104 to 2 × 106, wherein the flow around the airfoil is characterized by complex phenomena such as boundary layer transition, flow separation and reattachment, and formation of laminar separation bubbles on either the suction, pressure or both surfaces of airfoil. The predictive capability of the transition-sensitive k-kL-ω model versus the fully turbulent SST k-ω model is investigated for all airfoils. The transition-sensitive k-kL-ω model used in this study is capable of predicting both attached and separated turbulent flows over the surface of an airfoil without the need for an external linear stability solver to predict transition. The comparison between experimental data and results obtained from the numerical simulations is presented, which shows that the boundary layer transition and laminar separation bubbles that appear on the suction and pressure surfaces of the airfoil can be captured accurately by the use of a transition-sensitive model. The fully turbulent SST k-ω model predicts a turbulent boundary layer on both surfaces of the airfoil for all angles of attack and fails to predict boundary layer transition or separation bubbles. Discrepancies are observed in the predictions of airfoil stall by both the models. Reasons for the discrepancies between computational and experimental results, and also possible improvements in eddy-viscosity models, are discussed.

Suction effects on transitional bubbles

“…For example, the peak was not observed in previous computational 33,34 or experimental studies, 26,[35][36][37] whereas it was detected in 2D DNS studies 23,38 and in an experimental study, 21 but with a much smaller magnitude in comparison to the suction peak in the current investigation and in the 3D DNS results of 27 . Previous simulations of low-Reynolds-number flows on airfoils using RANS with the k-k L -ω transition model showed the formation of large-scale unsteady flow structures in the separated shear layer at the post-stall angles of attack 16 whose breakdown and transition and the subsequent near-field turbulent flow are generally not well-resolved with RANS simulations. 39 Moreover, RANS with the k-k L -ω transition model was found to produce more pronounced reverse flow and turbulence intensity in the outer layer.…”

“…15 As mentioned in the Introduction, this transition model predicts the separation bubble and the performance of airfoils in low-Reynolds-number flows reasonably well. [16][17][18][19] Time step used during the transient calculations is 0.00025 s. Program is allowed to complete 30 maximum iterations per time step. Time-dependent calculations are allowed to run 4000 iterations (1 s).…”

“…15 This k-k L -ω model was shown to reasonably predict the separation-induced transition location, and the aerodynamic forces, surface pressure distribution, and skinfriction coefficient on flat plates with imposed pressure gradient and airfoils in the pre-stall range of angles of attack. [16][17][18][19] Comparing the performance of airfoils at low Reynolds numbers using RANS with the k-ω SST turbulence model, and k-k L -ω and k-ω SST transition models, showed that the k-k L -ω transition model showed better agreement with the experimental results. The model also performed better in predicting the locations of separation, transition, and reattachment of a laminar separation bubble.…”