UC Davis

The unsteady laminar separation and subsequent dynamic stall vortex (DSV) formation is investigated on a NACA 0012 airfoil section subject to a constant pitch rate motion using Delayed Detached Eddy Simulations (DDES) in NASA's OVERFLOW 2.3 solver. This study focuses on the complex flow features during the initial DSV formation and analyzes the distinct mechanisms from which the vortex is formed. It is shown that DDES accurately predicts the bursting of a laminar separation bubble (LSB), which triggers the onset of a DSV. In parallel to studying the feasibilty of DDES in terms of capturing distinct flow features compared to Large Eddy Simulation (LES) results, a turbulence model study is also carried out, analyzing the influence of state-of-the-art turbulent and transition models on the DSV formation and subsequent stall onset. These include the fully turbulent Spalart Allmaras (SA) turbulence model and three different transition models: SA Coder Amplification Factor Transport (AFT), SA Medida-Baeder γ - Reθt, and the Shear Stress Transport (SST) Langtry-Menter γ - Reθt. It is found that the Coder SA AFT model provides the closest results with LES and the SST Langtry-Menter model predicts the earlier onset of the DSV. The fully turbulent model shows an abrupt development of a turbulent separation bubble and the under-prediction of the lift coefficient at lower angles of attack. At higher angles of attack, after the collapse of the separation bubble, all the models provide similar trends with each other and LES results.