Large-Eddy Simulation Analysis of Unsteady Separation Over a Pitching Airfoil at High Reynolds Number
Final rept. 1 Sep 2010 - 31 Aug 2013
CARNEGIE-MELLON UNIV PITTSBURGH PA OFFICE OF SPONSORED RESEARCH
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Unsteady flow separation during dynamic stall often leads to unacceptably large vibratory loads and acoustic noise, and limit forward flight speeds and maneuverability. To gain a quantitative understanding of the unsteady separation process, large-eddy simulation LES of turbulent flow over a pitching airfoil at realistic Reynolds and Mach numbers is performed. Numerical stability at high Reynolds number simulation is maintained using an unstructured-grid LES technology, which obeys higher-order conservation principles and employs a global-coefficient subgrid-scale turbulence model. A hybrid implicit-explicit time-integration scheme is employed to provide a highly efficient way to treat time-step size restriction in the separated flow region locally refined with dense mesh. The present simulations confirm the stability and effectiveness of the presented numerical schemes for dynamic stall simulations at realistic operating Reynolds and Mach numbers and show the characteristics of flow separation and reattachment processes which are qualitatively congruent with experimental observation. To improve quantitative understanding of unsteady separation processes of turbulent boundary layers, direct numerical simulations are performed. The distinct characteristics of unsteady separating turbulent boundary layers are revealed by a systematic comparison with steady attachedseparated turbulent boundary layers. For this purpose, four different flow configurations are simulated 1 turbulent boundary layer flow with a zero-pressure gradient 2 turbulent boundary layer flow with an adverse-pressure gradient 3 steady separated turbulent boundary layer flow and 4 unsteady separating turbulent boundary layer flow. The present comparative study suggests physical phenomena during the unsteady separation process including unsteady boundary-layer detachment and reattachment, and production and dissipation of turbulent kinetic energy and vorticity.
- Fluid Mechanics