Turbulence modelling of unsteady separated flow over an airfoil

Turbulent flow simulations are carried out for a NACA 0012 airfoil in steady and unsteady cases. The objective is to assess the performance of the research code SPARC such that it may be used for more complicated fluid structure interaction and airfoil optimization problems. Steady simulations are performed from a zero angle of attack up to stall at a chord Reynolds number of 3 x 106 using the Spalart Allmaras one equation turbulence model and the Speziale k - tau two equation turbulence model. An extensive grid study shows that lift and drag predictions are very sensitive to the near wall resolution; wall functions are not used. Lift, pressure drag and friction drag results are found to be in good agreement with experimental data up until the stall. Steady state simulations did not converge in the stalled regime. Unsteady simulations are carried out at an angle of attack of 20 degrees and a chord Reynolds number of 105; the Reynolds number is lowered to keep the computational requirements reasonable. Though conventional aircraft operate at Reynolds numbers over 106 , there is much interest in low Reynolds number (105) airfoil data for gliders and unmanned aircraft operating at very high altitudes. The Spalart Allmaras and Speziale models are used again, along with the adaptive k - tau model of Magagnato. While the Spalart Allmaras model did not yield any unsteady phenomena, the Speziale model yielded a periodic lift and drag signal. The adaptive model yielded a more complex and irregular signal but also gave the most realistic results.