Investigation of dynamic stall onset

In a 2003 test campaign in the DLR-VAG test stand unsteady pressure distributions during dynamic stall have been measured on a 2D-wing model with OA312 airfoil section. To investigate the effect of transition on dynamic stall onset different tripping devices have been fixed at the leading edge of the model. Adhesive tapes of different thickness as well as corundum with various grains have been used as tripping material. In addition to the experiments numerical calculations have been carried out based on the solution of the unsteady 2D-RANS-equations including a turbulence and transition model. In the present investigation it is shown that different physical flow phenomena can be identified which are triggering the onset procedure of dynamic stall. Three different flow-scenarios have been studied in detail: (1) Free transition during dynamic stall, (M=0.2). (2) Fully turbulent flow, (M=0.2). (3) Flow at M=0.4, compressibility effects. Unsteady measured and calculated pressures have been compared for these cases and good agreement has been found. The calculations then are used for detailed flow investigations including the development of leading edge separation bubbles as well as trailing edge separation areas.