Water tunnel measurements of unsteady separation

The results of experiments in which a controlled region of flow separation was generated downstream of an oscillating fence-type spoiler located on one surface of an airfoil are discussed. The experiments were conducted in a low speed water tunnel under incompressible flow conditions over wide range dimensionless frequencies and Reynolds numbers. High speed motion pictures and still photographs of hydrogen bubbles formed at surface mounted electrodes were used to obtain data regarding the shape and growth characteristics of the separation zone. Under certain conditions an energetic vortex-like structure was observed to form behind the spoiler and evolve in a manner determined by the Reynolds number and dimensionless frequency parameters. Observations of flow development reveal that the global structure of the separated region during the formation process is qualitatively similar to the one encountered in classical dynamic stall.