Experiments on a Separation Bubble Over an Eppler 387 Airfoil at Low Reynolds Numbers using Thin-Film Arrays

Preliminary experiments were carried out at the NASA Langley Low-Turbulence Pressure Tunnel on an Eppler 387 airfoil at several angles of attack for Reynolds numbers of 100,000 to 300,000 using surface pressure taps and surface-mounted thin-film arrays. Data are presented for a laminar separation bubble with turbulent reattachment primarily for the case of Reynolds number = 200,000 and angle of attack = 4°. Wavespeeds, wave direction and whether the flow was locally turbulent or laminar were documented and phenomena such as frequency doubling at stagnation regions and wave packets that led to breakdown are presented. There were some efforts to eliminate the bubble over several angles of attck and Reynolds numbers. The pressure and thin-film data along with the Drela-Giles airfoil design/analysis code and the oil-flow visualization results are compared with each other. All methods agree well in demarking the beginning and end of separation as well as showing where transition occurs within the bubble. Several data-reduction techniques were used to calculate wavespeeds and wave direction, and then compared. Spectra were used to check for unstable frequency bands and to document through what region in transition the flow was going.