The two-dimensional, time-resolved structure of laminar separation bubbles over an SD7003 airfoil under the influence of elevated background turbulence levels is presented using smoke visualization. The separation bubbles are visualized for angles of attack \(\alpha = 4^{\circ }\) and \(8^{\circ }\) , Reynolds numbers \(Re = 60,000\) and 100,000, and turbulence intensities \(T_u = 0.02\%\) and \(0.24\%\) . High-speed images captured the formation of shear layer roll-up vortices in the aft portion of the separation bubble for \(T_u = 0.02\%\) , which are compared with the \(T_u=0.24\%\) case. On increasing the background turbulence ( \(T_u = 0.24\%\) ), the bubble is eliminated for \(\alpha = 4^{\circ }\) , while its height and chordwise extent are considerably decreased for \(\alpha = 8^{\circ }\) . The images show that the large-scale turbulent structures in the freestream interact with the shear layer, causing a greater variance in the size and shedding period of the roll-up vortices. Increased dissipation rates cause greater diffusion and distort the roll-up vortices. Increasing the Reynolds number for \(T_u=0.24\%\) does not affect the size and shedding wavelength of the roll-up vortices.
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