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This study examines the effect of various forms of vortex generators on the laminar separation bubble of a two-dimensional low Reynolds number Liebeck LA2573A airfoil. The objective of this research was to determine the effects that different generator sizes and spacings have upon the separation bubble and the drag. Windtunnel measurements were made on several generator configurations at Reynolds numbers ranging from 200,000 to 600,000 at angles of attack less than the stall angle where the separation bubble can provide a significant contribution to the airfoil drag. The vortex generators used were constructed small enough to be contained completely within the laminar boundary layer. Wind-tunnel data included airfoil drag and mean and fluctuating velocity measurements in the laminar and turbulent boundary layers. Results have shown that the use of vortex generators provides a measurable decrease in airfoil drag at the lower range of Reynolds numbers tested. At the airfoil's design condition and Reynolds number of 235,000, the submerged vortex generators were shown to decrease the airfoil drag by a maximum of 38% at C/ = 0.572. NomenclatureC d = airfoil drag coefficient per unit span Cf = airfoil lift coefficient per unit span c -airfoil chord H = maximum height of a vortex generator L = streamwise length of a vortex generator Re c = Reynolds number based on the airfoil chord edge = boundary-layer edge velocity U x = freestream velocity W = spanwise width of a vortex generator x = chordwise position downstream of the airfoil leading edgê 30% -separation bubble height parameter a = angle of attack Az= center-to-center spanwise generator spacing Az cr = critical center-to-center generator spanwise spacing 8* = displacement thickness 6 = momentum thickness Introduction S EVERAL current airfoil applications operate in the low Reynolds number regime, including remotely piloted vehicles, high altitude aircraft, compressor blades, and wind turbines. Typically, many of these airfoils operate at a chord Reynolds number of Re c < 10 6 and experience a laminar separation bubble for angles of attack less than the stall angle. The separation bubble is formed slightly downstream of the beginning of the adverse pressure gradient where the laminar boundary layer separates and forms an unstable shear layer which rapidly transitions to a turbulent shear layer. The strong turbulent mixing downstream of the transition promotes the reattachment of the turbulent shear layer. The flow then continues as an attached turbulent boundary layer to the trailing edge.

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Research has been performed to determine the u| accuracy of neutrally buoyant and near-neutrally-buoyant v s bubbles used as flow tracers in an incompressible potential v~ flowfield. Experimental and computational results are presented xp to evaluate the quantitative accuracy of neutrally buoyant yb bubbles using a commercially available helium bubble ys generation system. A two-dimensional experiment was y conducted to determine actual bubble trajectories in the F stagnation region of a NACA oo12 airfoil at o ~ angle of attack. Pbfs A computational scheme evaluating the equation of motion for P a single bubble was also used to determine the factors which a affect a bubble's trajectory.

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