Spanwise variations in profile drag for airfoils at low Reynolds numbers

In a nominally two-dimensional flow, extensive wake surveys were performed on two airfoils at low Reynolds numbers to quantify the profile drag variation along the airfoil model span. Wake profile measurements were made at 57 spanwise stations spaced 2% chord apart and 1.25 chord lengths downstream of the trailing edge. Results at a Reynolds number (Re) of 2 X 10' show that variations on the order of 5-40% are typical. In an extreme case, however, over a spanwise distance of less than 12% chord, the profile drag coefficient changed from approximately 0.006 to 0.016, which illustrates the sometimes rather dramatic three-dimensional nature of the flow. Measurements taken at higher Reynolds numbers and closer to the trailing edge showed significant reductions in the spanwise drag variation, which suggests that the laminar separation bubble and the developing wake play an important role. A zigzag boundary-layer trip and an isolated roughness element were also investigated to examine their effects as compared with their respective undisturbed cases. Finally, the results suggest that the lack of good agreement in profile drag measurements between different wind-tunnel facilities can partly be traced to the associated measurement techniques used as weD as the flowfield three dimensionality. Differences in agreement between facilities can be expected when only one spanwise wake profile is taken per angle of attack, as has often been done in the past.