Shear Stress Measurements in a Laminar Separation Bubble on an SD7003 Airfoil at Low Reynolds Numbers

Sutton, D. M.* and Lavoie, P. University of Toronto Institute for Aerospace Studies Toronto, ON Interest in low Reynolds number airfoils has increased in recent years due to the growing number of applications for unmanned... [ view full abstract ]

Sutton, D. M.* and Lavoie, P.
University of Toronto Institute for Aerospace Studies
Toronto, ON
Interest in low Reynolds number airfoils has increased in recent years due to the growing number of applications for unmanned aerial vehicles (UAVs) in civilian and military sectors. One of the primary topics of research in this area is laminar boundary layer separation and the formation of a laminar separation bubble (LSB). LSBs have strong negative effects on the performance of airfoils at low Reynolds numbers (1). It is of interest to better understand the fluid mechanics governing the laminar separation bubble, particularly those governing the transition process, in order to improve low Reynolds number airfoil design methodology and develop LSB flow control strategies.
The experiments were performed in the low-speed, low-turbulence intensity wind tunnel at the University of Toronto Institute for Aerospace Studies (UTIAS). The airfoil section used was the SD7003, an airfoil commonly used in experimental studies of LSBs (2). The model was manufactured using stereolithography (SLA) and houses 39 built-in surface pressure taps. The model was mounted in the wind tunnel between two circular endplates to reduce three-dimensional effects (3).
Using oil film interferometry (OFI), a direct shear stress measurement technique, in conjunction with surface pressure measurements and wake surveys, the characteristics of the LSB and the aerodynamic performance of SD7003 (CL, CM, and CD) were documented over a range of chord Reynolds numbers (from Rec = 60 x 103 to Rec = 250 x 103) and angles of attack. Estimates of separation, transition, and reattachment points are made using both OFI and surface pressure measurements. The impact of increased freestream turbulence intensity on the LSB and overall airfoil performance was also investigated by means of a turbulence-generating grid installed at the entrance to the test section. A more turbulent freestream was shown to reduce the size of, and in some cases fully supress, the LSB.
Image processing to extract quantitative shear stress measurements from the interferograms, generated during OFI runs, has been completed following the method of Naughton et al. (4). The results agree qualitatively with recent computational results (5). In conjunction with the surface pressure and wake survey data, the shear stress measurements provide insight into the physics of the transition process that occurs in the separated shear layer of the LSB on an airfoil. The limitations of OFI in low Reynolds number work are also discussed.

Figure 1 – A typical interferogram showing light and dark fringe lines used to calculate surface shear stress. Flow reattachment is located at the point from which the oil spreads in both downstream and upstream directions.

Figure 2 – Coefficient of skin friction distribution measured on an SD7003 airfoil at Rec = 250 x 103 and α = 8o. A peak in negative shear stress is observed, marking the location of the transition process. This is followed by a positive peak in shear stress shortly downstream of reattachment.

WORKS CITED

1. Low-Reynolds-Number Airfoils. Lissaman, P. 1983, Annual Review of Fluid Mechanics, Vol. 15, pp. 223-239.
2. Comparison of laminar separation bubble measurements on a low reynolds number airfoil at three facilities. Ol, M.V., et al., et al. Toronto : AIAA, 35th AIAA Fluid Dynamics Conference and Exhibit. 2005.
3. Effects of end plates and blockage on low-Reynolds-number flows over airfoils. Boutilier, M. S. H. and Yarusevych, S. 2012, AIAA Journal, pp. 1547-1559.
4. Oil-film interferometry measurement of skin friction - analysis and summary of MATLAB program. Naughton, J. W., Robinson, J and Durgesh, V. 2003, IEEE, pp. 169-178.
5. Low-Reynolds-Number Aerodynamic Performance of the NACA 0012 and Selig-Donovan 7003 Airfoils. Counsil, J. N. N. and Boulama, K. G. 2013, Journal of Aircraft, pp. 204-216.