Comment on "Residual Stresses in 2024-T81 Aluminum Using Caustics and Moire Interferometry"

Abstract

turbines, the value of C? is positive due to a significant component of lift that increases with a in the chordwise direction. For symmetrical airfoils, the values of Cc are negative in the poststall region III. range of incidence for which the values of Cc are negative in this region increases with the increase in the thickness ratio of the airfoil. In region IV the values of Cc are positive again. In this region, the chordwise coniponenets of aerodynamic forces are dominated by the suction around the leading edge. Wortmann FX63-137 airfoil data show much larger values of Cc, which are positive in regions II, III, and IV. Cc vs a curve for a symmetrical airfoil has a direct implication in the design of a Wells air turbine for self-starting characteristics.' When such as turbine starts from rest, there is sufficient torque to start the turbine rotating due to the fact that at a = 90 deg, Cc is positive. As the turbine speed increases, a decreases. In order to move into the Operational regime (region II), the turbine has to go through a region of negative torques (region III), which results in a turbine speed much lower than the operational speed. This phenomenon, known as crawling, can be eliminated by increasing turbine solidity, which effectively results in a* positive value of Cc in region III. References 1 JacobSi E. N. and Abbot* I. H., Section Data Obtained in the NACA Variable Density Tunnel, NACA TR 669, 1939. Jacobs, E. N. and Sherman, A., Airfoil Section Characteristics Affected by Variation of Reynolds Number, NACA Rept. 586, 1937. Blackwell, B. F. and Sheldahl, R. E., Characteristics of Fbur Symmetrical Aerofoil Sections Through 180 deg Angle of Attack at Reynolds Number, Sandia Labs., Albuquerque, NM, Rept. 87115, 1976. Beans, E. W. and Jakubowski, G. S., Method for Estimating the Aerodynamic Coefficients fo Wind Turbine Blades at High Angle of Attack, AIAA Journal, Vol. 7, June 1983, pp. 747-749. Muller, T. j. and Jansen, B. J., Measurements at Reynolds Numbers, AIAA Paper 82-0598, 1982. Muller, T. J., Low Reynolds Number Vehicles, AGARD-AG288, 1985, pp. 1-69. Raghunathan, S; and Tan, C. P., Performance of the Wells Turbine at Starting, Journal of Energy, Vol. 6, 1982, pp. 430-431. Raghunathan, S. and Tan, C. P., The Aerodynamics of Wells Turbine, Journal of Energy, Vol. 7, 1983, pp. 226-230.