Compressibility Effects on Dynamic Stall of Airfoils Undergoing Rapid Transient Pitching Motion
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The research was carried out in the Compressible Dynamic Stall Facility, CDSF, at the Fluid Mechanics Laboratory (FML) of NASA Ames Research Center. The facility can produce realistic nondimensional pitch rates experienced by fighter aircraft, which on model scale could be as high as 3600 degrees/sec. Nonintrusive optical techniques were used for the measurements. The highlight of the effort was the development of a new real time interferometry method known as Point Diffraction Interferometry-PDI, for use in unsteady separated flows. This can yield instantaneous flow density information (and hence pressure distributions in isentropic flows) over the airfoil. A key finding is that the dynamic stall vortex forms just as the airfoil leading edge separation bubble opens-up. A major result is the observation and quantification of multiple shocks over the airfoil near the leading edge. A quantitative analysis of the PDI images shows that pitching airfoils produce larger suction peaks than steady airfoils at the same Mach number prior to stall. The peak suction level reached just before stall develops is the same at all unsteady rates and decreases with increase in Mach number. The suction is lost once the dynamic stall vortex or vertical structure begins to convect. Bases on the knowledge gained from this preliminary analysis of the data, efforts to control dynamic stall were initiated. The focus of this work was to arrive at a dynamically changing leading edge shape that produces only 'acceptable' airfoil pressure distributions over a large angle of attack range.
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Chandrasekhara, M.S.; Martin, P.B.; Tung, C. ;The control of compressible dynamic stall using a variable droop leading edge airfoil is described. The leading 25% of a VR-12 airfoil is drooped as it executes sinusoidal pitch oscillations such that the leading portion ...
Sahin, Mehmet; Sankar, Lakshmi N.; Chandrasekhara, M.S.; Tung, Chee (American Institute of Aeronautics and Astronautics Inc., 2000);Dynamic stall calculations were carried out for an airfoil with a deformed leading edge shape at a freestream Mach number of 0.3. The surface deformations were done about a baseline NACA 0012 airfoil, effectively ...
Sahin, M.; Sankar, N.L.; Chandrasekhara, M.S; Tung, C. ;Dynamic stall calculations were carried out for an airfoil with a dynamically deformed leading-edge (DDLE) shape at a freestream Mach number of 0.3. The surface deformations were done about a baseline NACA 0012 airfoil, ...