STUDY OF DYNAMIC STALL USING REAL-TIME INTERFEROMETRY

CONTROL and utilization, or alleviation, of the dynamicstall-induced aerodynamic loads which appear on helicopter rotor blades, and on rapidly moving conventional aircraft wings or control surfaces, will require a much greater understanding of the character of the unsteady flowfield that occurs on these aerodynamic surfaces than is currently available. The complexity and rapidity of the flow development during dynamic stall as well as the large pressure gradients that form near the leading edge make quantitative measurement very challenging and difficult; the need for a clearer understanding of the effect of compressibility on the dynamic stall process further complicates this difficult task. However, traditional experimental techniques for analyzing aerodynamic flows are limited at compressible flow speeds; experimental data at these conditions on dynamically stalling airfoils has usually been restricted to surface measurements of pressure and skin friction. Interferograms showing the flow away from the surface of rapidly pitching airfoils have been obtained using holographic techniques.1 However, the post-processing associated with the creation of the requisite interferograms has usually limited the utility of holography to single interferograms of selected test conditions, with the interferograms created after the test has been completed. Realtime techniques such as Mach-Zehnder interferometry systems2 require expensive optics, and need massive structure to reduce sensitivity to vibration. A new technique, based on the use of a vibration-insensitive real-time interferometry technique known as point diffraction interferometry (PDI), avoids these difficulties, while producing real-time interferograms, thus permitting on-line, detailed analysis and documentation of the dynamic stall process. © 1994 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.