Schlieren Studies of Compressibility Effects on Dynamic Stall of Airfoils in Transient Pitching Motion

Compressibility effects on the flowfield of an airfoil executing rapid transient pitching motion from 0 - 60 degrees over a wide range of Mach numbers and pitching rates were studied using a stroboscopic schlieren flow visualization technique. The studies have led to the first direct experiments] documentation of multiple shocks on the airfoil upper surface flow for certain conditions. Also, at low Mach numbers, additional coherent vortical structures were found to be present along with the dynamic stall vortex, whereas at higher Mach numbers, the flow was dominated by a single vortex. The delineating Mach number for significant compressibility effects was 0.3 and the dynamic stall process was accelerated by increasing the Mach number above that value. Increasing the pitch rate monotonically delayed stall to angles of attack as large as 27 degrees.AFOSR-MIPR-87-0029 and 88-0010NAVAIRAR

Aero-Optical and Hot-Wire Measurements of the Flow Around the Hemispherical Turret with a Flat Window

Extensive investigation of the flow over the semispherical turret with the flat window was performed in order to document optical distortions over the window using 2-dimensional wavefront sensor and the Malley probe, complemented with simultaneous Malley probe- single hot-wire measurements of streamwise component velocity’s profiles normal to the window at several points across the window’s aperture for different azimuthal angles and a range of Mach numbers. The results provide the levels of unsteady optical aberration across the window’s aperture, as well as the local thickness, intensity and a convective speed of the separated flow over the window. Results reveal that the optical distortions grow approximately as a square of the incoming Mach number multiplied by a freestream density, OPDrms ~ ρM2. I. Motivation. When an otherwise-collimated laser beam passes through a variable-index-of-refraction turbulent flow its wavefront becomes dynamically (unsteady) aberrated. These aberrations degrade the beam’s ability to be focused in the far field, thereby reducing the system utility of the beam that may be used for communication, interrogation and targeting or as a directed-energy weapon. When the laser platform is an aircraft, the two main causes of beam degradation are the thin-layer and immediate air flow around th