111 research outputs found
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
- …