Tip Vortex Trajectory Research Articles

Laser sheet technique for visualizing a periodic rotor wake

Introduction L sheets have been used to illuminate cross sections of steady flowfields for several years. Intense sheets of light only a few millimeters in thickness can be produced by either expanding a single laser beam with a cylindrical lens or by rapidly sweeping the beam by means of a vibrating mirror or other optical device.' Generally, smoke or other seed particles which scatter light as they pass through the laser sheet are introduced upstream of the region of interest. Streamlines and vortical structures are identified respectively as streaks and swirl patterns in the illuminated region.' Recent experiments conducted in the John J. Harper 2.31 m x 2.74 m (7 x 9 ft) low-speed wind tunnel at the Georgia Institute of Technology employed a strobed laser sheet to visualize the periodic flowfield between a model rotor and an airframe in forward flight. A nonintrusive method was developed to determine accurately tip vortex trajectories in a uniformly seeded flow. Unlike Schlieren, shadowgraphic, or interferometric methods, the laser sheet technique is well-suited for visualizing the incompressible (low Mach number) flows common to rotorcraft. Quantitative data valuable for use in the interpretation of vortex/airframe interaction measurements were gathered using this technique, as reported in Ref. 5. Used in conjunction with laser velocimeter measurements, the laser sheet flow visualization approach provides a powerful means for documenting vortex jitter or other flow instabilities that can affect measurements. The methods described are adaptable to many situations requiring visualization of a periodic flow structure.

Rotor Tip Vortex Geometry Measurements Using the Wide‐Field Shadowgraph Technique

Rotor tip vortex geometry data have been acquired using the wide-field shadowgraph technique. Shadowgraphs were taken of the wakes of two model main-rotor systems in hover. These shadowgraphs provided detailed tip vortex trajectories from which axial and radial vortex coordinates were measured. Using these coordinates, prescribed-wake parameters were determined and input into a hover-performance prediction code. The resultant predictions were compared with experimental performance data to help assess the capabilities of this code. Another result of the experimental work was the development of a method to predict the visibility of rotor tip vortices on shadowgraphs. Using this method, the ability of the shadowgraph technique to visualize tip vortices in the rotor wake was quantified.