Laser Sheet Flow Visualization Developed for Lewis' Icing Research Tunnel

A 15-W argon-ion laser coupled with sheet-generating optics allows flow to be visualized in the IRT. Fiber-optic cables transport light from the remotely located laser into the tunnel test section. The laser rests on a vibration isolation table that floats on a layer of air. The intensity of the light is controlled by a local controller, and the laser is cooled by circulating water. An optical box made of wavelength-limiting tinted acrylic material is attached to the laser housing. This filtered Plexiglas allows observers to see the optics inside the box, but not the laser beam itself. The box has an interlocked hinged top so that only qualified operators can open it to align the laser.

The sheet-generating optics allow the thickness and the fanning of the sheet to be controlled very easily. The optics are mounted on remotely controlled traverses to aid in positioning the laser sheet in the test section.

Cameras can be placed on up to three sides of the tunnel test section to record flow-visualization images. The most common views are an overhead view and a side view of the model. Because both areas have exposed laser light, they are isolated from nonqualified personnel with interlocks. The windows to the IRT control room are covered with wavelength-limiting, tinted acrylic material to protect the tunnel operators and researchers from the laser light. High-speed, low-light digital cameras, low-light video cameras, 35-mm cameras, and intensified, digital still cameras are used to image the flow. For most tests, the laser power is limited to 3 to 4 W to prevent blooming in the imaging.

As just mentioned, the IRT's spray system generates the seed for the flow visualization. The most commonly used spray condition was selected to produce frozen ice particles in the 10- to 15-micrometer range. This spray condition is used because the resultant particles do not produce any significant ice accretion on the model.

The photograph and drawing depict the wing tip vortex and wake produced by a swept-wing model. The dark bands correspond to regions where the water droplets have been swept away by the vortex or wake. The bright regions correspond to areas where the water droplets have accumulated.

Find out more about Lewis' Icing Research Tunnel.