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IIT/FDRC High-Speed Jet Facility Screech Screech is the term given to discrete acoustic tones produced by supersonic jets operated at off-design conditions. The phenomenon is a result of a resonant feedback condition involving downstream traveling shear-layer disturbances and upstream traveling acoustic waves. Disturbances originate at the nozzle exit which convect through the shock cell system of the supersonic plume. When these disturbances reach a sufficient size they interact with the shock cells to produce acoustic waves which propagate upstream. Shear-layer disturbances are again generated at the nozzle lip through receptivity thus closing the feedback loop. This process selects and amplifies a single disturbance mode resulting in a fundamental tone and harmonics.
Anechoic chamber The anechoic chamber is 176 inches in length from inlet to exit and has a square cross-section of 100 inches. The inner chamber surfaces and the laboratory walls outside the open ends of the chamber are lined with sound absorbing acoustic foam. The chamber has a total of 6 removable sections, two of which have optical inserts to allow access for the shadowgraph/schlieren flow visualization system. High-speed shadowgraph/schlieren The high-speed shadowgraph/schlieren system uses an Nd-Yag Q-switched laser as a light source to obtain exposure times of 5 to 7 nanoseconds. The laser beam is expanded using a spatial filter which also removes noise and generates a clean Gaussian beam distribution. The beam is then collimated using a plano-convex lens and passes through the jet plume. A second plano-convex lens focuses the beam past a knife edge in the case of the schlieren technique and directly onto a 4 x 5 inch photographic plate.
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