Development of a high-speed rotating bar mechanism
White, David Allen
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A high-speed rotary device was designed to generate shock waves in a transonic blowdown wind tunnel cascade. The rotary device (Rotating Bar Mechanism) will be used in research conducted at Virginia Tech to study the effects of unsteady aerodynamic flow and heat transfer (resulting from upstream shock wave / wake passing) on turbine blades. The rotating bar mechanism (RBM) consists of: a rotor with flexible cable “bars” attached to the rim of the disk, a disk housing, a bearing housing, a driving air turbine, and a turbine mounting housing. The RBM is mounted to the side of the wind tunnel so that only the bars enter and exit the tunnel test section through a small slot. As the bars translate through the test section, the bars create shocks / wakes similar to those shed from the trailing edges of nozzle guide vanes of a transonic turbine. Considerable design effort was required for the RBM due to its relatively high operating speed. As the result of a finite element stress analysis, a unique method of securing the disk to the shaft was developed. This unique method reduced the stress concentration factor at the disk hub from 2.9 to 1.7. In addition to the stress analysis, a rotordynamic study was also performed. The study revealed that the RBM could not be designed to operate below the first natural frequency. A critical speed of 14,000 RPM was predicted for the rotary device. This prediction was later verified by testing. An integral component of the overall design was the development of a computer code to predict the RBM’s speed under various loading conditions. The loading on the device is due primarily to the aerodynamic drag on the flexible cable bars. Since the mechanism was designed to facilitate bars of different diameters, the prediction code was an essential design tool. The speed prediction code was also verified by testing. The RBM was tested to wind tunnel operating speeds in a spin pit filled with argon to verify the mechanical design. Based on test performance, it was concluded that the RBM is suitable to generate shock waves in a transonic wind tunnel.
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