Bluff bodies immersed in a fluid stream are susceptible to flow-induced vibrations. Depending on the body dynamic characteristics and flow conditions, different types of flow-induced vibrations may occur. The failure of a blade in a large mixing vessel in a chemical plant raised the question of the response of a parabolic cross-section bluff body to the flow excitation. Experiments were conducted in a wind tunnel using two- dimensional “sectional” models. Models with parabolic, semi-elliptic and semi-circular cross-section were investigated. In the dynamic experiments, flow velocity was increased from 0 to 22 m\s, and the oscillating amplitude and wake response were monitored. Vortex-induced vibrations were observed with Strouhal numbers for parabolic and semi-circular cross-sections of 0.13 and 0.12, respectively. Steady lift force and fluid moment for different angles of attack were monitored in the static experiments. From these results, lift and moment coefficients were calculated. For the closed semi-circular cross-section, Reynolds number had a strong influence on the lift coefficient. With an increase in Reynolds number, the lift coefficient decreased. The largest difference was noted at an angle of attack a = 45°. In contrast, the open semi-circular model lift coefficient was independent of Reynolds number. In the experiments where the elastic axis of the model coincided with the model centre of gravity, galloping was not observed in the plunge mode. When the model elastic axis was moved to a position 90 mm behind the test model centre of gravity, galloping was observed for the semi-elliptic and parabolic models. The onset of galloping coincided with the vortex-induced resonance. Changing the model elastic axis position introduced a combination of plunge and torsional motion, and latter is believed to be responsible for the existence of galloping. The parabolic model was modified in an attempt to eliminate galloping instability. Fins were added at the separation points to widen the wake and prevent the reattachment of the flow to the afterbody. With these changes, galloping was not observed, although the oscillation amplitudes remained unacceptably high.
The present investigation revealed previously unknown characteristics of semi-elliptical and parabolic cross-section bluff body behaviour in fluid flow. At the same time, it laid a foundation for the solution to the practical problem encountered when a parabolic cross-section bluff body was used as a mixing blade. / Thesis / Master of Engineering (ME)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/24650 |
| Date | January 2001 |
| Creators | Veljkovic, Ivan |
| Contributors | Weaver, D. S., Mechanical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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