Self-Excited Vibration of Propeller Nozzles Experienced on Ocean-Going Tugboats : 2nd Report

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In order to determine the cause of propeller nozzle vibration attaining 8,000 Gal experienced on two ocean-going tugboats constructed by IHI and ranking among the world's largest, the data acquired on the actual ships were supplemented by model tank tests for hydrodynamic force measurements, flow visualization, reproduction of self-excited vibration, and cavitation tests, using 2-dimensional profiles representing the propeller nozzle, and 3-dimensional model of propeller-nozzle assembly. The mechanism causing the inordinate vibration was identified as "galloping"- self-excited vibration due to negative damping- of the nozzle, from the evidences observed of : (1 ) Negative hydrodynamic damping of nozzle exerted hydrodynamically on the structure. ascribable to a negative slope developing on the curve relating nozzle lift to incident angle, and which is induced by flow separation over entire outside surface of nozzle. (2) Galloping phenomenon reproduced in model experiment (3) Karman's vortex street discarded : The vortex-induced vibration developed in test with 2-dimensional model, but not with 3-dimensional model, the latter due to mutual counteraction between vortexes generated at points situated diametrically opposite each other on the nozzle circle. (4) The other conceivable factors causing the inordinate vibration such as cavitation, twin propeller wake interaction and propeller blade stall flutter were eliminated by model experiment and analysis of actual ship data. Measures considered effective for preventing the occurrence of galloping are : (1) In design, to select and specify nozzle profile and operating parameters such as to prevent flow separation over entire nozzle surface (2) As remedial measure, to ensure a vibration mode of large structural damping (3) In service, to reduce ship speed, so as to lower the absolute value of negative hydrodynamic damping of nozzle.
社団法人日本船舶海洋工学会の論文

Self-Excited Vibration of Propeller Nozzles Experienced on Ocean-Going Tugboats : 2nd Report

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