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Humanities
- ISSN: 2155-7993
- Journal of Modern Education Review
The Determining Torsional Vibration Damping Coefficients Algorithm for Computing Marine Shafting’s Vibrations
Nguyen Manh Thuong
(Faculty of Marine Engineering, Vietnam Maritime University, Vietnam)
Abstract: The most difficulty in the computations of ship shafting torsional vibrations is to determine such torsional vibration damping coefficients as those in main engines and shafting structures, or those of hydraulic resistances on propellers. The difficulties are due to the complex nature of hydrodynamical processes, that it is difficult to come up with an analytical method to determine these quantities. Even direct measurements of such quantities are difficult to make in practice.
Currently in Vietnam, these data usually are given by the manufacturers that doesn’t make sure about the accuracy, or they are calculated based on the semi-empirical formulae those are old and may be able not appropriate for the modern types of engines and equipments. Therefore, the aim of the author is to determine these quantities from results of torsional vibration measurements during the new built ship tests. Basing on the analysis of results obtained for shafting’s types with different features (for example, characteristic types of engines, propellers, shaft materials ...) and combining with the theoretical analysis we may generalize calculations of the above quantities. It allows to obtain the new calculating formulae for these quantities more certain and convenient for basic design calculation of ship shafting.
The paper presents a theoretical basis and algorithms of determining the torsional vibration damping coefficients of the engine structure, shaft material and hydrodynamic damping coefficient to propellers from results of vibration measurements. By programming with the help of Symbolic Math Toolbox package in Matlab software we will establish an analytical equations for amplitudes of elastic torque M(kω) caused by torsional vibrations in an arbitrary shaft section (where perform measurements conveniently). This equation will contain m unknown variables supposed to be unknown damping coefficients for each given shafting with given sizes, mass parameters and the finite number of masses. Therefore, to identify these m unknown variables, we must have at least m values of torque amplitudes corresponding to m values of kω measured in a position of shafting to make a system of m equations. However, the system of equations is not linear so to facilitate the calculations we need to have more than m values of the measured amplitudes. The program will indicate how many values of the measured amplitudes should be obtained, and after having the measurement results, it will determine the unknown damping coefficients.
To illustrate, the article presents the example calculation for a ship of series B 170-V designed by Polish Architecture Institute and built in Ha Long shipyard. The data on the shafting and the results of vibration measurements obtained from the documents submitted by Ha Long shipyard and the Designer to GL registry for
approval. The obtained results of calculation will explain the reason for deviations between real vibrations and
computed vibrations or show the appropriateness of assumptions about the damping coefficients used in computations of torsional vibrations. Basing on this profile, we can be able to improve the vibration calculation
model.
Key words: torsional vibrations, damping coefficient, relative and absolute damping coefficients, elastic torque.
