TS-91
Validation among Theoretical Calculations of the Propeller Harmonic Excitation and Experiments
Nicholas C. BUZBUCHl* and Valentin A. SOLOIU**
ABSTRACT
The paper is an attempt to the harmonic structure computation of the marine propeller torque and thrust, being predictive methods of propeller excitations induced into the marine engine shafting system in the design stage. Two quasi-steady methods using the lifting line theory, the first being semi-theoretical due to the wake field experimentally determined and based on the direct Prandtl theory, and the second one based on a developed Lerbs induction coefficients, an analytical method using only the propeller series diagrams and general empirical formulae for the wake and suction coefficients, have been presented.
The paper shows the simplicity and accuracy of the last method, as a primary information on the propeller excitations, and this procedure takes into account a non-dimensional expression of the circulation, as a two-dimension function of the blade radius and angle of rotation. A validation calculation with experiments of propeller excitations induced into the shafting system is analyzed, showing acceptable agreement between calculated and measured excitations.
Key words: Marine Engine, Shafting System, Propeller, Torque and Thrust, Computation, Experiments
1. INTRODUCTION
The permanent increasing of dimensions, speed and power per shaft of modem ships lead to a rush of propeller-induced vibrations knowledge. Nowadays the vibration performances of a new built ship have the same level of importance as speed performance and fuel consumption, since they may cause mechanical damage and may reduce comfort on board. The vibratory levels are mainly the result of the interaction between two parameters. i.e. excitations coming from one or several sources, and response of an elastic assembly to which the response of connected assembly can be added. This means that a search of the causes of troublesome ship vibrations must include not only the determination of the response characteristics, but also the study of the excitation sources due to the propeller, main engine and waves.
Among the excitation sources of vibration for the marine propeller shafting, the propeller itself plays a significant role. It is one of the excitation sources for the axial vibrations and a major contributor among the excitation sources for torsional vibrations. Thus, the possibility to predict the harmonic structure of the torque and thrust, in the design stage of the propeller system, has a great importance for the designer. In this direction, the paper presents two computation methods of calculation of the propeller torque and thrust, the first being a semi-analytic one, based upon the wake field diagrams experimentally determined, and the second based on the propeller series diagrams and general empirical formula for the wake and suction coefficients,
This second analytical method was presented more detailed, being a possible primary tool for the designer. A vortex model of the marine propeller, based on Prandtl lifting-line theory, has been developed. Considering the system of bounded and free vortices representing the propeller blade and its interaction with the incipient wake generated by hull motion, the velocity field around the blade profile is determined. Based upon a non-dimensional expression of the circulation, as a function of blade radius and angle of rotation, torque and thrust are computed and expressed as Fourier series.
Parallel to the theoretic studies, experimental investigations have been carried-out, both in cavitations tunnel and towing tank, showing an acceptable agreement between calculated and measured fluctuations of propeller excitations induced into engine shafting system.
1. THEORETICAL BACKGROUNDS
As mentioned above, the main cause of unsteady propeller forces when a ship is running in calm water is the wake non-uniformity.
Most theoretical methods, by considering the calm water running condition corresponding to the self-propulsion point, allow the time-average loads to be calculated with reasonable accuracy.
At present, several techniques using quasi-steady procedures, stripwise unsteady procedures and methods based on unsteady lifting surface theory have been developed for the estimation of unsteady loadings arising from the circumferential variation in the inflow velocity, All these procedures require the designer to be acquainted with the flow patterns at the propeller disc.
The hydrodynamic models of the marine propeller used in this paper are based upon the lifting line theory, considered in order to be able to compute the torque and thrust fluctuations.
