Hull Excitation Research Articles

Numerical Simulation and Experimental Study on Dynamic Characteristics of Gas Turbine Rotor System Subjected to Ship Hull Excitation

Numerical Simulation and Experimental Study on Dynamic Characteristics of Gas Turbine Rotor System Subjected to Ship Hull Excitation

Vibration analysis of super-yachts: Validation of the Holden Method and estimation of the structural damping

The hull excitation generated by marine propellers constitutes one of the most significant vibration sources affecting comfort on passenger ships. Consequently, the evaluation of the propeller-generated excitation through reliable numerical tools during the preliminary stage of ship design is fundamental. The Holden Method (HM) is an empirical tool utilized to calculate the propeller-induced pressure distribution on a ship hull. The present paper validates the HM, which is applied to a twin-screw, 54-m super-yacht. Finite Element Analysis (FEA) is used to benchmark the HM numerical predictions against a set of full-scale vibration measures. The outcomes show that the magnitude of the propeller-induced dynamic excitation predicted by the HM is overestimated. Thereafter, the calibrated propeller-induced forces and the diesel engine excitation are applied to the FE model to perform a series of forced vibration analyses and estimate the global structural damping coefficient of the super-yacht. The study highlights the necessity of developing new empirical methodologies, analogous to the HM, to be applied to modern small luxury vessels.

Destructive frequency of oblate spheroidal air-balloon for suppression of propeller cavitation induced hull excitation.

The air-balloon can effectively neutralize hull excitations induced by the propeller cavitation. For the design, it is essential to derive the destructive frequency of an oblate spheroidal air-bubble, which is elaborated on in this paper. Beginning with the exact modal-series solution proposed by Yeh [Ann. Phys. 468, 53-61 (1964)], an approximated form of the scattered pressure is set up by assuming that the acoustic wavelength is much larger than the size of the balloon in the low frequency ranges. An algebraic formula for the destructive frequency can then be written as a function of the resonance frequency and a spatial variable. It is well known that the resonance frequency of a deformed bubble is higher than that of an ideal spherical one with the same volume. In addition to this, the current investigation puts an emphasis on the fact that asphericity induces a more severe shift of the destructive frequency than the resonance frequency, and that its effect needs to be reflected in the balloon design.

풍선을 이용한 프로펠러 캐비테이션 유발 선체 가진력 저감

The purpose of this work is to give analytical evidence that air-balloon is able to replace the previous effort of air-injection for a mitigation of propeller cavity induced hull excitation. We exploit the acoustical nature of rubber-like material, which is similar to that of water; thus a rubber layer existing at water-to-air interface appears to be transparent in the propagation of acoustic wave. That is, the balloon could be anticipated to play only the role of air-packing without any deterrence of the desired acoustic phenomenon, i.e., the destructive interference. A simple design strategy that tunes the frequency of maximum destructive interference to an exciting frequency is also presented, which can be simply done by adjusting the size of balloon. Finally, the suggested scheme is verified by two experimental demonstrations.

Sea-trial verification of air-filled rubber membrane for mitigation of propeller cavitation induced hull excitation

At a certain frequency, an air-bubble excited by an acoustic wave produces a destructive interference, which plays a vital role in mitigating propeller cavitation induced pressure fluctuations on ship׳s hulls. By taking advantage of the acoustically transparent nature of rubber material, previous effort proposed employing an inflatable air-balloon so that the destructive interference could be implemented without any energy consumption. For its verification in a full-scale ship, this paper presents sea-trial measurements with the following major contribution. To inflate the balloon to the design size, and to keep the volume constant despite draught changes, a parametric relation is proposed by a semi-analytical method incorporated with the ideal gas law. This scheme is exploited in the full-scale measurements to attain a noticeable hull vibration reduction of 65% at the exciting frequency of interest. Finally, this simple device is expected to be an inexpensive alternative for resolving vibration issues that arise from propeller cavitation.

Possibility of air-filled rubber membrane for reducing hull exciting pressure induced by propeller cavitation

To mitigate hull excitation induced by the propeller cavity, our previous work proposed a single-nozzle air injection scheme based on the principle of acoustic destructive interference. Although inefficient energy consumption in a conventional air-carpet system could be reduced significantly, the proposed method was still hindered by the continuous usage of an air compressor and maintenance of the nozzle exposed to sea water.In this study, we take advantage of the acoustic properties of rubber-like materials, which are similar to that of water. Accordingly, a rubber layer existing at the water-to-air interface appears to be transparent in the propagation of acoustic waves. More specifically, a rubber membrane filled with air could be anticipated to act only the role of air-packing without influencing the desired acoustic phenomenon, i.e., destructive interference. Hence, the purpose of this work is to provide analytical evidence to prove that an air-filled rubber membrane is capable of replacing the previous effort of air-injection. A design strategy for tuning the frequency of maximum destructive interference to an exciting frequency is also presented, which can be accomplished by adjusting the rubber membrane size. Finally, two experimental demonstrations conducted in a water tunnel verified the suggested scheme.

Non-cavitating propeller noise modeling and inversion

Marine propeller is the dominant exciter of the hull surface above it causing high level of noise and vibration in the ship structure. Recent successful developments have led to non-cavitating propeller designs and thus present focus is the non-cavitating characteristics of propeller such as hydrodynamic noise and its induced hull excitation. In this paper, analytic source model of propeller non-cavitating noise, described by longitudinal quadrupoles and dipoles, is suggested based on the propeller hydrodynamics. To find the source unknown parameters, the multi-parameter inversion technique is adopted using the pressure data obtained from the model scale experiment and pressure field replicas calculated by boundary element method. The inversion results show that the proposed source model is appropriate in modeling non-cavitating propeller noise. The result of this study can be utilized in the prediction of propeller non-cavitating noise and hull excitation at various stages in design and analysis.

Propeller Excitation of Longitudinal Vibration Characteristics of Marine Propulsion Shafting System

The submarine experiences longitudinal vibration in the propulsion shafting system throughout most of run. A transfer matrix model of the propulsion shafting system, in which the dynamic characteristics of oil film within thrust bearing are considered, is established to describe the dynamic behavior. Using hydrodynamic lubrication theory and small perturbation method, the axial stiffness and damping of oil film are deduced in great detail, followed by numerical estimation of the foundation stiffness with finite element method. Based upon these values of dynamic parameters, the Campbell diagram describing natural frequencies in terms of shafting rotating speeds is available, and the effect on the 1st natural frequency of considerable variations in thrust bearing stiffness is next investigated. The results indicate that the amplitude of variation of the 1st natural frequency in range of low rotating speeds is great. To reduce off-resonance response without drastic changes in propulsion shafting system architecture, the measure of moving thrust bearing backward is examined. The longitudinal vibration transmission through propulsion shafting system results in subsequent axial excitation of hull; the thrust load acting on hull is particularly concerned. It is observed that the measures of structural modification are of little benefit to minimize thrust load transmitted to hull.

Passive and Active Control of the Radiated Sound Power from a Submarine Excited by Propeller Forces

To improve the stealth of a submarine, the sound power radiated from the hull can be reduced using active noise and vibration control. In this article, active control is applied to minimize hull excitation by propeller forces transmitted through the shaft. The dominant propeller axial force fluctuations are directly related to blade passage and the phase of the force is determined by the absolute angle of the propeller shaft. Because the propeller forces are deterministic, the application of feedforward control is appropriate. Numerical finite element/boundary element (FE/BE) models of a submarine have been developed to find the transfer functions. Different control strategies are investigated in which active control is applied to the propeller/shafting system and/or to the submarine hull. Active vibration control and discrete structural acoustic sensing based on the far field radiated sound power were considered for development of the cost function. In addition, the performance of a system in which active control is combined with passive control using a resonance changer is investigated. It is shown that the combination of tuned control actuators and a resonance changer results in a significant reduction of radiated sound power.

Influence of a resonance changer on the sound radiation of a submarine

In order to reduce the excitation of the submarine hull through the shaft, a vibration attenuation system, called a resonance changer, can be implemented in the propeller/propulsion system. The effectiveness of such a system in reducing the low frequency sound radiation characteristics of a submarine is investigated. Only sound radiation due to fluctuating propeller forces, which are generated by the operation of the propeller in a nonuniform wake, is considered. These fluctuating forces are transmitted to the submarine hull through the fluid, as well as through the propeller shaft. Both types of excitation cause hull vibration and sound radiation. The accordion modes of the pressure hull, are particularly efficient sound radiators. Parameters for the resonance changer system have been optimised previously by considering only excitation of the hull through the shaft. It is shown that the effectiveness of the resonance changer at different frequencies is modified significantly in a typical full-scale implementation, due to the sound radiation from the propeller. The effect on performance is increased further when the vibration of the propeller itself is taken into account. Therefore overall optimisation of any resonance changer system requires a comprehensive model. Some of the principal effects are explored in this paper.

Hull vibratory forces transmitted via the fluid and the shaft from a submarine propeller

Disturbing forces due to a rotating propeller are transmitted to a ship or submarine hull via both the propeller shaft and the pressure field that is created in the surrounding water. This pressure field has hydrodynamic and acoustic components, because water is a compressible medium with a finite speed of sound. The work described in this article is a pilot study to investigate the combination of forces generated by the propeller motion that are transmitted to a submerged body. Fluctuating propeller forces are transmitted along the shafting system to result in structural excitation of the submarine hull. These forces are also transmitted to the surrounding fluid and result in an external hull pressure field, because they correspond to acoustic dipoles. Both types of hull excitation contribute to hull vibration and underwater radiated noise. To demonstrate the fundamental nature of the problem, the case of a propeller located on the principal axis of a finite cylinder is considered, which represents a typical submarine arrangement. The analysis in this work has been simplified by considering only stationary fluctuating forces at the propeller hub and by regarding the propeller, hull and shaft as rigid structures. The relative magnitudes and phases of forces transmitted by the fluid and propeller shaft change with frequency in the range containing both low multiples of propeller blade passing frequency and broadband random components owing to turbulent flow in the submarine wake. The effects of a finite speed of sound are found to be significant at low frequencies, causing changes in the hull forces that would be computed by assuming that water is incompressible. Results are presented describing the properties of acoustic dipole sources, which have both hydrodynamic and acoustic pressure fields that change in relative importance with distance from the source.

Minimization of the vibration transmission through the propeller-shafting system in a submarine

Ships and submarines are efficient sources of underwater radiated noise. In the low-frequency range, the main sources of ship vibration and noise are the working machinery such as the diesel engines, gearboxes and generators, and the propeller/propulsion system. Rotation of the propeller results in discrete tonals at the blade passing frequency and its harmonics. In addition, in the case of a submarine, hydrodynamic forces acting on the propeller due to the fluctuating pressure field are transmitted through the propeller-shaft and thrust bearings, resulting in axial excitation of the submarine hull. A hydraulic dynamic absorber, also known as a resonant changer, is used to minimize the vibration transmission through the propeller-shafting system, in order to prevent excitation of the hull axial resonances. The present work is concerned with minimizing the vibration transmission to the hull by optimizing the virtual spring, mass, and damper parameters associated with the resonant changer in a submarine. The dynamic response of the propeller-shafting system, including the propeller shaft, thrust bearing, thrust block foundation, and resonant changer, are characterized using four pole parameters.

Modeling of Propeller Cavitation on Merchant Vessels

The time-varying cavitation on a marine propeller, which excites hull vibrations, is calculated. The analytical model is exercised to guide propeller design and operation in order to minimize this type of hull excitation. This model, as well as a more elaborate model under development at Massachusetts Institute of Technology, is compared with recent full-scale observations of propeller cavitation. The paper illustrates the importance of both accurate input data on scaled vessel wake velocities and the inclusion, in analytical models, of tip vortex cavitation.

Considerations on the hull excitation force induced by a cavitating propeller

A model to describe the velocity potential of a cavitating propeller is considered. Strip-wise quasi-steady application of free streamline theory leads to a source distribution on the blades, accounting for the time-dependent behaviour of the cavity

Recent developments in hull and shaft vibration analysis1

New problems with regard to hull and shaft vibration analy- sis are discussed in general. Recent developments in ship design and power requirements have created new priorities in the technology of vibrational analysis. Increased size of new ships and increased horsepower requirements, combined with the after location of machinery spaces, demands a change of thinking with regard to shaft, propeller and hull dynamics. Previous analyses determined the shafting as compared to the hull design, but now the hull is being designed with regard to the shaft. Transverse and axial vibrations of the shaft and bearings are mentioned in general, with rough calculations made on the natural vibration frequencies. Hull frequencies are also discussed and mention is made of the effect of shear lag. Slender beam concepts and hull excitation at maneuvering and service speed are covered in general.