Hull Pressure Research Articles

RANS simulation of cavitation and hull pressure fluctuation for marine propeller operating behind-hull condition

Simulations of cavitation flow and hull pressure fluctuation for a marine propeller operating behind a hull using the unsteady Reynolds-Averaged Navier-Stokes equations (RANS) are presented. A full hull body submerged under the free surface is modeled in the computational domain to simulate directly the wake field of the ship at the propeller plane. Simulations are performed in design and ballast draught conditions to study the effect of cavitation number. And two propellers with slightly different geometry are simulated to validate the detectability of the numerical simulation. All simulations are performed using a commercial CFD software FLUENT. Cavitation patterns of the simulations show good agreement with the experimental results carried out in Samsung CAvitation Tunnel (SCAT). The simulation results for the hull pressure fluctuation induced by a propeller are also compared with the experimental results showing good agreement in the tendency and amplitude, especially, for the first blade frequency.

Numerical and experimental investigation of conventional and un-conventional preswirl duct for VLCC

This paper shows the study of preswirl duct as an effective energy saving devices that have been devised and reviewed to support the propeller performance, especially for the ship of VLCC with large block coefficients. From the bare hull wake measurements, typical upper/lower asymmetry of hull wake at the propeller disk was found. The 2 kinds of pre-swirl duct, Unconventional half circular duct and Conventional circular pre-swirl duct have been designed and reviewed to recover the loss of propeller running in that condition. The general function of the pre-swirl duct was set to work against this asymmetry of wake and generate pre-swirled flow into the propeller against the propeller ro-tating direction. The optimum self propulsion tests with various angle configurations were carried out and the best configuration was decided. Accordingly, cavitation test was carried out with best configuration of unconventional half circular duct. The blade surface and tip vortex cavitation behaved smoother when the duct was mounted. The hull pressure amplitudes reflected this difference, so the hull pressure amplitude with duct was smaller than that of without duct.

An experimental investigation into cavitation behaviour and pressure characteristics of alternative blade sections for propellers

During the final quarter of the last century considerable efforts have been spent to reduce the hull pressure fluctuations caused by unsteady propeller cavitation. This has resulted in further changes in propeller design characteristics including increased skew, tip unloading and introduction of “New Blade Sections” (NBS) designed on the basis of the so-called Eppler code. An experimental study was carried out to investigate flow characteristics of alternative two-dimensional (2-D) blade sections of rectangular planform, one of which was the New Blade Section (NBS) developed in Newcastle University and other was based on the well-known National Advisory Committee for Aeronautics (NACA) section. The experiments comprised the cavitation observations and the measurements of the local velocity distribution around the blade sections by using a 2-D Laser Doppler Anemometry (LDA) system. Analysis of the cavitation tests demonstrated that the two blade sections presented very similar bucket shapes with virtually no width at the bottom but relatively favourable buckets arms at the suction and pressure sides for the NACA section. Similarly, pressure analysis of the sections displayed a slightly larger value for the NBS pressure peak. The comparative overall pressure distributions around the sections suggested that the NBS might be more susceptible to cavitation than the NACA section. This can be closely related to the fundamental shape of the NBS with very fine leading edge. Therefore a further investigation into the modification of the leading edge should be considered to improve the cavitation behaviour of the NBS.

Development of a time domain prediction method for hull pressure fluctuation induced by marine propeller sheet cavitation with consideration of scattering effects

Using a time domain acoustic analogy, we develop a method to predict the sound field and hull pressure fluctuation generated by unsteady sheet cavitation on marine propellers. Formulation 1A of Farassat is introduced to enhance theoretical understanding of this work, and it is applied to modeling the scattered sound field created by the fuselage boundary. To express the direct sound field resulting from sheet cavitation, a new solution is studied which considers the Doppler effect and also separately expresses the near and far fields. A small cube model is used to verify the method. Computed acoustic field pressures around the cube are compared with the boundary element method, and the numerical results show good agreement. Finally, the pressure fluctuation on a ship stern model is calculated.

Study of Microminiature Autonomous Underwater Vehicle Hull Form Based on CFD Technique

Based on CFD technique，this paper discusses the applicability of different turbulence models and mesh partition method which are used to predict the hydrodynamic performance of AUV. Firstly, the hydrodynamic performance prediction method was gotten and was validated, and three different shapes autonomous underwater vehicles were designed. The hydrodynamic performances of the three autonomous underwater vehicles were predicted. Then, the advantage and disadvantage of the four autonomous underwater vehicles were obtained by comparing the resistance and pressure distribution. Based on these, two other AUV hulls were designed which combined the advantages of them, and the hydrodynamic performance was predicted. The calculation results showed that the resistance and hull pressure distribution were improved remarkably comparing with the parent model. The resistance coefficient of optimized hull is reduced by 20% compared to the parent hull.

Non-cavitating propeller noise source inversion

Ship's propeller cavitation generates major inboard noise and vibration over the aft body surface of a ship. During the last decade, cavitation-induced hull pressure forces have been reduced considerably leading to reduced noise and vibration levels owing to cavitation, in which case the non-cavitating components of propeller excitations have to be considered: pressure fluctuations due to blade loading and blade thickness. In this work, an algorithm to invert for the non-cavitating propeller noise source parameters based on the experimental data in a cavitation tunnel is shown. The fluctuating hull pressure is estimated by forward modeling based on acoustic boundary element method (BEM). The propeller blade loading and thickness noise sources are modeled using rings of dipoles and quadrupoles, respectively. The inversion for these pseudo sources are carried out by matching the data obtained from a cavitation tunnel experiment, where several hydrophone are flush mounted on the ship's surface near the propeller. Proper inversion results are obtained when six or more dipole and quadrupole rings are placed at each blade. Using the inverted source parameters, hull pressures were estimated, which show good agreement with the experiment data.

대형 유조선의 저항 및 추진성능에 대한 축척효과의 수치적 연구

This article examines the scale effect of the flow characteristics, resistance and propulsion performance on a 317k VLCC. The turbulent flows around a ship in both towing and self-propulsion conditions are analyzed by solving the Reynolds-averaged Navier-Stokes equation together with the application of Reynolds stress turbulence model. The computations are carried out in both model- and full-scale. A double-body model is applied for the treatment of free surface. An asymmetric body-force propeller is used. The speed performances including resistance and propulsion factors are obtained from two kinds of methods. One is to analyze the computational results in model scale through the revised ITTC’78 method. The other is directly to analyze the computational results in full scale. Based on the computational predictions, scale effects of the resistance and the self-propulsion factors including form factor, thrust deduction fraction, effective wake fraction and various efficiencies are investigated. Scale effects of the streamline pattern, hull pressure and local flow characteristics including x-constant sections, propeller and center plane, and transom region are also investigated. This study presents a useful tool to hull-form and propeller designers, and towing-tank experimenters to take the scale effect into consideration.

폴리에틸렌 카누의 구조해석과 제조에 관한 연구

일반적으로 카누는 목재나 FRP 등으로 제작된다. 그러나 소비자들은 기존의 재료에 비하여 염가이면서 친환경 재료를 선호한다. 특히, 선진국에서는 일부 선박에 대하여 FRP 선체 제조를 금지하는 법안을 발표하고 이에 따른 국제협약을 제정하였다. 폴리에틸렌은 식료품 용기나 의료용 용기로 널리 사용되는 재료로서 리사이클 가능한 재료이다. 본 연구에서는 카누 선체 재료로 폴리에틸렌으로 선정하였으며 선형설계는 상용 선형 설계 프로그램인 3D Boat Design을 이용하여 수행하였다. 카누 구조하중은 우선 ANSYS CFX R12.1을 이용하여 선체에 작용되는 압력 분포를 구하고 이것을 ANSYS WORKBENCH R12.1로 넘겨 선체 압력 하중과 패들러 하중을 동시에 고려하였다. 카누 각 치수를 설계변수화하여 응력과 무게를 최소화하는 최적화 과정을 반응표면방법과 만족도 함수를 이용하여 수행하였다. 개발된 카누는 운항시험에서 직진성이나 안정성은 우수하나 운반성과 선회성 및 속도는 보통인 것으로 판단된다.

Experimental investigation of nonlinearities of ship responses in head waves

The present study focuses on building a systematic approach to identify, from experimental results, the nonlinearity in the dynamic system of a high-speed ship. The experimental program consists of tests in both regular and irregular head waves, and the measured quantities included wave elevation, vertical motions, and hull pressures. By contrasting these results to the quasi-linear behaviors of heave motion, the nonlinear behaviors of pressure are highlighted and presented. Three nonlinear assessments, the probability density function, and the variance spectra are provided. Based on these investigations, we conclude that the pressures, particularly, at the ship’s bow contain more nonlinearities than just the heave motion. They are identified mainly by the large amplitude of the higher harmonics and also by the large asymmetry in the measured signals. Furthermore, the coherence spectrum obtained from the third-order orthogonal frequency-domain Volterra model provides information regarding the magnitude of each order at the corresponding frequency, which serves as a strategy to simplify a complex problem or to achieve a balance between regular and irregular waves. The overall results show that the higher-order components are significant for the pressure system and the outcome of the proposed model can offer constructive feedback, which can lead to more practical applications.

Finite element vibration analysis and measurement of a 255 ft vehicle ferry.

Finite element analysis (FEA) methods have become part of the standard repertoire when designing a ship. This is particularly true for static analyses, although FEA methods are becoming increasingly utilized for analysis of low-frequency vibration as well. This paper discusses the FEA modeling process that was used for analyzing low-frequency (less than 80 Hz) habitability vibration on a 255 ft vehicle ferry. Deck vibration levels were measured after the vessel was built. Measured versus predicted vibration data show good correlation, particularly when the various sources of potential error and uncertainty are considered. The subject ferry was designed to meet the ABS Comfort Class (COMF) vibration criteria. Calculations of deck vibration were performed using NEiNastran to verify compliance with the criteria. Excitations due to propeller hull pressure excitation are highlighted here as these forces created the largest deck responses. However, it is noted that the main propulsion engines were resiliently mounted subsequent to initial analyses using the FE model. This paper discusses pertinent details of the model (element types, properties, etc.), application of hull pressures, damping, analysis methodology, and interpretation of results. Areas of uncertainty and impacts to the results are discussed.

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.

Acoustic excitation of hull surfaces by propeller sources

Acoustic boundary element models are used to solve the Helmholtz equation in order to explore the nature of fluctuating hull pressures due to propeller sources when the wavelength of the underwater sound is comparable to the hull dimensions. Sources are represented by stationary monopoles and dipoles near a rigid hull. The results for submerged and floating bodies are described. A simple ellipsoidal representation of submerged and floating bodies is used first to aid understanding of how hull pressure distributions are affected by the location and frequency of propeller sources. The sea surface is represented using image techniques, to ensure zero pressure there when the body is floating. Solid boundary factors are used to indicate the principal effects of source location, frequency, and the sea surface. Finally, the results from an independent model of a cruise liner hull are used to illustrate the combined effects of diffraction, interference, and flotation on hull forces, with explicit modeling of the free surface.

Performance assessment of a concept propulsor: the thrust-balanced propeller

This paper presents the results of a numerical performance analysis to demonstrate the worthiness of a recently patented new concept propulsor, the so-called “thrust-balanced propeller (TBP)”. The main advantage of this unconventional propulsor is its inherent ability to reduce the unsteady effect of blade forces and moments when it is operating in a non-uniform wake flow. The propulsor comprises a pair of diametrically opposed blades that are connected to one another and mounted so as to be rotatable together through a limited angle about their spindle axis. A quasi-hydrodynamic approach is described and applied to perform the numerical analysis using a state-of-the-art lifting surface procedure for conventional propellers. Performance comparisons with a conventional fixed-pitch propeller are made for the blade forces and moments, efficiency, cavitation extents and fluctuating hull pressures. Bearing in mind the quasi-static nature of the analyses, the results present favourable performance characteristics for the thrust-balanced propeller and support the worthiness of the concept. However, the concept needs to be proved through physical model tests, which are planned to take in a cavitation tunnel.

Experimental Verifications of a Theoretical Procedure for Propeller-Induced Hull Pressure Calculations

This paper describes the methodologies adopted by the Italian Ship Research Center to predict propeller-induced pressures. An outline of the developed computer program is given, and calculated pressure amplitude results for two ships are compared with the experimental values obtained from full-scale measurements. In particular, calculated pressure results, produced by three different propeller design solutions at several points on the stern of a new-generation single-screw roll-on/roll-off containership, are discussed and compared with the same quantities measured both on a ship model in the Depressurized Towing Tank of the Netherlands Ship Model Basin and on the full-scale ship fitted with the selected propeller.

Study of Prismatic Planing Model Spray and Resistance Components

The results of a study of prismatic planing model spray and resistance components are presented. The calculated total resistance values are shown to be in good agreement with the measured resistance. The features of the whisker spray and spray blister are discussed along with an experimental technique using wide-beam prismatic models for measuring the spray thickness. A derivation based on energy considerations of the prismatic planing hull pressure resistance component and the spray thickness is presented. The spray thickness values calculated from this formulation are in qualitative agreement with the measured spray thickness. The frictional resistance component obtained from the resistance measurements indicated an extensive laminar flow on the planing model bottom. An acetanilid film was sprayed on the planing model bottom which tripped the laminar-turbulent transition. The CF-values estimated from the acetanilid film pattern were found to be 75 to 90 percent of the CF-values estimated from the corresponding resistance tests.

Experiments and interpretation of propeller vibratory hull pressures on an elastic structure of a shipmodel1

Experiments and interpretation of propeller vibratory hull pressures on an elastic structure of a shipmodel1

Cavitation phenomena, propeller excited hull pressure amplitudes and cavitation scale effect

The cavitation research, described here and carried out with the laser-scattered-light-technique, demonstrates the large influence of the free air content on cavitation phenomena and propeller excited pressure fluctuations in a cavitation tunnel. Another result of this research, based on full scale investigations, is that in the free sea a large number of nuclei is always present. Therefore for equal propeller loadings substantial differences of the cavitation extent can apparently never occur in different sea regions. The experimental research was supported by theory, applied to hemispherical nosed bodies and model propellers. The comparisons between model and full scale cavitation phenomena which cause hull pressure fluctuations show remarkable differences. The reason is the scale effect of the cavitation due to different absolute pressures on the propellers in model and full scale. Other published model/full-scale comparisons considering this scale effect are discussed.

Calculation of Propeller-Induced Vibratory Hull Forces, Force Distributions, and Pressures; Free-Surface Effects

It is shown how a formula originally developed for calculating the total unsteady propeller-induced forces on a ship hull can be used for calculating the associated hull force distributions and pressures as well. Total vertical forces and the axial distribution of vertical force on a flat plate are calculated using the formula. Two different conditions on the water free-surface are investigated. These conditions correspond to approximations to the exact linearized free-surface condition in the extremes of high and low frequency. The calculations are also performed for circumferentially uniform and nonuniform propeller inflows.

A Measuring Method of Hull Pressure Resistance and Its Application

The paper deals with a method to measure and analyse hull surface pressure and with interpretation of the results for application to hull form design.A new and simple method to measure pressure in the proximity of free surface is introduced at.first, and then, integration of the measured pressure is introduced to examine a pressure drag. This new method of integration does not require any conventional correction term due to the sinkage and trim of a ship.Further, some application examples of the above methods for studying the following data are introduced.Wave profiles and pressure distribution on hull surface in breaking wave at the bowPressure distribution over the hull surface in the fore and aft bodySkin friction resistance derived by deducting the pressure resistance from the total resistanceViscous pressure resistance derived by deducting the wave pattern resistance (obtained througth the wave analysis) from the pressure resistancePressure components of thrust deduction obtained from the pressures measured with and without a propellerThe measurements for the above application were conducted at the Towing Tank of IHI (Ishikawajima-Harima Heavy Industries Co., Ltd.).