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1.
A practical method to account for the influence of sinkage and trim on the drag of a freely floating (free to sink and trim) common monohull ship at a Froude number F ≤ 0.45 is considered. The sinkage and the trim are estimated via two alternative simple methods, considered previously. The drag is also estimated in a simple way, based on the classical Froude decomposition into viscous and wave components. Specifically, well-known semiempirical expressions for the friction drag, the viscous pressure drag and the drag due to hull roughness are used, and the wave drag is evaluated via a practical linear potential flow method. This simple approach can be used for ship models as well as full-scale ships with smooth or rough hull surfaces, and is well suited for early ship design and optimization. The method considered here to determine the sinkage and the trim, and their influence on the drag, yields theoretical predictions of the drag of the Wigley, S60 and DTMB5415 hulls that are much closer to experimental measurements than the corresponding predictions for the hull surfaces of the ships in equilibrium position at rest. These numerical results suggest that sinkage and trim effects, significant at Froude numbers 0.25 < F, on the drag of a typical freely floating monohull ship can be realistically accounted for in a practical manner that only requires simple potential flow computations without iterative computations for a sequence of hull positions. 相似文献
2.
Neumann-Michell theory-based multi-objective optimization of hull form for a naval surface combatant
A numerical multi-objective optimization procedure is proposed here to describe the development and application of a practical hydrodynamic optimization tool, OPTShip-SJTU. Three components including hull form modification module, hydrodynamic performance evaluation module and optimization module consist of this tool. The free-form deformation (FFD) method and shifting method are utilized as parametric hull surface modification techniques to generate a series of realistic hull forms subjected to geometric constraints, and the Neumann-Michell (NM) theory is implemented to predict the wave drag. Moreover, NSGA-II, a muti-objective genetic algorithm, is adopted to produce pareto-optimal front, and kriging model is used for predicting the total resistance during the optimization process to reduce the computational cost. Additionally, the analysis of variance (ANOVA) method is introduced to represent the influence of each design variable on the objective functions. In present work, a surface combatant DTMB Model 5415 is used as the initial design, and optimal solutions with obvious drag reductions at specific speeds are obtained. Eventually, three of optimal hulls are analyzed by NM theory and a RANS-based CFD solver naoe-FOAM-SJTU respectively. Numerical results confirm the availability and reliability of this multi-objective optimization tool. 相似文献
3.
The concept of micro-bubble ejection is emerging as an attractive method for viscous drag reduction for high speed craft. Most of the drag reduction literature presents results for micro-bubble ejection above/below horizontal surfaces. Nevertheless, modern high-speed hull forms are slender with vertical surfaces. This paper presents results of model tests with micro-bubble ejection to the vertical sides of a high speed model with a non-wetting coating. The coating yielded drag reduction of 4–6% and micro-bubbles with coating reduced overall drag by 4–11%. The local coefficient of friction with micro-bubbles is analyzed and found comparable to vertical flat plate results of [Madavan et al., 1984]. 相似文献
4.
An inverse hull design approach in minimizing the ship wave 总被引：1，自引：0，他引：1
The Levenberg–Marquardt Method (LMM) and a panel code for solving the wave-making problem are utilized in an inverse hull design problem for minimizing the wave of ships. A typical catamaran is selected as the example ship for the present study. The hull form of the catamaran is described by the B-spline surface method so that the shape of the hull can be completely specified using only a small number of parameters (i.e. control points). The technique of parameter estimation for the inverse design problem is thus chosen. The LMM of parameter estimation, which is the combination of steepest descent and Newton’s methods, has been proven to be a powerful tool for the inverse shape design problem. For this reason it is adopted in the present study.In the present studies, the inverse hull design method can not only be applied to estimate the hull form based on the known wave data of the target ship but can also be applied to estimate the unknown hull form based on the reduced wave height. The optimal hull forms of minimizing wave for a typical catamaran in deep water at service speed and at the critical speed of shallow water are estimated, respectively. Moreover, a new hull form with the combining feature of the optimal hull forms for deep water and shallow water is performing well under both conditions. The numerical simulation indicates that the hull form designed by inverse hull design method can reduce the ship wave significantly in comparison with the original hull form. 相似文献
5.
6.
Significant drag reduction of large displacement vessels can be achieved by applying multi-wave air cavities arranged on the hull bottom. Waves generated on the air–water interface of air cavities impose requirements on the dimensions of a hull recess that accommodates the air cavity. An approximate model for calculating wave patterns in the critical upstream part of long air cavities in a simplified, horizontal-plane geometry is presented in this paper. The influence of the recess planform boundaries and other factors on the wave patterns is studied parametrically. Some hydrodynamic aspects of multi-wave air cavities are discussed. 相似文献
7.
Traditionally autonomous underwater vehicles (AUVs) have been built with a torpedo-like shape. This common shaping is hydrodynamically suboptimal for those AUVs required to operate at snorkeling condition near the free surface. In this case, the wave resistance associated to the wavy deformation of the sea surface induced by the motion of the platform is an important component of the drag. This work has investigated the optimum hull shape of an underwater vehicle moving near the free surface. Specifically a first-order Rankine panel method has been implemented to compute the wave resistance on a body of revolution moving close to the free surface. A simulated annealing algorithm was then employed to search those set of parameters defining the hull shape that minimize the wave resistance. The optimization was constrained to keep constant the total volume of the vehicle. The total drag of scaled models of the torpedo-like and resulting optimum shapes was measured in the naval tank of the University of Trieste. Measurements showed a smaller resistance of the optimized shape in the range of the considered Froude numbers. 相似文献
8.
Extensive use of autonomous underwater vehicles (AUVs) in oceanographic applications necessitates investigation into the hydrodynamic forces acting over an AUV hull form operating under deeply submerged condition. This paper presents a towing tank-based experimental study on forces and moment on AUV hull form in the vertical plane. The AUV hull form considered in the present program is a 1:2 model of the standard hull form Afterbody1. The present measurements were carried out at typical speeds of autonomous underwater vehicles (0.4-1.4 m/s) by varying pitch angles (0-15°). The hydrodynamic forces and moment are measured by an internally mounted multi-component strain gauge type balance. The measurements were used to study variation of axial, normal, drag, lift and pitching moment coefficients with Reynolds number (Re) and angle of attack. The measurements have also been used to validate results obtained from a CFD code that uses Reynolds Average Navier-Stokes equations (ANSYS™ Fluent). The axial and normal force coefficients are increased by 18% and 195%; drag, lift and pitching moment coefficients are increased by 90%, 182% and 297% on AUV hull form at α=15° and Rev=3.65×105. These results can give better idea for the efficient design of guidance and control systems for AUV. 相似文献
9.
本文通过物理模型试验研究了波流共同作用下珊瑚礁冠层附近平均流的分布特征以及阻力特性,分析了典型波浪工况下无潮流、正向潮流和反向潮流分别作用下平均流速、摩阻流速、阻力系数的沿礁变化规律。结果表明：无潮流时礁前斜坡及外礁坪上存在海底回流且在礁缘附近回流最强,在礁坪上冠层附近平均流表现为向岸流,且该流沿礁向海岸方向持续增大。相较于无潮流时,正向潮流作用下冠层内外均为向岸流,在礁坪上冠层内外的向岸流显著增大;反向潮流作用下冠层内外均为离岸流且在礁缘处达到最大,该离岸流在礁坪上逐渐减小然后趋于稳定。无潮流时礁坪上摩阻流速呈小幅波动;相较于无潮流时,正向潮流、反向潮流影响下礁坪上摩阻流速显著增大,其中正向潮流影响下增幅更大;无潮流时礁坪上水力粗糙度沿礁减小,正向潮流和反向潮流影响下水力粗糙度普遍有幅度不等的增加。三种工况下礁坪上的阻力系数均沿礁整体呈下降趋势,相较于无潮流时,正向潮流和反向潮流影响下礁坪上的阻力系数显著增大,且正向潮流作用时增幅更大。 相似文献
10.
Peter R. Payne 《Ocean Engineering》1984,11(5):513-524
Recent developments in slender planing theory permit rather precise estimates of forces and moments to be made. So using a methodology first suggested by Epshtein for flat plate wakes, the modern theory is employed to find the longitudinal shape of a flat plate's wake. The result is in better agreement with experiment than previous formulations. The same approach is then applied (for the first time) to a prismatic wake, and the agreement with experiment is found to be excellent. Finally, the equations developed are used to deduce the wave energy developed by the passage of the hull, and from this, its wave drag. Although there are no experimental data available for comparison, the ratio (wave drag)/(total pressure drag) follows the same trend with Froude number as earlier two-dimensional solutions by Sedov and Squire, and tends to zero as Froude number tends to infinity. 相似文献
11.
In the ship design optimization process, the neglect of the unavoidable uncertainty of parameters in the actual navigation and experimental observations, may lead to a bad result with some hidden dangers in practical applications. Considering the influence of the uncertainty, a new and effective hull form reliability-based robust design optimization (RBRDO) framework, including the hull form modification module and RBRDO module, has been developed and tested in the present work. Radial basis function method is utilized as the parametric hull surface modification technique to generate a series of smooth hull forms while combining polynomial chaos expansions (PCE) method with maximum entropy method (MEM) to conduct the uncertainty analysis for the prediction of the mean and the standard deviation of the objective and the failure probability of constraints. To verify the validity of the method, hull form design optimization of the bow of KCS model is implemented under the influence of the uncertainty. Numerical results indicate that the proposed RBRDO framework is effective compared with traditional Monte Carlo method. Meanwhile, compared with traditional DO case, RBRDO case has higher adaptability to the environmental uncertainty with the lower failure probability, which ensure the robustness and reliability of the optimal hull form. 相似文献
12.
Peter R. Payne 《Ocean Engineering》1982,9(4):373-384
Earlier papers (Payne, 1981 a,b,c) have developed what might be called a virtual mass theory which in principle permits the forces on any planing hull form to be calculated. In the present paper, this methodology is extended to calculate the thickness and momentum of the jet or spray sheet thrown off by the planing surface. For a two-dimensional flat planing plate—the only case where comparison is possible—the theory gives essentially the same result as that of Pierson and Leshnover (1948). For a three-dimensional flat plate and prismatic hulls, the results seem physically reasonable.For the small trim angles associated with efficient planing, on a weightless inviscid fluid the total pressure drag of any hull can be reduced to close to zero by deflecting the jet rearwards and parallel to the undisturbed surface, the residual resistance being due to the cross-flow force which varies as (trim angle)2. 相似文献
13.
Direct bed shear stress measurements in bore-driven swash 总被引：1，自引：0，他引：1
Direct measurements of bed shear in the swash zone are presented. The data were obtained using a shear plate in medium and large-scale laboratory bore-driven swash and cover a wide range of bed roughness. Data were obtained across the full width of the swash zone and are contrasted with data from the inner surf zone. Estimates of the flow velocities through the full swash cycle were obtained through numerical modelling and calibrated against measured velocity data. The measured stresses and calculated flow velocities were subsequently used to back-calculate instantaneous local skin friction coefficients using the quadratic drag law. The data show rapid temporal variation of the bed shear stress through the leading edge of the uprush, which is typically two–four times greater than the backwash shear stresses at corresponding flow velocity. The measurements indicate strong temporal variation in the skin friction coefficient, particularly in the backwash. The general behaviour of the skin friction coefficient with Reynolds number is consistent with classical theory for certain stages of the swash cycle. A spatial variation in skin friction coefficient is also identified, which is greatest across the surf-swash boundary and likely related to variations in local turbulent intensities. Skin friction coefficients during the uprush are approximately twice those in the backwash at corresponding Reynolds number and cross-shore location. It is suggested that this is a result of the no-slip condition at the tip leading to a continually developing leading edge and boundary layer, into which high velocity fluid and momentum are constantly injected from the flow behind and above the tip region. Finally, the measured stress data are used to determine the asymmetry and cross-shore variation in potential sediment transport predicted by three forms of sediment transport formulae. 相似文献
14.
《Coastal Engineering》2001,43(2):131-148
Four different expressions for wave energy dissipation by bottom friction are intercompared. For this purpose, the SWAN wave model and the wave data set of Lake George (Australia) are used. Three formulations are already present in SWAN (ver. 40.01): the JONSWAP expression, the drag law friction model of Collins and the eddy–viscosity model of Madsen. The eddy–viscosity model of Weber was incorporated into the SWAN code. Using Collins' and Weber's expressions, the depth- and fetch-limited wave growth laws obtained in the nearly idealized situation of Lake George can be reproduced. The wave model has shown the best performance using the formulation of Weber. This formula has some advantages over the other formulations. The expression is based on theoretical and physical principles. The wave height and the peak frequency obtained from the SWAN runs using Weber's bottom friction expression are more consistent with the measurements. The formula of Weber should therefore be preferred when modelling waves in very shallow water. 相似文献
15.
Improved data collection and processing technologies along with the use of high resolution spectral techniques soon will make it possible to obtain estimates of the Kelvin wave amplitude function A(θ), ship speed U, and ship heading α from synthetic aperture radar images of ship wakes. This paper presents a series of methods for deriving additional hull characteristics such as the length L, volume V, and offsets ζ(x,z) from this spectral and surface wave information. The first method estimates the ship length by taking the Fourier transform of the slope amplitude function |kA|. The remaining estimates make use of the hull inversion code developed at the University of Michigan by Wu in 1991. The accuracy of the hull offsets predicted by the code is first determined for various options for solving the linear inversion problem. In this case, both the magnitude and phase of A(θ) are known in addition to the hull draft H. Since the draft is not often known a priori, the accuracy of the code is determined next by predicting the volume of the ship for an approximate though plausible input value of H. Finally, the accuracy of the non-linear inversion problem of obtaining offsets along the entire hull is investigated when only the magnitude but not the phase of A(θ) is known. 相似文献
16.
The computer model for near shore wave propagation,SWAN,was used to study wave climates in Liverpool Bay,northwest England with various input parameters,including bottom friction factor,white capping,wind drag formulation and effects of tidal modulations.Results were compared with in-situ measurements and reveal the impacts from these inputs on the predictions of wave height and propagation distributions.In particular,the model results were found very sensitive to different input formulations,and tend to underestimate the wave parameters under storm conditions in comparison with the observations.It is therefore important to further validate the model against detailed field measurements,particularly under large storms that are often of the primary concern. 相似文献
17.
渔网水动力试验研究及分析 总被引：16，自引：1，他引：16
根据流体力学实验结果 ,研究了雷诺数 ,网的密实度 ,网型以及流向等对网的阻力影响 ,同时讨论了侧向力问题 ,对同样网面积的平面网及圆型网的阻力进行了比较 ,结果圆型网阻力较小。 相似文献
18.
In order to reduce the total resistance of a hull, an optimization framework for the bulbous bow optimization was presented. The total resistance in calm water was selected as the objective function, and the overset mesh technique was used for mesh generation. RANS method was used to calculate the total resistance of the hull. In order to improve the efficiency and smoothness of the geometric reconstruction, the arbitrary shape deformation (ASD) technique was introduced to change the shape of the bulbous bow. To improve the global search ability of the particle swarm optimization (PSO) algorithm, an improved particle swarm optimization (IPSO) algorithm was proposed to set up the optimization model. After a series of optimization analyses, the optimal hull form was found. It can be concluded that the simulation based design framework built in this paper is a promising method for bulbous bow optimization. 相似文献
19.
为提高母型船阻力性能,以船体阻力性能为优化对象,基于改造母型船法,研究船舶球鼻艏以及船尾线型的改变对船舶阻力性能的影响.采用高度集成化的Tribon系统、可视化绘图软件Auto CAD及CFD(Computational Fluid Dynamics)通用前处理软件ICEM联合建模的方法来建立船体模型.通过模拟计算结果与实验值的对比分析,验证CFD技术在船舶阻力性能预报中的合理性和有效性.通过对比3种不同球鼻艏时的船体阻力得知:从阻力性能方面考虑,对于低速丰满型船舶选用普通型球鼻艏以及中高速船舶采用上翘型球鼻艏均可以获取较好地减阻效果.同时比较不同航速下尾部线型对船体总阻力的影响表明,选优后的方形尾在相同的航速下阻力低、消耗的功率小、形状效应小、黏压阻力和摩擦阻力也相对较小. 相似文献
20.
The unsteady, two-dimensional Navier–Stokes equations and the exact free surface boundary conditions were solved to study the interaction of a solitary wave and a submerged dike. A piston-type wavemaker was set up in the computational domain to produce the incident solitary waves. The incident wave and the associated boundary layer flow in a wave tank with a flat bed were compared with the analytical solutions to verify the accuracy of this numerical scheme. Effects of the incident wave height and the size of the dike on the wave transformation, the flow fields, and the drag forces on the dike were discussed. Our numerical results showed that even though the induced local shear stress on the top surface of the dike is large at some particular locations, the resultant pressure drag is much larger than the friction drag. The primary vortex generated at the lee side of the dike and the secondary vortex at the right toe of the dike may scour the bottom and cause a severe problem for the dike. 相似文献