Wake Fraction Research Articles

Improved version of LeMoS hybrid model for ambiguous grid densities

Application of the LeMoS hybrid (LH) URANS/LES method for the wake parameters prediction is considered. The wake fraction coefficient is calculated for inland ship model M1926 under shallow water conditions and compared to results of PIV measurements. It was shown that due to lack of the resolved turbulence at the interface between LES and RANS zones the artificial grid induced separations can occur. In order to overcome this drawback, a shielding function is introduced into LH model. The new version of the model is compared to the original one, RANS k−ω SST and SST-IDDES models. It is demonstrated that the proposed modification is robust and capable of wake prediction with satisfactory accuracy.

Numerical prediction analysis of the fluctuating pressure and rudder force of full-scale hull-propeller-rudder system

In order to analyze the fluctuating pressure and rudder force characteristics of a full-scale hull-propeller-rudder system, a hybrid structured/unstructured grid was combined with the Reynolds-averaged Navier-Stokes (RANS) and volume of fluid (VOF) methods to perform a numerical prediction of model and full-scale performance for the Kriso Container Ship (KCS) case. A numerical simulation for the self-propulsion test of a full-scale ship was first performed to obtain the propeller velocity under the self-propulsion point, the results of which indicated a notable wake fraction and velocity scale effect at the self-propulsion point. Transient two-phase flow calculations were then performed for the model and full-scale KCS hull-propeller-rudder systems. According to the monitoring data, the hull surface fluctuating pressure and rudder force fluctuated periodically over time and the full-scale fluctuating pressure exhibited a higher time-average value as compared to the model-scale pressure. The frequency spectrum curves were also generated by the fast Fourier transform algorithm. The analysis of the frequency spectrum data indicated that the peaks of the fluctuating pressure and rudder force reached their respective maximums at the blade passing frequency (BPF), after which the peak gradually decreased and the full-scale ship reduced more quickly. Furthermore, fluctuating amplitude of full-scale ship was bigger than model-scale.

Numerical prediction of effective wake field for a submarine based on a hybrid approach and an RBF interpolation

A hybrid approach coupled with a surface panel method for the propeller and a Reynolds averaged Navier-Stokes (RANS) model for the hull with the propeller body forces are presented for predicting the self-propulsion performance and the effective wake field of underwater vehicles. To achieve a high accuracy and simplicity, a radial basis function (RBF) based approach is proposed for mapping the force field from the blade surface panels to the RANS model. The effective wake field is evaluated in two ways, i.e., by extrapolation from the flat planes upstream of the propeller disk, and by direct computation in a curved surface upstream of and parallel to the blade leading edges. The hull-propeller system of a real propeller geometry is further simulated with the sliding mesh model to numerically verify the hybrid approach. Numerical simulations are conducted for the fully appended SUBOFF submarine model. The high accuracy of the RBF-based interpolation scheme is confirmed, and the effective wake fraction predicted by the hybrid approach is found consistent with that obtained by the sliding mesh model. The effective wake fractions predicted by the two methods are, respectively, 4.6% and 3% larger than the nominal one.

Numerical Analysis of Wake Field over a Submarine with Full Appendages Based on STAR-CCM+

Numerical calculation of wake field over SUBOFF submarine with full appendages was made in the paper based on STAR-CCM+. By comparing the computed results with the experimental data. The influence of prism layer mesh and turbulent model on the computed results was analyzed systematically. It is shown that, in the numerical calculation on submarine model wake field, the SST k- model is the relatively more suitable turbulent model, the thickness of prism layer mesh is equal or greater than 15mm, and the non-dimensional distance between the first computational mesh node and the model surface is need to be set in the range from 25 to 170. Moreover, numerical calculation of submarine wake field was made by using a sliding mesh. by refining mesh and reducing time step , the axial wake fraction was in good agreement with that computed by steady method.

Shuttle tanker fuel consumption numerical analysis for improving ship efficiency

The fuel consumption analysis of a Suezmax tanker customized to the offloading operation in the brazilian coast is performed in order to verify the possible savings produced by the so-called “slow steaming” technique during navigation. This ship is equipped of a single engine/propeller but there is a trend of building new vessels considering an equivalent two engines–two propellers for better safety during navigation and offloading operations, therefore a comparison regarding the propulsive efficiency, fuel consumption, and operational conditions (max engine power, rotation, and cavitation limitations) is performed in order to verify the benefits of this new concept. The methodology applied is based on a mixed approach considering numerical simulations using CFD (computational fluid dynamics) and regression models available in the literature: the first one applied to compute the ship resistance and nominal wake fraction in the propeller plane and the second one applied for the propulsive efficiency prediction, as the propeller curves based on Wageningen B-series. The specific fuel oil consumption curves were obtained from the engine manufacturer catalogue.

Effects of Parallel-Middle-Body Relative Length and Stern Form on the Wake Fraction and Ship Resistance

Effects of parallel-middle-body relative length and stern form of a 10450-DWT tanker on the wake fraction and ship resistance are studied by using computational fluid dynamics (CFD). Calculations were done with varying ratios of parallel-middle-body length to ship length (Lmb/L) and with varying stern forms. The same ship displacement is maintained in all the calculations. It is found that a larger value of Lmb/L for ships with the same stern form results in a larger ship resistance but does not affect the wake fraction significantly. A more abrupt stern form results in a larger wake fraction and in a much larger ship resistance. In the former case, the increase of ship resistance is ascribed to the increase of the wetted surface area (WSA) of the ship and in the latter case particularly to the much larger wake fraction.

Numerical prediction analysis of propeller bearing force for full-scale hull–propeller–rudder system

The hybrid grid was adopted and numerical prediction analysis of propeller unsteady bearing force considering free surface was performed for mode and full-scale KCS hull–propeller–rudder system by employing RANS method and VOF model. In order to obtain the propeller velocity under self-propulsion point, firstly, the numerical simulation for self-propulsion test of full-scale ship is carried out. The results show that the scale effect of velocity at self-propulsion point and wake fraction is obvious. Then, the transient two-phase flow calculations are performed for model and full-scale KCS hull–propeller–rudder systems. According to the monitoring data, it is found that the propeller unsteady bearing force is fluctuating periodically over time and full-scale propeller's time-average value is smaller than model-scale's. The frequency spectrum curves are also provided after fast Fourier transform. By analyzing the frequency spectrum data, it is easy to summarize that each component of the propeller bearing force have the same fluctuation frequency and the peak in BFP is maximum. What's more, each component of full-scale bearing force's fluctuation value is bigger than model-scale's except the bending moment coefficient about the Y-axis.

Numerical study on scale effect of nominal wake of single screw ship

A 4000TEU container ship was studied without considering free surface effect, and the viscous flow fields of ship at different scales were solved numerically by RANS method, numerical uncertainty analysis was employed according to factors of safety method for Richardson extrapolation, scale effect of axial nominal wake field was analyzed in details. It shows that the reciprocal of mean axial wake fraction of propeller disc exhibits a near linear dependence on Reynolds number in logarithmic scale. For the single screw ship without bilge vortex, linear function is fit perfectly for the relationship between the reciprocal of mean axial wake fraction at each radius, reciprocal of amplitude of wake peak right above propeller disc and Reynolds number in logarithmic scales. In inner area of propeller disc, the reciprocal of amplitude of wake valley and wake peak right down propeller disc reveal nearly linear dependence on Reynolds number in logarithmic scales. While in outer area, the amplitude of wake peak and valley decline rapidly to potential wake fraction, and the wake width reveals negative exponent power function dependence on Reynolds number in logarithmic scales. On this basis, an extrapolated wake field scaling method is proposed.

The Effect of Ship Coefficients on the Efficiency Gain f Propeller-Vane System

This paper discusses a correlation between non dimensional parameter of the ship (L/B,B/T, wake fraction, ratio between induced and advance velocity, thrust loading coefficient) and energy gain by applying vane-turbine in the propeller slipstream. The data based on the basis of statistical data (numerical) and model testing. The correlation data can be calculated by quantifying the contribution of causal variables to a targeted effect variable directly and indirectly through other variables and this would be examined by Path analysis. By using this coefficient, it is possible to demonstrate which variable has the main contribution on the efficiency gain. The Data analysis of Microsoft Office Excel software is used to approach the calculation. It is found that L/B and CT affected indirectly the efficiency gain of vane-turbine

Development of energy-saving devices for a full slow-speed ship through improving propulsion performance

Energy-saving devices for 317K VLCC have been developed from a propulsion standpoint. Two ESD candidates were designed via computational tools. The first device WAFon composes of flow-control fins adapted for the ship wake to reduce the loss of rotational energy. The other is WAFon-D, which is a WAFon with a duct to obtain additional thrust and to distribute the inflow velocity on the propeller plane uniform. After selecting the candidates from the computed results, the speed performances were validated with model-tests. The hydrodynamic characteristics of the ESDs may be found in improved hull and propulsive efficiencies through increased wake fraction.

On the scale effect of thrust deduction in a judicious self-propulsion procedure for a moderate-speed containership

We studied the scale effect of the thrust deduction factor on an innovative self-propulsion procedure. In contrast to the International Towing Tank Conference (ITTC) committee documents, which adopt a skin friction corrector and also state that the thrust deduction factor is the same for both the model and full scales, complex interaction remains unresolved. Since the skin friction corrector is always calculated by an equation, to date no verifications of this equation have been published. In addition, the scale effect of the thrust deduction factor has seldom been studied because it is minor compared to that of the wake fraction. In the first section of this paper, the existence of the scale effect on the thrust deduction factor is proven and the misinterpretation of the equality is highlighted. Second, a balanced self-propulsion procedure, which does not use the skin friction corrector to isolate the propeller loading effect is proposed and compared to the conventional standard ITTC procedure. The study derived a simplified model for predicting the full-scale values for the thrust deduction factor correction. The new procedure was applied to a moderate-speed containership by CFD software and several characteristics were demonstrated. The scale correction model was verified from the computational results.

Propulsive factors in waves: A comparative experimental study for an open and a ducted propeller

The estimation of propulsive factors in waves is essential to understand the propulsion characteristics for a ship in actual sea conditions. While the propulsion factors of open propellers in waves have been established by significant experimental work, similar study for the ducted propulsion case couldn’t be found. The propeller–duct interaction has a considerable influence on the propulsive factors for a ducted propeller, and hence demands a separate study for understanding the effect of waves. This paper presents the results of model propulsion tests with a 120m cargo vessel at two Froude numbers under different propeller loadings for a series of head sea conditions, carried out with both a ducted and an open propeller. Comparison of propulsive factors in waves shows that, there are differences in both magnitude and in some cases—also trend between an open and a ducted propeller, most notably in the effective wake fraction, where the ducted propeller shows a stronger influence of the waves. The thrust deduction fraction is found to be independent of propeller loading in a broad range of loadings. The propeller efficiency doesn’t seem to be much influenced by the waves, except for the effect of change of propulsion point.

A multi-objective optimization design framework integrated with CFD for the design of AUVs

This paper presents a multi-objective optimization design framework that is integrated with the Computer Aided Design (CAD) for geometric variation and Computational Fluid Dynamics (CFD) software for hydrodynamic computations for the design of Autonomous Underwater Vehicles (AUVs). The optimization model utilizes the ‘Non-dominated Sorting Genetic Algorithm (NSGA-II)’. In the present model hull geometric parameters (i.e. length of nose (Ln), length of the parallel middle body (Lm), length of the tail (Lt), maximum diameter (Dmax), and two shape variation coefficients of nose (nn) and tail (nt)) are considered as the design parameters and minimization of viscous drag, and maximization of nominal wake fraction and total volume are considered as the objective functions for the integrated design approach. CFD software (Shipflow∗™) is used to evaluate the viscous drag and it is integrated with the CAD definition. The optimization framework NSGA-II is implemented in MATLAB∗∗™. Finally, we present a design example of an existing AUV Cormoran and show that the integration of NSGA-II with CFD and CAD is effective for AUV hull form design. Our reported results show that for the given bounds on the design parameters, the optimization design framework is able to produce more efficient hull forms than the existing design.

Twisted rudder for reducing fuel-oil consumption

Three twisted rudders fit for large container ships have been developed; 1) the Z-twisted rudder that is an asymmetry type taking into consideration incoming flow angles of the propeller slipstream, 2) the ZB-twisted rudder with a rudder bulb added onto the Z-twisted rudder, and 3) the ZB-F twisted rudder with a rudder fin attached to the ZB-twisted rudder. The twisted rudders have been designed computationally with the hydrodynamic characteristics in a self-propulsion condition in mind. The governing equation is the Navier-Stokes equations in an unsteady turbulent flow. The turbulence model applied is the Reynolds stress. The calculation was carried out in towing and self-propulsion conditions. The sliding mesh technique was employed to simulate the flow around the propeller. The speed performances of the ship with the twisted rudders were verified through model tests in a towing tank. The twisted versions showed greater performance driven by increased hull efficiency from less thrust deduction fraction and more effective wake fraction and decreased propeller rotating speed.

An experimental assessment of resistance reduction and wake modification of a KVLCC model by using outer-layer vertical blades

In this study, an experimental investigation has been made of the applicability of outer-layer vertical blades to real ship model. After first devised by Hutchins and Choi (2003), the outer-layer vertical blades demonstrated its effectiveness in reducing total drag of flat plate (Park et al., 2011) with maximum drag reduction of 9.6%. With a view to assessing the effect in the flow around a ship, the arrays of outer-layer vertical blades have been installed onto the side bottom and flat bottom of a 300k KVLCC model. A series of towing tank test has been carried out to investigate resistance (C TM ) reduction efficiency and improvement of stern wake distribution with varying geometric parameters of the blades array. The installation of vertical blades led to the C TM reduction of 2.15~2.76% near the service speed. The nominal wake fraction was affected marginally by the blades array and the axial velocity distribution tended to be more uniform by the blades array.

Coefficients of Propeller-hull Interaction in Propulsion System of Inland Waterway Vessels with Stern Tunnels

Propeller-hull interaction coefficients - the wake fraction and the thrust deduction factor - play significant role in design of propulsion system of a ship. In the case of inland waterway vessels the reliable method of predicting these coefficients in early design stage is missing. Based on the outcomes from model tests and from numerical computations the present authors show that it is difficult to determine uniquely the trends in change of wake fraction and thrust deduction factor resulting from the changes of hull form or operating conditions. Nowadays the resistance and propulsion model tests of inland waterway vessels are carried out rarely because of relatively high costs. On the other hand, the degree of development of computational methods enables’ to estimate the reliable values of interaction coefficients. The computations referred to in the present paper were carried out using the authors’ own software HPSDKS and the commercial software Ansys Fluent.

Research on the Application of Body Force in Calculations for Submarine Powering Performance

Using CFD software, the mean wake fraction and thrust deduction fraction are obtained by simulating the flow field around submarine with equally distributed body force adopted to simulate the affection of propeller. The open water performance of propeller 4118 was applied to calculate the rates of propeller rotation required by the submarine at different speeds and corresponding propulsive efficiencies. The comparison between these data and the calculated results of self-propulsion test indicates that a more accurate prediction of the submarine powering performance can be made with the mean wake fraction obtained near the leading edge of the propeller, and the computational error of propulsive efficiency can be reduced to 2%. Research of this paper shows that submarine powering performance can be calculated effectively with the propeller replaced by body force.

A new methodology for the CFD uncertainty analysis

With respect to the measurement uncertainty, this paper discusses the definition, the sources, the classification and the expressions of the CFD uncertainty. Based on the orthogonal design and the statistics inference theory, a new verification and validation method and the related procedures in the CFD simulation are developed. With the method, two examples of the CFD verification and validation are studied for the drag coefficient and the nominal wake fraction, and the calculation factors and their interactions which would significantly affect the simulation results are obtained. Moreover, the sizes of all uncertainty components resulting from the controlled and un-controlled calculation factors are determined, and the optimal combination of the calculation factors is obtained by an effect estimation in the orthogonal experiment design. It is shown that the new method can be used for the verification in the CFD uncertainty analysis, and can reasonably and definitely judge the credibility of the simulative result. As for CFD simulation of the drag coefficient and the nominal wake fraction, the results predicted can be validated. Although there is still some difference between the simulation results and the experiment results, its approximate level and credibility can be accepted.

船外機付き高速滑走艇の推進性能評価法の構築

The purpose of this study is to propose the propulsive performance estimation method for high-speed planing craft with outboard engine. In order to understand the characteristics of propulsion, a fully captive model test with propulsion is carried out. Based on the results, an estimation method taking the self-propulsion factors, which are the changes of hydrodynamic forces acting on hull with lower-hull of outboard engine caused by propeller action, the effective wake fraction and the relative rotative efficiency, is proposed. Using the proposed method, running attitude, thrust (or THP), crank shaft torque (or BHP), maximum forward speed and fuel consumption of a real craft are estimated. As the results, it is found that the effective wake fraction and the relative rotative efficiency of the self-propulsion factors are significant to estimate maximum forward speed of the craft.

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.