Elsevier

Applied Ocean Research

Volume 104, November 2020, 102367
Applied Ocean Research

CFD prediction of stern flap effect on Catamaran seakeeping behavior in long crest head wave

https://doi.org/10.1016/j.apor.2020.102367Get rights and content

Highlights

  • An inhouse CFD code with ship-motion predicting model and dynamic overset grid method is used to evaluate the influence of stern flap's dimension on resistance and motions of the catamaran in long crest head wave.

  • Presented numerical method is verified and validated by the uncertainty analysis for catamaran advancing in calm water and regular wave.

  • Both model-scale and full-scale simulations are conducted to compare stern flap effect on the catamaran at model scale and full scale

Abstract

The paper presents the numerical studies of seakeeping behavior for a high-speed catamaran with stern flap advancing in long crest head wave. Both model-scale and full-scale simulations have been carried out in sea state 6 using unsteady Reynolds-averaged Navier-Stokes (URANS) solver. Uncertainty analyses were performed to verify the accuracy of the numerical solver. Model tests were conducted for the catamaran including resistance tests in calm water and seakeeping tests in regular waves to validate simulation results. The comparisons between simulation results and experimental results show good agreement. Seakeeping behaviors of models with/without stern flaps were predicted for the comparison and evaluation. Model-scale simulation results indicate that using stern flap could reduce catamaran's total resistance and the effects on heave and pitch motions are obvious in sea state 6. The installation of stern flap shows significant effect on the reduction of vertical acceleration's amplitudes, especially for reducing the occurrence of slamming impact due to the effective control of model's trim angle. Full-scale simulations have also been conducted and compared with results at model scale. Results show that effects of stern flap on heave and pitch motions are the same for both model-scale and full-scale catamaran. Residuary resistance coefficients on full-scale catamarans with/without stern flap are almost the same with model-scale simulations.

Introduction

Improvement of ship's seakeeping performance is one of the most important point in design procedure. A ship is usually hydrodynamically optimized in calm water condition. However, a good performance of the hull in calm water could not guarantee the good behavior in the seaway. As a result, the engineering must put resistance, propulsion, maneuvering and seakeeping together during conducting hydrodynamic optimization for ship. With particular regard to the seakeeping performance of a ship, the ship advancing in waves has been investigated by experimental fluid dynamics (EFD) and computational fluid dynamics (CFD) methods, with focus on the influences of waves on ship motions.

EFD procedure has been performed for the research about the seakeeping behavior of ships in regular and irregular waves traditionally. Tiao [1] pursued model tests in both regular and irregular head waves to identify the nonlinearity in the dynamic system of a high-speed ship. The test 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. Bouscasse et.al. [2] performed several seakeeping tests in transient, regular and irregular waves for a fast catamaran advancing in head sea. These tests allow identifying the Froude number at which the maximum vertical response occurs and an analysis of natural heave and pitch frequencies. The whole set of measurements is a valuable database for both hydrodynamic studies of high-speed catamarans and CFD validation. Seo et.al. [3] investigated the resistance and seakeeping performance of a high-speed mono-hull vessel through a series of model tests in a towing tank. The spray rails were designed to control the flow direction and induce a hydrodynamic lift force on the hull bottom to reduce trim angle and increase rise of the hull. The vertical acceleration at the fore perpendicular was reduced by 11.3% after attaching the rails on the optimum location. Sigmund and Moctar [4] experimentally and numerically studied the effects of head waves on propulsion characteristics of a single and a twin screw ship. Computational results were compared to experimental results obtained from physical model tests and a fair agreement between numerical and experimental results was obtained. Sun et.al. [5] investigated seakeeping characteristics of a Small Waterplane Area Twin Hull (SWATH) vehicle equipped with fixed stabilizing fins using experimental and numerical methods. The numerical methods range from viscous CFD simulation based on an unsteady RANS approach to Boundary Element Method (BEM) based on Three-Dimensional Translating-pulsating Source Green Function (3DTP). Numerical simulations have been validated by the comparisons with experimental tests. Zou et. al. [6] conducted experimental and numerical studies to explore the influence of the flap mounting angle coupled with the steps. The simulated hull flow field showed a good agreement with the testing data. Park et.al. [7] carried out the experimental studies to evaluate the interceptor performances on vertical motion control by calm water tests and seakeeping tests. An improvement on the resistance by the interceptor is seen at the speed less than Froude number 1.16. The pitch motion is decreased by 41.3% in the regular wave and 32.4% in the irregular wave by the controllable interceptor system. Ravinthrakumar et.al. [8] investigated the ship and moonpool responses using experimental method. The hydrodynamic coupling of the ship and moonpool responses was studied to understand nonlinear effects under regular and irregular wave conditions. Tello et.al. [9] studied the seakeeping performance of a set of fishing vessels, aimed to identify the seakeeping criteria and vessel conditions that limit the operability of the fishing vessels in certain sea states using the transfer functions of the hull forms. The obtained results show that roll and pitch criteria are most critical for seakeeping performance.

In recent years, plenty of investigations on ship's seakeeping behavior have been conducted using CFD method. The applications of CFD method were generally based on potential flow theory or viscous RANS method. Orihara [10] used unsteady RANS code to simulate the tank model over a range of wave conditions. CFD simulation results were compared with experimental data. It is shown that the simulation results agree well with the experimental data. Guo et.al. [11] performed systematic validation and verification works of the numerical method to predict the added resistance and ship motions of KVLCC2 in head waves. The comparisons with theoretical calculations based on strip theory and experimental results show that RANS method predicts the added resistance better in all wavelengths. Ship pitch and heave motions in regular head waves can be estimated accurately by both CFD and strip theory. Winden et.al [12] used a RANS solver coupled with an actuator disk model for the propeller to simulate a self-propelled ship in waves. The simulation results show promising results and need more validation data to judge the accuracy of the method. Wang and Soares [13] used a numerical method to study the ship motions, slamming occurrence probability and slamming loads on the bow of a ship hull in irregular waves. The probabilities of slamming occurrence at the bow are studied numerically and statistically, and compared with the experimental data. Kim and Kim [14] compared the performances and trends of seventeen seakeeping analysis codes for the benchmark test of 6750-TEU container ship. Kim et.al. [15] predicted the added resistance and attainable ship speed under actual weather conditions. Three different methods: two-dimensional (2D) and three-dimensional (3D) potential flow methods and a CFD with an unsteady RANS approach were used to predict the added resistance and ship motions in regular head and oblique seas. Kim et. al. [16] used an unsteady RANS method to investigate the wave resistance and 2 degrees of freedom (DOF) motions (heave and pitch) for KVLCC2 and its modified hull form. The numerical method was validated when compared with KRISO towing tank experimental results under the identical condition. The modified hull form gives about 8% of energy saving in sea state 5 condition. Weems et.al. [17] describes the continued development of a high-speed simulation code which could provide a very fast but qualitatively reasonable model to predict large-amplitude ship motions. The description includes the extension of the code to 6DOF and results of its cross validation with the Large Amplitude Motions Program (LAMP). Taghva et.al. [18] studied seakeeping performance of the S-175 container ship under irregular wave conditions using numerical calculation and artificial neural networks (ANNs). Strip theory is employed to calculate the response amplitude operator (RAO) and wave resistance. Predictive equations based on length of vessel, the breadth, draft and wave encounter angle, are presented to estimate the seakeeping performance by using ANNs. Hou et.al [19] used a combination of random decrement technique (RDT) and support vector regression (SVR) to identify the nonlinear damping and restoring moments in the mathematical model simultaneously by using the random rolling responses of ships in irregular waves. Hirotada et. al. [20] conducted a validation of the unsteady RANS CFD method based on the experimental test on surf-riding/broaching event in irregular waves. The comparison results indicated good agreement between CFD results and the experimental results except for the wave-induced roll moment. Niklas and Pruszko [21] conducted full-scale CFD seakeeping analysis for a case study vessel in two variants: V-shaped bulbous bow hull form (as built) and innovative hull form (X-bow type). Selected numerical results were validated based on experimental tests in a towing tank and showed good agreement with experimental results. Song et.al. [22] analyzed stern flap effects on resistance and propulsion performance for the high-speed surface ship numerically. The simulation results showed that stern flap could not only reduce the ship resistance but also improve the propulsion performance. Lakshmynarayanana and Temarel [23] used a two-way implicit coupling between RANS/CFD and Finite Element Method (FEM), STARCCM+ and Abaqus, to investigate the symmetric motions and wave-induced loads of S-175 containership. The numerical predictions of symmetric motions, acceleration and vertical bending moment are compared against experimental measurements and other available numerical predictions.

The emerging particle method has also been used for various hydrodynamic researches of ship seakeeping behavior in waves. It was a particle-based Lagrangian method more efficient for handling large free-surface deformations than grid-based Eulerian method [24]. Amicarelli et al. [25] presented a 3D Smoothed Particle Hydrodynamics (SPH) numerical scheme to reproduce the transport of rigid bodies in free surface flows. This model has been validated on a sequence of 2D and 3D test cases, compared with measurements, RANS results, analytical and theoretical solutions. Kawamura et al. [26] conducted SPH simulations to predict the ship motions of a fishing vessel in severe water-shipping condition. The accuracy of this SPH method has been investigated by comparisons with captive and free motion tests, showing good potential for future development of quantitative safety assessment of fishing vessels. Servan-Camas et al. [27] carried out the numerical simulations in the time domain of ship seakeeping performance using a SPH solver coupled with a FEM diffraction-radiation solver. The numerical results agreed well with the experimental data. Zhang and Wan [28] applied an inhouse particle solver based on improved Moving Particle Semi-Implicit (MPS) method to simulate the roll motion of a 2D floating body in low-amplitude regular waves. The solver showed good capacity in predicting the seakeeping performance of floating body in regular waves. Sun et al. [29] used the modified MPS and modal superposition methods for the time-domain hydroelasticity computation. This method has been validated by the rigid cylinder water problem and the flexible cylindrical shell water entry simulation. Rijas et al. [30] used Improved Meshless Local Petrov-Galerkin method with Rankine Source function (IMLPG_R) with variable spacing resolutions to simulate the motion of a 2D rectangular freely floating body under waves. The results showed that this method is efficient for simulating steep wave interaction with floating body problems.

In the present study, inhouse viscous CFD code HUST-Ship has been used for the numerical studies. Based on URANS method, HUST-Ship is developed especially for ship hydrodynamic performance studies. This code contains 6DOF ship-motion model using structured overset multiblock grid technology. Structured dynamic overset grid technology could be used to simulate the rigid-body motions for different sea conditions, especially for the large-amplitude motions in severe wave conditions. Overset grid technology could also simplify the grid generation procedures. Verification and validation (V&V) studies in calm water and regular head wave conditions have been carried out to validate the numerical method. The experimental data was obtained from the model tests conducted at the Laboratory of Ship Model Towing Tank of HUST.

This paper presents a numerical study of seakeeping behavior for catamaran with/without stern flap advancing in long crest head wave using HUST-Ship. Both model-scale and full-scale seakeeping simulations have been carried out. Statistical analyses of unsteady motions and resistance in wave of the catamaran have been performed to investigate stern flap effects on seakeeping behavior of the catamaran at model scale and full scale.

The rest of paper is structured as follows. After introduction, Section 2 provides numerical and mathematical theories involved in this CFD code. The objectives of numerical study and simulation conditions are described in Section 3. Section 4 presents the verification and validation works for the CFD code. Model tests of the catamaran in calm water and regular wave have been conducted to validate the numerical method. In Section 5, numerical results of the model-scale catamaran with/without stern flap advancing in sea state 6 have been shown, as well as the comparisons and discussions. The accuracy of long crest wave generated using HUST-Ship has been validated. Full-scale simulation results of catamaran's vertical motions are also presented and the results are compared with model-scale results. Finally, the main results are summarized, and conclusions are drawn briefly in Section 6.

Section snippets

Computational method

HUST-Ship, an inhouse viscous CFD code, has been applied on CFD prediction of the catamaran's seakeeping behavior in long crest head wave. This CFD code solves the URANS equations in liquid phase of the free surface flow. Single-phase level-set method is used for capturing free surface. Governing equations are discretized in space by applying the finite difference method. The SST (shear stress transport) k-ω turbulence model is used to compute the turbulent viscosity, thus closing the governing

Catamaran model and stern flap

The present study analyzes stern flap effect on seakeeping behavior of a high-speed catamaran, including numerical simulations and experimental tests. A 1/10 scaled model based on Froude number (Fn) and a full-scale model have been chosen as the study objective. This catamaran consists of two symmetrical demi-hulls with a sharp bow and a transom stern. Fig. 1 and Table 1 show the geometry and principal dimensions of the catamaran model with stern flap.

Dimensions of stern flaps used in this

Verification and validation

Verification and validation are carried out following ASME V&V procedure [39]. Numerical modelling error is estimated requiring validation uncertainty and comparison error between numerical result and experimental data. Validation uncertainty includes uncertainties from experimental result, numerical solution, and input parameters. It is assumed that all input parameters are deterministic and the uncertainty is zero. Uncertainty UD in experimental result could be estimated using experimental

Stern flap effect in long crest head wave

The following section will outline the simulation results of model-scale and full-scale catamaran with/without stern flap in long crest head wave achieved during this study. Comparisons of the results are provided for discussions of stern flap effect on catamaran model in long crest wave and scale effect of stern flap. Before the discussions of results, numerical modeling is described in detail.

CRediT authorship contribution statement

Liwei Liu: Software, Data curation, Writing - original draft. Xianzhou Wang: Supervision, Writing - review & editing. Ran He: Methodology, Resources. Zhiguo Zhang: Conceptualization, Validation. Dakui Feng: Investigation.

Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests

Acknowledgements

This research was sponsored by the Advanced Research Common Technology Project of CHINA CMC (41407010401, 41407020502). This essential support is greatly acknowledged.

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