Proceedings of the 11th International Conference

on Hydrodynamics (ICHD 2014)

October 19 – 24, 2014Singapore
Editors: TAN Soon Keat, WANG Xikun, GHO Wie Min & Joy CHUA

Session Papers (61-80)

61.  Studies on hydrodynamics and modeling for a supercavitating body in transition phase.

Seonhong Kim, Nakwan Kim.

Department of Naval Architecture and Ocean Engineering, Seoul National University,Seoul, Republic of Korea.

Abstract :

A supercavitation is a modern technology that can reduce the frictional resistance of an underwater vehicle. To reach supercavity condition, which is when the cavity envelops the entire vehicle body, a vehicle passes through the transition phase from fully-wetted to supercaviting operation, where unsteady hydrodynamic forces and moments are created by partial cavity. This paper presents analytical and numerical investigations into the dynamics of a supercavitating vehicle in the transition phase. In this phase, the modeling for a supercavitating vehicle is performed with the cavity, the cavitator which generates the cavity, the fin, and the hydrodynamic and hydrostatic forces acting on the wetted body. The immersion depths of the fins and body are defined by the cavity profile to calculate the hydrodynamical effects on the body. The hydrodynamical effects are the added mass effect, damping forces and the planing force which is generated by the interaction between body and water wall. Numerical simulations have been performed by integrating each model to the steady state after launch and the physical feasibility and characteristics of the modeling are analyzed using the simulation.

62.  Simulation of Parametric Roll by Using a Semi-Analytic Approach.

J.-H. Lee, Y. Kim, K.H. Song.

Department of Naval Architecture and Ocean Engineering, Seoul National University, Seoul, KS013, Korea.

Abstract :

In this study, a semi-analytic approach is applied to simulate parametric roll  phenomena of large containerships in the longitudinal waves. This simplified approach is a 1.5 degree of freedom (DOF) model which includes the change of transverse stability of ship induced by heave and pitch motions as well as the wave elevation. Not only the check for susceptibility of parametric roll but also the quantitative prediction of roll motion is carried out by approximating the nonlinearity of GZ curve. In particular, the relationship between the righting arm (GZ) and the metacentric height (GM) in waves is examined to formulate a more proper model in comparison with the conventional models. Furthermore, the transfer functions of the mean value and the first harmonic component of GM value are introduced to approximate the change of stability. The assumption of linearity of these transfer functions, which is proven by numerical simulation, enables the model to be adopted for the prediction of parametric roll in irregular wave excitations by using the application of the principle of superposition. To validate the results of the proposed model, the comparison with those of the weakly nonlinear time domain method is conducted. As the time domain method, the impulse response function (IRF) method is developed by applying the rigorous transformation of the frequency domain hydrodynamic coefficients obtained by strip theory. According to the validation, the accuracy and efficiency of the approach are investigated, and stochastic characteristics of the parametric roll excitation in irregular waves are also discussed in the consideration of the sensitivity and uncertainty of the model.

63.  Experimental Study on Added Resistance for Different Bow Shapes of KVLCC2.

J. Lee, D.M. Park, Y. Kim.

Naval Architecture and Ocean Engineering, Seoul National University, Seoul, Korea.

Abstract :

The effects of the different bow shapes on added resistance are studied by towing tank experiment. For the comparison of added resistance, three different KLVCC2 hull forms are considered: one original hull form and two modified hull forms called Ax-bow and Leadge-bow. Ax-bow type hull form has the same shape with original hull form below mean water level but a different bow shape above mean water level. Meanwhile, Leadge-bow type hull form has rather different shapes along the whole bow section compared with original hull form. Test condition includes ten wave lengths for fixed wave amplitude in regular head sea condition at a design speed. For each case, added resistance is evaluated based on ITTC recommendation, subtracting the still water resistance from the mean total resistance in waves. Test results of the original hull formare compared with the published experimental data conducted in other institute and shows similar reasonable band for added resistance and motion responses. Finally, the experimental results of added resistance and motion responses are compared for three different hull forms and the effects of the bow shapes on added resistance are discussed.

64.  Aero-hydroelastic instabilities on an Offshore Fixed-Bottom Wind Turbine in severe sea state.

E. Marino, C. Lugni, C. Borri.

University of Florence, Dept. of Civil and Environmental Engineering, Via di Santa Marta, 3 - 50139 Firenze, Italy.
CNR-INSEAN, National Research Council, Via di Vallerano, 139 - 00128 Roma, Italy.

Abstract :

Hydrodynamic loads associated with nonlinear wave kinematics have important effects on the structural behaviour of offshore wind turbines. Recent literature has shown that steep non-breaking waves are responsible for triggering resonant vibrations of the tower with significant implications in terms of structural safety. In this paper, a new fast hydrodynamic solver, which combines a linear and a fully nonlinear high-order boundary-element wave solver within a domain decomposition strategy, is integrated into a full hydro-aero-elastic algorithm in order to assess the effects of nonlinear contributions on the whole system, including the rotor blades. High-frequency blades oscillations caused by the wave impacts produce high-frequency load cycles in the tower loads, revealing a complete hydro-elastic coupling of the whole system. Standard linear wave theory leads to dangerous inaccuracies, not only in the tower butalso in the blades responses.

65.  Wave interaction with floating flexible circular cage system.

S. Mandal, T. Sahoo.

Department of Ocean Engineering and Naval Architecture, Indian Institute of Technology, Kharagpur -721 302, India.

Abstract :

The present study deals with the hydroelastic analysis of surface gravity wave interaction with floating flexible circular cage system. The problem is analyzed in finite water depth under the assumption of small amplitude water wave theory and structural response. The flexible cage system is modeled as a surface-piercing porous flexible cylinder having a porous flexible bottom and is of uniform mass and is acting under uniform tension. Using the Fourier Bessel series solution and least square approximation method, the mathematical problem is handled for solution by matching the velocity and pressure at the fluid-structure interface. Further, the flexible cage system is assumed to be kept fixed at the free surface and the submerged end. The efficiency of the cage system is studied by analyzing the hydrodynamic force, effect of structural porosity and deflection of the flexible cage for various wave and structural parameters.

66.  Trapping of surface waves by a submerged trapezoidal breakwater near a wall.

H. Behera, T. Sahoo.

Department of Ocean Engineering and Naval Architecture, Indian Institute of Technology, Kharagpur -721 302, India.

Abstract :

In the present study, trapping of surface gravity waves by a submerged rigid trapezoidal breakwater in front of a rigid wall in the presence of a porous barrier is analyzed under the assumption of linearized long wave theory. As a special case, the problem of wave trapping by a sloping step having a rigid vertical end-wall is analyzed in the presence of the porous barrier. The role of sloping angle, the submergence depth of the structure, structural porosity and the distance between the porous barrier and rigid wall, in minimizing wave reflection and load on the structure are analyzed.

67.  A Cavitation Aggressiveness Index (C.A.I) within the RANS methodology for Cavitating Flows.

G. Bergeles, P. Koukouvinis, M. Gavaises.

Department of Engineering and Mathematical Science, City University, Northampton Square, London, EC1V0HB, UK.

Abstract :

The paper proposes a methodology within the Reynolds AveragedNavier Stokes (RANS) solvers for cavitating flows capable of predicting the flow regions of bubble collapse and the potential aggressiveness to material damage. An aggressiveness index is introduced, called Cavitation Aggressiveness Index (CAI) based on the total derivative of pressure which identifies surface areas exposed to bubble collapses; the index is tested in two known cases documented in the open literature and seems to identify regions of potential cavitation damage.

68. A time domain panel method for the prediction of nonlinear hydrodynamic forces.

R. Datta, C Guedes Soares, J. M. Rodrigues.

Dept. of OE & NA, IIT-Kharagpur, Kharagpur, West Bengal, 721302, India.

Abstract :

Prediction of nonlinear wave forces within engineering accuracy is one of the most challenging topics in the domain of numerical ship hydrodynamics for many years. In the present work, a time domain panel code is extended to deal with large amplitude motions by incorporating non-linear representations of the hydrostatic and Froude-Krylov forces. The formulation is based on earth fixed co-ordinate system using transient free surface Green’s function. The wetted surface is re-defined at each time step and a suitable algorithm is developed to predict the free surface and to remesh the wetted surface. The hydrostatic and Froude-Krylov forces are then calculated on exact wetted surface. However, the waves are generated using linear wave theory. To determine the robustness and efficiency of the newly developed nonlinear code, rigorous numerical results are obtained and are validated with published results. The fk-forces are obtained with increasing wave amplitude. The study shows that the non linear effects are important in case of large amplitude wave and effects are very clearly visible.

69.  A Propeller Design Procedure Considering the Interaction between Ship Hulls and Propulsion Devices.

Ching-Yeh Hsin, Ching-Pu Lee, Tung-Ching Chen And Le-An Lin.

Department of Systems Engineering and Naval Architecture, National Taiwan Ocean University, Keelung, Taiwan, China.

Abstract :

In this paper, a coupled viscous/potential flow computational method to design a propeller in the effective inflow is presented. The interactions between the ship hull and the propeller have usually been ignored when designing propellers even it may result in inadequate designs. In the presented method, the viscous flow and the potential flow computational methods are coupled to consider the interaction between the ship hull and the propeller. In this propeller design method, the traditional potential flow propeller design method is carried out, and the “body force” is used to represent the propeller in the RANS computations including the ship hull. The propeller designs therefore can include the effects of the interactions with ship hulls. In this paper, two propellers designed in the uniform inflow and in the effective inflow respectively show that their designed geometries reflect the inflow. The performances of two propellers are very close atthe self-propulsion point; however, the pressure distributions are different. To judge the performances from the pressure distributions, both propellers show a better quality in their designed inflows. Currently, propeller designers usually adjust their designs in the uniform inflow based on the ship wake. From the design results presented in the paper, we believe that the design method in the effective inflow proposed here is applicable, and designers can obtain a real “wake adapted” propeller geometry using this method.

70  Development of the Horizontal Axis Marine Current Turbine Blade Design Procedure.

An-Chieh Kuo, Ching-Yeh Hsin, Fan-Ting Hong, etc.

Department of Systems Engineering and Naval Architecture, National Taiwan Ocean University, Keelung, Taiwan, China.

Abstract :

In this paper, a current turbine blade design procedure similar tothe propeller designs is presented. In this procedure, the lifting line method with the Lagrange multiplier method is used for the design of the optimum circulation distribution, and the lifting surface method is adopted for the blade geometry design. Both the BEM and RANS methods are then used to verify the designs. The presented potential flow boundary element method is a perturbation potential based method, and a wake alignment numerical scheme is established for the current turbine. The commercial software STAR-CCM+ is used for the RANS computations. In order to extract the current energy from the Kuroshio, the horizontal axis marine current turbines are designed and studied. Both the potential flow boundary element method and the viscous flow RANS method are used for the analysis, and the results are compared to each other. The results from two methods are very close, and it assures that the BEM can be applied to the designs with reliable results. Two design examples are demonstrated in the paper, and the design results show that the developed design procedure is applicable, and it can be used for the current turbine blade designs in the future.

71. Sigma-ZED: A Computationally Efficient Approach to Reduce the Horizontal Gradient Error in the EFDC’s Vertical Sigma Grid.

P.M. Craig, D.H. Chung, N.T. Lam, P.H. Son, N.X. Tinh.

Dynamic Solutions-International, LLC, 535/24 Lac Long Quan Str. Hanoi, 10000, Vietnam.

Abstract :

The Environmental Fluid Dynamics Code (EFDC), originally developed at Virginia Institute of Marine Science, is a well-known and widely applied three dimensional hydrodynamic model used for many environmental applications (Hamrick, 1996). In the horizontal, the EFDC numerical scheme uses an orthogonal curvilinear grid. In the vertical, EFDC uses a sigma coordinate transformation that uses the same number of layers for all cells in the domain. To accommodate the varying depths over the model domain, the thickness of the layers vary from cell to cell but the number of layers and fraction of depth for each layer are constant. This approach introduces a well-known error in the density gradient terms, otherwise known as the pressure gradient error (Mellor, 1994). These errors are most pronounced in regions with steeply varying bathymetry. A new vertical layering approach that is computationally efficient has been developed and applied to the EFDC model, thereby reducing these pressure gradient errors. The vertical layering scheme has been modified to allow for the number of layers to vary over the model domain. Each cell can use a different number of layers, though the number of layers for each cellis constant in time. As with the Sigma stretch approach, the thickness of each layer varies in time to accommodate the time varying depths. The z coordinate system varies for each cell face, matching the number of active layers to the adjacent cells. The new version is computationally more efficient than a similarly configured sigma stretch grid, thus making models with 20 to 50 layers or more practical for typical projects. This new approach has been tested with several hypothetical test cases. The model has been applied to Lake Washington (Seattle,WA, USA), which has steep bottom gradients and sharp thermoclines. The results indicate that the vertical variation of temperature and the thermal stratification are more accurately reproduced and provide a significant improvement compared to the earlier sigma coordinate transformation method.

72.  Measurement of velocity field around a circular cylinder near plane boundary undergoing vortexinduced vibration.

S.-C. Hsieh, Y.-M. Chiew, Y.M. Low.

School of Civil and Environmental Engineering, Nanyang Technological University, Nanyang Avenue, Singapore.

Abstract :

Vortex-induced vibration (VIV) has been studied extensively, but when the cylinder isplaced near a plane wall, the flow around the cylinder becomes more complex. Most ofthe aforementioned investigations focus on the effects of response amplitude of the vibrating cylinder and categorizations of the wake with different gap ratios G/D. This paper aims to study the flow characteristics around the freely vibrating cylinder near the boundary with different G/D(included the wall-effect-free region, the intermediate region, and the vortex-shedding suppression region) using high time-resolved PIV technique. The mean velocity fields and turbulence characteristics were obtained by ensemble averaging repeated velocity measurements. Based on comparing the mean velocity fields of the cases with different G/D, the gap flow between the cylinder and plane boundary have a significant effect. The characteristics of the turbulence intensity, formation and transmission of the vortices are also discussed in the study.

73.  Shear stress acting on the bed with verticalcircular cylinders inopen-channel flow.

Kenjirou Hayashi, Ryou Saitou, Tuyoshi Tada.

Dept. of Civil and Environmental Eng., The National Defense Academy, 1-10-20, Hashirimiz,
Yokosuka, Kanagawa 209-8686, Japan

Abstract :

The shear stress acting on the smooth bed around a vertical circular cylinder installed in an open channel with multiple rows is measured directly using a shear stress sensor which is one component load cell. The velocity and Reynolds stress distributions on the bed with vertical circular cylinders are also measured by using a Laser Doppler anemometer with 2 velocity component systems (2D-LDV). The close agreement between the shear stress measured by the shear stress sensor and the estimated values from the velocity and Reynolds stress distributions are obtained inthe case of open channel flow without circular cylinders. The values of Manning’s roughness coefficient in the part of bed increases with increase of arrangement density ratio λ of multiple rows of vertical circular cylinders in the open channel.

74.  Calibration of MEMS Shear-Stress Sensors Array in Water Flume for Underwater Applications.

Tian Yu-Kui, Xie Hua, Huang Huan, Sun Hai-Lang, Zhang Nan.

National Key Laboratory of Science and Technology on Hydrodynamics, China Ship Scientific Research Center (CSSRC), P. O. Box 116, Wuxi, Jiangsu, 214082, China.

Abstract :

The measurement of skin friction on hydraulic smooth surface is ofimportant value for the design of advanced naval technology. Various MEMS shear stress sensors are continuously developed with great efforts and will play a more and more significant role in such underwater applications and, above all, correct calibration of the sensors array is indispensable to any practical measurement. For this purpose, a well-defined fully developed 2D flow is modeled in a water flume and a comprehensive measurement as well as CFD simulation adopted to characterize the inner flow and stream-wise distribution of wall shear stress inside the test channel. Under the investigated input conditions, the calibration of a typical thermal MEMS shear stress sensors array is conducted in the bulk velocity range of 1~5 m/s. And based on the calibration results, the optimal adjustment coefficients are determined for experimental applications with the array.

75.  A combined viscous and potential method for the computation of added resistance in head waves.

Chengsheng Wu, Jiang Lu, Daijun Yan, Shuxia Bu, Gengyao Qiu.

China Ship Scientific Research Center, Wuxi, 214082, China.

Abstract :

Prediction of added resistance in waves is now a hot topic for ship performance in seaway, especially for the added resistance in short waves. In order to provide a reliable and practical method for the prediction of added resistance in waves, a combined viscous and potential method is used for the computation of added resistance in head waves. In this method, the viscous part is based on Reynolds Averaged Navier-Stokes Equations (RANSE), while the potential part is Maruo method. The added resistance of KVLCC2 advancing in regular head waves is computed by the method mentioned above. Inwhich RANSE based method is used for the computation of added resistance when wave-length to ship-length is smaller than 0.6, while Maruo method is used in cases of wave-length to ship-length larger than 0.6. The comparison of the computed results of added resistance with the experimental data shows rather good agreement. This means thatthe combination of viscous and potential tool could provide a reliable and practical method for the prediction of added resistance in waves.

76.  Hydroelasticity analysis of ships in waves.

R Datta, M R Sunny.

Dept. of Ocean Engineering and Naval Architecture, IIT-Kharagpur, India.

Abstract :

In the present study, a coupled boundary element method (BEM)-finite element method (FEM) based approach is developed to predict the hydroelastic response of the ships in seaway. The 3Dlinear panel method is used to solve the hydrodynamic problem whereas time dependent finite element method is used for structural analysis. The computed results are validated with other published results for simple geometry such as rectangular barge. The results show the efficiency and the correctness of the proposed scheme. Also the effect of the flexible modes for bending is studied extensively.

77.  The base ratio perturbation on transient waves in a 3D regular tank due to oblique horizontal excitation.

C.H. Wu, B.F. Chen, T.K. Hung.

Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis North, 138632, Singapore.

Abstract :

A finite difference method with appropriatecoordinate transformations is developed to simulate fluid motion in a threedimensional tank. The developed numerical scheme is verified by the rigorous benchmark tests. The experiment measurement of liquid sloshing in a 3D tank was also carriedout in this study to further validate the accuracy of the present numerical results. The effect of various oblique horizontal excitation directions of the tank on liquid sloshing is discussed in this work. It is naturally for transient waves to change their types in the time domain, especially for a tank excited by resonant frequencies. The influence of different base ratios of a rectangular tank on kinematic and dynamic responses of sloshing fluid is also explored in this work. It is found that the natural mode system of the partially-filled tank is varied with the base ratio perturbations. The occurrences of different types of sloshing waves due to variousaspect ratios of tank cross-section on liquid sloshing are totally different from those in a square-base tank. The coupling effect of resonant modes of sloshing waves in x and z axes would induce the coexistence of various sloshing waves for a rectangular tank under a certain excitation frequency. As a result, the aspect ratio of tank cross-section has a significant influence on the kinematic and dynamic responses of transient waves.

78.  Numerical and semi-analytical methods for optimizing wave energy parks.

M. Göteman, J. Engström, M. Eriksson, J. Isberg.

Dept. of Engineering Sciences, Uppsala University, Box 534, 751 21 Uppsala, Sweden.

Abstract :

For the design of effective and economically viable wave energy parks, it is desirable to reduce the large power fluctuations and thereby eliminate the need for expensive and oversized electrical systems. Other challenges include minimizing the used ocean area and the costs associated with devices and sea cable. Simulations of the interactions between devices in wave energy parks can provide guidelines for optimal configurations with regard to maximizing produced electricity while minimizing fluctuations and costs. Parameters that may influence the performance include the dimensions and number of wave energy converters, the local and global geometry of the park, the wave climate and incoming wave direction, etc. However, the complexity of the model increases rapidly with growing number of interacting units, and the numerical simulations become a challenge that calls for new methods. Here we study large arrays of point-absorber wave energy converters (WECs) based on a numerical and an approximate analytical method. The WECs are driven by measured time-series of irregular waves. In the approximate analytical method, only interaction by radiated waves between the devices are taken into account, and wave energy parks with more than a thousand units can be modelled with relatively low computational cost. The approximation still gives good agreement with the numerical method, where full hydrodynamical interactions are accounted for. To enable accurate modelling of very large parks, an interaction distance is introduced, such that hydrodynamical interaction is neglected between devices that lie too far from each other. An overall aim of this paper is to optimize wave energy parks.

79.  Experimental study of drag reduction on rough cylinders.

B. Zhou, X.K. Wang, S.K. Tan And W. M. Gho.

Maritime Research Centre, School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore.

Abstract :

The main purpose of this study is to establish a better understanding of the relationship between drag reduction and surface roughness. In a range of Reynolds numbers, 2 ×104 < Re < 8 ×104, new experiments were performed to measure the turbulent flow around single cylinders with smooth, rough surfaces in tow tank. The rough surface is made by covering a net mask on the cylinders. It is found that there is a significant influence of surface roughness on the mean drag coefficient even at very large Reynolds numbers.

80.  Force and flow characteristics of a circular cylinder with grooved surface.

B. Zhou, X.K. Wang, S.K. Tan And W. M. Gho.

Maritime Research Centre, School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore.

Abstract :

New experiments were performed to measure the force and flowcharacteristics of a circular cylinder with grooved surface roughness in a range of Reynolds numbers, 7.43×103 to 7.99×104. The main aim of this study is to obtain a better understanding of the drag reduction mechanism of surface roughness, thorough quantitative measurement of the flow field around the cylinder using Particle Image Velocimetry (PIV). The findings of the study revealed that a grooved cylinder produces a lower drag coefficient that was about 0.75 times of that of a smooth cylinder.