China Ship Scientific Research Center Research Articles

Investigations on Flexural Strength of a Columnar Saline Model Ice under Circular Plate Central Loading

The properties of ice strength have a significant impact on the design and safety of structures in ice-infested waters. To analyze the flexural strength of columnar saline model ice, we conducted circular plate center loading tests at the Small Ice Model Basin of the China Ship Scientific Research Center (CSSRC SIMB) in China. The tests involved varying the loading rate and ice temperature, and a numerical model was developed using FEM and LS-DYNA for validation and comparison. The results of the tests revealed the crack propagation process, stress distribution, load response, and failure mode of the model ice. The model ice displayed typical brittle failure, and the flexural strength was linearly related to ice temperature but not significantly correlated with loading rate. The porosity of the model ice affected the load response and time of failure but not the failure mode. The model ice with 7% porosity had a 7.8% reduction in load response compared to the nonporous model ice. This study provides a reliable method for measuring and analyzing the flexural strength of model ice. It also serves as a foundation for further research on the interaction between structures and ice sheets.

An example of numerical ice tank based on DEM simulation and physical model testing

The determination of ice loads on polar ships and offshore structures is of great significance for ice-resistant design, safe operation, and structural integrity management in ice-infested waters. The physical model testing carried out in ice tank/basin is usually an important technical approach to evaluate the ice loads, however, the high cost and time consumption make it difficult to perform multiple repetitions or large number of trials for this purpose. Recently, the rapid development of high-performance computation techniques provides a usable alternative where the numerical methods represented by the discrete element method (DEM) have made remarkable contributions to the ice load predictions. On basis of DEM simulation validated by physical model testing, numerical ice tank can be developed as an effective supplement to its counterpart. In this paper, such an example of numerical ice tank adopting GPU computational mechanism and DEM modelling algorithm was established with respect to the small ice model basin of China Ship Scientific Research Center (CSSRC-SIMB). The numerical ice tank was calibrated and further optimized with physical model tests on typical structures of vertical cylinder and inclined flat plate in level ice sheets by making agreements of both globe value and time history of the ice loads. Then it was practiced for modelling the tests of Wass bow advancing in level ice performed in SIMB separately. It is demonstrated by the comparisons of ice failure details and ice loads that the numerical ice tank can precisely simulate the ice-structure interactions and determine the ice loads under the same initial conditions of physical model testing. In the end, the advantages as well as the challenges of the numerical ice tank are discussed.

Development of a Numerical Ice Tank Based on DEM and Physical Model Testing: Methods, Validations and Applications

The determination of ice loads on polar vessels and offshore structures is important for ice-resistant design, safe operation, and management of structural integrity in ice-infested waters. Physical model testing carried out in an ice tank/basin is usually an important technical approach for evaluating the ice loads. However, the high cost and time consumption make it difficult to perform multiple repetitions or numerous trials. Recently, the rapid development of high-performance computation techniques provides a usable alternative where the numerical methods represented by the discrete element method (DEM) have made remarkable contributions to the ice load predictions. Based on DEM simulations validated by physical model tests, numerical ice tanks can be developed as an effective complement to their counterparts. In this paper, a numerical ice tank based on 3D spherical DEM was established with respect to the small ice model basin of China Ship Scientific Research Center (CSSRC-SIMB). Based on spherical DEM with parallel bond model, the model tests of typical structures (vertical cylinder and inclined plate) in level ice sheets were established in the numerical ice tank, and the ice–structure interaction process under the same initial conditions was simulated. The accuracy of the simulations is verified by comparing the simulated ice loads with the measured ice loads from the model tests in the CSSRC-SIMB. Furthermore, the application of the numerical ice tank was extended to simulate the navigation of a Wass bow in level ice and broken ice conditions. The value of the break resistance of the Wass bow in level ice was evaluated, and the numerical ice tank produced results that were found to be consistent with those obtained from Lindqvist’s formula. The statistical properties of the bow load for different broken ice fields with the same initial physical conditions are analyzed by performing a repeatability test on the broken ice fields.

The first International Conference on Mechanical System Dynamics grandly convened in Nanjing, China

The first International Conference on Mechanical System Dynamics grandly convened in Nanjing, China

Numerical simulation analysis of flexural strength of columnar saline ice model

Objective As flexural failure is one of the main failure modes of ships navigating in level-ice region, research into flexural strength has great significance for engineering practices. Methods First, numerical prediction calculation is carried out for a model ice in a CSSRC （China Ship Scientific Research Center） ice tank; second, the discrete element method (DEM) is used to carry out the numerical simulation of the flexural strength of a columnar saline ice model, the calculation parameters are obtained through ice mechanics experiments, the rectangular element is used to discretize the ice layer, and cell array is used to transfer the control force. The calculation results are then verified using the flexural strength test data to ensure the reliability of the numerical simulation model. Results The variation law of flexural strength with velocity, ratio of width to thickness and ratio of length to thickness is obtained through the numerical model, and the basic scope of the flexural strength of the ice model is obtained through the error of the calculated results. Conclusion This numerical simulation method can provide references for predicting the mechanical properties of ice models.

Numerical Simulation of Ship Maneuvers through Self-Propulsion

The typical maneuvering of a ship can reflect its maneuvering characteristics, which are closely related to the safety and economy of its navigation. The accurate prediction of a ship’s maneuvering characteristics is essential for its preliminary design. This paper adopts the overset grid method to deal with multibody motion and the body-force method to describe the thrust distribution of the propeller at the model scale, as well as to obtain the changes in the hydrodynamic load and the characteristic parameters in a computational fluid dynamics (CFD) maneuver simulation. Then, the paper compares the results with those of a self-propulsion experiment conducted at the China Ship Scientific Research Center. The numerical results show that the maneuverability characteristics obtained from the CFD simulation are in satisfactory agreement with the experimental values, which demonstrates the applicability and reliability of the combination of the overset grid with the body-force method in the numerical prediction of the typical maneuvering of a ship. This provides an effective pre-evaluation method for the prediction of a ship’s maneuvering through self-propulsion.

Study of a Container Ship with Breaking Waves at High Froude Number Using URANS and DDES Methods

The KRISO container ship model is used for numerical simulations to investigate hydrodynamic performance under high speeds. Unsteady Reynolds-Averaged Navier-Stokes (URANS) and delayed detached eddy simulation (DDES) approaches are used to resolve the flow field around the ship model. High-resolution Volume of Fluid (VOF) technique in OpenFOAM is used to capture the free surface. The present work focuses on the wave-breaking phenomena of high-speed ships. To study the speed effects on the phenomenon of ship bow wave breaking, three different speeds, i.e., Fn = .26, .35, and .40, are investigated for a fixed ship model in calm water. Predicted resistance and wave patterns under Fn = .26 are validated with available experimental data, and a good agreement is achieved. The breaking wave phenomena can be observed from both URANS and DDES results for Froude numbers greater than .35. And the Fn = .40 case shows more violent breaking bow waves. The process of overturning and breaking of bow wave is more complex in the DDES results, and some small-scale free surface features are also captured. The predicted bow wave is compared with the experiment conducted at the China Ship Scientific Research Center. It shows that the DDES results are more accurate. Wave profiles and vorticity field at several cross sections are presented to illustrate the relationship between bow waves and vortices. It is found that the free surface vorticity dissipates quickly in the URANS simulation, which leads to the difference compared with the DDES results.

Hydroelasticity of the barge near island affected by bathymetry based on THAFTS software

Floating structures are sometimes deployed in shallow water areas close to reefs and islands as facilities serving tourists. In contrast to the open sea, the marine environment, such as wave and current, in the nearshore shallow water area will be affected by the non-uniform complex seabed bottom, and the hydrodynamic interaction between the floating structure and the seabed bathymetry should be investigated, instead of being ignored as the case when a uniform seabed is assumed. A hydroelastic analysis method for slowly varying water depths is proposed to calculate the hydroelastic responses of the floating structures, considering the seabed as a fixed boundary condition. This method was integrated into the software THAFTS (Three Dimension Hydroelastic Analysis of Floating and Translating, developed by China Ship Scientific Research Center). Based on the THAFTS software, the three-dimensional hydroelastic motion response analysis of a barge is carried out, in which the effects of different water depths and the complex bathymetry are investigated by comparing the results of the motion and structural responses of the barge. The stress distribution of the barge with the effect of the bathymetry taken into account is presented in this article as well as the modal stresses. The strength evaluation of the barge indicates that the effect of the inhomogeneous seabed plays a large role in the dynamic responses of the barge when it is deployed near reefs.

Three-dimensional hydroelasticity theory of ships in waves and ocean-acoustic environment with applications

The three-dimensional (3D) hydroelasticity theory of floating structures established in 1984 embodies the predictions of seakeeping behaviors, structural loads, dynamic strength, fatigue, and vibration of a surface ship or an underwater vehicle travelling in the sea in a unified generalized fluid-structure interaction theory. After more than thirty years of development, this theory has been further extended to two main branches: the 3D hydroelasticity theories for examining the wave excited dynamic responses of ships, and the 3D hydroelasticity theories for investigating the machinery or propeller excited vibrations and underwater acoustic radiations of a ship. The latter is also called the 3D sono-elasticity theory of ships. Drawn by the momentum gained from the comprehensive demand of marine applications, China Ship Scientific Research Center (CSSRC) has been devoted to the systematic innovative research in creating the linear and non-linear 3D hydroelasticity theories of ships, the 3D hydroelasticity theory of floating structures deployed in complicated ocean geographical environment near islands and reefs, the 3D sono-elasticity theory of ships in ocean acoustic environment etc. together with developing the numerical methods and corresponding software. These have been extensively employed to solve the key technical problems, such as motion and structural response predictions, behavior and safety assessment, design check and optimization of surface ships, underwater vehicles and other marine structures, This paper is not to present the state of the art of the worldwide development of hydroelasticity theories, but to concisely described the concept and framework of the 3D hydroelasticity and sono-elasticity theories of ships which are mainly developed by the research group of CSSRC. Typical application examples of the hydroelastic analyses of a 500000 DWT ultra large ore carrier and an 8-module very large floating structure, as well as the sono-elastic analysis of a simplified submarine model are also briefly illustrated. Finally, simply mentioned are the important directions of future research in the field of hydroelasticity and sono-elasticity of ships.

Experimental measurement of tip vortex flow field with/without cavitation in an elliptic hydrofoil

In this paper, recent measurements of tip vortex flow with and without cavitation carried out in Cavitation Mechanism Tunnel of China Ship Scientific Research Center (CSSRC) are presented. The elliptic hydrofoil with section NACA 662-415 was adopted as test model. High-speed video (HSV) camera was used to visualize the trajectory of tip vortex core and the form of tip vortex cavitation (TVC) in different cavitation situations. Laser Doppler velocimetry (LDV) was employed to measure the tip vortex flow field in some typical sections along the vortex trajectory with the case of cavitation free. Stereo particle image velocimetry (SPIV) system was used to measure the velocity and vorticity distributions with and without cavitation. Series measurement results such as velocity and vorticity distributions, the trajectory of tip vortex core, the vortex core radius, cavity size and cavitation inception number were obtained. The results demonstrated that the minimum pressure coefficient in the vortex core obtained by flow field measurement was quite coincident with the tip vortex cavitation inception number obtained under the condition of high incoming velocity and low air content. And TVC would decrease the vortex strength comparing with the case without cavitation.

Acoustic and vibrational characteristics of a propeller–shaft–hull coupled system based on sono-elasticity theory

The three-dimensional sono-elasticity method recently developed by Zou ((2014) Three-dimensional sono-elasticity of ships. PhD Dissertation, China Ship Scientific Research Center, China) and Wu ((1984) Hydroelasticity of floating bodies. PhD Dissertation, Brunel University, UK) is employed to explore the acoustic and vibrational characteristics of a propeller–shaft–hull coupled system. The acoustic field can be solved by introducing Green’s function for the ideal compressible fluid together with the Price–Wu generalized fluid–structure interface boundary conditions. The vibration of a ship structure is governed by the generalized equations, including added mass, damping and restoring coefficients. In order to discover the mechanisms underlying the acoustic and vibrational characteristics of the propeller–shaft–hull coupled system, numerical models for hull structures with a shaft and without a shaft are designated. Through modal analysis, the correlations of the line spectra of acoustic radiation and the corresponding vibration modes of the hull are clearly identified. Through further numerical analysis, the appropriate location of the base for the thrust bearing and installation schemes are recommended.

Combined experimental observation and numerical simulation of the cloud cavitation with U-type flow structures on hydrofoils

Sheet/cloud cavitation is an important topic that is a very common type of cavitation in turbo-machinery and marine propeller. Up to now we still have limited understanding of the cavitation shedding dynamics and cloud cavity formation and development. The present study used experimental and numerical studies to gain a better understanding of the complex physics involved in this problem. A series of experimental observations around hydrofoils are carried out in the cavitation tunnel of the China Ship Scientific Research Center (CSSRC) to illustrate the spatial–temporal evolution of the cloud cavity in detail. The results demonstrate that U-type flow structures are common in cloud cavities and can be divided into three stages and the closure line in a sheet cavity often has a convex–concave profile. Reentrant flows occur in the convex region with the jet direction normal to the contour edge so the shedding is mainly caused by the converging reentrant flows. Further analysis demonstrated that there was a striking difference with the cavity growth suppressed substantially in the twisted hydrofoil case if compared with straight hydrofoil and the effect of side entrant jets might make the cavity more uniform across the span. Numerical simulations were used to simulate the formation and development of the cloud cavity. The results show that the strong adverse pressure gradient in the stagnation region at the downstream end of the attached cavity forces the re-entrant flows into the vapor structure with a radially-diverging re-entrant jet and a pair of side-entrant jets, which causes the cavity shedding. Further analyzes of the local flow fields show that the interactions between the circulating flow and the shedding vapor cloud may be the main reason for the formation of the U-type cloud cavity structures.

Dynamic Asynchronous Coupled Analysis and Experimental Study for a Turret Moored FPSO in Random Seas

The dynamic coupling analysis between floating platform and mooring/risers is one of the challenging topics in development of oil and gas exploration in deepwater area. In this paper, the dynamic coupled analysis model for a turret-moored floating production storage and offloading (FPSO) is developed. In order to improve the computational efficiency, the proposed asynchronous coupling model is extended to application for FPSO coupled dynamic analysis. In order to validate the developed dynamic coupling analysis model, the vessel global motion and mooring tensions for the turret moored tanker KVLCC2 were tested in China Ship Scientific Research Center (CSSRC) wave basin. In this paper, the numerically predicted coupled response is directly compared with those from model test in time domain. The research focus is in the following: (1) Through comparison of free decay surge motion in calm water/regular wave, the damping effect from mooring on drift motion is validated and confirmed; (2) The physical phenomena of wave drift damping effect on slowly varying surge motion are theoretically explained making use of the related hydrodynamic model and physical model test; (3) In random seas, the simulated vessel motions and mooring tensions generally show fairly good match in time records with model test. But there is phase difference between simulation and measurements after tens of minutes in model test scale, the exact reason is not known yet; and (4) Another interesting phenomena in model test are the existence of unstable horizontal sway and yaw motion, which are not well predicted by current numerical model, the further research is needed.

Verification of a hybrid model test method for a deep water floating system with large truncation factor

The hybrid model test method has been regarded as the most reliable method to investigate the dynamics of deepwater floating systems when the wave basin is not large enough to conduct the full water depth model tests at reasonable model scales. In order to get the reliable experimental results, the truncation factor should be less than 5. However, as the offshore oil and gas industry moves towards deeper waters, the model tests of floating structures carried out at the wave basin with reasonable model scales need to be revamped. The objective of the present study is to investigate the feasibility of largely truncated hybrid model test for the deep water floating systems. The hybrid model test techniques and the dynamic behaviors of the truncated mooring lines are also studied. In order to achieve the static and dynamic similarities easily in the largely-truncated model tests, an optimization design method of the truncated mooring system is proposed, and the similarities of platform motion responses as well as the mooring lines dynamics are investigated in the largely-truncated model tests. To verify the present method, a deepwater semisubmersible system operated in 1500m and 1000m water depth is truncated to 200m water depth based on the static equivalent principle (the truncation factors are 7.5 and 5 respectively), and the dynamically similar design is implemented to the model tests which are carried out at China Ship Scientific Research Center wave basin. The motion responses of the platform and the mooring line dynamics are obtained. These experimental results are used to compare with the numerical simulation, which is based on the full time domain coupled dynamic analysis method. The comparisons show that the numerical results are in good agreement with the model measurements.

Hydroelastic analysis and model tests on the structural responses and fatigue behaviours of an ultra-large ore carrier in waves

To provide guidance on structural design for the life cycle safety of an ultra-large 500,000 deadweight tonnage ore carrier, the springing-induced high-frequency responses in waves, the resultant structural loads and their influences on the structural fatigue damage were investigated by employing the three-dimensional linear and second-order non-linear hydroelasticity theories of ships and the corresponding numerical methods. The theoretical predictions of the structural loads and responses and the comparisons with the model tests carried out in the towing tank of the China Ship Scientific Research Center are presented in this paper. The results for the hydroelastic responses of the ship at a forward speed and at zero forward speed in regular and irregular waves are illustrated, which provide the quantitative relations for the wave-motion-induced and the springing-induced structural loads, and their linear and non-linear components of the ship in different sea states, especially when the first flexible body resonant frequency is in the wave frequency region. The results show that the springing responses may occur in waves with any significant wave heights for such a large ship when the characteristic wave periods are less than a certain amount. The short-term and long-term distributions of the wave-induced and rigid-body-motion-induced structural loads and the springing-induced high-frequency loads are predicted. Based on the hydroelastic predictions of the structural loads, the accumulated fatigue theory is applied to assess the fatigue damage of the typical deck and bottom structural components of a ship serving in three shipping lines.

China's research sub dives deep

The country's first manned science sub goes for global bragging rights – When a team of Chinese engineers set out to build the deepest-diving submersible in the world, they had never laid eyes on a manned deepsea vehicle. "We had only read some reference papers that had pictures; there was no chance for us to see a real manned submersible," says Cui Weicheng, deputy director of the China Ship Scientific Research Center and project manager for the new sub. "We were starting from the very beginning."

Research on the noise generated by turbulent boundary layer flow over an axisymmetric body with a flat nose

The noise generated by hydrodynamic flow over an axisymmetric body with a flat nose is described quantitatively. Flush-mounted hydrophones were used to measure that part of the turbulent boundary-layer pressure fluctuations that propagates as true sound. Power spectra of sound pressure were measured in a 200-Hz to 150-kHz frequency range over a wide range of Reynolds numbers (U∞D/ν 2.52×10, where D is the diameter of the body) for the model operating in the China Ship Scientific Research Center 03B water tunnel. The power spectra of the noise measured in the laminar flow regions correspond little to those measured in the transition and fully developed turbulent regions of the flow. Free-stream turbulence intensity and transition regions on the model were measured by using a laser-Doppler velocimeter. The transition regions have been compared with the computation results and found to agree rather well with experiment. In addition to the experiment, 12 axisymmetric bodies are calculated and compared with the experiment. Results indicate that when the prismatic coefficient and DF/D are fixed, the model has fairly good flow properties. When DF/D is fixed and the prismatic coefficient is decreased, or the prismatic is fixed and the DF/D is decreased, the separation point or transition point is delayed.

The noise generated by turbulent boundary layer flow over an axisymmetric body with a flat nose

The noise generated by hydrodynamic flow over an axisymmetric body with a flat nose is described quantitatively. Flush‐mounted hydrophones were used to measure that part of the turbulent boundary‐layer pressure fluctuations that propagates as true sound. Power spectra of sound pressure were measured in a 200‐Hz to 150‐kHz frequency range over a wide range of Reynolds numbers (Ux D/ν = 2.52 × 10, where D is the diameter of the body) for the model operating in the China Ship Scientific Research Center 038 water tunnel. The power spectra of the noise measured in the laminar flow regions correspond little to those measured in the transition and fully developed turbulent regions of the flow. Free‐stream turbulence intensity and transition regions of the model were measured by using a laser doppler velocimeter. The transition regions have been compared with the computation results and it was found that they agreed rather well with the experiment. In addition to the experiment, 12 axisymmetric bodies were calculated and compared with the experiment. Results indicate that when the prismatic coefficient and DF/D is fixed, the model has fairly good flow properties. When DF/D is fixed and the prismatic coefficient is decreased, or the prismatic coefficient is fixed and the DF/D is decreased, separation point or transition point is delayed.

Cavitation Inception Tests on Axisymmetric Headforms

Six axisymmetric headforms were tested in the 350-mm dia water tunnel at China Ship Scientific Research Center (CSSRC) for observation and measurement of cavitation inception. Among them, three of the flow patterns were displayed and analyzed by holography. Results of the tests show that cavitation inception on the headforms was closely related to the structure of the near-wall flow past the body with different flow regimes corresponding to different features of cavitation inception. By carefully fitting a trip wire of suitable height on a hemispherical headform, it was observed that under conditions of low air content, this artificial means of boundary transition may bring about a significant reduction in cavitation inception number.