2022 Vol. 17, No. 3
2022, 17(3): 1-28.
doi: 10.19693/j.issn.1673-3185.02757
Abstract:
Water-air-bubble mixed flow is a complex flow generated by the intense interaction between marine structures and surrounding fluids. It involves a large span of temporal and spatial scale, and has many influencing factors on its generation and evolution. Combining the flow fields of ship and ocean engineering, water-air-bubble mixed flow affects the performance of structures in many aspects, giving the subject strong research significance. This paper reviews the research progress of the mechanism exploration and numerical simulation methods. In terms of mechanism exploration, the present scientific understanding of the formation and evolution mechanisms and the analysis of the phenomenon characteristics of water-air-bubble mixed flow around marine structures are introduced. In terms of numerical simulation methods, the development route and key technical problems of the algorithms are introduced according to insights from elaboration to modeling. Finally, future research prospects are proposed.
Water-air-bubble mixed flow is a complex flow generated by the intense interaction between marine structures and surrounding fluids. It involves a large span of temporal and spatial scale, and has many influencing factors on its generation and evolution. Combining the flow fields of ship and ocean engineering, water-air-bubble mixed flow affects the performance of structures in many aspects, giving the subject strong research significance. This paper reviews the research progress of the mechanism exploration and numerical simulation methods. In terms of mechanism exploration, the present scientific understanding of the formation and evolution mechanisms and the analysis of the phenomenon characteristics of water-air-bubble mixed flow around marine structures are introduced. In terms of numerical simulation methods, the development route and key technical problems of the algorithms are introduced according to insights from elaboration to modeling. Finally, future research prospects are proposed.
2022, 17(3): 29-48.
doi: 10.19693/j.issn.1673-3185.02758
Abstract:
High-speed hydrodynamics and its corresponding complex fluid-structure interactions (FSI) are challenging topics associated with naval architecture and ocean engineering, typically characterized by large deformations, moving boundaries, strong convection and multiple fluid media. Since traditional mesh-based numerical methods possess limited ability to accurately simulate such strongly nonlinear problems, it is imperative to develop meshless numerical schemes with high fidelity and robustness to tackle this dilemma. As one of the most promising truly meshless methods, smoothed particle hydrodynamics (SPH) shows apparent advantages in high-speed hydrodynamics problems thanks to its Lagrangian nature. In the present paper, the attention is particularly focused on the latest advances of several SPH techniques with respect to the following high-speed hydrodynamics problems: vessel-induced waves and wakes, the water entry process of projectiles, and underwater explosion and its resulting structural damage; in addition, the future prospects of SPH are provided in the last part of the paper.
High-speed hydrodynamics and its corresponding complex fluid-structure interactions (FSI) are challenging topics associated with naval architecture and ocean engineering, typically characterized by large deformations, moving boundaries, strong convection and multiple fluid media. Since traditional mesh-based numerical methods possess limited ability to accurately simulate such strongly nonlinear problems, it is imperative to develop meshless numerical schemes with high fidelity and robustness to tackle this dilemma. As one of the most promising truly meshless methods, smoothed particle hydrodynamics (SPH) shows apparent advantages in high-speed hydrodynamics problems thanks to its Lagrangian nature. In the present paper, the attention is particularly focused on the latest advances of several SPH techniques with respect to the following high-speed hydrodynamics problems: vessel-induced waves and wakes, the water entry process of projectiles, and underwater explosion and its resulting structural damage; in addition, the future prospects of SPH are provided in the last part of the paper.
2022, 17(3): 49-57, 84.
doi: 10.19693/j.issn.1673-3185.02430
Abstract:
Objective The ice resistance of an ice breaker on the Yellow River is predicted in the context of ice disasters caused by the freezing of the river in winter. Methods First, a ship-ice-water coupled numerical model is introduced on the basis of the smoothed particle hydrodynamics method (SPH), which includes the numerical process of the constitutive equation, yield criterion and coupling model of ice. The prediction results for the ice resistance of the icebreaker under different conditions are then given. Results The results show that the prediction error of ice resistance in level ice cases is less than 17.6% compared with empirical formulas; the ice resistance of the ice breaker increases with the increase in bending strength and ice thickness, and the thickness of the ice layer is the most important factor for the ice load. Conclusion The ship-ice-water coupled SPH method proposed in this paper can predict the ice resistance of an ice breaker in the Yellow River channel, providing references for the actual operation of ice breakers on the Yellow River.
2022, 17(3): 58-66.
doi: 10.19693/j.issn.1673-3185.02701
Abstract:
Objectives The cohesive zone length is the sum of the cohesive element length at the failure edge and the lengths of the other cohesive elements connected to it. The cohesive zone length determines the maximum size of the mesh. Therefore, the accurate estimation of cohesive zone length and reasonable mesh division are important factors affecting calculation accuracy. Methods Based on several J-integral assumptions and existing research results, a modified function on length thickness ratio value is added to the original formula. The modified formula is then applied to an ice mechanics model. Based on the finite element method, a model of a double cantilever beam is established to verify the accuracy of the formula through comparison with the experimental results. Results The results show that there must be at least four cohesive elements in a cohesive zone length to describe the fracture process accurately. This conclusion is also applied to the numerical simulation of a three-point bending experiment. The error of the limit load is 2.9%, and that of the fracture point is within a reasonable range. Conclusion It is concluded that the modified cohesive zone length formula is more suitable for ice materials.
2022, 17(3): 67-77, 101.
doi: 10.19693/j.issn.1673-3185.02490
Abstract:
Objectives This paper seeks to grasp the influence of a submarine's acceleration and deceleration on its wake spectrum characteristics in stratified flow, and provide a theoretical basis for submarine stealth. Methods The accuracy of CFD technology in simulating the motion of the submerged body making waves on the free surface is first verified, and then analyses are made of the wave-making on the free surface, its convergence and divergence field, and internal wave velocity field under the acceleration and deceleration of a real-scale submarine. By calculating the divergence of the velocity field of the free surface, the influence of submarine acceleration and deceleration on the free surface and jump layer is determined in depth. Results The results show that when the submarine is in unsteady motion, the shear wave and scattered wave distribution of the wake field are completely different from those under uniform motion. Combining the wave height and velocity divergence field, the acceleration and deceleration of the submarine causes the free surface convergence and divergence effect. Conclusions When the submarine depth and stratification mode are the same, the wake field disturbance, wave height and roughness can be reduced when the submarine decelerates, and the acceleration state can significantly increase the probability of near-field disturbances leading to the detection of the submarine.
2022, 17(3): 78-84.
doi: 10.19693/j.issn.1673-3185.02728
Abstract:
Objective Ship capsizing induced by pure loss of stability is an important issue for research on the second-generation intact stability criteria proposed by IMO. Methods A CFD solver based on the viscous theory is developed in combination with the dynamic overset approach and feedback controller for ship maneuver behavior, thereby simulating the course-keeping of a free-running ship with rudders and propellers in stern quartering waves. 6-DOFs motions are predicted for the ship under pure loss of stability with stability failure mode and capsizing assessment. Results The results indicate that large amplitude roll motion occurs with the continuous loss of stability, and the extreme roll eventually leads to the ship capsizing. The yaw angle increases significantly with the variation in roll angle, which indicates that the rudder deflection is unable to control the ship's course effectively, resulting in the broaching phenomenon. Conclusions The results of this study demonstrate that the CFD approach can accurately simulate the stability failure mode and capsizing of a ship, providing references for research on the second-generation intact stability criteria, and technical support for the development of direct stability assessment under pure loss of stability.
2022, 17(3): 85-92.
doi: 10.19693/j.issn.1673-3185.02331
Abstract:
Objectives Stern flaps have a significant impact on the water performance of high-speed amphibious platforms. This paper discusses the influence of stern flaps on the motion and hydrodynamic characteristics of a high-speed amphibious platform in a sliding state. Method Using the SST k-ω turbulence model and overset grid technology, the CFD numerical simulations of an amphibious platform's high-speed navigation under static water conditions are performed, and the motion response and stability of the platform with different stern flaps are analyzed. Considering the influence of the velocity, center of gravity and angle of the stern flaps on the longitudinal motion of the platform, supporting vector machines are adopted to classify and recognize the boundary of its motion stability. Results The results show that the stern flaps reduce the sailing trim angle by changing the pressure distribution on the underside of the platform, and the larger the rotation angle of the stern flaps, the more significant the influence on motion stability; when the position of the center of gravity is the same, the maximum speed of the platform in a stable state is increased. Conclusion The application of stern flaps and the improvement of motion stability are of great significance for the development of high-speed amphibious platforms.
2022, 17(3): 93-101.
doi: 10.19693/j.issn.1673-3185.02726
Abstract:
Objective In order to forecast a ship's maneuverability more realistically, it is necessary to conduct the direct simulation of the full-scale ship's maneuver. Methods In this paper, based on overset grid technology, calculations of the full-scale ship towing, supporting propeller open water and full-scale ship self-propulsion are first produced, and the calculations are then compared with the experimental results. On this basis, numerical simulations of the full-scale ship's 10/10 standard zigzag maneuvers are carried out to analyze changes in the ship's motion, hydrodynamics, flow field and vortex structure. Results The self-propulsion results show that the numerical method used in this paper is reliable. The torque coefficient decreases due to the propeller scale effect. When maneuvering, the ship's motion is more intense. Due to the interference of the ship-rudder system, the post-propeller vortex structure is more complicated and produces a certain angle of offset with the change in drift angle. Conclusions The ship's motion, hydrodynamic characteristics and flow field information can be accurately obtained using this numerical method to simulate full-scale ship maneuvers, it can be used as an effective pre-evaluation tool and provide references for ship design.
2022, 17(3): 102-111.
doi: 10.19693/j.issn.1673-3185.02742
Abstract:
Objective This paper aims to explore the hydrodynamic characteristics of underwater vehicles under the action of internal solitary waves. Methods Based on the internal solitary wave mKdV theory under the strong stratification assumption, the incompressible Navier-Stokes （N-S） equation is discretized by the finite volume method. Combining the velocity entrance wave-making and overset grid methods, a numerical model of an underwater vehicle coupled with internal solitary waves in stratified flow is established. Through this model, the coupling process of internal solitary waves and fixed and suspended vehicles in different internal wave environments is simulated, and the variation law of the hydrodynamic load is obtained by numerical solution. Results The amplitude of the force and moment of the underwater vehicle increases with the increase in wave amplitude. When the position of the fixed vehicle is close to the interface of two layers of fluid, it is more severely affected. When the suspended vehicle is close to the wave trough, the impact is more significant. The increase in the initial pitch angle will lead to a sharp increase in the horizontal force amplitude of the vehicle. In addition, the "falling depth" phenomenon occurs when a vehicle with initial speed is affected by the main wave and coda wave of an internal solitary wave. Conclusion The results obtained in this paper have reference value for safe operation of underwater vehicles.
2022, 17(3): 112-118, 134.
doi: 10.19693/j.issn.1673-3185.02748
Abstract:
Objectives In order to reveal the mechanism of perforated fixed and floating structure through-type flow fields in local high turbulence, breaking waves and fluid separation, three types of typical active/passive perforated structures (gushing floating breakwater, forced rolling flooded compartment in calm water and international standard model DTMB 5415 damaged ship) are used to study the coupled/uncoupled problem of through-type flow field and motion in perforated structures. Methods Volume force technology is introduced, the dynamic overset grid method based on the unsteady and incompressible Navier-Stokes equation is adopted, and the k-ε turbulence model is combined to establish a calculation model of the gushing floating breakwater inflow and outflow in waves, and a calculation model of the transient inflow and steady-state sloshing of a forced rolling empty flooded compartment in calm water and a prediction model of nonlinear damaged ship motion in waves. The numerical simulation of active/passive perforated fixed and floating structure through-type turbulent flow fields is carried out. Results The results show that the simulated rolling, flooded compartment wave elevations and rolling motion response of a damaged ship are consistent with the relevant experimental data, which verifies the feasibility and accuracy of typical perforated structure through-type flow and motion coupled/uncoupled calculation models. Conclusions The numerical simulation of the through-type turbulent flow fields of different structures provides new technical support for the safe navigation and survivability assessment of damaged ships, and the development of gushing breakwater.
2022, 17(3): 119-125.
doi: 10.19693/j.issn.1673-3185.02733
Abstract:
Objective To investigate ship power characteristics and the difference between the towing model and self-propulsion model for ship motion response in waves, numerical simulations of ship self-propulsion performance in waves are carried out. Methods In this paper, the KCS ship model and KP505 propeller model are selected, and the unsteady Reynolds-averaged Navier-Stokes (URANS) method is used to carry out computational fluid dynamics (CFD) simulations of ship self-propulsion in waves. The in-house URANS solver HUST-Ship and in-house structured dynamic overset grid code HUST-Overset are combined to solve the motions of the self-propelled ship in waves, and the improved body-force model is selected as the propulsion model. Towing simulations for KCS with two-degrees-of-freedom (DOF) in waves and self-propulsion simulations with 3-DOFs under different wave conditions are carried out, and the differences between these methods are discussed in detail. Finally, the components and their specific proportions of added power during ship self-propulsion in waves are analyzed in detail using the logarithmic analysis method. Results Regarding the added power of a self-propelled KCS in waves, the added resistance is responsible for 74%-77% while propulsive efficiency accounts for 23%-26%, that is, the added resistance occupy a larger proportion. Conclusion Reducing ship motion to decrease added resistance is the most effective approach to reducing added power.
2022, 17(3): 126-134.
doi: 10.19693/j.issn.1673-3185.02417
Abstract:
Objective In this paper, the hydrostatic resistance of a planing craft is studied using the high-precision numerical simulation method to improve the numerical prediction accuracy. Methods The three-dimensional viscous flow field of a planing craft in calm water is numerically simulated using the computational fluid dynamics (CFD) method combined with the Savitsky method and overset grid technique, and the flow field characteristics of the craft under different load coefficients and speeds are analyzed. Results The calculated results of the resistance, sinkage and trim angle of the planing craft are in good agreement with the experimental results, and the spray phenomenon and distribution of water and air on the bottom of the craft are simulated normally, which shows that this method can accurately and effectively predict the resistance performance of planing craft. With the increase in the load coefficient, the peak value of the pressure coefficient on the keel increases, and the position of the pressure center moves forward. With the increase in speed, the peak value of the pressure coefficient on the keel decreases, the position of the pressure center gradually moves towards the stern, the angle between the stagnation line and the longitudinal section in the center plane decreases, the depth of the cavity behind the transom decreases, and the length of the cavity increases. Conclusions This study provides an accurate and effective numerical calculation method for the resistance prediction of planing craft, and can provide technical support for the numerical study of the hydrodynamic performance of such craft.
2022, 17(3): 135-144.
doi: 10.19693/j.issn.1673-3185.02753
Abstract:
Objective In order to reduce the wave-making resistance of high-speed ships, this paper proposes an innovative hydrofoil appendage with a drag reduction effect. Methods An NACA 0012 airfoil is selected for the hydrofoil appendage cross-section, and a series of studies on the drag reduction effects of bow-mounted hydrofoil appendages of different sizes and installation positions is carried out using the numerical calculation method verified by convergence and reliability tests. The drag reduction effect of adding a T-wing to the stern of a ship is further studied. Results The results show that the maximum drag reduction is 7.38% and 6.82% at high speeds of Froude number 0.45 and 0.494 when the hydrofoil appendage is only installed at the bow. The drag is further reduced after adding the T-wing to the stern, and the total drag reduction is 9.35% and 11.13% at high speeds of Froude number 0.45 and 0.494. Conclusions The hydrofoil appendage with a drag reduction effect proposed in this paper greatly reduces the total drag of high-speed ships, improves their speed and provides new technical means for further improving their navigation performance.
2022, 17(3): 145-152.
doi: 10.19693/j.issn.1673-3185.02694
Abstract:
Objective In order to achieve the accurate prediction of the amplitude response of a vortex-induced vibrating cylinder under the sub-critical Reynolds number, a method for establishing a Cl-A/D (lift coefficient-amplitude ratio)model of the forced vibration of the cylinder by numerical simulation is proposed. Methods Based on the Realizable k-ε model, a two-dimensional numerical simulation of the forced vibration of a cylinder is carried out using the finite volume method. The calculated lift coefficient curves under different amplitude ratios A/D in the range of excitation frequency ratio fe/fn=1 are obtained. The lift coefficient corresponding to the maximum vibration velocity of the cylinder is then selected to establish the Cl-A/D model. Results The results show that the overall trend of the Cl-A/D fitting curve is in good agreement with the predicted results of SHEAR7. At the same time, it is found that the "zero lift coefficient" points under each excitation frequency ratio fe/fn are all located near the amplitude ratio A/D=0.8, and the wake shedding mode changes around A/D=0.8 from "P+S" to "2P" (P represents a pair of vortex shedding with opposite rotation directions, and S represents a single vortex shedding). In the vortex-induced vibration experiment of a single cylinder, the maximum amplitude when "lock-in" occurs is around 0.8D. Conclusions The amplitude ratio corresponding to the "zero lift coefficient" of the Cl-A/D model of forced vibrating cylinder under sub-critical Reynolds number is consistent with the maximum response amplitude ratio of the cylinder under vortex-induced vibration, and the shedding mode of the wake vortex changes under this amplitude ratio.
2022, 17(3): 153-159.
doi: 10.19693/j.issn.1673-3185.02275
Abstract:
Objective The viv-FOAM-SJTU solver is used to carry out the numerical simulation of the vortex induced vibrations of a riser under different top oscillation excitation amplitudes. Methods The flow field is computed by the strip theory and Reynolds-averaged Navier-Stokes (RANS) method, while the Bernoulli-Euler bending beam theory and finite element method are employed to model the structure of the riser. Results The simulation results show that the cross-flow vibrations transform from multi-mode to single mode with the decrease in oscillation amplitude. At the same time, the inline vibrations maintain single mode, but the amplitude of the mode vibration decreases. Conclusion With the variation in top oscillation excitation amplitudes, the washing out of the riser by shed vortices affects the characteristics of vortex induced vibrations.
2022, 17(3): 160-169.
doi: 10.19693/j.issn.1673-3185.02749
Abstract:
Objective This article studies the characteristics of the flow-induced vibration (FIV) of a cylinder with passive turbulence control and nonlinear springs in a wide range of Reynolds numbers in order to use the FIV of the PTC-cylinder to improve the efficiency of power generators. Methods The FIV of a PTC-cylinder supported by linear and nonlinear springs is studied by solving the 2-D URANS equations in combination with the Spalart-Allmaras turbulence model with dynamic mesh and user-defined-function (UDF) in FLUENT. The numerical simulation results are compared with the experimental results. Results Compared with linear springs, the amplitude of a cylinder with piecewise-linear springs is enhanced significantly, and the vortex mode becomes more complicated. When the spring stiffness of the first segment is greater than that of the second segment, the amplitude of the cylinder becomes larger, the oscillation frequency becomes smaller and the vortex mode of the cylinder becomes more complicated as the piecewise point of the piecewise-linear function becomes smaller. Conclusions The FIV of a PTC-cylinder with nonlinear springs is different from that with linear springs. The fluid-structure interaction is enhanced effectively with the reasonable selection of the piecewise point, providing theoretical and technical support for improving the efficiency of hydrokinetic energy conversion devices.
2022, 17(3): 170-177, 195.
doi: 10.19693/j.issn.1673-3185.02491
Abstract:
Objective Aiming at the mapping relationship between combined hydraulic components and performance, the hydrodynamic performance prediction method of a podded propulsor for cruise ship based on Reynolds-averaged Navier-Stokes (RANS) is studied. Methods Taking a scale model of a podded propulsor as the research object, the open water performance test of the propulsor is carried out in a deepwater towing tank, and the accuracy of the prediction method is verified. The effects of pod geometry, disk ratio and blade number on the hydrodynamic performance of the podded propulsor are simulated and analyzed. Results The results show that the geometry of the pod has little effect on the open water performance of the propulsor. Increasing the disk ratio of the propeller causes the thrust coefficient and open water efficiency to first increase and then decrease, while the torque coefficient only increases within a certain range. Increasing the number of propeller blades will first increase and then decrease the open water performance of the propulsor, and the number of propeller blades has little effect on the open water efficiency of the podded propulsor under low advance coefficient. Conclusion The results of this study can provide reference value for the optimal design of cruise ship podded propulsors.
2022, 17(3): 178-186, 204.
doi: 10.19693/j.issn.1673-3185.02358
Abstract:
Objective When the cavitation number drops to a certain critical value, the pressure at the leading edge of an underwater hydrofoil will decrease and cavitation will occur. In model test studies, it is difficult for a scale model hydrofoil to meet the Reynolds number of a real scale hydrofoil, so the critical cavitation number during cavitation initiation will be changed. Methods To this end, with reference to the NACA 0012 rudder used in the standard KCS ship with a speed of 24 knots, the hydrofoil cavitation characteristics are numerically simulated in the paper using the SST k-ω turbulence model and Schnerr-Sauer (S-S) cavitation model based on STAR-CCM+ software. According to different scale models, the surface flow field and cavitation distribution of the hydrofoil are calculated by changing the environmental pressure of the hydrofoil at different attack angles. In this way, the critical cavitation number corresponding to cavitation initiation is obtained, and the influence mechanism of the scale effect on the critical cavitation number is analyzed. Results Through the analysis of the calculation results, it is concluded that with the decrease in scale, the size of the critical cavitation number at the corresponding attack angle of the hydrofoil will decrease, indicating that the scale effect caused by the difference in the Reynolds number is intensified. Conclusion Therefore, in model tests, in order to prevent the influence of the scale effect on the initial cavitation number, a hydrofoil of a smaller scale should not be selected.
2022, 17(3): 187-195.
doi: 10.19693/j.issn.1673-3185.02387
Abstract:
Objective This paper aims to explore the suitability of mesh density and subgrid-scale model for the numerical simulation of three-dimensional twisted hydrofoil. Methods The large eddy simulation (LES) method and Schnerr-Sauer (S-S) cavitation model are used to simulate the unsteady cavitation flow of a Delft Twist11N three-dimensional twisted hydrofoil. Three sets of grid with different density and three types of different subgrid-scale models are mainly studied to identify the effects on the Twist11N hydrofoil cavitation evolution process, cavitation shedding frequency and time-averaged lift and drag coefficients. Results The results show that appropriate grid refinement can not only capture more unsteady cavitation evolution phenomena such as the shedding of smaller cavities and the inception and collapse of horse-shoe-shaped cloud cavities, but also obtain more exact cavity shedding frequency, time-averaged lift and drag coefficients, and time-averaged pressure distribution. Among the three subgrid-scale models, compared to the algebraic wall-modeled LES model (WMLES) and Smagorinsky-Lilly (SL) model, the wall-adapting local eddy-viscosity (WALE) model better captures the evolution of sheet and cloud cavitation, and has better accuracy in predicting the frequency of cavity shedding,time-averaged lift, drag and pressure coefficients. Conclusion It is recommended to adopt the LES method with the WALE subgrid-scale model for the numerical simulation of unsteady cloud cavitation.
2022, 17(3): 196-204.
doi: 10.19693/j.issn.1673-3185.02607
Abstract:
Objective The adaptability of the accuracy of finite space cavitation flow described by the verification and validation (V&V) method needs to be analyzed quantitatively. To this end, flow under different cavitation numbers in a Venturi tube is subject to large eddy simulation (LES) to implement the V&V method, thereby expanding the application scope of verification and methods. Methods First, the LES and Zwart-Gerber-Belamri (ZGB) cavitation model are used to simulate the flow in the Venturi tube under different cavitation numbers. The accuracy orders and numerical benchmarks of the pressure ratio and pressure coefficient under different working conditions are then calculated using the five-equation V&V method, and the verification uncertainties are calculated. Finally, the validation uncertainties are calculated and compared with the comparison error of the simulation values and experimental results. Results Under the three cavitation numbers, the verification uncertainties of the pressure ratios are small, and the pressure ratios are validated at the uncertainty levels. When the cavitation number is σ=0.263 at x/Dth=9 and x/Dth=13, the verification uncertainties of the pressure coefficients are high, and the pressure coefficients are not validated. The verification uncertainties of the pressure coefficients at other monitoring points and working conditions are small, and the pressure coefficients are validated. Conclusions The five-equation V&V method can smoothly reflect the error of the LES of Venturi flow, but the consumption is high, and the V&V method for the LES of internal cavitation flow in finite space requires further improvement.
2022, 17(3): 205-212.
doi: 10.19693/j.issn.1673-3185.02595
Abstract:
Objective This paper aims to study the application effects of heterogeneous supercomputing platforms based on domestic processors in the field of ship hydrodynamics. Methods The direct numerical simulation of the flow around a 3D finite square cylinder with Re = 250 using the Sunway Taihu Light supercomputer is implemented, in which the maximum number of grids is 245.76 million ( t = 600 s, dt = 0.001), and the simulation results are analyzed and verified. The parallel programming method of MPI+Threads is used with a maximum parallel size of 133 120 cores. Results According to the statistics, the calculation can be completed in a few days under the current parallel scale using 245.76 million grids. In addition, the simulation results show that the vortex shedding is synchronous in different cross-sections of a 3D finite square cylinder. When the characteristics of flow around the finite square cylinder with slenderness ratios of 2, 3 and 4 are compared, it is found that when the slenderness ratio is 2, the wake vortex structure of the wake is a long straight streamwise vortex secondary structure, or else it is an antisymmetric Karman vortex. Conclusion These results indicate that multilevel parallel computing using the Sunway Taihu Light supercomputer can effectively reduce the time-consumption caused by grid scale increase in small-scale grids, which has broad application prospects in the field of ship hydrodynamics.
2022, 17(3): 213-220.
doi: 10.19693/j.issn.1673-3185.02300
Abstract:
Objective This paper aims to explore the influence of rudder-fin combined structure on the induced noise. Methods To this end, the flow field and sound field of a rudder are calculated using the two-way fluid-structure interaction theory, and the sound vibration laws caused by the change of the rudder blade material and rudder stock position are studied. Results The results show that, among the three materials of aluminum alloy, titanium alloy and steel, the fluctuation of the fluctuating pressure caused by the steel rudder is the smallest, and the first peak point of the steel rudder appears last; the first-order peak frequency and amplitude increase with the backward movement of the rudder stock position, while the second-order frequency decreases with the backward movement of the rudder stock position; the forward movement of the rudder stock will reduce the total sound pressure level within 100 Hz. Conclusions Further analysis indicates that, the sound vibration characteristics of the steel rudder are the best among the three materials, and the forward movement of the rudder stock position can improve the sound vibration characteristics of the rudder-fin structure within a certain range. This study has certain guiding significance for the acoustic design of submarine rudder structures in order to improve the acoustic stealth performance of submarines to a certain extent.
2022, 17(3): 221-227.
doi: 10.19693/j.issn.1673-3185.02406
Abstract:
Objective The shape of a sail influences the airflow characteristics around it, and flexible sails are prone to large nonlinear deformation under wind pressure. As such, this paper studies the effects of sail deformation on aerodynamic performance. Methods A fluid-structure interaction (FSI) study on flexible sail deformation is performed. The initial shape model and material model of a sail are established, an explicit finite element method (FEM) such as AUTODYN is applied to calculate the large nonlinear deformation of the sail, and the shape of the deformed sail is constructed. STAR-CCM+ is applied to predict the performance of the initial and deformed sail. Results The results show that positive pressure on the windward side of the deformed sail increased, and the flow separation generated around the leeside of the sail leech became more severe in upwind conditions with a relative wind angle of 20°. The increase of the lift force due to the rise of the camber was relatively small, resulting in a decrease in overall lift force. Conclusion FSI studies on flexible sail deformation are suitable for the precise evaluation or optimization of sail performance.
2022, 17(3): 228-236.
doi: 10.19693/j.issn.1673-3185.02743
Abstract:
Objective In order to optimize the shape of the floating buoy of a multi-degree-of-freedom wave energy converter to improve its energy conversion efficiency, a method for shape optimization based on a genetic algorithm (GA) is proposed. Methods A B-spline surface is used to define the shape of the floating buoy, and the GA is applied to take all control points of the surface as individual variables in the iterative population. AQWA and WEC-Sim software are then used to establish a time-domain numerical model for each shape generated by the iteration, and the value of the objective function is calculated. The evolution curves of the objective function and optimized shape are then obtained. The effects of different schemes of shape definition and objective functions on the optimization results are studied and analyzed respectively. Results The optimization method can significantly improve the energy conversion efficiency of a multi-degree-of-freedoms (DOFs) wave energy converter, and the shape definition scheme and objective function have a great impact on the final optimization results. Conclusion The results can provide a feasible design method for the shape optimization of the floating buoy of a multi-DOFs wave energy converter.
Trajectory tracking control of underwater vehicle based on hydrodynamic parameters calculated by CFD
2022, 17(3): 237-245, 272.
doi: 10.19693/j.issn.1673-3185.02739
Abstract:
Objective This paper aims to analyze the horizontal trajectory tracking of an underwater vehicle. Methods First, the 3-DOFs motion equation is derived based on the plane trajectory tracking control target of the underwater vehicle, and the damping term parameters and additional mass term parameters of the vehicle are obtained through the commercial CFD software STAR-CCM+ and ANSYS AQWA respectively. Combined with the dynamic configuration of the vehicle and calculated hydrodynamic parameters, a control strategy is designed on the basis of backstepping and sliding mode control technology. Finally, using the designed control strategy, the trajectory tracking simulation calculation is carried out on the Matlab/Simulink platform for both the linear and nonlinear motion trajectories of the vehicle on the horizontal plane. Results The numerical simulation results show that the underwater vehicle can achieve good tracking effects for both kinds of motion trajectories, and the propeller thrust changes smoothly. Conclusions The controller designed in this paper can enable an underwater vehicle to track its target trajectory quickly and maintain a good continuous tracking effect.
2022, 17(3): 246-252.
doi: 10.19693/j.issn.1673-3185.02463
Abstract:
Objective In order to study the blowing and drainage performance of a submarine main ballast tank system, the simulation and experiment of main ballast blowing based on the short circuit blowing model are carried out. Methods First, the short circuit blowing model is modified to simulate the process of air release and water drainage. Next, an equal scale model test of a compressed air blowing ballast tank is carried out, and the effects of air source volume, air source pressure and sea opening area on the blowing effect are analyzed. Finally, the reliability of the short circuit blowing model is experimentally verified. Results The modified model has high accuracy, especially in the prediction of low air source pressure and small sea opening diameter conditions. When the air source pressure is less than 15 MPa, the relative error of peak pressure prediction is less than 15%. The occurrence time of air peak pressure is different under different sea opening diameters, under small diameter conditions, the air pressure reaches its maximum when the compressed air first enters the tank, while under large diameter conditions, the peak pressure appears before the accumulated air pressure is released. When the accumulated air pressure is released, the tank air pressure drops sharply, which can be used as a criterion to stop blowing. Conclusion The results of this study can provide references for the blowing operation of the main ballast tank in practical engineering applications.
2022, 17(3): 253-263.
doi: 10.19693/j.issn.1673-3185.02503
Abstract:
Objective In order to improve the anti-shock perfomance of ships subjected to underwater explosion, this paper studies the energy absorption and impact resistance of the new protective structure consisted of carbon fiber reinforced plastic (CFRP)-lattice aluminum sandwich plates. Methods First, finite element software ABAQUS is used to establish the numerical simulation model of CFRP-lattice aluminum sandwich plates under non-explosive and non-contact underwater explosion load, and its reliability is verified. Single variables are then controlled to analyze the influence of the fiber layer thickness of the upper and lower panels and the rod diameter of the sandwich lattice structure on the energy absorption characteristics and structural deflection of the CFRP-lattice aluminum sandwich plates. Finally, based on the above three design parameters, a surrogate optimization model is established using the experimental design method and numerical simulation methodology to optimize the energy absorption of the CFRP-lattice aluminum sandwich plate structure. Results The results show that when the mass of the CFRP-lattice aluminum sandwich plates is constant, the specific absorption of the optimized results can be increased by 284%. In full consideration of the deformation of the lower plates, the specific energy absorption of the optimized results can be increased by 59%. Conclusions This study shows that the proposed optimized structure of CFRP-lattice aluminum sandwich plates can effectively improve their energy absorption capacity, and the response surface method is an optimization method that can effectively improve the energy absorption characteristics of the structure.
2022, 17(3): 264-272.
doi: 10.19693/j.issn.1673-3185.02431
Abstract:
Objective In order to improve the calculation accuracy of the influence of slamming load on ship fatigue damage, and evaluate ship fatigue life more effectively, a direct calculation method of ship fatigue damage which accounts for slamming load is proposed. Methods First, the time domain calculation of nonlinear load is combined with the spectral analysis method of linear frequency domain, and the stress time history of the ship in a short-term sea state is obtained based on beam theory. Second, the damage of the check points is calculated using the rain flow counting method and S-N curve, the contribution and influence coefficients of nonlinear slamming load on fatigue damage are calculated with reference to the CCS guidelines, and the corresponding stress response transfer function is corrected by combining the spectral analysis method. Finally, the stress spectrum is calculated and fatigue damage accounting for nonlinear slamming is obtained. Results The results show that compared with traditional spectral analysis method, it is found that the ship fatigue damage obtained by the direct calculation method accounting for slamming load is about 10%–50%. Conclusion The proposed method can improve the accuracy of load calculation when evaluating the fatigue strength of ships sailing in harsh sea states.
