The occurrence of a tip leakage vortex (TLV) is a special phenomenon of ducted propellers, which ... more The occurrence of a tip leakage vortex (TLV) is a special phenomenon of ducted propellers, which has a significant influence on the propeller’s hydrodynamic performance and efficiency. The inception, evolution, and instability of the TLV under different tip clearance sizes have a direct impact on the cavitation and acoustic characteristics. A simulation was set up to calculate the open-water performance of a standard ducted propeller. The open-water characteristics (OWCs) were compared with the experimental data to verify the feasibility of the method. Furthermore, to capture the influence of tip clearance size on the vortex structure evolution and wake dynamics, the improved delayed detached eddy simulation (IDDES) method was adopted to simulate four groups of ducted propellers with different tip clearances. The results showed that with the increase in the gap-to-span ratio (GSR), KTD and η0 gradually decreased, while KQ and KTB increased, but a peak point existed. Moreover, the TL...
The paper explores Kriging-based surrogate model combined with Weighted Expected Improvement appr... more The paper explores Kriging-based surrogate model combined with Weighted Expected Improvement approach and for the ship hull form optimization. The training dataset of the Kriging-based surrogate model is obtained by sampling the design space (Design of Experiments, DOE) and performing expensive high-fidelity computations on the selected points. Expected Improvement (EI) is used as a criterion to select one additional sample point in each iteration. The Weighted Expected Improvement (WEI) is derived from EI by adding a tunable parameter which can adjust the weights on exploration and exploitation in the Efficient Global Optimization (EGO). The proposed method selects more than one new sample point by changing the weight parameter for each optimization iteration, thus it can be performed by parallel computation or multi-computer runs which improves the computational efficiency distinctly. This makes it possible not only to improve the accuracy of the surrogate model, but also to explo...
The Moving Particle Semi-implicit (MPS) method has been proven effective to simulate violent flow... more The Moving Particle Semi-implicit (MPS) method has been proven effective to simulate violent flows such as dam-break flow, liquid sloshing and so on. But the low computational efficiency is one disadvantage of MPS. In the field of scientific computations, GPU based acceleration technique is widely applied to reduce the computation time of various numerical methods. In this paper, an in-house solver MPSGPU-SJTU is developed based on modified MPS method and GPU acceleration technique. A three-dimensional (3-D) dam-break flow is simulated by present solver and the validity and accuracy of GPU code are investigated by comparing the results with those by other researches. By comparisons, the flow field of GPU-based calculation is in better agreement with the experiment. In addition, the computation times of GPU and CPU solvers are compared to demonstrate the effect of GPU acceleration technique on the computational efficiency of MPS method.
Limited by the space and topographic factors, wake effect cannot be avoided in the wind farm. Due... more Limited by the space and topographic factors, wake effect cannot be avoided in the wind farm. Due to the wake of upstream wind turbine, the inflow wind speed for downstream wind turbine will be reduced. As a result, the wind farm has a significant decrease of annual power output. Considering the high construction and maintenance cost of the wind farm, it is necessary to optimize wind turbine positions in order to reduce the cost of power generation. In the present work, an optimization approach based on wake model and optimization algorithm is proposed for wind farm layout optimization problem. The Bastankhah wake model is applied to calculate the wake velocity field and predict the power output of wind farm. The wake velocity predicted by the Bastankhah model is also validated by CFD results. In addition, the Kriging model is selected to construct the surrogate relationship between the wind turbine positions and the power output of wind farm. To quickly obtain optimal wind farm lay...
The interaction between structures and waves is a typical phenomenon in naval architecture and oc... more The interaction between structures and waves is a typical phenomenon in naval architecture and ocean engineering fields. In this paper, numerical simulations are carried out to study the interactions between a two-dimension horizontal plate and solitary wave by our in-house CFD solver MLParticle-SJTU. The simulation procedure is following the experiments performed at Dalian University of Technology. During these computations, the plate is submerged under water and the solitary wave with given amplitude is generated by a piston-type wave maker. Free surface elevations of the wave and the pressure impacting on the plate are recorded and compared with experimental data respectively. The predicted pressure agrees well with the benchmark data. Finally, the influence of wave height on the interaction between the plate and solitary wave is analyzed.
A number of locks have been under consideration or construction in recent years. The growing numb... more A number of locks have been under consideration or construction in recent years. The growing number of large vessels can only be used if sufficient infrastructure, like locks, is available. Especially for large vessels that are most critical, the hydrodynamic phenomena induced by a lock entry are important, since the ship’s movements are strongly influenced by those phenomena. For a better understanding of the mechanism of the ship behavior and the effects of eccentricity on the ship behavior while entering a lock, the unsteady viscous flow around the 12000TEU ship model while entering a lock with different eccentricity is simulated and the hydrodynamic forces, vertical displacement are calculated by solving the unsteady RANS (Reynolds Averaged Navier–Stokes) equations in combination with the k-ω SST turbulence model. Overset grid technology is used to keep grid orthogonality. To make our prediction more reasonable, the effects of the free surface is taken into account. A benchmark ...
Yaw is the most common working condition of a wind turbine and the key of reducing the fatigue lo... more Yaw is the most common working condition of a wind turbine and the key of reducing the fatigue loads and improving the performance of a wind farm is to understand the wake characteristics of a wind turbine in yaw condition. A neutral boundary layer flow in the atmosphere is simulated by the LES technique using the solver developed based on OpenFOAM and the wake flow of a yawed wind turbine modeled by the actuator line is studied. The timeaverage velocity field proves the practicability of the yaw control operation in optimizing the total power output of a wind farm, but from the cross section contours, the velocity distribution in the wake of a yawed turbine is not completely symmetric and the vertical wake deflection cannot be neglected, which is the main source of errors of the analytical wake models based on the gaussian distribution assumption. The time history curves and frequency spectrum of the wake meandering gained from the filtered flow data indicate that the yaw condition...
In the present study, interaction between regular waves and free roll motion of a 2D floating bod... more In the present study, interaction between regular waves and free roll motion of a 2D floating body is investigated by our in-house particle solver MLParticle-SJTU based on modified Moving Particle Semi-Implicit (MPS) method. Numerical wave tank is developed to calculate the interaction between waves and floating body, including wave-maker module and free roll motion module. The comparison between the numerical wave elevation and analytical solution shows that the MLParticle-SJTU can provide acceptable accuracy of wave making. Roll motion and force acting on the floating body in waves are in good agreement with experimental results. Profiles of the wave surface surrounding floating body are presented.
Moving particle semi-implicit (MPS) method is a Lagrangian particle method, first proposed by Kos... more Moving particle semi-implicit (MPS) method is a Lagrangian particle method, first proposed by Koshizuka and Oka for incompressible flows. Due to its Lagrangian nature, MPS is capable of computing large deformed free surface flow, such as violent liquid sloshing, green water and dam breaking. However, MPS suffers from high computational cost. This significantly limits its applications in 3D flows which have to involve a large number of particles. In the present work, a parallel strategy for GPU (Graphics Processor Units) based acceleration is developed, aiming to extend MPS to compute practical 3D flows. Simulation of 3D violent sloshing and 3D dam breaking is performed. Speed-up obtained by using GPU is satisfied. The solution of PPE and search of neighboring particle are accelerated up to 10 times and 6 times faster respectively based on the present parallel strategy, than serial computation. However, performance on different GPU indicates that neighbour list is a memory bound prob...
In this paper, the effects of the location of one horizontal baffle on liquid sloshing under angu... more In this paper, the effects of the location of one horizontal baffle on liquid sloshing under angular excitation are investigated. An in-house solver MLParticle-SJTU developed based on improved moving particle semi-implicit (MPS) method is used for numerical simulation. Firstly, a rectangular tank without baffles is modeled to show the validity of solver. Then, one horizontal baffle is installed in the tank and the location of it is changed. The results show that the arrangement of horizontal baffle makes significant effects on the impact pressure on the tank walls and the deformation of free surface.
Liquid sloshing in a 3D rectangular tank under multiple degrees of freedom (DOF) excitations is s... more Liquid sloshing in a 3D rectangular tank under multiple degrees of freedom (DOF) excitations is simulated by the in house solver MLParticle-SJTU based on Moving Particle Semi-Implicit (MPS) method. Firstly, liquid sloshing under horizontal and angular motion is carried out to validate the present particle solver MLParticle-SJTU respectively. The comparison between the numerical flow pattern and impact pressure and the corresponding experimental data shows that MLParticle-SJTU is reliable and efficient to solve 3D sloshing problem. In addition, the present model is extended to 3D liquid sloshing with multiple degrees of freedom motions. The evolution of sloshing waves and impact pressure are analyzed to investigate the coupling effect of different DOF excitations. Results show that surge motion and rolling motion have weak interaction with each other. Sloshing can be damped if the amplitude and phase difference between surge motion and pitch motion are appropriate.
The bubbly flows have always been a key research object in the field of multiphase simulations. H... more The bubbly flows have always been a key research object in the field of multiphase simulations. However, when the bubbly flows become violent to cause the large deformations of bubbles, complex interfaces may be generated and bring great challenges to the traditional mesh-based methods. To overcome this challenge, one of the most commonly used mesh-free particle methods-moving particles semi-implicit (MPS) method, is adopted in this paper for the numerical simulations of bubbly flows with complex interfaces. Firstly, a multiphase MPS method is developed by combining the single-phase improved MPS (IMPS) method proposed in our previous study with different multiphase models, including the improved density smoothing scheme, interparticle viscosity model, continuous acceleration model, surface tension model, incompressible-compressible model and multiphase collision model. Then, the deformations of 2D/3D square droplets are simulated to validate the surface tension model, and the multiphase MPS method is verified through the benchmark cases of 2D/3D single bubble rising with large density and viscosity ratios (up to 1000 and 100, respectively). Finally, a series of bubbly flows with increasing complexity are performed, including the bubble rising and breaking at a high Reynolds number, the co-axial coalescence of two bubbles with different radiuses, and the violent interactions between a set of bubbles. The MPS results keep fair agreements with other numerical and experimental results, through which the advantages of the present multiphase MPS method in bubbly flows with complex interfaces can be well demonstrated.
Vortex ring thruster (VRT) is a new type of propulsion device which adopts impulse propulsion mod... more Vortex ring thruster (VRT) is a new type of propulsion device which adopts impulse propulsion mode. The idea of impulse propulsion comes from bionics. It imitates cephalopods like squid and can achieve high efficiency. Vortex ring is a typical phenomenon during the pulse process and it can help improve the propulsion efficiency. Many previous studies believe that VRT can obtain large thrust and efficiency but few articles study the efficiency problem systematically. In addition, experiment is the main research method adopted at present. In this paper, an axisymmetric piston-nozzle computational model is established to simulate the thruster. Fluid is solved by CFD method based on the open source platform OpenFOAM. Dynamic mesh technology is adopted to control piston motion. This paper focus on two aspects: vortex ring evolution and propulsion efficiency characteristics. In the first part, simulations of pulse process with different strokes are carried out and compared with standard experiments. The two results are in good agreement. The vortex ring evolution is then studied with different background flows. In the second part, a reasonable open-water characteristics calculation method for VRT is proposed. Thrust coefficient and propulsion efficiency are obtained in the calculation. The results indicate that the VRT can achieve high efficiency near to 1 in the case of high advance ratio. In addition, efficiency comparison is carried out between four kinds of asymmetric velocity programs and three kinds of symmetric velocity programs to find out a more reasonable velocity program design. Finally, the impact of piston stroke is investigated. Results show that smaller stroke can obtain larger efficiency because pulsation effect is more prominent.
The occurrence of a tip leakage vortex (TLV) is a special phenomenon of ducted propellers, which ... more The occurrence of a tip leakage vortex (TLV) is a special phenomenon of ducted propellers, which has a significant influence on the propeller’s hydrodynamic performance and efficiency. The inception, evolution, and instability of the TLV under different tip clearance sizes have a direct impact on the cavitation and acoustic characteristics. A simulation was set up to calculate the open-water performance of a standard ducted propeller. The open-water characteristics (OWCs) were compared with the experimental data to verify the feasibility of the method. Furthermore, to capture the influence of tip clearance size on the vortex structure evolution and wake dynamics, the improved delayed detached eddy simulation (IDDES) method was adopted to simulate four groups of ducted propellers with different tip clearances. The results showed that with the increase in the gap-to-span ratio (GSR), KTD and η0 gradually decreased, while KQ and KTB increased, but a peak point existed. Moreover, the TL...
The paper explores Kriging-based surrogate model combined with Weighted Expected Improvement appr... more The paper explores Kriging-based surrogate model combined with Weighted Expected Improvement approach and for the ship hull form optimization. The training dataset of the Kriging-based surrogate model is obtained by sampling the design space (Design of Experiments, DOE) and performing expensive high-fidelity computations on the selected points. Expected Improvement (EI) is used as a criterion to select one additional sample point in each iteration. The Weighted Expected Improvement (WEI) is derived from EI by adding a tunable parameter which can adjust the weights on exploration and exploitation in the Efficient Global Optimization (EGO). The proposed method selects more than one new sample point by changing the weight parameter for each optimization iteration, thus it can be performed by parallel computation or multi-computer runs which improves the computational efficiency distinctly. This makes it possible not only to improve the accuracy of the surrogate model, but also to explo...
The Moving Particle Semi-implicit (MPS) method has been proven effective to simulate violent flow... more The Moving Particle Semi-implicit (MPS) method has been proven effective to simulate violent flows such as dam-break flow, liquid sloshing and so on. But the low computational efficiency is one disadvantage of MPS. In the field of scientific computations, GPU based acceleration technique is widely applied to reduce the computation time of various numerical methods. In this paper, an in-house solver MPSGPU-SJTU is developed based on modified MPS method and GPU acceleration technique. A three-dimensional (3-D) dam-break flow is simulated by present solver and the validity and accuracy of GPU code are investigated by comparing the results with those by other researches. By comparisons, the flow field of GPU-based calculation is in better agreement with the experiment. In addition, the computation times of GPU and CPU solvers are compared to demonstrate the effect of GPU acceleration technique on the computational efficiency of MPS method.
Limited by the space and topographic factors, wake effect cannot be avoided in the wind farm. Due... more Limited by the space and topographic factors, wake effect cannot be avoided in the wind farm. Due to the wake of upstream wind turbine, the inflow wind speed for downstream wind turbine will be reduced. As a result, the wind farm has a significant decrease of annual power output. Considering the high construction and maintenance cost of the wind farm, it is necessary to optimize wind turbine positions in order to reduce the cost of power generation. In the present work, an optimization approach based on wake model and optimization algorithm is proposed for wind farm layout optimization problem. The Bastankhah wake model is applied to calculate the wake velocity field and predict the power output of wind farm. The wake velocity predicted by the Bastankhah model is also validated by CFD results. In addition, the Kriging model is selected to construct the surrogate relationship between the wind turbine positions and the power output of wind farm. To quickly obtain optimal wind farm lay...
The interaction between structures and waves is a typical phenomenon in naval architecture and oc... more The interaction between structures and waves is a typical phenomenon in naval architecture and ocean engineering fields. In this paper, numerical simulations are carried out to study the interactions between a two-dimension horizontal plate and solitary wave by our in-house CFD solver MLParticle-SJTU. The simulation procedure is following the experiments performed at Dalian University of Technology. During these computations, the plate is submerged under water and the solitary wave with given amplitude is generated by a piston-type wave maker. Free surface elevations of the wave and the pressure impacting on the plate are recorded and compared with experimental data respectively. The predicted pressure agrees well with the benchmark data. Finally, the influence of wave height on the interaction between the plate and solitary wave is analyzed.
A number of locks have been under consideration or construction in recent years. The growing numb... more A number of locks have been under consideration or construction in recent years. The growing number of large vessels can only be used if sufficient infrastructure, like locks, is available. Especially for large vessels that are most critical, the hydrodynamic phenomena induced by a lock entry are important, since the ship’s movements are strongly influenced by those phenomena. For a better understanding of the mechanism of the ship behavior and the effects of eccentricity on the ship behavior while entering a lock, the unsteady viscous flow around the 12000TEU ship model while entering a lock with different eccentricity is simulated and the hydrodynamic forces, vertical displacement are calculated by solving the unsteady RANS (Reynolds Averaged Navier–Stokes) equations in combination with the k-ω SST turbulence model. Overset grid technology is used to keep grid orthogonality. To make our prediction more reasonable, the effects of the free surface is taken into account. A benchmark ...
Yaw is the most common working condition of a wind turbine and the key of reducing the fatigue lo... more Yaw is the most common working condition of a wind turbine and the key of reducing the fatigue loads and improving the performance of a wind farm is to understand the wake characteristics of a wind turbine in yaw condition. A neutral boundary layer flow in the atmosphere is simulated by the LES technique using the solver developed based on OpenFOAM and the wake flow of a yawed wind turbine modeled by the actuator line is studied. The timeaverage velocity field proves the practicability of the yaw control operation in optimizing the total power output of a wind farm, but from the cross section contours, the velocity distribution in the wake of a yawed turbine is not completely symmetric and the vertical wake deflection cannot be neglected, which is the main source of errors of the analytical wake models based on the gaussian distribution assumption. The time history curves and frequency spectrum of the wake meandering gained from the filtered flow data indicate that the yaw condition...
In the present study, interaction between regular waves and free roll motion of a 2D floating bod... more In the present study, interaction between regular waves and free roll motion of a 2D floating body is investigated by our in-house particle solver MLParticle-SJTU based on modified Moving Particle Semi-Implicit (MPS) method. Numerical wave tank is developed to calculate the interaction between waves and floating body, including wave-maker module and free roll motion module. The comparison between the numerical wave elevation and analytical solution shows that the MLParticle-SJTU can provide acceptable accuracy of wave making. Roll motion and force acting on the floating body in waves are in good agreement with experimental results. Profiles of the wave surface surrounding floating body are presented.
Moving particle semi-implicit (MPS) method is a Lagrangian particle method, first proposed by Kos... more Moving particle semi-implicit (MPS) method is a Lagrangian particle method, first proposed by Koshizuka and Oka for incompressible flows. Due to its Lagrangian nature, MPS is capable of computing large deformed free surface flow, such as violent liquid sloshing, green water and dam breaking. However, MPS suffers from high computational cost. This significantly limits its applications in 3D flows which have to involve a large number of particles. In the present work, a parallel strategy for GPU (Graphics Processor Units) based acceleration is developed, aiming to extend MPS to compute practical 3D flows. Simulation of 3D violent sloshing and 3D dam breaking is performed. Speed-up obtained by using GPU is satisfied. The solution of PPE and search of neighboring particle are accelerated up to 10 times and 6 times faster respectively based on the present parallel strategy, than serial computation. However, performance on different GPU indicates that neighbour list is a memory bound prob...
In this paper, the effects of the location of one horizontal baffle on liquid sloshing under angu... more In this paper, the effects of the location of one horizontal baffle on liquid sloshing under angular excitation are investigated. An in-house solver MLParticle-SJTU developed based on improved moving particle semi-implicit (MPS) method is used for numerical simulation. Firstly, a rectangular tank without baffles is modeled to show the validity of solver. Then, one horizontal baffle is installed in the tank and the location of it is changed. The results show that the arrangement of horizontal baffle makes significant effects on the impact pressure on the tank walls and the deformation of free surface.
Liquid sloshing in a 3D rectangular tank under multiple degrees of freedom (DOF) excitations is s... more Liquid sloshing in a 3D rectangular tank under multiple degrees of freedom (DOF) excitations is simulated by the in house solver MLParticle-SJTU based on Moving Particle Semi-Implicit (MPS) method. Firstly, liquid sloshing under horizontal and angular motion is carried out to validate the present particle solver MLParticle-SJTU respectively. The comparison between the numerical flow pattern and impact pressure and the corresponding experimental data shows that MLParticle-SJTU is reliable and efficient to solve 3D sloshing problem. In addition, the present model is extended to 3D liquid sloshing with multiple degrees of freedom motions. The evolution of sloshing waves and impact pressure are analyzed to investigate the coupling effect of different DOF excitations. Results show that surge motion and rolling motion have weak interaction with each other. Sloshing can be damped if the amplitude and phase difference between surge motion and pitch motion are appropriate.
The bubbly flows have always been a key research object in the field of multiphase simulations. H... more The bubbly flows have always been a key research object in the field of multiphase simulations. However, when the bubbly flows become violent to cause the large deformations of bubbles, complex interfaces may be generated and bring great challenges to the traditional mesh-based methods. To overcome this challenge, one of the most commonly used mesh-free particle methods-moving particles semi-implicit (MPS) method, is adopted in this paper for the numerical simulations of bubbly flows with complex interfaces. Firstly, a multiphase MPS method is developed by combining the single-phase improved MPS (IMPS) method proposed in our previous study with different multiphase models, including the improved density smoothing scheme, interparticle viscosity model, continuous acceleration model, surface tension model, incompressible-compressible model and multiphase collision model. Then, the deformations of 2D/3D square droplets are simulated to validate the surface tension model, and the multiphase MPS method is verified through the benchmark cases of 2D/3D single bubble rising with large density and viscosity ratios (up to 1000 and 100, respectively). Finally, a series of bubbly flows with increasing complexity are performed, including the bubble rising and breaking at a high Reynolds number, the co-axial coalescence of two bubbles with different radiuses, and the violent interactions between a set of bubbles. The MPS results keep fair agreements with other numerical and experimental results, through which the advantages of the present multiphase MPS method in bubbly flows with complex interfaces can be well demonstrated.
Vortex ring thruster (VRT) is a new type of propulsion device which adopts impulse propulsion mod... more Vortex ring thruster (VRT) is a new type of propulsion device which adopts impulse propulsion mode. The idea of impulse propulsion comes from bionics. It imitates cephalopods like squid and can achieve high efficiency. Vortex ring is a typical phenomenon during the pulse process and it can help improve the propulsion efficiency. Many previous studies believe that VRT can obtain large thrust and efficiency but few articles study the efficiency problem systematically. In addition, experiment is the main research method adopted at present. In this paper, an axisymmetric piston-nozzle computational model is established to simulate the thruster. Fluid is solved by CFD method based on the open source platform OpenFOAM. Dynamic mesh technology is adopted to control piston motion. This paper focus on two aspects: vortex ring evolution and propulsion efficiency characteristics. In the first part, simulations of pulse process with different strokes are carried out and compared with standard experiments. The two results are in good agreement. The vortex ring evolution is then studied with different background flows. In the second part, a reasonable open-water characteristics calculation method for VRT is proposed. Thrust coefficient and propulsion efficiency are obtained in the calculation. The results indicate that the VRT can achieve high efficiency near to 1 in the case of high advance ratio. In addition, efficiency comparison is carried out between four kinds of asymmetric velocity programs and three kinds of symmetric velocity programs to find out a more reasonable velocity program design. Finally, the impact of piston stroke is investigated. Results show that smaller stroke can obtain larger efficiency because pulsation effect is more prominent.
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