PySPH: A Python-based Framework for Smoothed Particle Hydrodynamics

PySPH is an open-source, Python-based, framework for particle methods in general and Smoothed Particle Hydrodynamics (SPH) in particular. PySPH allows a user to define a complete SPH simulation using pure Python. High-performance code is generated from this high-level Python code and executed on either multiple cores, or on GPUs, seamlessly. It also supports distributed execution using MPI. PySPH supports a wide variety of SPH schemes and formulations. These include, incompressible and compressible fluid flow, elastic dynamics, rigid body dynamics, shallow water equations, and other problems. PySPH supports a variety of boundary conditions including mirror, periodic, solid wall, and inlet/outlet boundary conditions. The package is written to facilitate reuse and reproducibility. This article discusses the overall design of PySPH and demonstrates many of its features. Several example results are shown to demonstrate the range of features that PySPH provides.

Research on rolling stability of the flexible waverider based on computational fluid dynamics/computational structural dynamics/rigid body dynamics coupling methodology

The shape of hypersonic aircrafts represented by waveriders is becoming more slender and flatter, thereby greatly reducing the structural rigidity. This innovation is applied to satisfy the demand of long-range flight. The rolling stability of the waveriders is poor due to the slender shape. Therefore, the effect of the elastic deformation on the rolling stability cannot be ignored. The effect of the elastic deformation on the stability of rolling and forced pitching/free rolling coupling motions of the waveriders is studied through computational fluid dynamics (CFD)/computational structural dynamics (CSD)/rigid body dynamics (RBD) coupling methodology. Comparison results of numerical simulation indicate that the elastic deformation of the structure increases the local angle of attack, thereby enhancing the static stability of the waveriders. The rolling motion of the waveriders changes from point attractor to periodic attractor when the vibration velocity due to elastic deformation is considered. The rolling oscillation frequency of the flexible model is higher than that of the rigid model. For the forced pitching/free rolling motion, stability theory based on the rigid body hypothesis is unsuitable when the elastic effect is taken into consideration.

Analysis of propulsion performance of wave-propelled mechanism based on fluid-rigid body coupled model

Wave-propelled mechanisms are applied to propel unmanned marine vehicles such as Wave Glider and wave-powered boats, which can convert wave energy directly into propulsion. In this paper, a fluid-rigid body coupled dynamic model is utilized to investigate the propulsion performance of the wave-propelled mechanism. Firstly, the coupled dynamic model of the wave-propelled mechanism is developed based on relative motion principle by combining rigid body dynamics model and CFD method. Then, the motion responses of wave-propelled mechanism are calculated. The relationship between the propulsion force, heave and pitch motion of hydrofoil are analyzed by using phase diagrams and the actual operation conditions of propulsion mechanism are obtained. Besides, the effects of restoring spring stiffness and wave heights on the propulsion performance are also investigated, and the vortex evolution is illustrated at different moments of movement and different restoring stiffness. These works can be helpful for the design and optimization of different kinds of wave-propelled vehicles.

A numerical study of the hydrodynamics of an offshore fish farm using REEF3D

In this paper, the hydrodynamics of and non-linear interaction between the large offshore fish farm “ShenLan 1” and regular waves are investigated using the open-source CFD toolbox REEF3D. The framework consists of a rigid body dynamics solver for the frame structure coupled to a fluid solver including the shielding effects of the nets. The solver and grid independence are validated using a 2D numerical wave tank, a free decay test and a study of the wave loads on a rigid net panel. Then, the effects of regular wave parameters, the thickness of the vertical outer columns of the structure, and the variations of the aspect ratios on the loads, responses and maximum mooring tension forces are studied. It is concluded that the response motion is sensitive to the wave period rather than the wave height due to the longer duration of unidirectional wave loads acting on the frame. The frequent events of partial submersions and wave overtopping in rather steep waves are confirmed through the capturing of the free surface. The net system accounts for about 30% of the total drag but does not influence the structural response to a larger extend. The effect of the aspect ratio on the hydrodynamics is more distinct than that of the frame thickness. As a result of the study, the first step towards a systemic evaluation of the importance of different structural parts of an offshore fish cage for the expected responses is provided.

Effects of Nonlinearity of Restoring Springs on Propulsion Performance of Wave Glider

Wave glider is an unmanned surface vehicle which can directly convert wave energy into forward propulsion and fulfill long-term marine monitoring. Previous study suggested that the wave motion and stiffness of restoring springs mounted on the hydrofoil are main factors affecting the propulsion performance of wave glider. In this paper, the dynamic responses and nonlinear characteristics of underwater propulsion mechanism considering the nonlinear stiffness of restoring springs are investigated based on a fluid-rigid body coupled model. Firstly, the models of propulsion mechanism with different kind of restoring spring are proposed, and the linear and nonlinear characteristics of restoring spring are considered. Then, a fluid-rigid body coupled model of wave glider is developed by coupling the rigid body dynamics model and hydrodynamic model. Dynamic responses are simulated by numerical analysis method and the nonlinear characteristics with different restoring springs are illustrated by time/frequency domain motion response and phase diagram analysis. The effects of wave excitation frequency and wave heights on the propulsion performance of wave glider are analyzed. The results show that, multi-frequency responses occurred in propulsion system. And the study suggests that the nonlinear restoring spring on the hydrofoil can be suitable for different sea condition and better propulsion performance can obtained than linear stiffness spring, which provides a reference for developing propulsion mechanism with high performance in complex marine environment.

A hierarchical approach for rigid-body dynamics model simplification of a high-speed parallel robot by considering kinematics performance

Considering the real-time control of a high-speed parallel robot, a concise and precise dynamics model is essential for the design of the dynamics controller. However, the complete rigid-body dynamics model of parallel robots is too complex for online calculation. Therefore, a hierarchical approach for dynamics model simplification, which considers the kinematics performance, is proposed in this paper. Firstly, considering the motion smoothness of the end-effector, trajectory planning based on the workspace discretization is carried out. Then, the effects of the trajectory parameters and acceleration types on the trajectory planning are discussed. But for the fifth-order and seventh-order B-spline acceleration types, the trajectory will generate excessive deformation after trajectory planning. Therefore, a comprehensive index that considers both the motion smoothness and trajectory deformation is proposed. Finally, the dynamics model simplification method based on the combined mass distribution coefficients is studied. Results show that the hierarchical approach can guarantee both the excellent kinematics performance of the parallel robot and the accuracy of the simplified dynamics model under different trajectory parameters and acceleration types. Meanwhile, the method proposed in the paper can be applied to the design of the dynamics controller to enhance the robot's performance.

A convolution neural network based semi-parametric dynamic model for industrial robot

Robot dynamic model is widely applied to control, collision detection and motion planning. Accurate dynamic model can achieve better performance for the above applications. Traditional dynamic models have several limitations, such as the complex hypotheses for friction model and the requirement of additional joint torque sensors. This article constructs a convolution neural network (CNN) based semi-parametric dynamic (SPD) model by only using the motor encoder signals and motor currents. The SPD model not only contains the physically feasible parameters but also compensates the dynamic model by CNN. The parametric and non-parametric parts constitute the SPD model. A lightweight CNN is proposed to simultaneously ensure the accuracy and computational efficiency. To effectively train the CNN model, a dataset generation method, which expands the excitation trajectory and only uses a continuous trajectory to record data, is proposed. The CNN-based SPD model is verified on a 6-DoF laboratory-developed industrial robot only with the proprioceptive sensors. Compared with the traditional rigid body dynamics (RBD) model, the average error of the CNN-based SPD model is reduced by 9.23% in terms of the experimental results. Meanwhile, the proposed CNN-based method achieves better performance than other supervised methods.