Numerical Modelling of the Interaction of Moving Fish Nets and Fluid

The significant difference in length scales between the flow around a moving fish net and the flow around each twine of the net prevents the resolution of the complete structure within a discrete fluid domain. In this paper, this issue is overcome by calculating the net and fluid dynamics separately and incorporate their interaction implicitly. The forces on the net are approximated using a screen force model, and the motion of the net is computed with a lumped mass method. Here, a linear system of equations is derived from the dynamic equilibria and kinematic relations. The net model is coupled to the CFD solver REEF3D which solves the incompressible Navier-Stokes equations using high-order finite differences in space and time. Several numerical calculations are provided, and the comparison of loads and velocity reduction with available measurements indicates the good performance of the proposed model.

Modeling and Analysis of Amplitude-Frequency Characteristics of Torsional Vibration for Automotive Powertrain

In the present paper, the amplitude-frequency characteristics of torsional vibration are discussed theoretically and experimentally for automotive powertrain. A bending-torsional-lateral-rocking coupled dynamic model with time-dependent mesh stiffness, backlash, transmission error etc. is proposed by the lumped-mass method to analysis the amplitude-frequency characteristic of torsional vibration for practical purposes, and equations of motive are derived. The Runge–Kutta method is employed to conduct a sweep frequency response analysis numerically. Furthermore, a torsional experiment is performed and validates the feasibility of the theoretical model. As a result, some torsional characteristics of automotive powertrain are obtained. The first three-order nature torsional frequencies are predicted. Torsional behaviors only affect the vibration characteristics of a complete vehicle at low-speed condition and will be reinforced expectedly while increasing torque fluctuation. Gear mesh excitations have little effects on torsional responses for such components located before mesh point but a lot for ones behind it. In particular, it is noted that the torsional system has a stiffness-softening characteristic with respect to torque fluctuation.

Numerical Modelling of the Interaction of Moving Fish Nets and Fluid

The significant difference in length scales between the flow around a moving fish net and the flow around each twine of the net prevents the resolution of the complete structure within a discrete fluid domain. In this paper, this issue is overcome by calculating the net and fluid dynamics separately and incorporate their interaction implicitly. The forces on the net are approximated using a screen force model, and the motion of the net is computed with a lumped mass method. Here, a linear system of equations is derived from the dynamic equilibria and kinematic relations. The net model is coupled to the CFD solver REEF3D which solves the incompressible Navier-Stokes equations using high-order finite differences in space and time. Several numerical calculations are provided, and the comparison of loads and velocity reduction with available measurements indicates the good performance of the proposed model.

Experimental Test and Numerical Analysis of a Structure Equipped with a Multi-Tuned Liquid Damper Subjected to Dynamic Loading

Tuned liquid dampers (TLDs) have many advantages in controlling building vibrations, among which multi-tuned liquid dampers (MTLDs) appear to have better stability and effectiveness. However, the tank wall was assumed to be rigid in the past by ignoring the fluid-structure interaction (FSI) at the interface, resulting in simplified calculation for the design of the TLDs. Moreover, the fluid in the tank was considered to be separate from the structure. This paper presents two numerical methods to control the responses of the frame under the dynamic loadings: (1) the lumped mass method for quickly designing the TLDs, and (2) the finite volume method/finite element method (FVM/FEM) for analyzing the fluid and solid domains of the TLDs in a single computational 3D model. In addition, the multi-field interaction between the structure-fluid-tank walls is considered by solving the coupling equations at the interfaces. A steel frame is fitted with an MTLD and tested experimentally on a shaking table to investigate its dynamic response. Numerical results are verified with the experimental ones, which show good agreement.

A Dynamic Model for Continuous Lowering Analysis of Deep-Sea Equipment, Based on the Lumped-Mass Method

The installation of subsea equipment is a critical step in offshore oil and gas development. A dynamic model to evaluate the lowering process is proposed. The cable–payload system is discretized as a series of spring dampers with the lumped-mass method. For the first time, not only the lowering velocity but also the rope’s structural damping and the nonlinear loads, such as drag force and snap load, are considered. The lowering velocity of the cable is considered through a variable-domain technique. Snap loads are considered by setting the internal forces in the elements to be zero when the cable slacks. A series of simulations reveals that the lowering velocity has great effects on the dynamic force in the cable. However, the structural damping of the cable has little effect on the system response. The snap load may occur in the cable when subjected to rapid downward heave motion, and decreases with the lowered depth increasing. The cable stiffness affects the system’s resonance depth, but has little effect on the peak dynamic force. The present work should be a valuable reference for future subsea equipment installation analysis.

Theoretical analysis of the vibration modes of the star herringbone gear transmission system

The star herringbone gear transmission system has a high load-carrying capacity, and is widely used in aviation, marine power drives, off-road vehicles, and hybrid electric-drive vehicles. Vibration and noise are the key concerns with this transmission system. The lumped mass method was adopted to establish the dynamic model and equations of this system. The modes of the system were analyzed and classified, and the eigenvalues and their multiplicities were determined. The results showed that the system has four typical vibration modes: (1) a lateral-rotational coupled vibration mode (multiplicity m = 1), (2) star gear compound mode (multiplicity m = N-3, N > 3), (3) center component lateral vibration mode (multiplicity m = 2), and (4) star gear and center gear-coupled mode (multiplicity m = 2). The contribution of this paper lies in the discovery of the coupling vibration modes in the star herringbone gear transmission system and the multiplicities of these modes. This work provides the foundation for further research on vibration suppression for the star herringbone gear transmission system and the theory of planet phasing.

Identifying the Design Alternatives and Flow Interference of Tuna Purse Seine by the Numerical Modelling Approach

Dynamic responses of tuna purse seine to currents were numerically studied with regard to the vertical subsidence and global load distribution, and determinant attributes were identified. We rebuilt the submerged geometry of a purse seine net using the lumped mass method with hydrodynamic coefficients obtained from measurements of the prototype material, as well as a mesh grouping method, which corrects the twine diameter and netting material density for the equivalent net by introducing the compensation coefficient. Uneven tension distribution showed that it was vulnerable to high loads in the bunt area at shooting and along the lead line at pursing. High loads were present at the convex sections of net circles in the direction of the current. Higher shooting speeds resulted in well-balanced sinking, while a reduced hanging ratio of netting panels was beneficial to faster sinking. While large mesh sizes reduce water resistance and increase sinking velocity, extensive use should be cautioned in terms of the sensibility to stress. Comparing two options of different proportions of large-mesh panels, mesh sizes doubled for 15 strips versus 5 strips, suggesting that the 15 strip option would pose a higher risk of strand vulnerability, while the 5 strip option may be a more balanced alternative with a lower mesh density and a lower mesh stress.

Vibration Modelling and Verification for Rotor-Support-Casing Coupling System with Misalignment Fault

Misalignment is one of the common malfunctions that occur in rotating machines. Effects of misalignment on the casing vibration response of a rotor-support-casing (RSC) coupling system is investigated in detail. The model of an RSC coupling system is established using the lumped mass method. The coupling effects between the rotor, support and casing are fully considered. A misalignment model is proposed, and equivalent misalignment force is applied on corresponding lumped mass points. Nonlinear factors of bearing, such as the clearance of bearing, oil film force, nonlinear Hertzian contact force, and the varying compliance vibration are developed. The influences of oil film thickness and bearing size are considered in the nonlinear oil film force model. By using a numerical method, the governing equations of the system are solved to obtain the steady-state vibration. The simulation results from a coupling model are compared with the experimental results and the effectiveness of the new model is verified. It has been found that spectrums and orbit plots are effectively used to reveal the unique nature of misalignment faults, leading to reliable misalignment diagnostic information.

Numerical Modelling of the Mooring Line Failure Induced Performance Changes of a Marine Fish Cage in Irregular Waves and Currents

This study aims to investigate the performance changes resulted from a mooring line failure of a marine fish cage exposed to irregular waves and current. A numerical model based on the lumped mass method and Morison equation was extended to simulate the mooring line failure scenario. In this study, the failed resulting changes were compared with its normal counterpart in both the time domain and the frequency domain. After one upstream anchor loss, the maximum tension on the remaining anchor has increased significantly, as well as the drift distance of the rearing part (net chamber, floating collar, and tube-sinker) of the fish cage. The resulting changes can also be seen in both the wave-frequency and the low-frequency region in the spectra, including mooring tensions and body motions.