Numerical Analysis of Hydrodynamic Behaviors of Two Net Cages with Grid Mooring System Under Wave Action

Based on rigid kinematics theory and lumped mass method, a mathematical model of the two net cages of grid mooring system under waves is developed. In order to verify the numerical model, a series of physical model tests have been carried out. According to the comparisons between the simulated and the experimental results, it can be found that the simulated and the experimental results agree well in each wave condition. Then, the forces on the mooring lines and the floating collar movement are calculated under different wave conditions. Numerical results show that under the same condition, the forces on the bridle ropes are the largest, followed by forces on the main ropes and the grid ropes. The horizontal and the vertical float collar motion amplitudes increase with the increase of wave height, while the relationship of the horizontal motion amplitude and the wave period is indistinct. The vertical motion amplitude of the two cages is almost the same, while on the respect of horizontal motion amplitude, cage B （behind cage A, as shown in Fig. 4） moves much farther than cage A under the same wave condition. The inclination angle of the floating system both in clockwise along y axis and the counter one enlarges a little with the increase of wave height.

Dynamic Analysis of Hydrodynamic Behavior of A Flatfish Cage System Under Wave Conditions

This paper presents a simulation model based on the finite element method. The method is used to analyze the motion response and mooring line tension of the flatfish cage system in waves. The cage system consists of top frames, netting, mooring lines, bottom frames, and floats. A series of scaled physical model tests in regular waves are conducted to verify the numerical model. The comparison results show that the simulated and the experimental results agree well under the wave conditions, and the maximum pitch of the bottom frame with two orientations is about 12o. The motion process of the whole cage system in the wave can be described with the computer visualized technology. Then, the mooring line tensions and the motion of the bottom frame with three kinds of weight are calculated under different wave conditions. According to the numerical results, the differences in mooring line tensions of flatfish cages with three weight modes are indistinct. The maximum pitch of the bottom frame decreases with the increase of the bottom weight.

Experimental investigation of tubing collar's influence on hydrodynamic behavior of annular duct flow

Tubing collars’influence on hydrodynamic behavior of annular duct flow has been investigated using Particle Image Velocity(PIV)technology.PIV has become an efficient method for complex transient flows visualization.A water flow loop with two replaceable variable cross-sections(VCS),75-90 mm and 90-110 mm,in a 129 mm inner diameter(ID)pipe was used.The whole field of the variable cross-section annulus(VCSA)was visualized,including forward-facing step(FFS),narrow annulus(NA),and backwardfacing step(BFS)flow.The VCSA ratio and Reynolds(Re)number influence on streamline distribution,velocity distribution,and turbulence intensity were discussed.Results showed that the recirculation is easier to form in BFS than FFS flow under the same condition.The VCSA ratio affects the formation of recirculation zones and the location of the reattachment point.Reynolds number mainly affects BFS flow by influencing the main velocity component-axial velocity.The turbulence intensity is relatively high in BFS than FFS flow and is larger at y/h>1.0 than y/h<1.0.Furthermore,the streamwise cohere nt structures reveal that the first two modes are predominant and represent the main characteristics of the flow by proper orthogonal decomposition(POD)method.

Interaction of Linear Waves with Infinitely Long Horizontal Cylinders Studied by Boundary Element Method

The two-dimensional problems concerning the interaction of linear water waves with cylinders of arbitrary shape in two-layer deep water are investigated by use of the Boundary Integral Equation Method （BIEM）. Simpler new expressions for the Green functions are derived, and verified by comparison of results obtained by BIEM with those by an analytical method. Examined are the radiation and scattering of linear waves by two typical configurations of cylinders in two- layer deep water. Hydrodynamic behaviors including hydrodynamic coefficients, wave forces, reflection and transmission coef- ficients and energies are analyzed in detail, and some imeresting physical phenomena are observed.

Hydrodynamic behavior of an airlift reactor with net draft tube with different configurations: Numerical evaluation using CFD technique

In terms of gas holdup,liquid velocity,and volumetric mass transfer coefficient for oxygen(KLa),the hydrodynamic behavior of four configurations of an airlift reactor(ALR)with a net draft tube(NDT)of different net mesh sizes(ALR-NDT-3,6,12,and ALR)have been numerically simulated for a range of inlet air flow rates.The effect of various levels of ratio of height(H)to inner tube diameter(D)of the net draft tube(H/D:9.3,10.7,17.5,and 20)and ratio of inner cross-sectional area of the riser(Ar)to the inner cross sectional area of the downcomer(Ad)(Ad/Ar:1.3 and 7)for different air flow rates is also evaluated for each reactor configuration operating with an air-water system.The two-fluid formulation coupled with the k-εturbulence model is used for computational fluid dynamics(CFD)analysis of flow with Eulerian descriptions for the gas and liquid phases.Interactions between air bubbles and liquid are taken into account using momentum exchange and drag coefficient based on two different correlations.Trends in the predicted dynamical behavior are similar to those found experimentally.A good agreement was achieved suggesting that geometric effects are properly accounted for by the CFD model.After a comparison with experimental data,numerical simulations show significant enhanced gas holdup,liquid velocity,and Kta for the ALR-NDTs compared with the conventional ALR.Higher gas holdup values are achieved for ALR-NDT-3 than that for the other ALRs because it acts like a bubble column reactor as the holes present in the NDT are large.Maximum liquid velocities are seen in ALR-NDT-12,which operates like a conventional ALR.Moreover,the interaction between the NDT and upward gas flow leads to cross flow through the net,small bubbles,and high interfacial area as well as good mass transfer.This was significant in ALR-NDT-6 with maximum Kta value of 0.031 s-1.The applied methodology provides an insightful understanding of the complex dynamic behavior of ALR-NDTs and may be helpful in optimizing the design and scale-up of reactors.

EXPERIMENTAL STUDY ON INSTANTANEOUS DISCHARGE OF UNSORTED PARTICLE CLOUD IN CROSS-FLOW

A series of extensive laboratory experiments were conducted to investigate the transport and mixing of instantaneous discharge of unsorted particle cloud in cross-flow. The following experimental results were obtained： （1） the vertical frontal position and the longitudinal width of the particle cloud in the cross-flow is much larger than those in stagnant water, （2） the smaller cross-flow velocity will normally cause the larger non-dimensional frontal position for the same particle size range and initial volume, （3） the non-dimensional longitudinal width of the particle cloud in the cross-flow increases with the increase of cross-flow velocity. The empirical constants （α1, α3 and α2） and their variance range, which can be used to determine the relationships of non-dimensional vertical frontal position and time, longitudinal width and time, and vertical frontal position and longitudinal width were also obtained through the analysis of experimental data.

Hydrodynamic modeling strategy for dense to dilute gas-solid fluidized beds

When investigating the hydrodynamic behavior of gas–solid flow systems, there are several options for the drag function, viscosity model, and other parameters. The low accuracy obtained with a random trial and error modeling strategy has led researchers to develop new drag models that are fine-tuned for their specific studies. However, besides the drag functions, an appropriate viscosity model together with radial distribution function have a great impact on the hydrodynamic modeling of fluidized beds. In this study, a detailed validation and verification task is conducted using three different experimental datasets to derive a modeling strategy for predicting hydrodynamic behavior in dense to dilute flow regimes of various fluidized beds. For this purpose, the steady-state Reynolds-averaged Navier–Stokes equations are solved in a finite volume scheme using the twoPhaseEulerFoam solver in the OpenFOAM 2.1.1 software. A comparative study of different drag and viscosity models enables an optimal modeling strategy to be determined for the accurate prediction of the bed pressure drop, bed expansion ratio, time-averaged solid hold-up, and bed height in various dense and dilute flow regimes. Our results show that the modeling strategy prescribed in this study is widely applicable for identifying the hydrodynamic characteristics of various gas–solid fluidized beds with different operating conditions.