Identifying the turbulent flow developing inside and around the bottom trawl by Electromagnetic Current Velocity Meter approach in the flume tank

The Knowledge of turbulent flow developing inside and around the bottom trawl net is of great importance not only for improving the hydrodynamic performance of the gear but also for the selectivity via the fish response,such as the herding response or escape behavior.The 3-D Electromagnetic Current Velocity Meter(ECVM)measurements were performed to investigate the effect of turbulent flow on the bottom trawl net performance and to analyze the turbulence intensity and velocity ratio inside and around different parts of the trawl net.Proper orthogonal decomposition(POD)method was applied in order to extract the phase averaged mean velocity field of turbulent flow from each available ECVM instantaneous velocity.The results demonstrated the existence of turbulence flow,consisting of turbulent boundary layer flow and the turbulence due to the trawl wake developing all inside and around the bottom trawl net.Increasing input streamwise velocity results in faster trawl movement and a significant turbulent flow.The maximum turbulence intensity inside and around trawl wing,square part,first belly,second belly,third belly,cod-end is 0.95%,1.34%,3.40%,4.10%,4.25%and 3.80%,respectively.It was found that the mean velocity field in a turbulent flow inside and around trawl net cod-end recovered on the average was~77.58%of the input streamwise velocity.It is~12.92%,~13.07%,~11.40%,~13.00%and~0.45%less than that inside and around trawl wing,square part,first belly,second belly,and third belly of the bottom trawl net,respectively.The turbulent flow behavior depends strongly on the structure oscillation,input streamwise velocity and,porosity of the net structure.It is necessary to take into account the velocity reduction inside and around a different part of the trawl net to improve the entire drag force determination,cod-end design,and further selectivity control of the fishing gear.

Observations and analyses of floc size and floc settling velocity in coastal salt marsh of Luoyuan Bay, Fujian Province, China

In coastal environments, fine-grain sediments often aggregate into large and porous flocs. ElectroMagnetic Current Meters （EMCM） and Laser In Situ Scattering and Transmissometry （LISST-ST） have been deployed within a Spartina alterniflora marsh of the Luoyuan Bay in Fujian Province, China, to measure the current velocity, the floc size and the settling velocity between 15 and 22 January 2008. During the observations, the near-bed water was collected in order to obtain the suspended sediment concentration （SSC） and constituent grain size. Data show that： （1） the nearbed current velocities vary from 0.1 to 5.6 cm/s in the central Spartina alterniflora marsh and 0.1–12.5 cm/s at the edge; （2） the SSCs vary from 47 to 188 mg/dm 3 . The mean grain size of constituent grains varies from 7.0 to 9.6 μm, and the mean floc sizes （MFS） vary from 30.4 to 69.4 μm. The relationship between the mean floc size and settling velocity can be described as： w s =ad b , in which w s is the floc settling velocity （mm/s）, a and b are coefficients. The floc settling velocity varies from 0.17 to 0.32 mm/s, with a mean value of 0.26 mm/s, and the floc settling velocity during the flood tide is higher than that during the ebb tide. The current velocity and the SSC are the main factors controlling the flocculation processes and the floc settling velocity.

A hybrid forecasting model for depth-averaged current velocities of underwater gliders

In this paper,we propose a hybrid forecasting model to improve the forecasting accuracy for depth-averaged current velocities(DACVs) of underwater gliders.The hybrid model is based on a discrete wavelet transform(DWT),a deep belief network(DBN),and a least squares support vector machine(LSSVM).The original DACV series are first decomposed into several high-and one low-frequency subseries by DWT.Then,DBN is used for high-frequency component forecasting,and the LSSVM model is adopted for low-frequency subseries.The effectiveness of the proposed model is verified by two groups of DACV data from sea trials in the South China Sea.Based on four general error criteria,the forecast performance of the proposed model is demonstrated.The comparison models include some well-recognized single models and some related hybrid models.The performance of the proposed model outperformed those of the other methods indicated above.

Analytical solutions for transverse distributions of stream-wise velocity in turbulent flow in rectangular channel with partial vegetation

The theory of poroelasticity is introduced to study the hydraulic properties of the steady uniform turbulent flow in a partially vegetated rectangular channel. Plants are assumed as immovable media. The resistance caused by vegetation is expressed by the theory of poroelasticity. Considering the influence of a secondary flow, the momentum equation can be simplified. The momentum equation is nondimensionalized to obtain a smooth solution for the lateral distribution of the longitudinal velocity. To verify the model, an acoustic Doppler velocimeter （ADV） is used to measure the velocity field in a rectangular open channel partially with emergent artificial rigid vegetation. Comparisons between the measured data and the computed results show that the method can predict the transverse distributions of stream-wise velocities in turbulent flows in a rectangular channel with partial vegetation.

Application of computational fluid dynamics simulation for submarine oil spill

Computational fluid dynamics （CFD） codes are being increasingly used in the simulation of submarine oil spills. This study focuses on the process of oil spills, from damaged submarine pipes, to the sea surface, using numerical models. The underwater oil spill model is developed, and a description of the governing equations is proposed, along with modifications required for the particalization of the control volume. Available experimental data were introduced to evaluate the validity of the CFD predictions, the results of which proved to be in good agreement with the experimental data. The effects of oil leak rate, leak diameter, current velocity, and oil density are investigated, by the validated CFD model, to estimate the undersea leakage time, the lateral migration distance, and surface diffusion range when the oil reaches the sea surface. Results indicate that the leakage time and lateral migration distance increase with decreasing leak rates and leak diameter, and increase with increasing current velocity and oil density. On the other hand, a large leak diameter, high density, high leak rate, or fast currents result in a greater surface diffusion range. The findings and analysis presented here will provide practical predictions of oil spills, and guidance for emergency rescues.

Hydrodynamics and Associated Sediment Transport over Coastal Wetlands in Quanzhou Bay,China

Coastal salt marshes represent an important coastal wetland system. In order to tmderstand the differences between boundary layer parameters of vegetated and unvegetated areas, as well as the mechanisms of sediment transport, several electromagnetic current meters （AEM HR, products of Alec Electronics Co. Ltd.） were deployed in coastal wetlands in Quanzhou Bay, China, to measure current velocity. During the low tide phase, the surficial sediment was collected at 10 m intervals. In situ measurements show that the current velocities on the bare flat were much higher than those in the Spartina alterniflora marsh. Current velocity also varied with distance from marsh edge and plant canopy height and diameter. Around 63% of the velocity profiles in the tidal creek can be described by a logarithmic equation. Over the bare flat and Spartina alterniflora marsh, a logarithmic profile almost occurs during the flood tide phase. Sediment analysis shows that mean grain size was 6.7 Ф along the marsh edge, and surface sediments were transported from bare fiat to marsh; the tidal creeks may change this sediment transport pattern. The hydrodynamics at early flood tide and late ebb tide phases determined the net transport direction within the study area.

Characterizing ship-induced hydrodynamics in a heavy shipping traffic waterway via intensified field measurements

Ship-induced hydrodynamics play an important role in shaping the cross-sectional profile of inland waterways and produce a large amount of pressure on the fluvial environment.This study aimed at quantifying the characteristics of ship-induced waves and currents in a heavy shipping traffic waterway via intensified field measurements conducted in the Changzhou segment of the Grand Canal,in Jiangsu Province,China.Based on the processed hydrodynamic data,waves and currents caused by single ships and multiple ships were investigated.For single ships,the shipinduced wave heights estimated with empirical formulas were not consistent with the observations.Categorized by the loading conditions of barges,the drawdown height was characterized by the ratio of ship speed to its limit speed.The maximum non-dimensional ship-induced wave height was parameterized by a nonlinear combination of the depth Froude number and a blockage coefficient.For multiple ships,when ships closely followed each other or interlaced each other's paths,it was difficult to characterize the superposition of several ship wakes.The magnitudes of current velocities induced by single ships and multiple ships were respectively nine and six times as large as those of natural flow.This may result in more severe sediment(re)suspension than natural flows.©2020 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Cross-shore suspended sediment flux in the salt marsh pioneer zone of Chongming eastern beach in the Chang- jiang Estuary in China

Between April 2002 and April 2003,in situ measurements of water depth,current velocity and suspended sediment content were carried out in edge region of East Chongming salt marsh and neighboring bald flat in the Changjiang （Yangtze） Estuary under different weather conditions.Cross-shore suspended sediment flux was calculated and analyzed.The results show that under calm weather conditions,the current velocity process in bald field and salt marsh area varied differently during semidiurnal tidal cycles.Owing to current velocity asymmetry,mean SSC during flood tide phase was 1.8 times higher than that of ebb tide phase.As a result,net onshore sediment flux controlled cross-shore suspended sediment transport process and salt marsh pioneer zone was generally accreting.There was significant positive correlation between total sediment flux and quartic power of maximum water depth.It indicates that tidal ranges dominate suspended sediment transport and sedimentation process in the salt marsh pioneer zone under the calm weather condition.The sedimentation rate on the adjacent mudflat was higher than the salt marsh,which induced stable accreting of salt marsh towards the sea.The wind events enhanced SSC and current velocity during the semidiurnal tides.And the remarkable onshore net sediment flux could occur on the high marsh and mudflat close to the marsh fringe during the short period under the rough weather condition.

Numerical analysis of pipeline in J-lay problem

The pipe configuration and internal loads along the pipeline during the pipeline laying process have long been the focus of engineers. Most researchers simplify the seabed to be rigid and the water to be calm, ignoring the pipe embedment into the seabed and the influence of ocean currents. In this paper, a novel numerical approach is proposed for the laying of pipelines in the so-called J-lay method, taking into account the importance of both pipe embedment and ocean currents. The pipeline is divided into two parts, one part suspended in water, and the other laid on the seabed. The continuity of the two parts at the touch down point (TDP) is guaranteed to make a whole. The feasibility of the model is proved by the comparison between the present model and an analytical model, which shows good agreement in both pipeline configuration and bending moment distribution. Finally, parametric study was performed to consider the influence of current velocity, water depth, top inclination angle, and seabed stiffness, and conclusions are drawn.