Main Current Direction Research Articles

Investigation on the keyhole/molten pool dynamic behavior during adjustable ring-mode laser welding of medium-thick aluminum alloy

Adjustable ring-mode laser welding (ARM) provides a new solution for the pore defects of aluminum alloy joints. However, the inhibition mechanism of ARM for porosity defects in the aluminum alloy welds with medium-thickness is still lacking. In this work, the dynamic behavior of the keyhole/molten pool and the formation/transport process of bubbles during adjustable ring-mode laser welding of medium-thick aluminum alloy are investigated by experiments and simulations. The results display that the laser power ratio of central-ring beam of 6:4, the porosity is 4.19 ± 2.32 %, which is reduced by 52.2 % compared with the weld by single laser welding (SLW) of the same penetration depth. The addition of ring beam helps to enlarge the tilt angle of the keyhole rear wall, which increases the ability of the keyhole wall to resist the impact of the molten pool. Besides, it also lowers the radial pressure gradient and the local variation of the keyhole wall. These reasons reduce the formation of pores. Furthermore, the main eddy current direction of the molten pool changes less frequently, and the melt flow velocity impacting the keyhole wall only changes in the range of 0–0.5 m/s (SLW: 0.25–0.75 m/s). This diminishes the fluid dynamic pressure which is the primary driving force of the bubbles. The velocity of the bubble moving away from the bottom of the keyhole is decreased and it is more likely to remelt the keyhole and escape under the action of the laser beam at the next moment. This work provides engineering application and theoretical reference value for the suppression of pore defects in laser welding of aluminum alloy in medium-thick plates.

Statistical analyses of eddies in the Western Mediterranean Sea based on Synthetic Aperture Radar imagery

We analysed 7188 Sentinel-1A Synthetic Aperture Radar (SAR) images acquired in a 3-year period, between October 2014 and September 2017, in the Western Mediterranean Sea and found imprints of about 14,300 oceanic eddies with diameters ranging from 0.4 km to 160.1 km. Those eddies manifest on the SAR imagery either through the accumulation of surfactants or through wave-current interaction, and considering the favourable wind-speed ranges for both mechanisms we performed statistical analyses to gain insight into the eddies' spatio-temporal distribution, their typical sizes and shapes, and into the predominant mechanisms that are responsible for their formation. The vast majority of the detected eddies is elliptical, with a tendency for only large mesoscale eddies towards a circular shape. We found no general influence of the wind on the eddies' orientation, although mesoscale eddies tend to orient into the main current direction in each subregion. Submesoscale eddies do not show such tendency. Submesoscale eddies in the Western Mediterranean Sea are predominantly cyclonic (>90%), and with increasing eddy diameter there is a transition from cyclonic to anticyclonic sense of rotation. In general, we found highest densities of submesoscale eddies in autumn, when the biological activity is still high and when mean regional winds are already strong, but variable. In addition, instabilities of thermal fronts during summer and autumn and instabilities of the main and secondary currents influence the formation of submesoscale eddies. Some of our results confirm those previously obtained for other regions.

Analysis of the population structure of a gorgonian forest (Placogorgia sp.) using a photogrammetric 3D modeling approach at Le Danois Bank, Cantabrian Sea

The presence of gorgonian forests and deep-sea sponge aggregations in the Le Danois Bank promoted its declaration as the “El Cachucho” Marine Protected Area (MPA) by the Spanish Ministry of Environment, and its inclusion in the Natura 2000 network. Both habitats are considered vulnerable, so follow-up surveys are being performed to monitor their conservation in compliance with the EU Habitats Directive. The use of a non-invasive methodology, which does not cause damage or alterations on benthic communities, is particularly necessary in vulnerable ecosystem studies and MPA monitoring.This study analyzed the assemblage structure of a Placogorgia sp. population using a 3D photogrammetry-based method. The study was carried out through the analysis of the video transects obtained at the Le Danois Bank, using the Politolana underwater towed vehicle during the July 2017 ECOMARG survey. Recent developments in specific software of photogrammetric image analysis allowed extracting valuable information from these video transects. Using the Pix4D Mapper Pro software, 3D point clouds were obtained, and the size and morphometry of yellow fan-shaped gorgonian population structure could be evaluated. Due to gorgonian's high structural complexity, the use of length (i.e. height) as the morphometric descriptor of the real size of the colonies is not appropriate. Instead of length, the fan surface area covered by each gorgonian colony was selected as a suitable parameter of size. The direct measurement of this parameter was possible through a complete 3D reconstruction of the gorgonian forest.A total of 426 colonies of Placogorgia sp. were digitalized to obtain surface measurements and fan spread orientation calculations in 3D models. The results show that gorgonian populations were mostly composed of a high proportion of small colonies (0–0.10 m2). The population structure distribution shows a high proportion (~27%) of recruits (<0.05 m2) and also many (19%) large colonies (>0.5 m2). In 78% of the gorgonian colonies, facing angles were grouped inside the first quadrant (0°-90°), in accordance with the main current direction in this zone. Colony distribution and fan orientation inside the gorgonian forest can be used as data sources to improve monitoring and management programs of these unique habitats in MPAs.

Spatial and temporal patterns of seed dispersal in eelgrass Zostera marina: A case study in Moon Lake, Shandong, China

Seed dispersal plays an important role in the spread of eelgrass (Zostera marina). In order to study the temporal and spatial patterns of eelgrass seed dispersal in Moon Lake, Shandong, China, a continuous eelgrass meadow without outlying patches in a subtidal area was selected and assumed as the source of seed dispersal in the experiment. A square plot of 8 m by 8 m was set up inside the meadow and another two plots of the same size were also set up outside the meadow. Seed traps made of artificial turf were used to collect seeds transported by currents during the study period. Results showed that the potential seed production of the selected bed was 10,028 ± 4874 seeds m−2 and seed dispersal lasted from July to September. In all experimental plots, the highest densities of seed rain during a half-month period were observed in July. After two months, the average density of seed rain across all transects inside the meadow was 154 ± 27 seeds m-2, while outside the meadow, the density decreased nearly to zero. Over the entire study period, the average seed rain density across all transects inside the meadow was 1634 ± 94 seeds m−2, while at a distance of 8 m away from the meadow either downstream or perpendicular to the main tidal current direction, the average accumulated density of seed rain was 515 ± 80 and 203 ± 42seeds m−2, respectively. Our results indicate that though most of the seed rain falls within the parent bed in Moon Lake, tidal currents can disperse the seeds at a distance of 8 m, and besides tidal currents, seeds might also be dispersed in other ways, such as seed predators, wind-driven waves and other processes.

How marine currents influenced the widespread natural overseas dispersal of reptiles in the Western Indian Ocean region

In a recent contribution to this journal, Wilme et al. (2016) proposed that the giant tortoises of the islands of the Western Indian Ocean (WIO: Aldabra, the Mascarenes, and the Granitic Seychelles) might have originated from translocation by early Austronesian sailors. Prompted by this paper we review recent literature and show that natural overseas dispersal was remarkably widespread in the colonization history of terrestrial reptiles in the WIO region. Almost 90% of the successful colonization events are supported by prevailing marine surface currents. However, these currents may change over geological (and evolutionary) time‐scales, and eddies and counter‐currents may facilitate transport against the main current direction. We review the cases of the extant and extinct WIO giant tortoises and suggest that the current distribution of all lineages can be convincingly explained by overseas dispersal.

CURRENT CHARACTERISTICS IN THE WATERS OF KHANH HOA PROVINCE DURING SOUTHWEST MONSOON PERIOD

This paper describes the characteristics of spatial and temporal variation of the currents in the Khanh Hoa province waters through observation during southwest monsoon period (July - August 2010). The main current direction is northeastern. The current speed decreases with depth and from south to north. The maximum current speed is 106cm/s. The tidal current is a main component of the general current. The residual current is weak with speed of about 3.9cm/s and northwestern direction. The distribution features of current in the study area indicate the structure of circulation system in the strong upwelling area of South Vietnam waters. Besides, there is an eastward current system on the shelf of Nha Trang bay exists in the study area with speeds of about 10 to 26cm/s.

CURRENT CHARACTERISTICS IN THE WATERS OF KHANH HOA PROVINCE DURING SOUTHWEST MONSOON PERIOD

This paper describes the characteristics of spatial and temporal variation of the currents in the Khanh Hoa province waters through observation during southwest monsoon period (July - August 2010). The main current direction is northeastern. The current speed decreases with depth and from south to north. The maximum current speed is 106cm/s. The tidal current is a main component of the general current. The residual current is weak with speed of about 3.9cm/s and northwestern direction. The distribution features of current in the study area indicate the structure of circulation system in the strong upwelling area of South Vietnam waters. Besides, there is an eastward current system on the shelf of Nha Trang bay exists in the study area with speeds of about 10 to 26cm/s.

거금수로에 시설된 사각형 인공어초 주변의 조류에 의한 세굴 퇴적 변화

인공어초 시설해역 1개 정점에서 15일 연속조류를 관측하였다. 정점의 최강유속은 약 82.4 ㎝/sec로 나타났으며, 평균유속은 최강유속보다 대략 5.9 ~ 19.0 ㎝/sec 정도 작은 값을 보였다. 조시별 평균유속은 12.8 ~ 28.0 ㎝/sec로 나타났다. 어초군 주변 4방위에 대해 시료를 채취하여 분석한 결과, 주조류방향에 따라서 서로 다른 저질로 구성되어 있는 것을 볼 수 있었다. 이와 같은 결과로 볼 때, 인공어초 전면 및 후면에서 퇴적 및 세굴이 지속적으로 발생하기 때문에 주조류 방향에 따라서 서로 상이한 저질이 분포하는 것으로 판단된다.

Third order approximation to capillary gravity short crested waves with uniform currents

A third-order analytical solution for the capillary gravity short crested waves with uniform current (the main current direction is along the vertical wall) in front of a vertical wall is derived through a perturbation expanding technique. The validity and advantage of the new solution were proved by comparing the results of wave profiles and wave pressures with those of Huang and Jia [H. Huang, F. Jia, The patterns of surface capillary gravity short-crested waves with uniform current fields in coastal waters, Acta Mech. Sinica 22 (2006) 433–441] and Hsu [J.R.C. Hsu, Y. Tsuchiya, R. Silvester, Third-order approximation to short-crested waves, J. Fluid Mech 90 (1979) 179–196]. The important influences of currents on the wave profiles, wave frequency, the ratio of maximum crest height to the total wave height, and wave pressure are investigated for both small-scale (for example, the incident wave wavelength is 9.35 cm) and larger-scale (for example, the incident wave wavelength is 5 m) short crested wave. By numerical computation, we find wave frequency of short crested wave system is greatly affected by incident wave amplitude, incident angle, water depth, surface tension coefficient and the strength of the currents for small-scale incident wave. Furthermore, for the larger-scale short crested wave system, the higher-order solution with uniform current is particularly important for the prediction of wave profile and wave pressure for different water depth and incident angle. Computational results show that with the increase of the current speed, the crests of wave profile and wave pressure become more and more steep. In some cases, the crest value of wave pressure with strong current would be larger about six times than that of no current. Therefore, ocean engineers should consider the short crested wave-current load on marine constructs carefully.

Tidal influence on the hydrodynamics of the French Guiana continental shelf

This study investigates the circulation on the French Guiana continental shelf under tidal influence. Indeed, hydrodynamics are characterised by a weak salinity tongue located in the middle of the shelf and induced by the Amazon River, a coastal current flowing from the southeast, and a tidal standing wave whose co-range lines are parallel to the coast.In addition to field observations, a numerical model also is used to evaluate the tidal influence on coastal circulation. The model makes use of the MOBEEHDYCS code, a three-dimensional free surface time-splitting model whose domain is bounded with a closed coastal boundary, two active boundaries (offshore and lateral) and a passive boundary. The boundary configuration and hydrodynamics require a careful choice of passive open boundary conditions. The initial and boundary conditions come from field data.The tidal currents are essentially cross-shore and do not have a great influence on the main current direction on the offshore part of the shelf. The offshore currents remain parallel to the coast. In the inner shelf, the tidal influence is found to be much more important and the tidal currents can reach 0.45m/s. Vertically, the tidal currents are barotropic, in spite of the high stratification and they induce a horizontal cross-shore migration (about 3km) of the weak salinity tongue and vertical oscillations of the halocline without complete mixing.

Deformation of a modern alluvial plain

The alluvial plain of the Waal River (The Netherlands), over 4000 m 2, showed large flow-lobe like structures after it had fallen dry following floodings. The upstream-directed parabola-shaped folds had mutual distances from < 1 m to about 10 m and amplitudes of up to a few decimeters. The deformation is ascribed to a rare coincidence of hydrological, topographic, and weather conditions. Local high-energy currents developed opposite to the main river current direction, dragging the top layer along over several meters. A drilled section indicates that no true detachment plane but rather a detachment zone is present. Similar structures formed under comparable conditions have not been described previously in the literature.

Three-dimensional modeling of electron quasiviscous dissipation in guide-field magnetic reconnection

A numerical study of guide-field magnetic reconnection in a three-dimensional model is presented. Starting from an initial, perturbed, force-free current sheet, it is shown that reconnection develops to an almost translationally invariant state, where magnetic perturbations are aligned primarily along the main current flow direction. An analysis of guide-field and electron flow signatures indicates behavior that is very similar to earlier, albeit not three-dimensional, simulations. Furthermore, a detailed investigation of electron pressure nongyrotropies in the central diffusion region confirms the major role the associated dissipation process plays in establishing the reconnection electric field.

Shape and surface variations of syphon openings during complete tidal cycles in Mya arenaria in the intertidal zone

Since the degree of syphonal opening can be an index of the pumping activity, the shape and the surface of syphon openings in Mya arenaria were filmed with an underwater camera at two stations in the middle of the intertidal zone. The individuals were perpendicularly oriented to the main current direction or parallel with the inhalant syphon upstream during flood tide, causing refiltration risks during ebb tide. The surface of the inhalant opening (SI) was strongly reduced with increasing current speeds. Its shape (XI) varied with the individual's orientation and had a tendency to become more circular with time. The surface of the exhalant opening (SE) decreased and its shape (XE) flattened with increasing current velocity and with time. However, variations of XI and XE were weak. Current direction had no significant effect on SI, SE, and XE, but did cause a strong decrease of the SE/SI ratio during ebb in individuals exposed to important refiltration risks during ebb tide. Significant negative correlations between stomach content in phaeopigments and SI and SE suggest that a syphonal constriction could contribute to more efficient feeding. For parallel oriented individuals, the decreases of SE, SE/SI, and XE during ebb can then be interpreted as an attempt to deviate or increase the excurrent velocity relative to the incurrent so as to limit refiltration. We suggest that decreases in opening surface and shape may serve, above all, to increase syphon current jet velocity in syphonate bivalves.

Sediment transport and bedforms in a carbonate tidal inlet; Lee Stocking Island, Exumas, Bahamas

ABSTRACTAn active oolitic sand wave was monitored for a period of 37 days in order to address the relationship between the direction and strength of tidal currents and the resultant geometry, and amount and direction of migration of bedforms in carbonate sands. The study area is situated in a tidal channel near Lee Stocking Island (Exumas, Bahamas) containing an estimated 5.5 to 6 × 105 m3 of mobile oolitic sand. Tidal ranges within the inlet are microtidal and the maximum current velocity at the studied site is 0.6 m s−1. At least 300–400 m3 of mostly oolitic sand are formed within, or brought into, the channel area every year. The tidal inlet is subdivided into an ocean‐orientated segment, in which sand waves are shaped by both flood and ebb tides, and a platform‐orientated segment, where sand waves are mainly shaped by flood tides. The studied sand wave lies on the platformward flood‐tide dominated segment in a water depth of 3.5.4.5 m. During the 37 days of observation, the oolitic and bioclastic sand wave migrated 4 m in the direction of the dominant flood current. The increments of migration were directly related to the strength of the tide.During each tidal cycle, bedforms formed depending on the strength of the tidal current, tidal range and their location on the sand wave. During flood tides, a steep lee and a gentle stoss side formed and current ripples and small dunes developed on the crest of the sand wave, while the trough developed only ripples. The average lee slope of the sand wave is 24.2°, and therefore steeper than typical siliciclastic sand waves. During ebb tides, portions of the crest are eroded creating a convex upward ebb stoss side, covered with climbing cuspate and linguoid ripples and composite dunes. The area between the ebb‐lee side and the trough is covered with fan systems, sinuous ripples and dunes.The migration of all bedforms deviated to a variable degree from the main current direction, reflecting complex flow patterns in the tidal inlet. Small bedforms displayed the largest deviation, migrating at an angle of up to 90° and more to the dominant current direction during spring tides.

Transport of Fine Sands by Currents and Waves

The transport process of fine sand with a median diameter of about 100 μm in combined current and wave conditions in a laboratory basin was studied. Irregular waves with a single-topped spectrum and peak period of 2.5 s were generated. The significant wave height was varied from 0.07 m to 0.14 m. The water depth was about 0.4 m in all tests. The depth-mean current velocities were varied from 0.1 m/s to 0.3 m/s. The angles between the wave and current directions were 60°, 90°, and 120°. Instantaneous fluid velocities were measured by an acoustical probe and an electromagnetic probe. The acoustical probe was also used to measure the instantaneous sand concentrations. Time-averaged and bed-averaged concentrations were measured by a pump sampler. Current-related and wave-related transport rates were determined by decomposing, multiplying, and time-averaging instantaneous signals. The current-related transport rate in the main current direction was found to be largest when the waves were propagating normal to the current. High-frequency wave-related transport rates were found to be directed onshore because of wave asymmetry effect. Low-frequency wave-related transport rates were found to be directed offshore because of the bound long waves.

Model for current patterns in physical systems with two charge carriers.

The temporal and spatial evolution of patterns in physical systems due to electrical current flow can be described for a certain class of systems, which includes certain semiconductor and gas discharge systems, by a two-layer model in terms of the electrical current density and the electrical potential. An equation for the nonlinear layer is derived, the characteristic parameters of which can be obtained from experiments at the respective systems without inhomogeneous patterns lateral to the main current direction. The resulting equation takes into account diffusion and drift effects in the nonlinear layer. It turns out that the drift effects occur only if there is a ``net space charge.'' Two special cases are derived depending on which of the two layers gives the main contribution to the displacement current density, and the typical static and dynamic behavior is studied by numerical calculations. Furthermore, the application of the model to real devices is discussed, and a comparison is made with experimental results obtained with a dc glow-discharge system.

A sand wave field in the entrance to Bahia Blanca Estuary, Argentina

Sand waves in the mouth of the Bahía Bllanca Estuary show differences in wavelength, crestlength and crest shape characteristics between two sectors of the field. These differences may be explained by assuming that the residual ebboriented currents are stronger in the southern sector. Crest bifurcations on the centerline are suggestive of intermediate energy conditions and the development of secondary flows oblique to the main current direction. The large lee slopes and the presence of megaripple fans are indicative of flow separation, a condition seldom mentioned for subtidal sand waves. Boundaries of the sand wave field may be controlled by the thickness of the surficial sand layer, rapid reduction of the water depth as produced by a large marginal shoal, and variations in grain size. An average migration rate of 33 m yr −1 was calculated from repeated bathymetric surveys. Assuming that sand waves preserve shape and size, the established net sediment flux over the crests is 0.025 cm 3 cm −1 s −1. Current measurements made during four individual tidal cycles yielded a net bedload sediment transport of 0.048 cm 3 cm −1 s −1.