Bottom currents

Different processes like eddy turbulence, bottom current, stagnation of flows, and storm-water events can be simulated, using either laminar or turbulent flow model for simulation. All processes are displayed in real-time graphical mode (history, contour graph, surface, etc.) you can also record them to data files. Thanks to innovative sparse matrix technology, calculation process is fast and stable a large number of layers in vertical and horizontal directions can be used, as well as a small time step. You can hunt for these on the Web. 

Fig. 5. Illustration of the real and superficial current density. Top Current ditribution over lines on an inert substrate. Middle Current distribution over an array of trenches. Bottom Current distribution over a branching aggregate.

By combining the findings of Cacchione, Drake and the results reported here, a coherent model can be proposed to explain the deposition inventory of the radionuclides. The down-canyon current transports large quantities of sediment toward the radioactive waste disposal site at 4000 m. Within the upper canyon, fine material is transported the furthest. Near the mouth of the canyon, sediment erosion of the walls occurs due to the down-canyon currents meeting a proposed opposing on-shore bottom current. The eroded material from the walls is transported and the finer material is deposited in eddies formed where the two currents meet. 

If material which contains nutrients, 210Pb and 239,240pu on particulate matter is raining down from the water column to the sediment and is initially deposited on ridges at the ocean bottom, currents may subsequently transport part of this material and deposit it at other locations. The low deposition rate found at Stations 5, 7, and 15 (Table 2) may be due to the initial deposition on ridges at the bottom where only a small fraction of the... 

Particulate matter that reaches the seafloor becomes part of the blanket of sediments that lie atop the crust. If bottom currents are strong, some of these particles can become resuspended and transported laterally until the currents weaken and the particles settle back out onto the seafloor. The sedimentary blanket ranges in thickness from 500 m at the foot of the continental rise to 0 m at the top of the mid-ocean ridges and rises. Marine scientists refer to this blanket as the sedimentary column. Like the water column, the sediments contain vertical gradients in their physical and chemical characteristics. Similar to the vertical profile convention used in the water column, depth in the sediments is expressed as an increasing distance beneath the seafloor. 

When particles first become incorporated into the sediments, quite a bit of seawater is usually present between adjacent grains. This is termed pore water or interstitial water. In some cases, it is difficult to define exactly where the bottom of the ocean stops and the seafloor begins, especially if bottom currents are resuspending a lot of particles. As pelagic sedimentation adds particles to the sediment, layers deposited at an earUer time are eventually buried. This produces distinct horizontal layers if the types of particles collecting on the seafloor vary over time. 

The slowest growth rates are found in the Fe-Mn oxides that have formed predominantly by precipitation of solutes from seawater, being on the order of 1 to a few millimeters per million years. Because of slow formation rates, these hydrogenous precipitates tend to form only in areas where sedimentation rates are slow, such as the abyssal plains of the mid-Pacific Ocean, or where bottom currents are strong enough to prevent sediment accumulation, such as on submarine seamounts and plateaus. 

Even the nodules growing at the fastest rates ought to be buried but are not. Several theories have been advanced to explain how the nodules maintain their position on the sediment surface. For example, earthquakes could rock the nodules frequently enough to keep their tops clear of sediment. Bottom currents could be strong enough to sweep the nodules free of sediment. This explanation is supported by the presence of Fe-Mn nodule pavements in areas subject to fast bottom currents. 

The effects of wind winnowing by bottom currents could also explain why 25% of the nodules are buried in the sediments. Variations in local rates of sediment deposition and erosion could cause the periodic burial and exposure of nodules. Growth would be expected only during periods of exposure. This would also explain why buried nodules appear unaffected by diagenesis (i.e., they are not buried long enough to undergo extensive alteration). 

A second example of convective dissolution is the dissolution of a solid floor or roof. Forced convection means that the fluid is moving relative to the solid floor or roof such as magma convection in a magma chamber, or bottom current over ocean sediment. Free convection means that there is no bulk flow or convection, but the interface melt may be gravitationally unstable, leading to its rise or fall. 

Figure 5.5a Breadboard step response (bottom, current 100 mA/Div top, select voltage step).

Some amounts of sand riverine sediments are transported in channels in the form of sand ripples, waves and dunes formed on the river bed under the influence of the near-bottom currents. This sediment load (called bedload) usually comprises about 10% of the suspended sediment load. Taking into account this fact, we find that the total river sediment input to the Black Sea may range up to 84 x 106 tyear-1. 

The photoconductivity increases when the a-Si H is lightly doped with phosphorus (Anderson and Spear, 1977). However, phosphorus doping causes very slow decay of photoresponse. The photoresponse characteristic for the phototconductive sensor using undoped a-Si H is shown in Fig. 3. The illumination is the modulated light from a GaP LED. The modulation ratio is defined as M = (it — i2)/i2, where is the peak photocurrent and i2 is the bottom current just prior to the next pulse. Figure 4 shows the modulation ratio of a-Si H versus the pulse width T, compared to that of the CdS-CdSe photoconductive sensor. The CdS-CdSe sensor modulation ratio decreases as the repetition time becomes shorter. On the other hand, in the a-Si H photoconductive sensor, the modulation ratio does not decrease... 

High-speed linear-sweep voltammetry (LSV) or linear potential sweep chronoamperometry (top) potential waveform (bottom) current response. The areas between the solid lines and the dotted lines measure approximately the charge transferred in the oxidation or reduction. 

Fig. 6C Primary current distribution and potential profiles for a parallel-plate configuration, with the Luggin capillary placed close to the working electrode. K = 50 mS c/ r. Top equipotential lines. Bottom current lines. Reprinted with permission from Landau, Weinberg and Gileadi, J. Electrochem. Soc. 135, 396. Copyright 1988, the Electrochemical Society.

Figure 4.15. Top Cross section of a MISS diode. The device can be regarded as a reverse-biased metal-insulator-semiconductor diode in series with a for-ward-biased n-p Junction. It then exhibits two stable states separated by an unstable negative resistance region. Bottom Current-voltage characteristics for a GaAs-(j -TA MISS device. The LB film thickness is approximately 9 nm...

Figure 3.15 (A) Dynamic polarization curves obtained with a carbon disk (A = 0.46cm2, top) of a Pt carbon RRDE (N = 0.175) and corresponding Pt ring (bottom) currents recorded in 1.5 mM Co(lll)(cyclam) in 0.5 M HCI04aqueoussolutionsintheabsence (curve 1) and in the presence of 02 at 900 rpm (curve 2), that is, Jc0(cyclam)/o2 small, for Ering = 0.6 (curve 3) and 1.0V versus SSCE (curve 4) both in the presence of 02. (B) Corresponding dynamic polarization curves recorded with the same...

On an average, the flow in the Fehmambelt below 20 m depth is governed by topography. Salinity data indicate that most of the time the near-bottom currents observed in the deep... 

We hypothesize that the development of nonuniform distribution of both linear and sediment accumulation rates is related to the course of the rim current at the Gotland Deep. Empirical measLuements of near-bottom currents are rare (e.g., Hagen and Feistel, 2001 Hagen and Feistel, 2004). Hence, near-bottom velocities were derived from a 3D model (Fig. 14.9 Schmidt, personnel communication). 

Besides the decreasing trend of Hg contents in surface sediments, this hot spot is still active, that is, serving as a source for Hg-rich suspensions participating in a basinwide natural tracer experiment. The 2005 mapping shows a very vivid example of a mercury distribution pattern (geochemical aureole) reflecting the sediment dynamics, including the effects of near-bottom currents. 

An analysis of the contributions to the skin friction shows that the surface waves play the dominant role. The shear stress induced by bottom currents exceeds the low fluff threshold only occasionally, especially during inflow events of saline water through the narrow channels. Therefore, an estimation of the resuspension potential of bottom sediments may be based on waves only, as, for instance, done by Jonsson (2006). However, without bottom currents some events will be missing, and, more important, no conclusions about the transport paths are possible. 

In a tank with radial impellers, suitable baffles will produce strong top-to-bottom currents from the radial discharge. The installation of baffles generally increase the power consumption [65]. For axial flow impellers, the need for baffling is not as great as for radial flow impellers, thus axial flow impellers also consume less power than radial impellers. Baffles are normally used in turbulent mixing only. 

The surface circulation of the western basin apparently remains anticyclonic, and the deep circulation cyclonic, under the predominant winds. As far as the relatively fast surface currents are concerned, the transversal spatial scale of today s sea is too small for the Coriollis force to be significant, so that the direct wind drag matters rather than the Ekman transport. On the other hand, the bottom layer circulation seems to immediately follow the sea surface slopes in the classic barotropic manner, so the Coriollis force is still effective for the slower, near-bottom currents. 

Fig. 5.1 Circumpolar view of the Arctic Ocean showing bathymetry, major surface current (heavy arrows) and bottom current (thin arrows) patterns, and locations of major river inflows.

Aller, J.Y. (1989) Quantifying sediment disturbance from bottom currents and its effect on benthic communities in a deep western boundary zone. Deep-Sea Research, 36, 901—934. 

Fig. 21 Top normalized electroluminescence spectra for (dotted line) 30, (dashed line) [Pt(C1 ANAN)(C CC6H4Me-4)], and (continuous line) 40 at 4% doping level and multilayer configuration of device. Bottom current density, voltage and luminance characteristics (inset luminescent efficiency vs current density) for OLED using 30 as emitter as 4% doping level (Reproduced with permission from [36a]. Copyright 2002 Royal Society of Chemistry)...

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