Bottom Velocity Research Articles

Effect of a void on the heat transfer in the head disk interface

The effect of a head void on the heat transfer in the air bearing between two parallel surfaces is investigated. The temperature distribution in the air bearing in the presence of the head void is calculated, and the heat flux in the air bearing is determined as a function of boundary pressure, boundary temperature, bottom surface velocity, and void size.

Circulation and mixing along the North Passage in the Changjiang River estuary, China

Simultaneous field observations were made of time series of water level, current velocity, salinity and suspended sediment concentration at ten hydrological gauging stations along the North Passage in the Changjiang River estuary on 17 to 24 February (dry season) and 12 to 18 August 2012 (wet season), respectively. Quantitative analyses of those data are attempted to understand circulation and mixing along the Deepwater Navigational Channel in this estuary. Landward subtidal flow only appears during the neap tide. The maximum bottom landward velocity is in the order of 0.05–0.1m·s−1 in the dry season and 0.15–0.2m·s−1 in the wet season, respectively. In both dry and wet seasons, calculated mixing parameter (M), which is the ratio of the tidal timescale to the vertical mixing timescale, is below the critical value (1.0) on a neap tide but above it on a spring tide within the middle and lower reaches of the Channel. This suggests that tidal variation of mixing may be able to generate the tidal straining circulation during the periodic stratified spring tide. The interaction between significant river shear flow and longitudinal density gradient, a process termed “river effect” in this study, is revealed as the major reason for seasonal variation of stratification. The competition between tidal stirring, gravitational circulation and river effect could produce an increase in tidal mean value of the potential energy anomaly (ϕ) from neap to spring tides. Tidal mean value of the Simpson number (Si) over a neap tide surpasses the critical value (8.4×10−1) within the middle reach of the Channel, suggesting the occurrence of persistent stratification there. Tidal mean Si over a spring tide is in the order of 8.4×10−1–8.8×10−2 within the middle reach, suggesting the occurrence of strain induced periodic stratification (SIPS). Shear prevails within the pycnocline in the middle and lower reaches of the Channel on a neap tide, with the value of squared shear (S2) exceeding 10−3s−2. Calculated gradient Richardson number (Ri) is small at the salt–fresh water interface, indicating the occurrence of Kelvin–Helmholtz instability there.

Energy Flux into Internal Lee Waves: Sensitivity to Future Climate Changes Using Linear Theory and a Climate Model

Abstract Internal lee waves generated by geostrophic flows over rough topography are thought to be a significant energy sink for eddies and energy source for deep ocean mixing. The sensitivity of the energy flux into lee waves from preindustrial, present, and possible future climate conditions is explored in this study using linear theory. The bottom stratification and geostrophic velocity fields needed for the calculation of the energy flux into lee waves are provided by Geophysical Fluid Dynamics Laboratory’s global coupled ocean–ice–atmosphere model, CM2G. The unresolved mesoscale eddy energy is parameterized as a function of the large-scale available potential energy. Simulations using historical and representative concentration pathway (RCP) scenarios were performed over the 1861–2200 period. The diagnostics herein suggest a decrease of the global energy flux into lee waves on the order of 20% from preindustrial to future climate conditions under the RCP8.5 scenario. In the Southern Ocean, the energy flux into lee waves exhibits a clear annual cycle with maximum values in austral winter. The long-term decrease of the global energy flux into lee waves and the annual cycle of the energy flux in the Southern Ocean are mostly due to changes in bottom velocity.

Wave exposure as a predictor of benthic habitat distribution on high energy temperate reefs

The new found ability to measure physical attributes of the marine environment at high resolution across broad spatial scales has driven the rapid evolution of benthic habitat mapping as a field in its own right. Improvement of the resolution and ecological validity of seafloor habitat distribution models has, for the most part, paralleled developments in new generations of acoustic survey tools such as multibeam echosounders. While sonar methods have been well demonstrated to provide useful proxies of the relatively static geophysical patterns that reflect distribution of benthic species and assemblages, the spatially and temporally variable influence of hydrodynamic energy on habitat distribution have been less well studied. Here we investigate the role of wave exposure on patterns of distribution of near-shore benthic habitats. A high resolution spectral wave model was developed for a 624 km2 site along Cape Otway, a major coastal feature of western Victoria, Australia. Comparison of habitat classifications implemented using the Random Forests algorithm established that significantly more accurate estimations of habitat distribution were obtained by including a fine-scale numerical wave model, extended to the seabed using linear wave theory, than by using depth and seafloor morphology information alone. Variable importance measures and map interpretation indicated that the spatial variation in wave induced bottom orbital velocity was most influential in discriminating habitat the classes containing canopy forming kelp Ecklonia radiata, a foundation kelp species that affects biodiversity and ecological functioning on shallow reefs across temperate Australasia. We demonstrate that hydrodynamic models reflecting key environmental drivers on wave exposed coastlines are important in accurately defining distributions of benthic habitats.

Impacts of hydrodynamic disturbance on sediment resuspension, phosphorus and phosphatase release, and cyanobacterial growth in Lake Tai

The objective of this study was to link hydrodynamic disturbance with sediment resuspension, phosphorus release, and algal growth in Lake Tai, a typical shallow lake located in the south of the Yangtze River Delta in China. With this regard, a sediment–water-algae laboratory experiment was conducted and extrapolated to the real situation in terms of field observations. The results show that the algal growth rate synchronically increased with dissolved total phosphorus (DTP) release rate. The DTP decreased with increase of bottom flow velocity, indicating that the phosphorus release rate was lower than its transfer rate into algal biomass. While all levels of hydrodynamic disturbances could increase sediment resuspension and phosphorus release, a low to moderate disturbance was beneficial, but a strong disturbance was harmful for algal growth. Also, a low to moderate disturbance caused the dissolved alkaline phosphatase activity (DAPA) to increase with time, which provided the enzyme for hydrolyzing a variety of organic phosphorus compounds from bed sediment into algae-needed nutritional DTP. The experiment proved to be an efficient means to understanding eutrophication mechanisms of large shallow lakes such as Lake Tai.

Ferrofluid measurements of bottom velocities and shear stresses

A novel direct measurement strategy of bottom velocities and shear stresses based on the use of ferrofluids is presented. Such a strategy overcomes some of the limits of state-of-the-art instruments. A preliminary experimental campaign has been carried out in the presence of currents in steady flow conditions in order to test the effects of ferrofluid quantity and of the controlling permanent magnetic force. An alternating current (AC) circuit and a direct current (DC) conditioning circuit have been tested. For velocities larger than 0.05 m/s, the near-bottom velocity-output voltage calibration curve has a monotone parabolic shape. The sensitivity of the instrument is increased by a factor of 30 when the DC circuit is used.

Optimization of thermal resistance and bottom wall temperature uniformity for double-layered microchannel heat sink

In this paper, a three-dimensional solid–fluid conjugate model coupled with a simplified conjugate-gradient method was employed to optimize the performance of double-layered microchannel heat sinks. Channel number, channel width, bottom channel height, and bottom coolant inlet velocity were selected as search variables to achieve the optimal heat sink performance. Firstly, two single-objective optimizations based on different objective functions (one is the maximum temperature change on the bottom wall ΔTw,b and the other is the overall thermal resistance R) were performed at a constant pumping power. Subsequently, the effects of total pumping power on the optimal ΔTw,b and R were analyzed, and the optimal search variables at various pumping powers were obtained. For single-objective optimization with the objective function of ΔTw,b, ΔTw,b is respectively decreased by 6.01, 5.29, and 2.99K when compared with three original designs. For the objective function of R, however, R is respectively decreased by 36.51%, 15.10%, and 16.67%. The results also indicate that R and ΔTw,b cannot achieve their optimal values simultaneously by the two single-objective optimizations. Thus, a multi-objective optimization was carried out, which demonstrates that when a set of desirable values of ΔTw,b and R is required by designers, the present multi-objective optimization could meet this requirement.

Numerical Simulation of the Bottom Hole Flow Field of Particle Impact Drilling

It is very difficult to obtain the bottom hole fluid field of particle impact drilling though theoretical and experimental method. A method of numerical simulation was used to study the bottom hole fluid field of particle impact drilling, and it has been proved a good way to interpret particle impact process. The numerical model and computing method was used for solve the problem. It was predicted that five fluid areas were in the bottom hole fluid field by the simulation results. The changed law of the bottom hole pressure field and velocity field by altering different parameter was worked out, and the motion curve of the particles was also obtained. The result of the numerical simulation can provided more material for increasing the efficiency of particle impact and optimizing the bottom hole fluid field of particle impact drilling. Key words: Bottom hole fluid field; Numerical simulation; Pressure and velocity field; Numerical model; Particle impact

Numerical simulation of flow patterns in the forebay and suction sump of Tianshan pumping station

Tianshan Pumping Station takes water from the Yellow River. A three-dimensional (3D) mathematical model of turbulence flow patterns in the forebay and suction sump was developed and a 3D turbulent flow simulation technique applied to numerical calculation of the flow pattern characteristics in both the original and rebuilt forebays of the pumping station. The numerical simulation results were analyzed and contrasted. The results showed that, with technical improvement, surface backflow was avoided, and bottom velocity in the forebay was increased while bottom velocity in the suction sump was unchanged. Because of this, there was no sediment deposition in the bottom of the forebay, and flow velocities in the forebay and suction sump were more evenly distributed.

Sandwaves and sand transport on the Barents Sea continental slope offshore northern Norway

We integrate morphological, geological and geophysical observations with oceanographic measurements and modelling results to interpret the processes behind the origin and evolution of sandwaves on the upper continental slope of the southwestern Barents Sea. Seven sandwave fields covering c. 130 km2 and with a volume of c. 72 × 106 m3 occur in water depths of c. 475–800 m. Individual sandwaves reach heights up to 6 m. The sandwaves are sinusoidal with asymmetric stoss–lee side relationships, and time lapse multibeam bathymetry analysis indicates that some sandwaves have migrated up to 10 m towards the NNW over a 4-year time period. This contour current parallel migration is counteracted by currents in several other directions, and the sand unit is thus not a simple contourite. Measurements show prominent diurnal period oscillations and clear spring-neap variations of current direction and speed with along-slope and cross-slope current velocities up to 75 cm s−1 and 65 cm s−1, respectively. Numerical ocean modelling results produce eddies travelling along the slope, positionally stable, daily recurring vortices and bottom current velocity up to 100 cm s−1 in the sandwave fields. Sandwave migration towards the SE is also observed, and the cross-slope currents are capable of transporting sand up and down the slope, as evidenced by ripples migrating normal to the sandwaves. Eroded sand accumulates in the sandwave fields along the boundary between North Atlantic Water and Norwegian Sea Arctic Intermediate Water. This is a consequence of the combined effect of the NNW directed along-slope Norwegian Atlantic Current and tidally induced topographic waves. The sandwave fields started to form 11 000–13 000 14C yrs BP or possibly later by current erosion of glacial sediments on the continental shelf and slope.

From the continent to the coast: the bedload transport across the lower sector of the Guadiana River Mouth (Spain-Portugal)

The estuary of the Guadiana River is located in the southwestern Iberian Peninsula and serves as border between Spain and Portugal for more than 40 km. This estuary consists of a narrow rock bounded channel with meandering morphology imposed by the geologic materials. Its mouth is affected by a mesotidal regime, with tidal ranges comprised between 3 and 1 m (spring vs neap tides). This work analyzes the sediment transport patterns across a section located in the low marine sector of the estuary. The transversal profile is composed by a deep channel and a lateral bar. The bed of the deeper area is constituted by well sorted medium-to-coarse sand, displaying large ebb oriented sand waves (mean wavelength of 12 m), in consequence can be deduced that serves as a bypassing channel. The bed of the lateral bar is composed of cohesive bad-sorted sediment with an important population of clayey silts including a variable amount of fine-to-medium sand. The most frequent bottom feature is the plane bed, but sand patches with bedforms of a mean wavelength of 2.5 m can be observed. The fieldwork was developed in July 2010 during two opposite (neap and spring) mean tides. During the surveys, near bottom water velocity was measured using an Acoustic Doppler Current Profiler (ADCP). The speed data were used to calculate values of potential bedload transport (Qb) according with the equations suggested by Bagnold (1963). The values of bed density and rugosity were determined by using of a Laser Difractometer in sediment samples taken with a Van Veen grab sampler. The final values of Qb were compared to the real bedload transport (Sb) obtained from the amount of sand gathered in a Poliakoff sediment trap. A preliminary analysis of the results of Qb shows as the transport in the contrary senses (ebb and flood) is nearly balanced. The values of Qb oscillate between the 69.8 gr/cm during a neap tidal cycle and 329.1 gr/cm during a spring tidal cycle, whereas Sb varies between 14.6 and 72.1 gr/cm during the same tides. We can conclude that the volumes of real transport are always minor than those of potential transport, being in an approximate relation of 1/5.

Effect of velocity regime on ontogenetic dispersal and habitat use of Kootenai River White Sturgeon (Acipenser transmontanus , Richardson, 1836) early life stages: An artificial stream study

Summary The effect was studied of two channel velocity regimes (slow regime = mean, 16.9 cm s−1; fast regime = mean, 23.4 cm s−1) on the behaviour and dispersal of Kootenai River white sturgeon Acipenser transmontanus early life stages (ELS = free embryos, larvae, early-juveniles) in artificial streams. A conceptual model incorporating velocity effects on ELS behaviour and dispersal follows that: (1) most free embryos hide under cover and do not disperse in any velocity; (2) the velocity regime has the greatest effect on peak dispersal by larvae − dispersal is intense and short (11 and 13 days in 16.9 and 23.4 cm s−1, respectively); and (3) the long peak and valley dispersal (alternating periods of intense and slow dispersal) by post-peak larvae and early-juveniles, which lasts to day 74 post-hatch (early-September) for some fish, is not affected by the velocity regime. Edge velocity habitat (about 4 cm s−1 between eddy and channel velocities) was used by 65% of the late-larvae foraging on drift during the day in either a slow or fast velocity regime, the edge being used even more at night (75% in fast velocity, 82% in slow velocity). Early-juveniles also used edge habitat. Regulation of river flow should maintain a minimum 23 cm s−1 bottom velocity regime in the larval dispersal reach to trigger an intense dispersal from the spawning reach and enable fish to disperse quickly downstream.

Influence of Seepage on Embedded Depth for Anchored Pile Supporting

In the present code, the embedded depth is determined on the multi-pivot support structure to consider the impact of seepage less, and it is verified by anti-seepage stability for single layer aquifer, while the embedded depth is determined not expressly provide for heterogeneity aquifer of the coefficient of permeability difference. However, in the northwest loess areas, the formation is complex, permeability coefficient is variations. To solve this problem, the arctic investigates change law that the impacts of embedded depth variation change on bending moment, the vertical displacement of bottom and maximum flow velocity under seepage, and made a qualitative research model. And the impact of seepage on the embedded depth cannot be ignored and a greater impact on the stability of the pit.

Heat fluxes across the Antarctic Circumpolar Current in Drake Passage: Mean flow and eddy contributions

AbstractIn contrast to a long‐standing belief, observations in the Antarctic Circumpolar Current (ACC) show that mean velocity vectors rotate with depth, thus suggesting a possible importance of the time‐mean flow for the local poleward heat transport. The respective contributions of the eddy and mean flows to the heat flux across the ACC in Drake Passage (DP) are investigated using recently acquired and historical time series of velocity and temperature from a total of 24 current meter moorings and outputs of a high‐resolution (1/12°) model with realistic topography. Only 11 out of the 24 depth‐integrated eddy heat flux estimates are found to be significant, and they are poleward. Model depth‐integrated eddy heat fluxes have similar signs and amplitudes as the in situ estimates at the mooring sites. They are mostly poleward or nonsignificant, with amplitude decreasing to the south. The cross‐stream temperature fluxes caused by the mean flow at the moorings have a sign that varies with location and corresponds to the opposite of the vertical velocity estimates. The depth‐integrated temperature fluxes due to the mean flow in the model exhibit small spatial scales and are of opposite sign to the bottom vertical velocities. This suggests that the rotation of the mean velocity vectors with depth is mainly due to bottom topography. The rough hilly topography in DP likely promotes the small‐scale vertical velocities and temperature fluxes. Eddy heat fluxes and cross‐stream temperature fluxes are integrated over mass‐balanced regions defined by the model transport streamlines. The contribution of the mean flow to the ocean heat fluxes across the Southern ACC Front in DP (covering about 4% of the circumpolar longitudes) is about four times as large as the eddy heat flux contribution and the sum of the two represent on the order of 10% of the heat loss to the atmosphere south of 60°S.

Wind‐modulated buoyancy circulation over the Texas‐Louisiana shelf

AbstractThis numerical investigation examines buoyancy‐driven circulation on the Texas‐Louisiana shelf modulated by seasonal winds. In wintertime, with downcoast (in the direction of Kelvin wave propagation) wind forcing, the Mississippi‐Atchafalaya River plume exhibits a bottom‐advected pattern. The plume is fairly wide and the horizontal density gradients span almost across the entire shelf inshore of 50 m. Within the plume, vertical shear of alongshore flow is in thermal wind balance with the cross‐shore density gradient, and the shear causes a slight reversal of alongshore flow near the bottom. An alongshore flow estimated by the thermal wind relation, along with an assumption of zero bottom velocity, generally well agrees with the actual flow near the surface in regions deeper than 20 m. In spring and summer, the thermal‐wind‐balance‐derived flow deviates from the actual alongshore flow, and an Ekman flow driven by strong onshore wind makes the major contribution to the deviation. Besides, the summertime upcoast wind component transforms the plume to a surface‐advected state, resulting in reduced cross‐shore density gradients and increasing the relative importance of wind‐driven, barotropic alongshore flow, which contributes to the remaining deviation.

Deciphering bottom current velocity and paleoclimate signals from contourite deposits in the Gulf of Cádiz during the last 140 kyr: An inorganic geochemical approach

AbstractContourites in the Gulf of Cádiz (GC) preserve a unique archive of Mediterranean Outflow Water (MOW) variability over the past 5.3 Ma. In our study, we investigate the potential of geochemical data obtained by XRF scanning to decipher bottom current processes and paleoclimatic evolution at two different sites drilled during IODP Expedition 339 through contourites in the northern GC: Site U1387, which is bathed by the upper MOW core, and Site U1389, located more proximal to the Strait of Gibraltar. The lack of major downslope transport during the Pleistocene makes both locations ideally suited for our study. The results indicate that the Zr/Al ratio, representing the relative enrichment of heavy minerals (zircon) over less dense alumnosilicates under fast bottom current flow, is the most useful indicator for a semiquantitative assessment of current velocity. Although most elements are biased by current‐related processes, the bromine (Br) record, representing organic content, preserves the most pristine climate signal rather independent of grain‐size changes. Hence, Br can be used for chronostratigraphy and site‐to‐site correlation in addition to stable isotope stratigraphy. Based on these findings, we reconstructed MOW variability for Marine Isotope Stages (MIS) 1–5 using the Zr/Al ratio from Site U1387. The results reveal abrupt, millennial‐scale variations of MOW strength during Greenland Stadials (GS) and Interstadials (GI) with strong MOW during GS and glacial Terminations and a complex behavior during Heinrich Stadials. Millennial‐scale variability persisting during periods of poorly expressed GS/GI cyclicities implies a strong internal oscillation of the Mediterranean/North Atlantic climate system.

Numerical modeling of the hydrodynamics of standing wave and scouring in front of impermeable breakwaters with different steepnesses

The aim of this paper is to numerically study the effects of breakwater steepness on the hydrodynamics of standing wave and scouring process in front of impermeable breakwaters. A two-dimensional hydrodynamics model based on the Reynolds Averaged Navier–Stokes (RANS) equations and the Volume of Fluid (VOF) method was developed and then combined with an empirical sediment transport model. Comparisons with an analytical solution and experimental data showed the present model is very accurate in predicting the near bottom velocity and capable of simulating the scour/deposition patterns consistent with experimental data. It was found that the additional terms of bottom shear stress in the momentum equations are necessary to produce a physical scouring pattern. Different breakwater steepnesses produce different characteristics of standing wave, the steady streaming system, and scouring pattern in front of the breakwaters, which also affects the correlations between them. An additional analysis of the turbulence field parameters and the sediment transport rate was also performed. All these important information will be presented in details in this paper and can be worthwhile for designing the breakwater in coastal areas.

A three-dimensional hydrodynamic model for aquaculture: a case study in the Bay of Fundy

Impacts of aquaculture on the local current field and the erosion of the bottom sedi- ment in the Bay of Fundy, Canada, have been investigated with a 3-dimensional hydrodynamic model. The model is evaluated against independent observations of the current. Model results show that the presence of fish cages restricts water flow and reduces the velocity in the surface layer occupied by the cages, but enhances the water velocity in the bottom layer beneath the cages. Sensitivity studies show that the change in the flow velocity beneath the cages is sensitive to variations in the drag coefficient and the height of the fish cages. As the drag coefficient increases, the bottom velocity also increases until a steady state value is reached. For the cage height, however, the tidal speed beneath the cages first increases with cage height and then sig- nificantly decreases with further increasing height. The maximum increase in velocity occurs when the cage height is about half the local water depth (H/H0 = 0.5, where H is the cage height and H0 is the water depth). The increase in bottom velocity significantly speeds up the erosion of the bottom sediment. The model results also indicate that there is an optimal drag coefficient and an optimal cage height for a specific farm site. By utilizing the optimal drag coefficient and height, it is possible to speed up sediment erosion beneath the cages and, thus, decrease the environ - mental problems caused by accumulated fish farm waste.

3-D Numerical Simulation of Flow Structures in a Sharp Bend of the Lower Yangtze River

The three-dimensional (3-D) flow structures in the Taiziji Waterway of the Lower Yangtze River were simulated by a 3-D numerical model, where the river plan form is a sharp bend and island-braided. The verification results of the 3-D model indicated that the simulated results agree with the measured data. The patterns of 3-D flow structures in this sharp bend with different discharge were analyzed based on the calculated results.The simulated results demostrated that strong 3-D features of flow structures can be found in the sharp bend of Taiziji Waterway, the bottom velocity vectors point to the convex bank while the surface vectors point to the concave bank. The velocity distribution alone the lateral direction in the sharp bend depends mostly on the flow discharge. The discharge ratio of the left branch increases as flow discharge increases.

An Experimental Study on the Wave-Induced Topographic Change in Artificial Shallows: Focusing on the Effects of Pore-Water Pressure on Sediment Transport

Dredging to maintain channels and berths produces large volumes of sand that need to be appropriately disposed of or used. The construction of artificial shallows and tidal flats has attracted attention as a way of effectively using dredged sand. However, there is still an issue to be resolved in that dredged sand with its fine sediments is vulnerable to wave action. In this study, the wave-induced topographic change in artificial shallows composed of fine sand was examined experimentally for bottom flow velocity on its surface and pore-water pressure on its surface layer. Experimental results showed that the relative vertical effective normal stress ratio calculated from pore-water pressure was negatively correlated with the signed Shields parameter defined as positive landward. Its relationship was unchanged despite a change in the surface profile of the shallows, implying an acceleration in seaward sediment transport and a reduction in landward sediment transport, regardless of the topographic change in the shallows. Furthermore, a signed Shields parameter modified to consider the effects of pore-water pressure gave a reasonable evaluation of the trend in the sediment transport. From these results, it was demonstrated that the effects of pore-water pressure must be considered when discussing the sediment transport.