Bottom Shear Velocity Research Articles

Flow and Heat Transfer on the Surface of Molten Steel Slag Layer in Continuous Casting Mold

Protective slag is coated on the surface of molten steel during continuous casting, the flow and heat transfer state of the protective slag is a decisive factor affecting the inflow and consumption of liquid slag and is also an important prerequisite for stabilizing and improving the quality of continuous casting billets. Based on the Navier Stokes fluid momentum conservation equation and energy equation, a two-dimensional longitudinal numerical model describing the flow/heat transfer of liquid protective slag on the surface of steel is established. The data comes from the equipment parameters and casting process of an arc shaped slab continuous casting machine in a domestic steel plant. The flow field and temperature field distribution of protective slag are calculated and analyzed, and the effects of factors such as slag layer thickness and shear speed on the flow and heat transfer status of the liquid slag layer are discussed. When the bottom shear velocity increases from 0.005 m/s to 0.2 m/s, the maximum flow velocity of liquid slag from the nozzle to the narrow surface in the center area of the model increases from 0.0012 m/s to 0.0617 m/s, and the average longitudinal flow velocity of liquid slag near the nozzle increases from 0.0012 m/s to 0.0627 m/s. The research results provide reference for investigating the complex metallurgical behavior of protective slag.

Effects of Bubble Plumes on Lake Dynamics, Near‐Bottom Turbulence, and Transfer of Dissolved Oxygen at the Sediment‐Water Interface

AbstractWe quantify the lake dynamics, near‐bottom turbulence, flux of dissolved oxygen (DO) across the sediment‐water interface (SWI) and their interactions during oxygenation in two lakes. Field observations show that the lake dynamics were modified by the bubble plumes, showing enhanced mixing in the near‐field of the plumes. The interaction of the bubble‐induced flow with the internal density structure resulted in downwelling of warm water into the hypolimnion in the far‐field of the plumes. Within the bottom boundary layer (BBL), both lakes show weak oscillating flows primarily induced by seiching. The vertical profile of mean velocity within 0.4 m above the bed follows a logarithmic scaling. One lake shows a larger drag coefficient than those in stationary BBLs, where the classic law‐of‐the‐wall is valid. The injection of oxygen elevated the water column DO and hence, altered the DO flux across the SWI. The gas transfer velocity is driven by turbulence and is correlated with the bottom shear velocity. The thickness of the diffusive boundary layer was found to be consistent with the Batchelor length scale. The dynamics of the surface renewal time follow a log‐normal distribution, and the turbulent integral time scale is comparable to the surface renewal time. The analyses suggest that the effect of bubble plumes on the BBL turbulence is limited and that the canonical scales of turbulence emerge for the time‐average statistics, validating the turbulence scaling of gas transfer velocity in low‐energy lakes.

The contribution of tidal currents in the sedimentation of the Strait of Avlida, Southern Evoikos Gulf (Greece)

The investigated area of Avlida (northern part of the South Evoikos Gulf), which obtained its present shape within Holocene, is characterised by shallow water depths (<12 m) and the presence of muddy sediments. The hydrological regime of the area is governed mainly by the tide (sea level fluctuation 0.25-0.4 m). The water column (in April 1998) found to be almost homogeneous with temperature and salinity to increase progressively with depth from 13.7 °C and 34.1 psu (surface) to 14.1 °C and 35.7 psu (near bed). Suspended sediment concentrations varied between 1 and 4 mg/1 with the highest values observed above the seabed (4-8 mg/I). These values of suspended sediment are relatively high when compared with other coastal areas, as that of Thermaikos Gulf (<1 mg/1). Furthermore, their presence is attributed primarily to the action of tidal currents assisted periodically by the wave activity and human interference i.e. navigation. Especially the high near bed values are associated with the near-bed activity of the tidal currents, which having usually velocities > 11 cm/s apply to the seafloor bottom shear velocities capable of causing resuspension of the surficial muddy sediments and inhibiting final deposition of the suspensates. Therefore, the fine-grained sediments are resuspended and subsequently advected by the tidal currents and eventually distributed all over the study area. Finally, the relatively coarse-grained sediments found to be more abundant at the southern part are related to the deltaic progradation within historical times of the torrential rivers Mégalo Rema and Lilantas which form the south-end part of the strait of Avlida.

Influence of emergent macrophyte (Phragmites australis) density on water turbulence and erosion of organic-rich sediment

The effects of the emergent macrophyte Phragmites australis on water turbulence, bottom shear velocity and water turbidity were experimentally studied with the stem densities 0 stems/m2-100 stems/m2 and using organic-rich sediment. It is found that the maximum root-mean-square (rms) velocity and shear velocity decrease linearly with increasing stem density. By affecting the height of peak turbulence values, emergent plants can thus lower the frequency of sediment erosion events. Additionally, the slope of the rms turbulence increment decreases with increasing stem density. Thus, also the duration of strong turbulence is affected by the density of emergent macrophytes. The critical shear velocity for sediment resuspension is 0.0035 m/s-0.0055 m/s and not affected by the stem density. In the absence of macrophytes, water flow causes a 6.3-fold increment in turbidity, but with macrophytes turbidity values remain lower. The experiments suggest that even relatively low densities of Phragmites can have substantial effects on water turbulence and consequently on water quality.

Effect of tidal resuspension on benthic–pelagic coupling in an experimental ecosystem study

To test the effect of sediment resuspension on the nutrient and ecosystem dynamics we performed a 4 wk experiment in three 1000 l shear-turbulence-resuspension-mesocosm (STURM) resuspension (R) tanks and three 1000 l non-resuspension (NR) tanks. All tanks contained defaunated muddy sediment and brackish estuarine water and had similar water-column turbulence intensities (~1 cm s -1 ), energy dissipation rates (~0.08 cm 2 s -3 ), and tidal cycles (4 h mixing-on and 2 h mixing- off). However, while bottom shear velocity (stress) was low in the NR tanks, high instantaneous bottom shear produced resuspension in the R tanks during the mixing-on cycles. Tidal resuspension in the R tanks resulted in concentrations of 120 to 220 mg l -1 total suspended solids when mixing was on, decreasing to between 10 and 20 mg l -1 when mixing was off. Particulate nitrogen, phosphorus, and carbon concentrations, as well as dissolved inorganic nitrogen, nitrate + nitrite, and phosphate levels were higher in the R tanks. Phytoplankton biomass was also higher in the R tanks, though light was limiting. Tidal resuspension affected water-column algal and zooplankton community composi- tion and induced significantly higher concentrations of brown tide Aureococcus anophagefferens. Microphytobenthos biomass was significantly higher in the NR tanks. Dissolved inorganic nitrogen sediment effluxes were similar in both tanks; however, polychaetes and amphipods developed in the NR, but not in the R tank sediments. Tidal resuspension shifted processes from the benthos to the water column. Regular tidal resuspension profoundly affected ecosystem structure and function, often through indirect pathways and linkages.

Prediction of longitudinal dispersion coefficients in natural rivers using genetic algorithm

A new expression for the prediction of longitudinal dispersion coefficient in natural rivers, using genetic algorithms, is proposed. The expression uses hydraulic and geometric characteristics of rivers, which are readily available. For performance evaluation, using published field data, results of coefficient prediction by the new expression and by the other reported expressions are compared. According to various performance indices, it is concluded that the new formula predicts the longitudinal dispersion coefficient more accurately. Sensitive analysis performed on input parameters indicates the ratio of the cross-sectional mean velocity to the bottom shear velocity to be the most influencing parameter for accurate prediction of the longitudinal dispersion coefficient.

Experimental Study on Temporal and Spatial Distributions of Bottom Shear Stresses in Surf Zone

The mechanical properties of bottom shear stresses in the surf zone is still uncertain. This study has investigated the temporal and spatial distributions of the bottom shear stresses in the surf zone. The experiments have indicated that the temporal and spatial distributions of the bottom shear stresses varies, depending on the water surface level and flow velocity. The time variations of the bottom shear stresses and velocity in the same phase of the water surface level changes. Thus, the water surface level changes in the same phase of the velocity in the surf zone. The maximum bottom shear stresses have shifted from the peak of the water surface level. The bottom shear stresses under the peak of waves tend to be larger than those under the trough of waves.

A new measure for direct measurement of the bed shear stress of wave boundary layer in wave flume

In this article, a shear plate was mounted on the bottom in a wave flume and direct measurements of the smooth and rough bed shear stress under regular and irregular waves were conducted with the horizontal force exerted on the shear plates by the bottom shear stress in the wave boundary layer. Under immobile bed condition, grains of sand were glued uniformly and tightly onto the shear plate, being prevented from motion with the fluid flow and generation of sand ripples. The distribution of the bottom mean shear stress varying with time was measured by examining the interaction between the shear plate and shear transducers. The relation between the force measured by the shear transducers and its voltage is a linear one. Simultaneous measurements of the bottom velocity were carried out by an Acoustic Doppler Velocimeter (ADV), while the whole process was completely controlled by computers, bottom shear stress and velocity were synchronously measured. Based on the experimental results, it can be concluded that (1) the friction coefficient groews considerably with the increase of the Reynolds number, (2) the shear stress is a function varying with time and linearly proportional to the velocity. Compared with theoretical results and previous experimental data, it is shown that the experimental method is feasible and effective, A further study on the bed shear stress under regular or irregular waves can be carried out. And applicability to the laboratory studies on the initiation of sediments and the measurement of the shear stress after sediment imigration.

Experimental Study of Sediment Transport in Meandering Channels

Sediment transport in a meandering channel is measured in a laboratory flume. The measurements are evaluated and compared with previous studies. The values of the calculated sediment transport rates using previous formulas are generally overestimated. Under these circumstances, in this research, it is noticed that the sediment transport rates as measured in the laboratory, and sediment transport capacities computed with the Einstein’s approach, behave in a quite similar way, seemingly closely related to the varying bottom shear velocity or stress. Einstein’s approach for bed load comparison is chosen in this experimental research because it has been tested to some extent.

Modeling Transport of Fluff Layer Material in the Baltic Sea

To estimate impacts of nutrients and pollutants discharged by river inflow on the ecosystem it is essential to quantify the sedimentation, resuspension and transport processes of fine particulate matter. The present study aims at the modeling of basic transport processes of fine material in the western Baltic. A three-dimensional model of the Baltic Sea, that is based on the Modular Ocean Model (MOM-3.1) (Pacanowski & Griffies 2000), was applied to study maximum bottom shear velocities. Comparing the calculated shear velocities with the critical value for resuspension of fluff layers allows an identification of potential areas of deposition and accumulation. Further, model experiments were conducted to study transport paths of fluff layers in the southwestern Baltic. The model results give information about the resuspension and accumulation areas of fine material and could provide indications of potential accumulation areas of diapause eggs or cysts.

Sediment Properties in the Western Baltic Sea for Use in Sediment Transport Modelling

BOBERTZ, B.; KUHRTS, C.; HARFF, J.; FENNEL, W.; SEIFERT, T., and BOHLING, B., 2005. Sediment properties in the western Baltic Sea for the use in sediment transport modelling. Journal of Coastal Research, 21(3), 588‐597. West Palm Beach (Florida), ISSN 0749-0208. To simulate transport of clastic material in the Baltic a sediment transport module is linked to a Baltic Sea Model that is based on the Modular Ocean Model—MOM3. In order to describe the properties of the seabed sediment parameters as mean grain size, critical shear velocity and bed roughness length must be provided as input data to the numerical model system. To obtain maps of these quantities for the Baltic Sea area the proxy-target concept is applied. As proxy-variable the mean grain size of the sediment types is used. For different sediment samples the critical shear velocity was measured and serves as the target variable. Using the relation between the sediment classifications based on the mean grain size (proxy) and the measured critical shear velocity (target) a map of the critical shear velocity in the Baltic is derived. In January 1993 several extreme strong storm events occurred in the Baltic. Using this period for a model calculation maximum values of current and wave induced bottom shear velocities were obtained. Comparing these model results with the critical shear velocity distribution provided by the proxy-target concept we identify potential erosion areas. Further we show the transport path of material initially deposited in the Mecklenburgian Bight.

Numerical study of flow and heat transfer in a molten flux layer

Numerical simulations are performed to study coupled fluid flow and heat transfer in a thin liquid slag or flux layer. The steady state Navier–Stokes equations are solved using the commercial finite volume code FLUENT. The combined effects of natural convection, bottom shear velocity and strongly temperature dependent viscosity are investigated. It is found that the variation of Nu with Ra for fluxes with strongly temperature dependent viscosities is analogous to correlations for fluids with constant viscosity, but the critical Ra number for the onset of natural convection is larger. For thin layers of realistic fluxes, natural convection is suppressed, and Nu increases linearly with increase of bottom shear velocity. The increase is greater for decreasing average viscosity. The increase of Nu is slight and is only due to end effects for the flat interface shape studied here.

Effect of oysters Crassostrea virginica and bottom shear velocity on benthic-pelagic coupling and estuarine water quality

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 271:61-75 (2004) - doi:10.3354/meps271061 Effect of oysters Crassostrea virginica and bottom shear velocity on benthic-pelagic coupling and estuarine water quality Elka T. Porter1,*, Jeffrey C. Cornwell2, Lawrence P. Sanford2 1University of Maryland Center for Environmental Science, Chesapeake Biological Laboratory, 1 Williams Street, Solomons, Maryland 20688, USA 2University of Maryland Center for Environmental Science, Horn Point Laboratory, PO Box 775, Cambridge, Maryland 21613, USA *Email: porter@cbl.umces.edu ABSTRACT: Increasing the biomass of bivalve suspensions feeders has been proposed as a means to improve water quality in eutrophic estuaries such as the Chesapeake Bay. However, water quality impacts are likely to be determined by the balance of bivalve feeding and the deposition and sediment regeneration of nutrients from particulate organic matter. In shallow-water environments, benthic and pelagic processes are closely coupled and water flow can regulate the supply of seston to the bivalves. In addition, such flow may regulate benthic-pelagic nutrient fluxes through mass transfer limitation and resuspension. We studied the interacting effects of juvenile oysters Crassostrea virginica and bottom shear velocity on phytoplankton biomass and on nutrient regeneration in a series of three 4 wk long comparative experimental ecosystem experiments. All mesocosms had a 1000 l water volume, a 1 m2 sediment surface area, and a 1 m water-column depth, and the same realistic water-column mixing (turbulence intensity 1 cm s-1). The systems included a multi-component mesocosm with moderate bottom shear velocity (0.6 cm s-1) and 2 standard cylindrical tanks with an unrealistically low bottom shear velocity (0.1 cm s-1). Oysters shifted processes to the sediments by decreasing phytoplankton biomass without stimulating additional blooms and by increasing light penetration to the bottom. Through complex and indirect relationships, the interaction of oysters and enhanced shear velocity significantly affected microphytobenthos biomass. Light, as enhanced by the oyster feeding on phytoplankton, increased microphytobenthos biomass; a moderate bottom-shear velocity eroded the biomass. Microphytobenthos biomass decreased nutrient regeneration from the sediments to the water column and may have implications for water quality in low-energy parts of shallow-water estuaries such as Chesapeake Bay. Enhanced bottom shear in more energetic parts of shallow estuaries negatively affects microphytobenthos biomass and may increase nutrient regeneration from the sediments.Erratum KEY WORDS: Benthic-pelagic coupling · Crassostrea virginica · Shear velocity · Microphytobenthos · Chesapeake Bay Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 271. Online publication date: April 28, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Combined water-column mixing and benthic boundary-layer flow in mesocosms: key for realistic benthic-pelagic coupling studies

We developed 2 scaled linked mesocosms that realistically mimicked both water-col- umn mixing and benthic boundary-layer flow, enabling more realistic benthic-pelagic coupling experiments. The first was a 'large' 1000 l system linking a mesocosm with an annular flume; the sec- ond a 'small' 100 l system linking a mesocosm with a Gust microcosm. We compared bottom shear velocity, flow speeds, and internal mixing energies between linked and isolated mesocosms that were the same in volume and shape, and compared them to nature. In addition, we performed scaled 4 wk long comparative ecosystem experiments with oysters in the large and small mesocosms to determine if a realistically mimicked benthic boundary-layer flow and system shape could signifi- cantly affect ecosystem processes. We scaled all 4 systems to have the same realistic water-column turbulence levels and increased bottom shear velocity to moderate levels in the linked mesocosms. Bottom shear remained unrealistically low compared to nature in the isolated tanks. In addition, the water column and the sediment-water interface were more realistically connected in the linked than in the isolated mesocosms. The linked mesocosms had a similar scaling relationship of turbulence intensity and bottom shear velocity of 1.6, as found in nature. System shape and bottom shear signif- icantly affected ecosystem properties through changes in light, microphytobenthos biomass growth and erosion, sediment inorganic nutrient fluxes, oyster growth, and water column nutrient dynamics. In this study we show that a commonly used system shape in ecosystem studies and unrealistically low bottom shear in mesocosms both produce significant artifacts in benthic-pelagic coupling stud- ies. We also demonstrate improved systems without these artifacts. System shape, bottom shear, water-column turbulence levels, and their ratios should all be considered in designing mesocosms to mimic natural processes.

The shaping of continental slopes by internal tides.

The angles of energy propagation of semidiurnal internal tides may determine the average gradient of continental slopes in ocean basins (approximately 2 to 4 degrees). Intensification of near-bottom water velocities and bottom shear stresses caused by reflection of semi-diurnal internal tides affects sedimentation patterns and bottom gradients, as indicated by recent studies of continental slopes off northern California and New Jersey. Estimates of bottom shear velocities caused by semi-diurnal internal tides are high enough to inhibit deposition of fine-grained sediment onto the slopes.

Boundary-shear velocities and fluxes in the MEERC experimental ecosystems

Diffusion to solid-water interfaces is directly related to shear velocity. Shear velocities at the walls and bottoms of experimental ecosystem enclosures of different sizes and shapes (the Mul- tiscale Experiment Ecosystem Research Center pelagic/benthic (MEERC P/B) tanks at the University of Maryland Center for Environmental Science, Horn Point Laboratory) were measured using hot- film sensors. Spatially averaged bottom and wall shear velocities were related to internal mixing, which was produced by rotating internal paddles and measured using a combination of gypsum dis- solution and direct turbulence measurements. Shear velocities always increased with increasing mix- ing, but relationships between mixing and shear velocity changed with tank volume and shape. Spa- tially averaged bottom shear velocities decreased with increasing tank volume at an internal mixing level of 2 cm s -1 , but average wall shear velocities were similar for most tanks. In contrast, the rate of increase in bottom shear velocity with increasing mixing was similar for most tanks, but the rate of increase in wall shear velocity with increasing mixing was lower for the larger tanks. A bulk impeller Reynolds number captured some, but not all, of the scale dependence of ratios of boundary shear velocity to internal mixing intensity; mixing design and tank geometry were also important. Levels of all shear velocities in the MEERC P/B tanks were lower than levels in natural coastal environments for equivalent internal mixing. Realistic levels of internal mixing in the tanks resulted in unrealisti- cally low boundary shear velocities. As a result, wall diffusive sublayer thicknesses were similar to those found in deep-sea environments, and benthic diffusive sublayer thicknesses were even larger. Most current mesocosm designs are likely to be affected similarly. The artificially low-energy benthic environment may have particularly important consequences for ecosystem processes affected by pelagic-benthic coupling.

Taxon-specific tidal resuspension of protists into the subtidal benthic boundary layer of a coastal embayment

Sediment resuspension has widespread effects on microbial processes, primary and secondary production, and nutrient cycles, but its influence on protists other than microalgae is largely unknown. Distributions and abundances of protists in subtidal benthic boundary layers (BBL), in particular, are poorly known. We measured vertical profiles of protists in the BBL and underlying sediment at a subtidal silty site in Buzzards Bay, Massachusetts, USA, to determine cell-resuspension patterns. Tidal flow produced maximal bottom shear velocities of 1.4 to 2.2 cm s -1 . Near-bottom turbidity increased during each slack tide, when the suspended load settled, and it decreased during tidal exchange, presumably after a thin veneer of sediment resuspended from the sediment-water interface (SWI) and mixed into the upper water column. Tidal periodicities in protistan vertical profiles were taxon- and functional-group specific. Heterotrophic nanoflagellates (HNan) and ciliates, including scuticociliates, oligotrichs, and hypotrichs of the genera Euplotes and Urostrongylum, showed periodicities in distribution consistent with cycles of resuspension and deposition. BBL concentrations of HNan and scuticociliates were elevated during tidal exchange by factors of ≤2.1 and 4.6, respectively, within 5 cm of the SWI; oligotrichs were found consistently in the BBL but were in the sediment only during slack tide; Euplotes was present consistently in the sediment but was in the BBL only during tidal exchange. Total resuspended cells in the bottom 1 m were of the order 10 8 to 10 9 HNan m -2 and 10 5 to 10 6 ciliates m -2 , and in some cases the measured cell disappearance from surficial sediment during tidal exchange balanced the increase in the BBL. In contrast, pigmented nanoflagellates, pennate diatoms, and ciliates, including karyorelictids and other hypotrichs, maintained constant profiles throughout tidal cycles. Specificity of results among protistan groups might be due to behavioral adaptations such as depth zonation in the sediment, associations with particles, and vertical migration. We know of no other documentation in the field of cyclical emergence of heterotrophic protists and re-entry into sediment. Our data suggest complex taxon-specific linkages between sedimentary and water-column protistan communities that may be controlled by flow in the BBL, potentially influencing food-web dynamics.

Bottom shear stresses and velocity profiles in stratified tidal planetary boundary layer flow from similarity theory

The paper presents the bottom shear stresses and velocity profiles in stratified tidal planetary boundary layer flow by using similarity theory. For a given seabed roughness length, free stream current velocity components, frequency of tidal oscillation, Coriolis parameter and stratification parameter, the maximum bottom shear stress is determined for flow conditions in the rough, smooth and transitional smooth-to-rough turbulent regime. Further, the direction of the bottom shear stresses and the velocity profiles are given. Comparison is made with data from field measurements of time-independent as well as tidal planetary boundary layer flow for neutral conditions, and the agreement between the predictions and the data is generally good. Further, an example of application for stable stratification is given, and qualitatively the predictions show, as expected, that the bottom shear stress and the thickness of the boundary layer become smaller for stable than for neutral stratification. Other features of the tidal planetary boundary layer flow are also discussed.

Modeling of seafloor wave propagation and acoustic scattering in 3-D heterogeneous media

Results from a 3-D finite difference code for the elastic wave equation are used to compare scattering from volume and surface heterogeneities at the seafloor. The 2-D and 3-D models for flat ocean crust containing volume heterogeneities, single facets, and statistically rough seafloors are compared. For both hard bottom (shear velocity greater than water velocity) and soft bottom (shear velocity less than water velocity) environments, heterogeneities scatter energy both forward and backward into the water column via three mechanisms: primary scattering from the direct compressional wave reflecting off the seafloor heterogeneities, secondary scattering from transmitted compressional and shear waves interacting with volume heterogeneities and from interface waves scattering from near surface heterogeneities, and tertiary scattering from interface waves spawning additional interface waves. There is significant secondary and tertiary scattering from rough, hard bottoms representative of basaltic seafloors. The results indicate that two-dimensional modeling techniques may be appropriate for modeling sub-bottom volume heterogeneities with velocity contrast less than 10% but may underestimate scattering from rough seafloors. It is also found that subsurface conversion to horizontally polarized shear waves is significant for both volume and surface heterogeneous crust in the three-dimensional model.

Disappearance of Reduced Manganese in Reservoir Tailwaters

Results of field studies conducted downstream of five reservoirs are used to develop an improved understanding of the disappearance of reduced manganese in tailwaters. A first-order removal formulation accurately describes the removal of dissolved manganese versus travel time downstream. Results support the theory that adsorption to the stream bottom is the primary removal mechanism. The first-order removal rate, which varied by an order of magnitude from 0.30 to 4.45 day −1 , is found to be highly correlated to the stream slope, which affects shear velocity. Using observed removal rates, a formulation based on the ratio of the stream-bottom mass-transfer velocity, which depends on bottom-shear velocity, and stream depth is developed for computing the removal-rate coefficient. The formulation also depends on substrate type (i.e., cobble or fine-grain sediments). Dissolved manganese concentrations computed with the developed mathematical model show good agreement with observations. Some questions still rem...