Horizontal Recirculation Research Articles

Analysis of Flow Pattern and Bubble Behavior in Slab Continuous Casting with Mold Electromagnetic Stirring

The electromagnetic stirring (EMS) and argon blowing are the widely recognized and extensively employed technologies to control the flow pattern in slab continuous casting, and their interaction is directly linked to the final product. To investigate the flow pattern, bubble characteristics under mold EMS and argon blowing, a mathematical model coupling the electromagnetic field, two‐phase flow is constructed. The population balance model (PBM) under the Eulerian frame is applied to describe the bubbles breakage and coalescence. The results show that the flow regime under the interaction of argon blowing and EMS transfers from bubble ascending flow to intermediate flow to horizontal recirculation flow as EMS current increases, and the large argon flow rate could suppress the formation of horizontal recirculation flow. Meanwhile, EMS can promote the coalescence of bubbles, with the increasing EMS current, the bubble numbers decrease first and then increase, and the bubble mean diameter increases first and then decreases. For obtaining the horizontal recirculation flow, the suitable argon flow rate and EMS current should be matched suitably.

Standing Eddies in Glacial Fjords and Their Role in Fjord Circulation and Melt

Abstract Glacial fjord circulation modulates the connection between marine-terminating glaciers and the ocean currents offshore. These fjords exhibit a complex 3D circulation with overturning and horizontal recirculation components, which are both primarily driven by water mass transformation at the head of the fjord via subglacial discharge plumes and distributed meltwater plumes. However, little is known about the 3D circulation in realistic fjord geometries. In this study, we present high-resolution numerical simulations of three glacial fjords (Ilulissat, Sermilik, and Kangerdlugssuaq), which exhibit along-fjord overturning circulations similar to previous studies. However, one important new phenomenon that deviates from previous results is the emergence of multiple standing eddies in each of the simulated fjords, as a result of realistic fjord geometries. These standing eddies are long-lived, take months to spin up, and prefer locations over the widest regions of deep-water fjords, with some that periodically merge with other eddies. The residence time of Lagrangian particles within these eddies are significantly larger than waters outside of the eddies. These eddies are most significant for two reasons: 1) they account for a majority of the vorticity dissipation required to balance the vorticity generated by discharge and meltwater plume entrainment and act to spin down the overall recirculation and 2) if the eddies prefer locations near the ice face, their azimuthal velocities can significantly increase melt rates. Therefore, the existence of standing eddies is an important factor to consider in glacial fjord circulation and melt rates and should be taken into account in models and observations.

An investigation on the switching of asymmetric wake flow and the bi-stable flow states of a simplified heavy vehicle

This study investigates the effect of aspect ratios (Ra*) on the wake bi-stability behind the GTS model using improved delayed-detached eddy simulation (IDDES) at a Reynolds number of 2.7×104. Eight cases with Ra* in the range of Ra*∈[0.5, 2.0] have been considered to investigate the asymmetric wake flow and the corresponding aerodynamic response. The primary objective of this study is to reveal the mechanism of the Ra* effects on the GTS’s flow topology, asymmetric wake flow and the bi-stable flow switching process. The accuracy of the numerical method has been validated by comparing the recirculation bubble, vortex core and aerodynamic drag with those obtained from the previous large-eddy simulation study and the water channel experiment. The results show that three typical flow topologies (FT-I, FT-II and FT-III) are classified according to the drag coefficient and the length relation between the vertical and horizontal recirculation region. The vertical and horizontal asymmetric characteristics can be transformed between two adjacent flow topologies. Moreover, a switching process of the bi-stable wake flow is also observed as Ra* is equal to 0.71. Additionally, the ground motion can reverse the dominated flow state and inhibiting the corresponding switching process.

The impact of land-sea breezes on the formation of Brown haze in an urban isthmus environment

In some international cities, winter air pollution can manifest into a local-scale brown air pollution haze which has been associated with negative health outcomes. Land-sea breezes are known to impact urban air quality through the recirculation of air pollution and the formation of internal boundary layers (IBLs). However, research into land-sea breezes has primarily focused on summer air pollution and little is known about the influence of land-sea breezes on local-scale winter brown haze. Using continuous data (including surface meteorology, surface air quality, satellite-derived sea surface temperatures, and ceilometer-derived boundary-layer depths) observed over seven winters from 2013 to 2019, we present a novel investigation of the influence of land-sea breezes on brown haze in Auckland, New Zealand. Severe brown haze days are significantly more likely to coincide with a land-sea breeze circulation simultaneously occurring at both the east and west coasts when compared with days on which brown haze is expected but not observed (based on favourable meteorology and high surface air pollution levels). Both severe brown haze and high surface level PM2.5 concentrations (previously associated with the presence of brown haze) are found to be associated with a high degree of horizontal recirculation at Auckland's east coast. No relationship is found between the occurrence of sea-breeze-induced IBLs and the formation of brown haze. The results presented in this study offer insights into the physical mechanisms that influence the formation and persistence of local-scale winter brown haze in a complex coastal setting with correspondingly complex land-sea breezes.

Collective influences of boundary layer process and synoptic circulation on particulate pollution: A new study in changsha-zhuzhou-xiangtan urban agglomeration of central china

Particulate matter pollution is increasingly serious in Changsha-Zhuzhou-Xiangtan urban agglomeration (CZT) due to rapid industrialization and urbanization. Particulate matter pollution and weather conditions are closely related. In this study, the relationship between the large-scale weather circulation types (CTs), horizontal recirculation wind field, boundary layer structure and particulate matter pollution (PM2.5) in CZT was systematically investigated. Firstly, the Allwine and Whiteman (AW) wind field classification method and Richardson method were used to calculate the horizontal recirculation index and the planetary boundary layer height (PBLH) in CZT. By analyzing the relationship among the recirculation index, the PBLH and the particle concentration, it was found that the recirculation process in CZT could be divided into horizontal recirculation and “pseudo recirculation”. Then, the circulation patterns affecting the CZT were divided into nine types (CT1-CT9) by using T-mode PCA method based on 925 hpa geopotential data. The CZT was prone to recirculation under the control of high pressure rear (CT4), weak high pressure edge (CT5) and southwest vortex (CT7, CT8). It was found that CT4 and CT5 accompanied with lower PBLH and poorer vertical diffusion conditions characterized by high particle concentration. While CT7 and CT8 accompanied with higher PBLH and better vertical diffusion conditions characterized by low particle concentration. Finally, the effect of CTs, recirculation index and boundary layer structure on paticle matter concentration were assessed during two recirculation processes in December 2016 and August 2017, repectively. The analysis was complemented with FLEXPART-WRF model simulations, which confirmed that the recirculation and “pseudo recirculation” can be effectively distinguished by introducing the PBLH, combined with the large-scale weather circulation.

Parameterised drag model for the underlying surface roughness of buildings in urban wind environment simulation

Knowledge of the flow field in the urban canopy layer (UCL) is of great significance in guiding the layout of buildings in blocks and mitigating local air pollution. Large eddy simulation (LES) with relatively high precision can simulate the flow field in the UCL. Considering the limitations of computational power, building resistance source parameterisation is an effective method of solving the average blocking effect of buildings on airflow and reducing the number of grids. Belcher's drag model has been widely used for calculating the average flow field in the UCL. However, the model does not consider the influence of the horizontal recirculation region and the variation of the canopy-drag length scale in the height direction. Therefore, a parameterised drag model for the resistance source of building(s) was improved by considering the horizontal recirculation region and effective roughness density. The simulation results suggest that the horizontal recirculation area of an isolated building is mainly affected by the spanwise width of the building at a certain height and presents a quadratic function distribution in the height direction, with a maximum at approximately z/H = 0.45. The validation of the isolated building and building arrays with different layouts demonstrated that the parameterised drag model could reveal the average flow field more accurately than Belcher's drag model.

Geometric Constraints on Glacial Fjord–Shelf Exchange

AbstractThe oceanic connections between tidewater glaciers and continental shelf waters are modulated and controlled by geometrically complex fjords. These fjords exhibit both overturning circulations and horizontal recirculations, driven by a combination of water mass transformation at the head of the fjord, variability on the continental shelf, and atmospheric forcing. However, it remains unclear which geometric and forcing parameters are the most important in exerting control on the overturning and horizontal recirculation. To address this, idealized numerical simulations are conducted using an isopycnal model of a fjord connected to a continental shelf, which is representative of regions in Greenland and the West Antarctic Peninsula. A range of sensitivity experiments demonstrate that sill height, wind direction/strength, subglacial discharge strength, and depth of offshore warm water are of first-order importance to the overturning circulation, while fjord width is also of leading importance to the horizontal recirculation. Dynamical predictions are developed and tested for the overturning circulation of the entire shelf-to-glacier-face domain, subdivided into three regions: the continental shelf extending from the open ocean to the fjord mouth, the sill overflow at the fjord mouth, and the plume-driven water mass transformation at the fjord head. A vorticity budget is also developed to predict the strength of the horizontal recirculation, which provides a scaling in terms of the overturning and bottom friction. Based on these theories, we may predict glacial melt rates that take into account overturning and recirculation, which may be used to refine estimates of ocean-driven melting of the Greenland and Antarctic ice sheets.

Achievement in ultra-low-load combustion stability for an anthracite- and down-fired boiler after applying novel swirl burners: From laboratory experiments to industrial applications

Deep peak shaving requires extremely high requirements for low-load combustion stability of boilers. In this study, a novel swirl burner (NSB) with an eccentric secondary air arrangement was proposed, and the validity and the progressiveness of the NSB in achieving ultra-low-load combustion stability for down-fired boilers (DFBs) were confirmed from laboratory experiments to industrial applications. Firstly, the cold-modelling experiments of gas/particle (GP) two-phase flow characteristics involving two combustion systems (i.e., the DFB with traditional swirl burners (TSBs) and the DFB with NSBs) were performed at an ultra-low load of 90 MWe. Compared with the original boiler with TSBs, the maximum horizontal recirculation velocity and the area of recirculation zone below arches significantly increase for the improved DFB with NSBs. The particle number concentration near furnace center for the improved DFB with NSBs is much higher than that for the original DFB with TSBs. The above two aspects will effectively guarantee timely ignition of anthracite. The downward depth of GP flows and the space utilization ratio of the lower furnace increase, which will be beneficial to promoting volume heat load and heat flux density in primary combustion zone. In addition, full-scale industrial-sized measurements aiming at a 300-MWe DFB improved by NSBs were carried out at ultra-low loads of 100 and 90 MWe. For the original DFB after improved by NSBs, the minimum load for stable combustion without oil support is reduced from 150 to 90 MWe, and the ultra-low-load combustion stability is achieved. For the improved DFB with NSBs, pulverized coal ignition distances for operating burners are about 2 and 2.6 m, respectively, and the signal strength of flame detectors for all operating burners is above 95% at 100 and 90 MWe. The flame fullness and combustion stability are good at the initial combustion stage. At ultra-low loads, furnace negative pressure, superheat steam pressure and oxygen concentration at furnace outlet fluctuate slightly. The temperature at air preheater inlet meets the needs of normal operation of denitrification system, and there is no problem of low temperature corrosion on the surface at flue gas side of air preheater. The unburned carbon in fly ash is about 4%. The maximum concentrations of NOx emission at furnace outlet are 714 and 687 mg/m3 (O2 = 6%) at 100 and 90 MWe, respectively, and ultra-low emission of NOx after denitrification system is achieved.

Lagrangian Views of the Pathways of the Atlantic Meridional Overturning Circulation

AbstractThe Lagrangian method—where current location and intensity are determined by tracking the movement of flow along its path—is the oldest technique for measuring the ocean circulation. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid‐20th century, technological advances in electronic navigation allowed oceanographers to continuously track freely drifting surface buoys throughout the ice‐free oceans and begin to construct basin‐scale, and eventually global‐scale, maps of the surface circulation. At about the same time, development of acoustic methods to track neutrally buoyant floats below the surface led to important new discoveries regarding the deep circulation. Since then, Lagrangian observing and modeling techniques have been used to explore the structure of the general circulation and its variability throughout the global ocean, but especially in the Atlantic Ocean. In this review, Lagrangian studies that focus on pathways of the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC), both observational and numerical, have been gathered together to illustrate aspects of the AMOC that are uniquely captured by this technique. These include the importance of horizontal recirculation gyres and interior (as opposed to boundary) pathways, the connectivity (or lack thereof) of the AMOC across latitudes, and the role of mesoscale eddies in some regions as the primary AMOC transport mechanism. There remain vast areas of the deep ocean where there are no direct observations of the pathways of the AMOC.

Experimental Investigation of Three‐Dimensional Flow Structure and Turbulent Flow Mechanisms Around a Nonsubmerged Spur Dike With a Low Length‐to‐Depth Ratio

AbstractFlume experiments were conducted to investigate the three‐dimensional flow structure and turbulent flow mechanisms around a nonsubmerged, sidewall‐attached rectangular spur dike with a low length‐to‐depth ratio. Velocity measurements show that the wake of the spur dike in the middepth region consists of a single, large recirculation zone, while that in the near‐bed region is composed of a horizontal recirculation zone and a corner vortex with its axis perpendicular to the flume sidewalls. A horseshoe vortex system observed in front of the spur dike is found to interact with the downstream recirculation zone, which results in the increase in lateral turbulent mixing especially in near‐bed regions. Another vortex, which rotates in the opposite direction to the horseshoe vortex, is found beneath the free surface in front of the spur dike, which is associated with increased free surface elevation and its fluctuation. Power spectra of the transverse velocity show evidence of a periodic behavior of the near‐wake shear layer emanating from the tip of the spur dike with a Strouhal number of about 2. It is also shown that the two experiments conducted at different Reynolds and Froude numbers show very similar flow fields in spite of large differences in free surface elevations, which indicates that the effects of free surface deformation on mean velocities and Reynolds stresses are marginal.

Two-dimensional Pollutant Transport Simulations in Natural Streams with Horizontal Recirculation Zone

In this study, flow and pollutant transport simulations were conducted in the tidal reach of the Han River where the horizontal recirculation zone induced by the tidal cycle causes the storage effect. The flow analysis results show that the flow direction was repeatedly changed due to the downstream water level and the horizontal recirculation zone was occurred at the point where the flow direction was changed. Thus, the pollutant cloud was advected back and forth by the periodically occurred reversal flow. The storage effects of the horizontal recirculation zone cause that the concentration-time curve shows oscillatory increasing patterns, and the retention time of pollutant cloud was also increased.

Electrical resistivity tomography and distributed temperature sensing monitoring to assess the efficiency of horizontal recirculation drains on retrofit bioreactor landfills

In bioreactor landfills, the recirculation of water can accelerate biodegradation and increase gas production. The dedicated infrastructure aims at increasing wastewater content over a wide area, with a long-lasting effect. To assess the efficiency of horizontal drains in bioreactor landfills, we use electrical resistivity tomography (ERT) and distributed temperature sensing (DTS) to monitor two injection experiments. The first monitoring experiment focuses on image resolution and takes advantage of a pseudo 3D ERT data set. This technique successfully highlights the waste horizontal anisotropy and the crucial role of existing gas wells, acting as vertical preferential flow paths. The observations are supported by borehole temperature logging. The second monitoring experiment focuses on temporal resolution and requires repeated 2D ERT measurements. The hourly acquisition frequency offers better insight on the water-flow dynamics, such as the flow direction and velocity and the water retention trough time. Temperature logging along the horizontal drain indicates that the injected water is distributed over the entire drain length. Altogether, the two recirculation experiments inform us on the suitability of large horizontal drains for water recirculation on bioreactor landfills. In conclusion, the two geophysical tools provide essential information to determine the most appropriate water-injection protocol in terms of frequency, volume, and flow rate.

Analysis of two-dimensional flow and pollutant transport induced by tidal currents in the Han River

Abstract We simulated two-dimensional pollutant transport in the tidal reach of the Han River and examined the mixing induced by reversed flow. We used the downstream water surface elevation to reflect the tidal constituents of the Yellow Sea when modeling the flow characteristics. We found that reversed flows were repeatedly created and destroyed as the tide varied. The reversed flow affected the spatial variation of the dispersion coefficients and caused the lateral fluctuations of the dispersion coefficient to increase. The change in the flow direction was related to oscillatory variations in the concentration–time curves, and the horizontal recirculation zone at the front of the reversed flow became a storage area where the pollutant cloud was trapped. The concentration–time curves showed oscillatory changes that corresponded with the temporal variations of the flow direction, and the amplitude of the oscillations gradually decreased. The increases in the time for which the polluted water was retained at the riverine parks can be explained by the oscillations on the falling limb of the concentration curves.

Monitoring extent of moisture variations due to leachate recirculation in an ELR/bioreactor landfill using resistivity imaging

Bioreactor or enhanced leachate recirculation (ELR) landfills are designed and operated for accelerated waste stabilization, accelerated decomposition, and an increased rate of gas generation. The major aspects of a bioreactor landfill are the addition of liquid and the recirculation of collected leachate back into the waste mass through the subsurface leachate recirculation system (LRS). The performance of the ELR landfill largely depends on the existing moisture content within the waste mass; therefore, it is of utmost importance to determine the moisture variations within the landfill. Traditionally, the moisture variation of the ELR landfill is determined by collecting samples through a bucket auger boring from the landfill, followed by laboratory investigation. Collecting the samples through a bucket auger boring is time consuming, labor intensive, and cost prohibitive. Moreover, it provides the information for a single point within the waste mass, but not for the moisture distribution within the landfill. Fortunately, 2D resistivity imaging (RI) can be performed to assess the moisture variations within the landfill and provide a continuous image of the subsurface, which can be utilized to evaluate the performance of the ELR landfill. During this study, the 2D resistivity imaging technique was utilized to determine the moisture distribution and moisture movement during the recirculation process of an ELR landfill in Denton, Texas, USA. A horizontal recirculation pipe was selected and monitored periodically for 2.5years, using the RI technique, to investigate the performance of the leachate recirculation. The RI profile indicated that the resistivity of the solid waste decreased as much as 80% with the addition of water/leachate through the recirculation pipe. In addition, the recirculated leachate traveled laterally between 11m and 16m. Based on the resistivity results, it was also observed that the leachate flow throughout the pipe was non-uniform. The non-uniformity of the leachate flow confirms that the flow of leachate through waste is primarily through preferential flow paths due the heterogeneous nature of the waste.

Flow structure caused by a local cross‐sectional area increase and curvature in a sharp river bend

AbstractHorizontal flow recirculation is often observed in sharp river bends, causing a complex three‐dimensional flow structure with large implications for the morphological and planimetric development of meanders. Several field observations in small‐scale systems show that sharp bends are often found in association with a strong increase in cross‐sectional area, the deposition of outer bank benches, and reattachment bars near the inner bank. Recent studies show that these bends can also occur in large‐scale systems. In this study, we present field measurements of a sharp bend in the Mahakam River, East Kalimantan, Indonesia. The cross‐sectional area increases by a factor of 3 compared with the reach‐averaged cross‐sectional area. Along a river reach of about 150 km, cross‐sectional area correlates strongly with curvature. The field measurements are analyzed together with the results from numerical simulation with a three‐dimensional finite element model, which yields a comprehensive view of the intricate flow structure. In turn, the model is used to validate a new equation that captures the water surface topography dependence on cross‐sectional area variation and curvature. The results show the importance of the increase in cross‐sectional area in the development of horizontal recirculation. Vertical acceleration of the flow into the scour causes the pressure to deviate from a hydrostatic pressure distribution. Strong downflow (up to 12 cm s−1) advects longitudinal momentum toward the bed, causing the flow to concentrate in the lower part of the cross section. This increases the velocity magnitude throughout the cross section, which is expected to maintain the large scour depth found in several bends along the Mahakam River.

Argon Blowing Technology Using in the LF Refining

The process of LF Refining is introduced in this paper. Because the LF ladle furnace has the advantages are widely used and developed in many companies. The argon is blown into the ladle, so the purpose of refining is achieved. The argon stirring can accelerate the chemical reaction between slag and steel. In the circulation process, large inclusion and deoxidation product in the lower region transfer into the slag. The material transfer between steel and slag can be accelerated. At the same time, argon stirring can make molten steel composition and temperature uniformity. Thus, the ladle of blowing argon can be roughly divided into three important flow region has been introduced. Two-phase region, the top horizontal flow zone and recirculation zone exists in the ladle.

Flow dynamics through a submerged bridge opening with overtopping

ABSTRACTThis paper reports the results of detailed numerical simulations of the flow over a submerged bridge. The method of Large Eddy Simulation (LES) is applied, which allows elucidation of the instantaneous flow and accurate quantification of its turbulence statistics. The in-house LES code employs a free surface algorithm based on the Level Set Method (LSM) to determine the complex water surface profile over the bridge, which is validated with data from a physical model of the bridge under analogous flow conditions. Numerically predicted water surface profiles show good agreement with measured data. The water surface resembles that of the flow over a broad-crested weir with a plunging nappe and a standing wave downstream of the bridge. The mean flow is characterized by a multitude of complex flow features including horizontal recirculation zones upstream and downstream of the bridge abutment and vertical recirculation zones of the separated plunging flow. The latter influences significantly the near-bed turbulence and the bed shear stress.

Research Status of Physical Simulation on LF Furnace Refining Ladle

The research status of physical simulation on LF furnace refining ladle is introduced in this paper.Since 1970, the research on the gas-liquid two-phase region has begun. The ladle of blowing argon can be roughly divided into three important flow region has been introduced. Two-phase region, the top horizontal flow zone and recirculation zone exists in the ladle.The establishment of physical model should include similarity principle and flow field measurement technique

Computational fluid dynamics modelling of different detention pond configurations in the interest of sustainable flow regimes and gravity sedimentation potential

AbstractThis study presents the results of the flow regime evaluation, by means of computational fluid dynamics (CFD), of a vegetated detention pond located at Waterlooville, Hampshire, UK. Alternative pond layouts were assessed for the same flow conditions on the basis of recommendations made in the literature. The results were validated by comparing the maximum computational velocities for the same case using different numbers of mesh elements. It was found that the development of a CFD model of detention ponds is intricate but feasible. The main findings were: (i) The present design performed well in terms of flood risk management, but the flow patterns could result in questionable treatment efficiency; (ii) vegetation seems to promote horizontal recirculation and turbulence; (iii) triangular and elliptical pond designs showed very poor performance; (iv) the most appropriate design for the given location and hydrological regime is an elliptical pond with a central emergent/submerged island.

Assessing the predictive capabilities of isotropic, eddy viscosity Reynolds‐averaged turbulence models in a natural‐like meandering channel

The predictive capabilities of an isotropic, eddy viscosity turbulence model for closing the unsteady Reynolds‐averaged Navier‐Stokes (RANS) equations are systematically investigated by simulating turbulent flow through a field‐scale meandering channel and comparing the computed results with the large‐eddy simulation (LES) of the same flow recently reported by Kang and Sotiropoulos (2011). To facilitate the comparison of the two turbulence models, both RANS simulation and LES are carried on exactly the same grid with the same numerical method. The comparisons show that while the RANS model captures the curvature‐driven secondary flow within the bend, it fails completely to predict other key flow features in the channel, which are predicted by the LES and also observed in flow visualization experiments. These features include the inner and outer bank shear layers, the outer bank secondary cell, and the inner bank horizontal recirculation zone. By analyzing the results of the LES, we conclusively show that flow features not predicted by the RANS calculation are located in regions of the flow with high levels of turbulence anisotropy. The extent of these regions and, consequently, the degree of disagreement between the RANS and LES predictions are shown to depend on the stream geometry and the flow rate. Our results underscore the major challenges confronting the computationally expedient, isotropic RANS models, which are widely used today in three‐dimensional hydrodynamic and morphodynamic simulations.