Downstream Side Research Articles

The Tip of Dendritic Crystal in an Inclined Viscous Flow

We study the flow around the tip of a dendritic crystal by an inclined stream of viscous incompressible liquid. The tip shape is chosen accordingly to recent theory [Phil. Trans. R. Soc. A 2020, 378, 20190243] confirmed by a number of experiments and computations [Phil. Trans. R. Soc. A 2021, 379, 20200326]. Our simulations have been carried out for a 0, 30, 60, and 90-degree flow slope to the dendrite axis. We show that the stream inclination has a significant effect on the hydrodynamic flow and shear stress. In particular, a transition from laminar to turbulent currents on the upstream side of the dendritic crystal may occur in an inclined hydrodynamic flow. This leads to the fact that the heat and mass transfer mechanisms on the upstream and downstream sides of a growing dendritic crystal may be different.

GPU simulation of wake effects at the Horns Rev 1 offshore wind farm using the CFD porous disk wake model

To verify the effectiveness of the GPU simulation of wake effects at a large-scale offshore wind farm, we ran an in-house large-eddy simulation (LES) solver with a CFD porous disk wake model for the Horns Rev 1 wind farm. For this numerical research, we prepared the latest workstation equipped with a Xeon W-2265 CPU and an NVIDIA RTX A6000 GPU. We clarified that the calculation speed of the single GPU of the NVIDIA RTX A6000 is approximately 10 times faster than the calculation speed of the Xeon W-2265. Careful data analysis and visualization of the unsteady turbulent flow fields obtained in the current LES study suggest that the mutual interference of the wakes developed by wind turbines may frequently form a local speed-up region around wind turbines, located on the downstream side of large offshore wind farms.

Study on collapse settlement and cracks of core wall rockfill dams under wetting deformation

AbstractThe safety and stability of core wall rockfill dams during impoundment are threatened by the wetting deformation of up‐stream shell materials. The serious wetting deformation not only aggravates the collapse settlement of upstream rockfill but also intensifies differential settlement of the dam crest during impoundment, and then causes cracks on the dam crest. On the basis of the proposed wetting deformation model and its simulation method of wetting deformation, this paper simulated the impoundment process of Guanyinyan core wall rockfill dam and studied the deformation characteristic of the dam during impoundment. In addition, the smeared cracking model was used to simulate the crack propagation on the dam crest, and the crack develop and spread mechanism was analysed. The results show that the simulated deformation can fit the in‐situ data well, and the simulated crack propagation is in good agreement with the actual situation. Once watered, the upstream rockfill and core wall have significant settlement, and the whole dam crest has obvious horizontal displacement towards the upstream. It is on the same order of magnitude that the increment of horizontal displacement and settlement at the top of the dam during impoundment. In the process of impoundment, the upper part of the dam tends to deform towards the reservoir area, which will lead to tensile cracks appearing in the rockfill areas on both upstream and downstream sides of the core wall of the dam crest, and the propagation direction of the cracks is basically parallel to the adjacent core wall surface. With the water level rising, the cracks on the downstream side of the dam crest mainly extend vertically, and the cracks on the upstream side of the dam crest not only extend vertically, but also extend horizontally.

Backwards wave breaking by flow separation vortices under solitary waves

In this work, the backward wave breaking process by the presence of flow separation vortices under a solitary wave is studied. Based on a set of non-dimensional variables defined from the Buckingham Π theorem, a set of numerical experiments are performed in order to analyze the effect of varying the submerged obstacle geometry on the surrounding flow and free surface by using a RANS-VOF model. Model simulations are tested against available experiments showing a good performance of numerical results. The structure-submergence ratio and the structure-based Reynolds number modulate the type of breaking (collapsing-plunging). The structure aspect ratio defines the location and number of backward breaking points for rectangular structures and the breaking wave direction for triangular structures (either forward or backward). Moreover, when the flow separation vortex diameter is comparable to the local water depth a backward wave breaking process is originated. The vortex behaves as a rigid body that accelerates the flow at the upper side of the vortex, accumulating mass at the downstream side where a very large slope on the free surface makes the water fall backward due to gravity.

Real-Time Leak Diagnosis in Water Distribution Systems Based on a Bank of Observers and a Genetic Algorithm

The main contribution of this paper is to present a novel solution for the leak diagnosis problem in branched pipeline systems considering the availability of pressure head and flow rate sensors on the upstream (unobstructed) side and the downstream (constricted) side. This approach is based on a bank of Kalman filters as state observers designed on the basis of the classical water hammer equations and a related genetic algorithm (GA) which includes a fitness function based on an integral error that helps obtaining a good estimation despite the presence of noise. For solving the leak diagnosis problem, three stages are considered: (a) the leak detection is performed through a mass balance; (b) the region where the leak is occurring is identified by implementing a reduced bank of Kalman filters which localize the leak by sweeping all regions of the branching pipeline through a GA that reduces the computational effort; (c) the leak position is computed through an algebraic equation derived from the water hammer equations in steady-state. To assess this methodology, experimental results are presented by using a test bed built at the Tuxtla Gutiérrez Institute of Technology, Tecnológico Nacional de México (TecNM). The obtained results are then compared with those obtained using a classic extended Kalman filter which is widely used in solving leak diagnosis problems and it is highlighted that the GA approach outperforms the EKF in two cases whereas the EKF is better in one case.

Health Risk Assessment during In Situ Remediation of Cr(VI)-Contaminated Groundwater by Permeable Reactive Barriers: A Field-Scale Study.

The presence of residual Cr(VI) in soils causes groundwater contamination in aquifers, affecting the health of exposed populations. Initially, permeable reactive barriers(PRB) effectively removed Cr(VI) from groundwater. However, as PRB clogging increased and Cr(VI) was released from upstream soils, the contamination plume continued to spread downstream. By 2020, the level of contamination in the downstream was nearly identical to that in the upstream. The study results show that during normal operation, the PRB can successfully remove Cr(VI) from contaminated groundwater and reduce the carcinogenic and non-carcinogenic risks to humans from the downstream side of groundwater. However, the remediated groundwater still poses an unacceptable risk to human health. The sensitivity analysis revealed that the concentration of the pollutant was the most sensitive parameter and interacted significantly with other factors. Ultimately, it was determined that the residual Cr(VI) in the soil of the study region continues to contaminate the groundwater and constitutes a serious health danger to residents in the vicinity. As remediated groundwater still poses a severe threat to human health, PRB may not be as effective as people believe.

A phase-gradient acoustic metasurface for broadband duct noise attenuation in the presence of flow

A phase-gradient acoustic metasurface is designed based on wavefront manipulation for achieving broadband noise reduction in a flow duct. It is constructed by eight periodically arranged varying-depth units filled with porous materials, forming a linear phase shift of reflected wave from 0 to 2π at 3000Hz. The linear reflected phase-shifting can be roughly maintained in a wide frequency range. By employing the broadband phase-shift feature of the units, the metasurface exhibits an enhanced sound absorption performance in terms of the absorption coefficient and bandwidth when comparing to the individual units, which originates from forcing the higher-order modes of reflection waves to be evanescent. Then, its transmission attenuation characteristics in the presence of background flow are assessed by installing it on the side wall of a flow tube. The effects of flow speed (Mach number is up to 0.3) and sound source position (at the upstream and downstream sides of the metasurface) are experimentally and numerically investigated. Results show that the attenuation performance of the metasurface decreases with flow speed for the upstream sound source while increasing for the downstream sound source. For both upstream and downstream sound sources, a high transmission loss (≥20dB) is achieved by the metasurface in the frequency range of 1500–3000Hz at different flow speeds. Hence, the proposed metasurface is promising for achieving broadband noise attenuation in the presence of flow.

On‐line melt temperature measurements for polymer injection molding through an instrumented annular duct

AbstractPolymer processing requires reliable monitoring equipment to ensure product quality, improve energy efficiency and apply advanced control strategies. This work tests an instrumented annular duct as a nozzle on the downstream side of the screw‐barrel to analyze the thermal behavior of the polymer flow in an injection molding machine. The plasticization step is first studied, and then virtual injection molding cycles are performed. The measurements via the instrumented annular duct show that the melt temperature decreases during fast production, due to the lack of residence time in the barrel for the material to be sufficiently heated by conduction. The instrumented annular duct also enables detection of changes in melt temperature over time during a single injection. Moreover, the experiments prove the robustness of the temperature measurements in an annular geometry for process monitoring.

Numerical Analysis of Leakage Performance of Brush Seal Based on a 2-D Tube Bank Model and Porous Medium Model Considering the Effect of Compressible Gas

This paper presented a formula for calculating the resistance coefficient of porous medium model of brush seals considering the effect of gas compressibility. Firstly, the 2-D tube bank model is used to calculate the axial resistance coefficient of brush seals. Secondly, the feasibility of the method is validated according to the existing experiment results. The validation results show that this formula is effective for brush seals. Lastly, the pressure distribution and leakage rate of the test brush seal are analyzed. With the presented porous medium model, the pressure drop through the bristle is computed, and the effect of groove of the back plate is addressed. The pressure drop is mainly concentrated on the downstream side of the bristle pack. The groove in the back plate can keep the pressure balance of the bristles back, causing the recovering of the pressure in the last row of bristles. The leakage rate of porous medium model is higher than the 2-d tube bank model, because the 2-D tube bank model only considers the leakage rate of the zone of the fence height. The rotation speed has a little effect on the reduction of the leakage rate.

Opposing-buoyancy mixed convection through and around arrays of heated cylinders

We numerically investigated the opposing-buoyancy mixed convection through and around square arrays of$10\times 10$heated circular cylinders with the solid fraction ($\phi$) ranging from 0.0079 to 0.66 and the Richardson number ($Ri$) varying from 0 to 1 at a fixed Reynolds number ($Re$) of 100. Our simulations revealed that the large mean recirculation in the far wake can be detached from or connected with the vortex pair in the near wake for different combinations of$Ri$and$\phi$. Also, it was found that the array with relatively small$\phi$can significantly promote flow instability even at moderate$Ri$. The instability, which is closely related to the fluctuating heat flux, develops from the lateral sides to the downstream side of the array and gives rise to the large mean recirculation in the far wake. The power spectra density of the array-scale force coefficients demonstrates that the flow undergoes different bifurcation behaviours under various parameter combinations, which reflects the interaction between the near-wake and far-wake vortexes. Interestingly, the Strouhal–Richardson number curves can be collapsed onto the same curve when$Ri$is increased by a$\phi$-dependent factor. Also, for$\phi \leqslant 0.22$, both the mean drag coefficient and the mean Nusselt number of the array were found to decrease linearly with$Ri$since the buoyancy within the array becomes prominent in this range of$\phi$.

Analysis of effects of aerodynamic interference on dynamic response of suspended monorail wind–vehicle–bridge system using joint simulation approach

A dynamic simulation model of a suspended monorail vehicle–bridge system was developed using a joint simulation approach based on ANSYS and SIMPACK. Moreover, the aerodynamic coefficients of the vehicle–bridge system were measured through a wind-tunnel test, in which a wind load was applied as an external excitation. The variation law of the dynamic response of a double-line suspended monorail vehicle–bridge system with respect to the line spacing under aerodynamic interference was systematically studied, and the following results were obtained. (1) The effect of the vehicle–bridge combination on the aerodynamic coefficient was significant. In the case of one vehicle, the vehicle on the downstream line was slightly less affected by the crosswind than that on the upstream line. For these two vehicles, the three-component force coefficient of the downstream vehicle was far smaller than that of the upstream vehicle. (2) In the case of two vehicles, an increase in the spacing ratio reduced the drag of the vehicles. When the spacing ratio exceeded 4.5, the change in the three-component force of the upstream vehicle was no longer obvious, but the drag value for the downstream vehicle changed from negative to positive. (3) With an increase in the spacing ratio, the vehicle acceleration generally increased, but the changes in the acceleration and displacement of the bridge were insignificant. (4) During the meeting of two vehicles, the vehicle running on the upstream side is easy to be affected by winds than on the downstream side, which leads to the result that the dynamic response of the upstream vehicle and bridge are larger than that of the downstream side.

Impact of Piping Erosion Process on the Temporal–Spatial Mechanisms of Soil

Earthen dams with greater fines are more susceptible to failure from the piping. This study employed the coupled computational fluid dynamics (CFD)-discrete element method (DEM) to investigate the impact of the piping erosion process. Results displayed that increasing the fine contents would reduce the particle velocity and drag force for enhancing the erosion resistance. Piping would reduce the stability of the structure at different positions of the sample with various fine contents. The representative volume elements (RVE) were selected to observe the local geometry and material behavior as the erosion progressed. Severe water flows significantly reduced the peak shear strength of the eroded soils for the interactive-underfilled soil, particularly for the downstream side. However, the interactive-overfilled specimen reduced the peak shear strength on the upstream side. Results indicated that the proposed weighted clustering coefficient is a better index to capture the shear strength of the eroded soil matrix, with a high magnitude corresponding to a high peak shear strength. Different local material behavior may lead to differential settlements and associated catastrophic consequences during the piping erosion process, which should draw special attention.

Utilization of Geogrid and Water Cushion to Reduce the Impact of Nappe Flow and Scouring on the Downstream Side of a Levee

Water overflowing from a levee generates scour holes on the toe, which progresses towards the backward crest of the levee and results in nappe flow generation. The direct collision of nappe flow on the downstream area causes levee failure. It is important to introduce a novel countermeasure against scouring caused by nappe flow. Hence, the present study utilized a new technique to reduce scouring due to nappe flow by introducing a combination of pooled water and geogrids. Herein, laboratory experiments were conducted with the three cases for rigid bed (R), named as NR, G1R, G2R (N, G1 and G2 represent no geogrid, geogrid 1 and geogrid 2, respectively), and moveable bed (M), named as NM (nothing moveable), G1M (geogrid 1 moveable), G2M (geogrid 2 moveable), to elucidate the effect of dimensionless pooled water depth (DP*), overtopping depth (DC*) and the aperture size of geogrids (d*) on flow structure and scouring. The results showed that the scour depth was reduced by around 17–31% during the NM cases, 57–78% during the G1M cases and 100% during the G2M cases by increasing the DP* from 0.3 to 0.45. Hence, the combination of geogrids with pooled water (G1M, G2M) performed a vital role in suppressing the scouring, but the results of G2M were more advantageous in terms of scouring countermeasures.

Analysis of the existence form of landfill leachate water level and its development pattern under the condition of considering internal source water: a case study.

In this paper, we study the existence form and development pattern of leachate level in the pile through a case of high water level landfill. The innovation of this paper is to consider not only the infiltration of rainfall, but also the internal source water of waste as the source of leachate. There is one main water level and multiple layers of stagnant water levels in this study area. The main water level of the pile first appears along the base slope of the landfill, and the trend of development is to connect from the bottom pile upwards layer by layer. The connection point of each layer gradually moves from the upstream side to the downstream side with time, and the main water level is formed by the gradual connection of the stagnant water level in each layer of the pile. Piles filled with slope as the base surface, and the stagnant water level in the pile first develops on the slope. And piles that are filled with a gentle base surface, and stagnant water level in the pile is uniformly developed on the base surface. At least one layer of stagnant water level exists in each layer of the pile, and multiple saturated zones occur in landfills with gentle base surfaces, and the saturation zone of the upper and lower waste pile will be penetrated. Depending on the form of leachate water level present in the pile, the location can be reasonably chosen for engineered precipitation of the landfill, and based on the development trend of each water level in the pile, it can provide a reference for the site selection and design of the landfill.

Impact of a dam construction on the intertidal environment and free-living nematodes in the Ba Lai, Mekong Estuaries, Vietnam.

The impact of high siltation and accumulation of organic and waste material in the intertidal of the dammed Ba Lai River in Vietnam as part of the Mekong estuarine system was investigated by means of marine free-living nematodes. Nutrients content (nitrate, ammonium, total phosphorus, total nitrogen), total suspended solids, total organic carbon, coliform, bacteria E. coli, pH, dissolved oxygen, total dissolved solids, methane and hydrogen sulfide concentration, and the nematode communities were characterized in sediment at selected stations along the river above and below the dam. Our results found elevated methane concentrations at the upstream side of the dam while hydrogen sulfide concentrations found to be highest in the downstream side of the dam. Furthermore, methane and hydrogen sulfide concentrations were correlated to nematode community characteristics such as trophic composition densities and genera composition. There was a clear difference between the communities above and below the dam. The discontinuous nematode community distribution indicated that the Ba Lai River is impacted by dam construction. Potentially the high deposition and eutrophication could turn the area into a methane-rich area related to predicted impact on nematodes.

Seismic Behavior of an Innovative System for Preventing Gas Leakage through Reinforced Concrete Shear Walls of Nuclear Facilities

ABSTRACT During extreme events such as earthquakes or explosions, cracks in reinforced concrete (RC) shear walls of facilities housing hazardous gases or radioactive substances may escape to outdoor environment through cracked concrete. For this reason, it is crucial to produce safe and reliable design and reinforcement details of such walls. This paper presents results of a comprehensive study that focuses on developing cost-effective and reliable gas barrier for RC walls used in underground hazardous facilities, such as nuclear plants, utilizing surrounding soil. The gas leakage experiments were performed in two phases. In the first phase, simulated leakage through cracks of reduced-scale RC shear wall specimens was examined and was used as a baseline for comparison with the proposed leak-tightening protection system. The second phase of the experimental program involved evaluation of the performance and leak-tightening efficiency of the proposed hybrid shear walls/soil system. During the second phase, several types of soils with different characteristics such as soil layer thickness, soil ingredients, moisture content, etc., were assessed experimentally. Results of this study indicated that in general, the presence of soil in the downstream side reduced the gas leakage remarkably. Results also showed that the use of sandy soil with smaller grain sizes has lower permeability as compared to those with larger grain sizes. Similarly, the gas leakage decreases, to some extent, when soil protective layer thickness increases. In addition, it was concluded that gas leakage is inversely proportion to soil’s moisture content.

Dynamics of gap winds in the Great Rift Valley, Ethiopia: emphasis on strong winds at Lake Abaya

Abstract. Lake Abaya, located in the Great Rift Valley (GRV) in Ethiopia, is affected by regularly occurring strong winds that cause water waves, which in turn affect the lake's ecology and food web. The driving forces for these winds, however, are yet unexplained. Hence, the main goal of this study is to provide a physical explanation for the formation of these strong winds in the GRV and especially at Lake Abaya. To this aim, two case studies were performed based on measurements, ERA5 reanalysis data and mesoscale numerical simulations conducted with the Weather Research and Forecasting (WRF) model. The simulations revealed that in both cases a gap flow downstream of the narrowest and highest part of the GRV (i.e. the pass) led to high wind speeds of up to 25 m s−1. Two types of gap flow were identified: a north-eastern gap flow and a south-western gap flow. The wind directions are in line with the orientation of the valley axis and depend on the air mass distribution north and south of the valley and the resulting along-valley pressure gradient. The air mass distribution was determined by the position of the Intertropical Convergence Zone relative to the GRV. The colder air mass was upstream of the GRV in both case studies. During the day, the convective boundary layer in the warmer air mass on the downstream side heated up more strongly and quickly than in the colder air mass. The most suitable variable describing the timing of the gap flow was found to be the pressure gradient at pass height, which corresponds roughly to the 800 hPa pressure level. In both cases the gap flow exhibited a strong daily cycle, which illustrates the importance of the thermal forcing due to differential heating over complex terrain in addition to the large-scale forcing due to air mass differences. The start, strength, and the duration of the gap winds within the valley depended on location. For both cases, the strongest winds occurred after sunset and in the ongoing night downstream of the gap and on the corresponding lee slope. The ERA5 reanalysis captures both events qualitatively well but with weaker wind speeds than in the mesoscale numerical simulations. Hence, ERA5 is suitable for a future climatological analysis of these gap flows.

Experimental study of film cooling characteristics of annular cascade endwall with a partitioned film cooling layout

Based on the heat transfer characteristic of the endwall and the distribution law of the turbine cascade vortex system, a partitioned film cooling layout was proposed on an annular cascade endwall. By the passage vortex lift-off line and the passage throat line, the endwall is divided into four regions, which were respectively upstream passage wedge region, upstream pressure side region, downstream pressure side region, and downstream passage wedge region. The endwall film cooling performances of the leakage flow and the partitioned film holes were studied experimentally by the PSP technique. Results indicate that the upstream passage wedge region can be cooled highly when the MFR of leakage flow is more than 1%. Owing to the film superposition effect, the η of the upstream pressure side region is lower than that of two downstream partitions. As the increased coolant MFR, the η of the upstream partition was increased, however, there was an opposite trend for the downstream partition. In addition, it is found that the leakage flow enhanced dramatically the film cooling characteristics of the partitioned film holes on the region of X/Cax ≤ 0.5 in the study of endwall full-coverage film cooling.

Assessing the bridge construction effect on river shifting characteristics through geo-spatial lens: A case study on Dharla River, Bangladesh

The key objective of the bridge structure on a river is to confirm and assist the continual communication. On the other hand, it also has a harmful effect on the hydrological and morphological behavior of the neighboring zone as of the river width contraction. The present study assessed the Bridge construction effect on river shifting characteristics for Dharla River. For doing so, this study followed two steps. First, Landsat imageries for both pre-bridge structure period (1988, 1993 and 1998) and post-bridge structure period (2003, 2008, 2013 and 2017), has been processed and used to determine the thalweg line. Then, geo-spatial environment has been used to assess the thalweg line shifting characteristics and channel width variations. The results revealed a frequent shifting of thalweg line towards both the east and west direction at the upstream side of this Bridge and also the dominating eastwards shifting at the downstream side of the Bridge. After constructing the Dharla Bridge on Dharla River (i.e. post-bridge structure period) the movement of the channel has been changed at a significant level. Particularly, it has been found that the east side shifting is higher than the west side. Thus, the existing of Dharla Bridge has an abundant effect on the shifting characteristics of thalweg line. The study concluded that the shifting of the thalweg lines of Dharla River undergoes a drastic change of study period and the River can be treated as a very dynamic one. The results of this study might supportive for the sustainable and future development of the rivers and adjacent flood plain in Bangladesh.

The extent and magnitude of edge effects on woody vegetation in road‐bisected treed peatlands in boreal Alberta, Canada

AbstractTreed peatlands can exhibit dramatic shifts in woody plant cover when they are bisected by roads, a product of change in the flow of surface and subsurface water; however, the edge effects that roads have on overstory cover remain poorly understood. We examined how road and environmental conditions influence woody cover in treed fens in northeastern Alberta, Canada. We used generalized linear mixed models to explain variation in cover as measured using airborne laser scanning (ALS) data obtained for 48 road‐bisected fens. Over half of the study fens had >10% differences in canopy cover between the upstream and downstream sides. Variation in cover was best explained by a complex interaction between road side, distance, and type, as well as distance to upland forest and open water, in both rich and poor treed fens. Substrate texture (fine vs. coarse) further explained cover in rich fens. Gravel roads appeared to have the most dramatic effect on cover adjacent to roads (0–20 m) in both fen types, with differences persisting beyond 100 m. In fens bisected by gravel and paved roads, differences in cover between road sides tended to be ameliorated within 200 m, except for unimproved roads where changes were more linear. This study demonstrates the complexity of landscape conditions under which roads built through peatlands can cause structural changes in woody cover and the usefulness of ALS data for studying this phenomenon.