Main Flow Direction Research Articles

Isolating influences of varying pitch from the effects of non-zero compound angles on effusion cooling

Abstract Effusion cooling is the state-of-the-art cooling technology for gas turbine hot-gas path components. Typically, effusion cooling holes across the entire combustor liner are aligned with the combustor axis, rendering a nominal zero compound angle between highly directional miniature effusion cooling jets and the main flow direction. The pitch of effusion cooling holes is optimised accordingly. However, the swirling main flow results in a non-zero compound angle and an effectively different pitch from the design. The directional effect of effusion cooling as a result of swirling main flow on the adiabatic film cooling effectiveness (AFE) is a combined effect of a non-zero compound angle and a varied pitch. The current experimental study aims to investigate the isolated effects of compound angle on AFE by excluding the influences of varying pitch. With an improved understanding of the sole effects of non-zero compound angles on AFE, the roles that a varied pitch plays in modifying AFE are further discussed to guide future effusion cooling designs under swirling main flow conditions. Binary pressure sensitive paint (PSP) was used to determine AFE experimentally.

An exact solution of the lubrication equations for the Oldroyd-B model in a hyperbolic pipe

An exact analytical solution of the lubrication equations for the steady, isothermal, incompressible flow of a viscoelastic Oldroyd-B fluid in a hyperbolic cylindrical contracting pipe is derived. The solution is valid for values of the Deborah number, De, up to order unity (De is defined as the ratio of the longest relaxation time of the polymer to the characteristic residence time of the fluid in the pipe), all values of the ratio of the polymer viscosity to the total viscosity of the fluid, η, and typical values of the contraction ratio, Λ, encountered in experiments and practical applications. It is provided in terms of the streamfunction only and is used in the momentum balance to derive a strongly non-linear ordinary differential equation of second order with unknown a function which corresponds to a modified fluid velocity along the main flow direction. The final equation is solved semi-numerically using a fully spectral (Legendre)-Galerkin approach to resolve the unknown function almost down to machine accuracy. The exact solution for the polymer extra-stresses, which is emphasized is not the full solution of the complete lubrication equations, allows for the derivation of a variety of theoretical expressions for the average pressure-drop along the pipe. In all cases, a decrease in the pressure drop compared to the Newtonian value with increasing De, η and/or Λ is predicted. The differences between the corresponding analytical solution for the planar geometrical configuration are also identified and discussed.

CRRT circuit venous air chamber design and intra-chamber flow dynamics: a computational fluid dynamics study

BackgroundVenous air trap chamber designs vary considerably to suit specific continuous renal replacement therapy circuits, with key variables including inflow design and filter presence. Nevertheless, intrachamber flow irregularities do occur and can promote blood coagulation. Therefore, this study employed computational fluid dynamics (CFD) simulations to better understand how venous air trap chamber designs affect flow.MethodsThe flow within a venous air trap chamber was analyzed through numerical calculations based on CFD, utilizing large eddy simulation. The working fluid was a 33% glycerin solution, and the flow rate was set at 150 ml/min. A model of a venous air trap chamber with a volume of 15 ml served as the computational domain. Calculations were performed for four conditions: horizontal inflow with and without a filter, and vertical inflow with and without a filter. Streamline plots and velocity contour plots were generated to visualize the flow.ResultsIn the horizontal inflow chamber, irrespective of filter presence, ultimately the working fluid exhibited a downstream vortex flow along the chamber walls, dissipating as it progressed, and being faster near the walls than in the chamber center. In the presence of a filter, the working fluid flowed uniformly toward the outlet, while in the absence of a filter the flow became turbulent before reaching the outlet. These observations indicate a streamlining effect of the filter.In the vertical inflow chamber, irrespective of filter presence, the working fluid flowed vertically from the inlet into the main flow direction. Part of the working fluid bounced back at the chamber bottom, underwent upward and downward movements, and eventually flowed out through the outlet. Stagnation was observed at the top of the chamber. Without a filter, more working fluid bounced back from the bottom of the chamber.ConclusionsCFD analysis estimated that the flow in a venous air trap chamber is affected by inflow method and filter presence. The “horizontal inflow chamber with filter” was identified as the design creating a smooth and uninterrupted flow throughout the chamber.

Comprehensive analysis of the performance of a microfluidic photoelectrochemical solar cell with heat exchanger

A dye-sensitized solar cell (DSSC) is a type of photoelectrochemical cell that converts sunlight directly into electricity using a photoanode with a dye-sensitized, nonporous semiconductor layer. This study investigates the impact of integrating microfluidic capabilities into DSSCs, resulting in a flow-cell configuration that introduces new electrical characteristics without compromising its performance. The primary goal of the work was to define and determine the values of heat, mass, and charge transport resistances, and to explore their relationships with the cell’s current–voltage characteristics, temperature, light intensity, and spectra. We employed both experimental methods and computational fluid dynamics using the Lattice-Boltzmann solver to analyze the cell’s dynamics. A novel multilayered microfluidic DSSC (µDSSC) was designed, featuring an innovative “gapless” configuration with a photoactive surface area exceeding 7 cm2, integrated directly with a microfluidic heat exchanger. This configuration allowed the µDSSC to achieve a power conversion efficiency (PCE) of at least 6 % under standard test conditions, with a fill factor reaching up to 68 %. By gradually covering the cell surface, the impact of series resistance (Rs) on PCE was minimized, enabling a maximum PCE of 19–22 %. A new integral definition of the fill factor (IFF) was proposed, which accurately describes the performance of cells constrained by ion mass transport resistance. For the gapless µDSSC, the IFF ranged from 58 % to 88 %. Additionally, we introduced a maximum cell efficiency (MCE) parameter to represent the cell’s efficiency at the theoretical limit of Rs = 0, determined to be 20.7 % for the gapless µDSSC. This MCE value is specific to the cell and is independent of the illuminated surface area and incident light intensity. The thermal efficiency of µDSSC cells is generally limited by heat conduction through the multilayer structure, but at practical cell surface temperatures of 50–80 °C, this limitation is 3 to 6 times less significant than the limitation imposed by the maximum solar energy flux reaching the cell surface. The integration of microchannels into the cell led to a 58 % reduction in hydraulic resistance along the main flow direction. While electrolyte flow typically reduces efficiency in wide-gap DSSCs, the gapless configuration minimizes mass transport resistance, ensuring that fluid flow does not adversely affect the performance of gapless µDSSCs.

Intensifying interfacial oscillations in falling film flows over rectangular corrugations

Unsteady film flows play an important role in intensifying heat and mass transfer processes, with applications, e.g., in falling film absorbers or reactors. In this context, the influence of surface structure modification on the wave dynamics of falling film flows is experimentally investigated based on localized film thickness time series data. Arrays of rectangular ridges oriented perpendicular to the main flow direction are considered, and an optimum ridge distance is identified, at which particularly strong interfacial oscillations are induced in the falling film. These potentially result from the interaction of the flow with a statically deformed base film under resonance-like conditions. The transient destabilization is amplified in the case of narrow ridge sizes, where inertia-driven flow features are particularly pronounced. With regard to mass transfer applications, the structure-induced increase in gas–liquid interfacial area may be of secondary importance compared to changes in internal flow conditions.

Fine-scale movement response of juvenile brown trout to hydropeaking

Juvenile fish are known to be the most impacted during hydropeaking events due to stranding or uncontrolled drift resulting from changes to water depth and flow velocity. To shed light on their response to such hydraulic alterations, we coupled flume experiments with image-based fish tracking and quantified the fine-scale movement behavior of wild (n = 30) and hatchery-reared (n = 38) brown trout (Salmo trutta) parr. We exposed fish to two distinct hydropeaking treatments in a laterally inclined (14 %) flume section stocked with real cobbles to create refuge and heterogeneous hydraulic conditions. Fish were individually acclimated (20 min) to baseflow (Q = 1.6 L s-1) and then exposed to three consecutive hydropeaking events, reaching peakflows tenfold larger than baseflow (Q = 16 L s-1). We found that, within just minutes, fish exhibited fine-scale movement responses to cope with the change of hydrodynamic conditions. Fish moved perpendicular to the main flow direction to shallow areas as these became submerged during discharge increase, holding position at low velocity zones. This resulted in a significant difference (p < 0.001) in lateral occupancy of the experimental section between baseflow and peakflow. During peakflow, fish occupied specific positions around cobbles and exhibited swimming behaviors, including bow-riding and entraining, that allowed them to hold position while likely minimizing energy expenditure. As a result, swimming distance reduced 60–70 % compared to baseflow. During the decrease in discharge following peakflow, fish abandoned areas falling dry by moving laterally. In the treatment with the larger down-ramping rate, the time to initiate relocation was lower while the relocation speed was higher. This study shows that, for the conditions investigated here, brown trout parr is capable of swiftly deploying multiple behavioral responses to navigate rapid changes in hydrodynamic conditions. These findings can be incorporated into habitat modeling and improve our capacity to inform hydropeaking mitigation efforts.

Ice slurry pigging technology in drinking water distribution system: From flow mechanisms to pipelines cleaning application

Drinking water distribution systems are prone to (bio)fouling over time, negatively impacting drinking water quality. Ice slurry pigging is a promising technology used to remove solids attached to the inner parts of the pipe walls with ice-water mixture. To understand the ice slurry pigging flow characteristics and optimize its operations, this study presents the development of a Computational Fluid Dynamics model (CFD) combined with the Kinetic Theory of Granular Flow model (KTGF) and a thermal model. Firstly, the model is validated with experimental data from the literature on solid-liquid Two-Phase Flow (TPF), including flow characteristics and thermal effects. Secondly, simulations for a typical ice slurry pigging process show uneven ice phase fraction and shear stress distributions in the main pipe flow and vertical direction. Results indicate that the main limitations of effective pipe cleaning are due to buoyancy and thermal effects. Moreover, ice slurry with 1.0 mm ice particles performs better in providing effective shear stress compared to 0.1 mm, 0.5 mm, and 1.5 mm ice particles. Additionally, the optimal injection ratios of ice slurry with 1.0 mm are determined to be over 0.3 and 0.26 when cleaning pipeline lengths within 500 m and 1000 m, respectively.

Thermal measurement of a unitized regenerative fuel cell during mode switching using thin film sensors

The research on temperature and heat flux during mode switching process of unitized regenerative fuel cells is importance. Thus, in this paper, the local temperature and heat flux are measured during mode switching using self-made thin film sensors, and heat transfer coefficient is also obtained. The results of the experiment exhibit that the temperature and heat flux change significantly during mode switching, and periodically change with periodic mode switching, the heat generation is greater than the heat dissipation inside the cell. During mode switching, the temperature and heat flux are higher in upstream, and the temperature and local heat transfer coefficient increase along main gas flow direction. The fuel cell current density has a great influence on heat transfer when cell mode switches. At different current density, the temperature and heat flux show different changing trend when the cell switches from electrolytic cell mode to the fuel cell mode. These experimental results can provide guidance to make fuel cell run more efficiently in practical application.

Segmented distribution of gas diffusion layer porosity and catalyst layer ionomer content in a polymer electrolyte membrane fuel cell

Gas diffusion layer porosity and ionomer content inside catalyst layer both can affect cell performance. Electrochemical reaction rate is non-uniform inside the cell. Non-uniform design of components or structures can improve the electrochemical reaction rate distribution and thus promote cell performance. Thus, the investigations on the optimal distribution are done using the optimization solver within COMSOL Multiphysics 5.3a to seek the optimal segmented distribution of porosity or ionomer content to obtain higher current density. The distributions in cathode side along three directions are all investigated. The optimized results show that the porosity increases along main gas flow direction and the ionomer content reduces. With voltage decreasing, the value of porosity around outlet region reduces and the value around outlet region increases, ionomer content shows opposite trend. The porosity under gas channel is higher than that under land. Ionomer content increases under land and reduces under gas channel with voltage decreasing. The porosity is high near catalyst layer and ionomer content near proton exchange membrane is higher at low voltages. The ionomer content distribution along thickness direction can improve current density by 5.596 % at 0.2 V. For better overall cell performance, the porosity changing along x-axis can be selected, the values from under land to under gas channel are 0.384, 0.499, 0.618. These results can provide theoretical guidance for cell components design to obtain higher cell output performance.

Influence of material in sheet-bulk metal forming from coil

AbstractSheet-bulk metal forming (SBMF) represents an innovative approach for the efficient manufacturing of functional integrated parts from sheet metal by applying bulk forming processes to sheet metal. It combines the advantages of part weight optimization and process chain shortening. In view of the increasing demand for functionally integrated components, the forming from coil offers an economical possibility to produce high output rates. Nevertheless, an underfilling of functional elements as well as an asymmetric part forming are current challenges. In order to apply SBMF from coil in industrial contexts, it is necessary to understand the causes of these process limits and to develop measures for improving the forming of functional elements and thereby push existing forming limits. In this paper, both a lateral and a backward extrusion process from coil for the forming of internal and external geared SBMF parts are investigated to develop a fundamental process understanding. In a combined numerical-experimental approach, the influence of the flow behavior of different materials as well as the impact of the main material flow direction of the process on the dimensional part accuracy is analyzed. This enables the evaluation of fundamental material-related dependencies and influences regarding the geometrical part accuracy, the mechanical component properties and the tool load for forming of the conventional deep-drawing steel DC04 (t0 = 2 mm), a high-strength steel material (HC260LA) and an aluminum alloy (AA6082 T6). Based on these findings, measures for material flow control are identified and investigated to counteract the challenges of SBMF from coil. These findings permit the identification of potential transferability of the identified cause-effect mechanisms to different processes, component geometries and materials. Thereby, in particular, an increase in coil width as well as lower a strain hardening behavior of the material offers the possibility to improve the geometrical accuracy of the components.

Use of process intensification concepts for targeted delivery of inhaled aerosolized medicines

Process intensification (PI) concepts have potential applications beyond the classic industrial problems solved by chemical engineering. This paper discusses the flow and mass transfer intensification using acoustic waves to enhance drug delivery to the nasal cavity. The efficiency of drug delivery to the nose still needs improvement, and the mechanisms related to using acoustic waves to increase it have yet to be studied. The influence of pressure pulsations induced by an acoustic wave (∼100 Hz) on the concentration, droplet size, and flow structure of the mists delivered from medical nebulizers was studied. The direct visualization showed that pulsations intensify aerosol deposition inside narrow channels due to particle displacement perpendicularly to the main flow direction. This motion also allows for particle penetration via narrow openings. UV-assisted observation of the intensified aerosol deposition inside the anatomical cast of the nasal cavity confirmed the importance of acoustic pulsations for intranasal drug delivery, also helping aerosol penetrate hard-to-reach areas (including the paranasal sinuses). A new idea regarding appropriate time control of acoustic pulsations has been proposed to improve the aerosol drug mass transfer in the nose. The study shows that PI has great potential to expand into new and interdisciplinary areas.

Modelled Water and Phosphorus Transports in the Archipelago Sea and through the Åland Sea in the Northern Baltic Sea and Their Links to Water Quality

The Baltic Sea is currently classified as ‘affected by eutrophication’. In this study, water exchanges and net phosphorus flows in the Archipelago Sea and through the Åland Sea from the Baltic proper was estimated with the aid of a 3-D hydrodynamic model for the years 2000–2021. The modelling configuration is based on the Copernicus regional reanalysis data. Water flowed from the Baltic proper to the Bothnian Sea at 669 km3/a and out from there at 879 km3/a. The inflow occurred in the deep-water layer (over 40 m), while the outflow occurred in the surface layer (0–40 m). With the inflow, 14,500 tons/a of phosphorus were transported during the years 2000–2014, but the amount increased to 20,300 tons per year from 2015 to 2021. At the same time, the winter-time concentrations of DIP in the Bothnian Sea almost doubled. In the Archipelago Sea, the main flow direction of water was from south to north from 2000 to 2014. From 2015 to 2021, the net flow direction reversed, and water flowed from the Archipelago Sea to the Baltic proper in the surface layer at a rate of 140 km3/a. At the same time, the background loading of phosphorus entering the Archipelago Sea with the flows decreased significantly and the chlorophyll-a concentration decreased below the threshold for a good ecological status. The U-turn in surface currents in the Archipelago Sea since 2015 may be related to variations in upwellings caused by climate change.

Thermal efficiency evaluation in shell-and-tube heat exchangers: A CFD-based parametric study

In this study, a novel approach was employed to enhance the performance of a Bowman brand EC-2028 shell-and-tube heat exchanger (STHE) by introducing perforated plates. The heat exchanger was first tested in a real system within a laboratory environment to establish baseline performance data. Computational fluid dynamics (CFD) analysis revealed that flow velocities in the middle tubes were higher due to their alignment with the main flow direction, resulting in decreased heat transfer efficiency. To address this issue, a perforated plate with concentric circles was introduced at the cold fluid entry. This plate, devoid of central holes, featured circles with radii of 9, 18, 27, 36, 45, 54, and 63 mm, each containing 6, 10, 17, 24, 30, 36, and 42 equidistant holes, respectively. The introduction of the perforated plate led to a decrease of approximately 0.33°C in the hot outlet temperatures, indicating an improvement in heat transfer efficiency. To further enhance performance, various configurations were tested by progressively closing holes from the innermost to the outermost circles. Eight different configurations, including the control, were evaluated under counterflow conditions. The CFD model was validated with experimental data before introducing the perforated plates, ensuring the accuracy of the simulations. The findings demonstrated significant improvements in heat transfer efficiency, with the optimized perforated plate configurations leading to more uniform flow distribution and reduced pressure drops. This study's novel approach of using perforated plates to modulate flow and enhance thermal performance highlights a new avenue for optimizing STHE designs, contributing to more efficient thermal management solutions in industrial processes.

Drag, lift, and torque correlations for axi-symmetric rod-like non-spherical particles in linear wall-bounded shear flow

This paper presents novel correlations to predict the drag, lift, and torque coefficients of axi-symmetric non-spherical rod-like particles in a wall-bounded linear shear flow. The particle position and orientation relative to the wall are varied to systematically investigate the influence of the wall on the hydrodynamic forces. The newly derived correlations for drag, lift, and torque on the particle depend on various parameters, including the particle Reynolds number, the orientation angle between the major axis of the particle and the main local flow direction, the aspect ratio of the particle, and the dimensionless distance from the particle centre to the wall. The impact of the wall on the hydrodynamic forces is accounted for as a function of a multiplication factor on the drag force in case of locally uniform flow, and an additional force contribution for the lift and the torque, modifying the resultant forces experienced by a particle in a locally uniform flow. The changes in the hydrodynamic forces prove to be substantial, emphasizing the necessity of accounting for wall effects across all particle types and flow conditions investigated in this study. The coefficients of the correlations are determined through a fitting process utilizing the data generated from our previous study on the interaction forces between a locally uniform flow and an axi-symmetric non-spherical rod-like particles, as well as from data of novel direct numerical simulations (DNS) performed in this work of flow past axi-symmetric rod-like particles near a wall. The proposed correlations exhibit a good agreement compared to the DNS results, with median errors of 2.89%, 5.37%, and 11.00%, and correlation coefficients of 0.99, 0.99, and 0.96 for the correlations accounting for the drag, lift, and torque coefficients of a non-spherical particle in wall-bounded linear shear flow profile, respectively. These correlations can be used in large-scale simulations using an Eulerian–Lagrangian or a CFD/DEM framework to predict the behaviour of axi-symmetric rod-like non-spherical particles in wall-bounded shear flow to locally uniform flow conditions.

Reynolds number effect on the Lagrangian evolution of hairpin vortices in turbulent channel flow

The evolution and generation of hairpins are studied in a Lagrangian perspective in turbulent channel flows. Direct numerical simulation database of turbulent channel flow at friction Reynolds numbers R e τ = 590, 390, and 180 is employed to perform tracking and interpolation algorithms of fluid particles on hairpins. These particles are carefully selected using a vortex line identification technique developed to extract the hairpin's core line. Interesting results are observed regarding the Reynolds number impact on the evolution and regeneration of hairpins. As the Reynolds number increases, the head and the vortical tongues are compressed and aligned in the main flow direction and the hairpin structures tend to orient more horizontally. The deformation shape of hairpins using the strain state parameter, which is correlated with the dissipation rate, is analysed using the probability density function. Most phenomena related to the generation of secondary hairpins, including the existence of vortical tongues, occur in the range 35 ≲ y + ≲ 50 . The results reveal that the Lagrangian perspective is a proper technique to address the extension of vortical tongues, generation of the secondary hairpin, and stretch of hairpin legs.

Hydro‐Sedimentary Processes of a Plunging Hyperpycnal River Plume Revealed by Synchronized Remote Imagery and Gridded Current Measurements

AbstractThe present knowledge of plunging hyperpycnal river plumes is mainly based on two‐dimensional (confined) laboratory experiments. Several hypotheses on three‐dimensional (unconfined) flow processes have been made, but not tested in situ. In this field study, the dominant three‐dimensional hydro‐sedimentary processes related to unconfined plunging were elucidated by synchronizing autonomous time‐lapse camera images with boat‐towed acoustic Doppler current profiler measurements. It was found that the flow field complies with two‐dimensional conceptualizations along the plume centerline. Perpendicular to the centerline, the plume slumped laterally due to its density excess and simultaneously converged laterally due to its vertical divergence. This combination led to a narrowing of the plume near the surface, resulting in a sediment‐rich triangle‐shaped pattern on the surface near the inflow, and a rather stable width near the bed. The formation of secondary flow cells transporting riverine water and suspended particulate matter (SPM) away from the plume near the bed, up toward the surface and back toward the plume near the surface, was revealed. An increase of the average SPM concentration and the SPM flux in the main flow direction indicates net sediment erosion under the investigated conditions of high discharge and sediment load. This suggests transient storage of sediment during conditions of lower discharge and sediment load, and high morphological activity in the plunging area. These findings allowed extending a classical conceptual plunging model to laterally unconfined sediment‐laden plunging inflows for conditions of well‐mixed ambient water in the plunging region and inflow densimetric Froude numbers exceeding 1, common in nature.

Numerical study on discharge capacity of piano key side weir with various ratios of the crest length to the width

Abstract A side or lateral weir can be defined as a longitudinal weir put in parallel to the main flow direction. A piano key side weir (PKSW) is one of the various side weirs used to control flow level, flow diversion, and flood harm prevention in dams and hydraulic systems. A side weir aims to keep the water level in the main channel at a specific level by discharging the overflow water into a side channel. The discharge coefficient of the PKSW was covered in this study by numerical modeling of a rectangular PKSW type B with various ratios of the crest length to the width in a straight channel. Results showed that the discharge coefficient of the PKSW was more affected by the L/W parameter when the other parameters were constant. It was noted that the PKSW discharge coefficient for L/W equal to 6 demonstrated a significantly higher level of performance and also found that increasing the upstream head above the side weir crest (h a/P) negatively affected the coefficient of discharge. It was concluded that a high capacity of the discharge coefficient required the (h a/P) ratio to be smaller than 0.75 or within the range (0.3 ≤ h a/P &lt; 0.75).

CYLINDRICAL CROSS-BARS FOR THERMAL PERFORMANCE AUGMENTATION IN AIR CHANNEL

Experiments of the heat transfer and pressure drop are performed in a rectangular channel employing eight different arrays of cylindrical cross-bars as inserts. Numerical simulations are also performed to examine the secondary flow caused by the cylinders. The objectives are to investigate the enhancement of the channel-wall heat transfer and pressure drop caused by the local flow, cylinder array geometry, and flow Reynolds number. Two different cylinder diameters of 1.0 mm and 2.0 mm are employed. The diameter of 2.0 mm is used to create four in-line arrays while the diameter of 1.0 mm is used to create four staggered arrays of the cylinders. The cylinder arrays employ different diameter-to-pitch ratios (0.025 to 0.2) and cylinder orientations (45&amp;deg;, 90&amp;deg;) relative to the main flow direction. The flow Reynolds number is varied between 600 and 13,000. Only two array geometries, one in-line and one staggered, with the cylinders oriented at 90&amp;deg; to the flow are modeled for the numerical study. The numerical results show that the local flow accelerates in the gap between the cylinder and channel wall. The vortex shedding downstream of the cylinders interacts with the channel wall. The Nusselt number on the channel wall and the friction factor are measured with and without the cylinders. The ratios of Nusselt numbers and friction factors increase with the Reynolds number when the Reynolds number is less than 3000. The ratios always increase with the diameter-to-pitch ratio.

Pore-scale insight into the effect of porous walls on fluid flow in rough-walled fractures

This study investigates the influence of porous walls on fluid flow behavior in single rough-walled fractures. A series of direct numerical simulations (DNS) were conducted to examine fluid flow through rough-walled fractures surrounded by porous walls under various hydraulic gradients. The simulation results clearly demonstrate the progressively significant impact of porous walls on flow characteristics as porosity increases. Regression analyses indicate that the Forchheimer's law, which characterizes the nonlinear flow of rock fractures, remains valid in the presence of porous walls. Moreover, increased porosity enhances fluid exchange and leads to an increased ability for single fractures to transmit fluid. Two double-parameter equations were established to quantitatively characterize the effects of porous walls on hydraulic conductivity of the fracture and fluid exchange between porous walls and the fracture. Further analysis revealed that the angle between the fracture profile and main flow direction profoundly influences the inflow/outflow exchange between the fracture and its adjacent porous walls. Moreover, the pore-scale results indicate that it is unreasonable to equate the effect of porous walls with slip boundaries as suggested in previous studies. The results and findings of this study deepen our understanding of flow behavior at the interface between the rough-walled fracture and adjacent porous walls, and they serve as a basis for accurately simulating large-scale fracture networks that consider the hydraulic contribution of fracture walls.

Study on the Impact of Offshore Wind Farms on Surrounding Water Environment in the Yangtze Estuary Based on Remote Sensing

Offshore wind farms (OWFs), built extensively in recent years, induce changes in the surrounding water environment. The changes in the suspended sediment concentration (SSC) and chlorophyll-a concentration (Chl-aC) induced by an OWF in the Yangtze River Estuary were analyzed based on Chinese Gaofen (GF) satellite data. The results show the following: (1) The flow near the wind turbines makes the bottom water surge, driving the sediment to “re-suspend” and be lost, deepening the scour pit around the bottom of the wind turbines, which is known as “self-digging”. The interaction between the pillar of a wind turbine and tidal currents makes hydrodynamic factors more complicated. Blocking by wind turbines promoting the scour of the bottom seabed of the OWF results in speeding up the circulation rate of sediment loss and “re-suspension”, which contributes to the change in the SSC and Chl-aC. This kind of change in sediment transport in estuarine areas due to human construction affects the balance of the ecological environment. Long-term sediment loss around wind turbines also influences the safety of wind turbines. (2) The SSC and Chl-aC are mainly in the range of 200–600 mg/L and 3–7 μg/L, respectively, in the OWF area, higher than the values obtained in surrounding waters. The SSC and Chl-aC downstream of the OWF are higher than those upstream, with differences of 100–300 mg/L and 0.5–2 μg/L. High SSC and Chl-aC “tails” appear downstream of wind turbines, consistent with the direction of local tidal currents, with lengths in the range of 2–4 km. In addition, the water environment in the vicinity of a wind turbine array, with a roughly 2–5 km scope (within 4 km during flooding and around 2.5 km during ebbing approximately) downstream of the wind turbine array, is impacted by the OWF. (3) In order to solve the problem of “self-digging” induced by OWFs, it is suggested that the distance between two wind turbines should be controlled within 2–3.5 km in the main flow direction, promising that the second row of wind turbines will be placed on the suspended sediment deposition belt induced by the first row. In this way, the problems of ecosystem imbalance and tidal current structure change caused by sediment loss because of local scouring can be reduced. Furthermore, mutual compensation between wind turbines can solve the “self-digging” problem to a certain extent and ensure the safety of OWFs.