Flow Characteristics In Channel Research Articles

Analysis of open channel flow with various layered vegetation using CFD, considering different near-wall treatment methods

PurposeThe purpose of this paper is to investigate the impact of near-wall treatment approaches, which are crucial parameters in predicting the flow characteristics of open channels, and the influence of different vegetation covers in different layers.Design/methodology/approachAnsys Fluent, a computational fluid dynamics software, was used to calculate the flow and turbulence characteristics using a three-dimensional, turbulent (k-e realizable), incompressible and steady-flow assumption, along with various near-wall treatment approaches (standard, scalable, non-equilibrium and enhanced) in the vegetated channel. The numerical study was validated concerning an experimental study conducted in the existing literature.FindingsThe numerical model successfully predicted experimental results with relative error rates below 10%. It was determined that nonequilibrium wall functions exhibited the highest predictive success in experiment Run 1, standard wall functions in experiment Run 2 and enhanced wall treatments in experiment Run 3. This study has found that plant growth significantly alters open channel flow. In the contact zones, the velocities and the eddy viscosity are low, while in the free zones they are high. On the other hand, the turbulence kinetic energy and turbulence eddy dissipation are maximum at the solid–liquid interface, while they are minimum at free zones.Originality/valueThis is the first study, to the best of the author’s knowledge, concerning the performance of different near-wall treatment approaches on the prediction of vegetation-covered open channel flow characteristics. And this study provides valuable insights to improve the hydraulic performance of open-channel systems.

The effect of compound channel shape for local scouring and flow dynamics using NaysCUBE-3D RANS at Downstream Tukad Unda River Bali Province

The prediction of sediment transport and flow dynamics are important issues in channel design as the local scouring has an important effect on the shape of the channel, especially on the compound channel. Tukad Unda River in Bali has river section in the form of compound channel. It consists of two sections with different dimension, the main channel and the flood plain. These dimensional differences cause two different flows. Transformation of flow velocity and turbulence cause movement of sediment transport, generates a complex 3D flow structure in the boundary of main channel and floodplain. This study uses morphology data in Tukad Unda River and simulated by using NaysCUBE. NaysCUBE is a three-dimensional numerical modeling which use the Reynold Average Navier Stoke equation (3D RANS model). The result in this study shows the different pattern of flow dynamics and local scouring in the boundary between main channel and floodplain as the concrete block located in that boundary compare to the nature compound channel. In the 40-minute simulation, the nature compound channel represented significant deformation of the river bed in the interface area, with a decrease in elevation of 0.01 meters higher than the compound channel with concrete blocks. while the increase in elevation in the main channel is around 0.09 meters lower than the compound channel with concrete blocks. Application of concrete blocks can be useful to reduce local scour at the interface as the impact of compound channel flow characteristics.

Incipient Motion of Bed Material in a Channel with Varying Width and Vegetated Channel Walls

This experimental study aims to investigate the characteristics of turbulent flow in channels with vegetated banks and varying channel width under the condition of the incipient motion of bed material. The natural reeds were used as emergent vegetation on the sidewalls of a laboratory flume. In total, nine experimental runs have been conducted with different experimental setups by using three different particle sizes of bed material and three different channel bed slopes. An Acoustic Doppler velocimetry (ADV) was used to acquire velocity components in three directions. The results of this study indicate that the streamwise velocities have the maximum and minimum values at the cross sections with the narrowest and widest width, respectively. When the aspect ratio is less than 5, the maximum velocity occurs below the water surface, due to presence of the secondary currents. It is found that, at all measurement points, the distribution of the Reynolds shear stress has a Z-shaped profile owing to presence of vegetation on the channel sidewalls. By extrapolating the profiles for flow velocity and Reynolds shear stress towards the surface of the channel bed, the near-bed incipient velocities and the corresponding shear stresses for the incipient motion have been determined. By increasing the channel bed slope, the estimated near-bed parameters for all particle sizes decreased, indicating the dominance of the gravity effect over the pressure gradient effect. It was also observed that the Shields method was invalid for assessing the incipient motion of bed material in the presence of vegetation on the sidewalls of a channel that has a varying width.

Reconciling bathymetric and stratigraphic expressions of submarine channel geometry

Modern submarine channels form distinctive morphological features on the seafloor and play a critical role in shaping the marine sedimentary record. Recent studies have captured the extremely diverse range of cross-sectional geometries in submarine channels from bathymetric data, which typically display aspect ratios markedly different to the stratigraphic record of ancient submarine channels. Here, we compare and reconcile the relationship between the geomorphic expression of submarine channels as observed on the seafloor and the geometry of their stratigraphic bodies as mapped in seismic-reflection data, using the Niger Delta slope an exemplar. For the same channels, our data allows us to contrast the distribution of widths, depths, and aspect ratios from bathymetric data and at two hierarchical scales in the underlying stratigraphy – the channel element and channel system scale. Channel characteristics are also contextualised with respect to two key variables, the underlying structural template and the relative timescale for which the studied systems have been active. Analysis of the seafloor bankfull geometries highlights substantial variability with widths ranging from ∼300 m to ∼4 km and aspect ratios from ∼10:1–100:1. In contrast, the geometry of stratigraphic channel element bodies remains remarkably consistent across the three channels with widths ∼480–620 m and aspect ratios of ∼9:1. At channel system scale stratigraphic width is comparable to that seen in the bathymetric data, but with aspect ratios of 6–23:1. Our results therefore highlight a marked disparity in the cross-sectional geometries on the present-day seafloor and for their associated channels in the stratigraphic record. We demonstrate that a large part of the disparity between modern and ancient submarine channel geometries may be explained by post-abandonment modification of the seabed channels where there is reduced Holocene activity and we argue this effect likely plays a role in the differences seen in global data sets. These results have significant implications for the use of bankfull process analogues when applied to bathymetric data to estimate submarine channel flow characteristics.

Numerical investigation of the effect of hot-water outlet inclination angle on the temperature dilution in open channel flow

In this study, the authors investigated the environmental impact of discharging hot water from a thermal power plant into a water source for cooling. Three hundred forty-three runs using two-dimensional numerical ANSYS Fluent simulation model with (k- ε) turbulence model and open channel flow characteristics was applied in this research. The single port submerged discharge inclination angle was studied with different scenarios for the hot-water outlet (velocity and temperature) to determine the optimum inclination angle to increase cooling system efficiency and reduce hot water's environmental impact. It was discovered that by increasing the inclination angle and velocity of the hot-water outlet, the efficiency of cooling system is increased, as it increased the dilution of the hot water and reduced its environmental impact. Specifically, inclination angle of 15° increased the dilution by 80% and 86% in x and y directions, respectively compared to angle 90°. Additionally, the temperature variation in water body induced by hot-water outlet was reduced by counter flow at angle −15°. The velocity profile showed a recirculation region at the right side of the hot-water outlet, which was reduced at higher inclinations. A mathematical equation was developed to estimate the temperature value in the open channel water body based on the study parameters. Overall, the study found that increasing the inclination angle of the hot-water outlet can significantly improve the cooling system's efficiency and reduce the environmental impact of the hot water discharge. Counter flow conditions were unsuitable because they have less dilution than the normal flow conditions.

Protracted melt-present deformation during the Rigolet phase of the Grenvillian Orogeny. Insights from geochronology along the highway 117 transect through the Grenville Province in western Quebec, Canada

In this contribution we use U-Pb zircon geochronology to investigate the age of melt-present deformation within the various structural levels of the Mesoproterozoic Grenville Orogen that are exposed along the Highway 117 transect in western Quebec, Canada. Samples include six syn- to late-deformational felsic leucosomes and injected veins. Four of them are physically linked with hornblende-bearing leucosome in their host gneiss and all six contain undeformed quartz grains, implying they were deformed while still partially molten. Zircon from all samples reveal complex morphologies defined by inherited cores and successions of texturally distinct zones, that mostly share trace element characteristics and yield overlapping Grenvillian 207Pb/206Pb weighted mean ages between ca. 1020–968 Ma. A crustal panel charactered by pervasive migmatites that present evidence for protracted suprasolidus deformation, are key characteristics of channel flow during the youngest phase (Rigolet) of the Grenville Orogen. Field data also indicate that, coeval to deformation in the lower structural level, felsic dykes brecciated mafic components within the upper level; these components were subsequently incorporated into the ductile channel as decametre-sized mafic boudins. Our findings are comparable to recent models advocating Rigolet ductile flow within the Parautochthonous Belt in eastern Quebec but differ from the current conceptual models viewing proposed for the evolution of the orogen during this phase.

Numerical study of bionic parallel mini-channel heat sink applied to the cooling of a linear Fresnel photovoltaic system

This paper proposes a parallel mini-channel with bionic structure (BPM-channel) heat sink for linear Fresnel reflector photovoltaic systems (LFR-PV system), which can be rotated axially to trace the solar ray. In this paper, the design of rectangular channel as well as a mini-channel with a bionic structure is optimized. The thermal enhancement factor of the horizontally placed BPM-channel and the rectangular channel, the influence of the rotation angles on the channel flow characteristics and liquid filling are investigated. The research results show that the thermal enhancement factor of the BPM-channel can be increased by up to 50%. The rotation angle has little effect on the inlet pressure, and the pressure change range is within 4%. Within the current operating condition (volume flow rate 70mL/min-190mL/min, the rotation angle ±45°), the smallest flow resistance can be found when the characteristic dimension of the bottom structure is 1mm (Dr=1mm) in the BPM-channel. The BPM-channel with Dr=2mm has the best liquid filling performance.

Numerical Investigation of the Effect of Rigid Vegetation at an Inclined Bank, on Streamflow Hydrodynamics

Abstract This study is a computational analysis of hydrodynamic alterations of flow such as secondary current, turbulent kinetic energy, and shear stress, in an open channel with an increased vegetation density at an inclined bank. Two arrangements of rigid vegetation at an inclined bank, linear and staggered, are investigated for five different vegetation density. The computational analyses are performed under constant flow and stable bed conditions. The variations of stream wise flow velocity, turbulent kinetic energy, and Reynolds stress along the main channel are evaluated and presented for each vegetation density. The results illustrated the impacts of vegetation density on the entire channel flow characteristics. These results are further analyzed based on average bed shear stress calculations. The main outcome of the study demonstrated that combined effect of inclined bank and effect of partly obstructed flow significantly generates Kelvin-Helmholtz type instability due to the presence of inflection point at different location along a vegetation cover. The calculated Reynolds stresses are higher at the main channel bed near the vertical bank, but shift to midflow depths near the boundary between the main channel and inclined bank. The turbulence kinetic energy profiles in the main channel and at the inclined bank exhibited results that were similar and parallel to the simulated stream wise velocities and Reynolds stresses. Furthermore, it is found that the ratio between bed shear stress and turbulent kinetic energy is almost 0.3 as the depth approaches the channel bed.

Numerical simulation of the unsteady flow field in the human pulmonary acinus

Understanding of airflow dynamics in the human pulmonary acinus is important for increasing targeted drug effectiveness and determining the health impact of toxic aerosols. However, there is a lack of quantitative data about the pulmonary airflow in realistic and flexible idealized geometries. This paper aims to numerically analyse the flow field of the pulmonary acinus using the computational fluid dynamics (CFD) model during transient breathing. Three-dimensional models with rhythmically expanding-contracting alveolar walls were developed for representing the pulmonary region of the human lung. Three different breathing scenarios were applied in the CFD simulations. The results showed that the transient flow conditions determined the transitions between flow types. The recirculating flow in the alveoli was observed for all cases and it was determined that its intensity depended on the breathing scenario. The flow velocity in the alveoli was slower than that of the channel flow. As we moved deeper into the lung, the flow pattern inside the alveoli exhibited a radial velocity profile. It was found that the alveolar flow exhibited a typical stenotic channel flow characteristics. As a result, the acinus models used in this study takes into account the alveolar wall motion based on physiological breathing conditions. To simulate or estimate the airflow dynamics, thus, the results obtained in this study can be easily utilized in the human lung airway models.

Numerical Modelling of Flow Characteristics in an Asymmetric Trapezoidal Compound Channel with Vegetation Patches

Vegetation is an important component of rivers, present naturally alongside the banks. In this research study, three-dimensional computational fluid dynamics (CFD) code ANSYS FLUENT was used to solve the Reynolds Averaged Navier-Stokes equations and to simulate flow features in an asymmetric vegetated compound channel. Three-dimensional flow characteristics and turbulence properties were captured well by the Reynolds stress turbulence model (RSM). A total of five flow cases were considered in this work. Among these, four cases were of circular vegetation patches present on the floodplain edge and one was of non-vegetated asymmetric trapezoidal compound channel. Impact of vegetation patch density, patch center to center (c/c) spacing and flow blockage area has been investigated on various flow features and compared with flow characteristics in a channel without vegetation. Flow features include mean and depth-averaged velocities, secondary flows, Reynolds shear and normal stresses, turbulence intensities and turbulent kinetic energy. The results show that the presence of vegetation patches along the floodplain imparted flow resistance which shifted the zone of higher velocities towards outer wall of main channel and floodplain. Increased velocities in the main channel up to 24.8% was observed for larger flow blockage (aD, where a is the density of the vegetation patch and D is the diameter of patch) and smaller normalized c/c spacing (L/D, where L is the c/c spacing between two patches and D is the diameter of patch) of patches. Large transverse shear stresses existed around the vegetated zone due to the gradient of velocities and vortices in the main channel and floodplain. Floodplain, especially vegetated zone acquires less Reynolds stresses and turbulence making it favorable for deposition of sediments and growth of ecological species.

Effect of Expansion Ratio on Fluid Flow Characteristics of Channel Flow with a Backward-Facing Step

Effect of Expansion Ratio on Fluid Flow Characteristics of Channel Flow with a Backward-Facing Step

Impact of sand mining on alluvial channel flow characteristics

Mining pit in a channel bed causes disturbance to the flow characteristics. To interpret the phenomenon of change in flow characteristics in a mining pit, experimental study was performed in a tilting laboratory flume. This research focuses on the impact of mining pit on different turbulent parameters, such as flow velocity, Reynolds shear stress distribution, turbulent kinetic energy fluxes, and higher order moment distribution. We have observed dropped in flow velocity in the pit and also significant reversal velocity layer at the pit bottom. The analysis of Reynolds shear stress and higher order moment distribution shows higher values of velocity fluctuations in the pit than the upstream section. The response of bursting events, which is identified by the third order moment, shows influence of sweep events near the bed in all four sections. Third order moment analysis also shows a rise in dominance of sweep events along the flow depth in the pit and it’s downstream, compared to the upstream section. The kurtosis distribution shows highly intermittent nature of turbulence close to the channel bed. The longitudinal turbulent kinetic energy fluxes and vertical turbulent kinetic energy fluxes are in downstream and downward directions for both inner and outer layer in the pit region, which shows an increase in bed particle mobility in that region, whereas it is present only in the inner layer at the upstream section. Additionally, improved bed load transport equation by incorporating pit geometry has been proposed for alluvial channel subjected to sand mining.

Flow Characteristics in Coaxial Channels with Three Helical Fins of Annular Gap

The helical coordinate system is used for mathematical description of laminar viscous fluid flows in channels with three helical fins of annular gap. The mathematical model in which the equations of motion are written in projection on the directions of natural basis of the helical coordinate system is suggested. The distributions of flow characteristics in channels with long triple screw inserts of various geometry are analyzed on the basis of the numerical results obtained for the Reynolds numbers up to 806.

Improvement of surface run‐off in the hydrological model ParFlow by a scale‐consistent river parameterization

AbstractWe propose an improvement of the overland‐flow parameterization in a distributed hydrological model, which uses a constant horizontal grid resolution and employs the kinematic wave approximation for both hillslope and river channel flow. The standard parameterization lacks any channel flow characteristics for rivers, which results in reduced river flow velocities for streams narrower than the horizontal grid resolution. Moreover, the surface areas, through which these wider model rivers may exchange water with the subsurface, are larger than the real river channels potentially leading to unrealistic vertical flows. We propose an approximation of the subscale channel flow by scaling Manning's roughness in the kinematic wave formulation via a relationship between river width and grid cell size, following a simplified version of the Barré de Saint‐Venant equations (Manning–Strickler equations). The too large exchange areas between model rivers and the subsurface are compensated by a grid resolution‐dependent scaling of the infiltration/exfiltration rate across river beds. We test both scaling approaches in the integrated hydrological model ParFlow. An empirical relation is used for estimating the true river width from the mean annual discharge. Our simulations show that the scaling of the roughness coefficient and the hydraulic conductivity effectively corrects overland flow velocities calculated on the coarse grid leading to a better representation of flood waves in the river channels.

Lattice Boltzmann simulations of liquid flows in microchannel with an improved slip boundary condition

Flow characteristics in channels are critical in chemical engineering. Slip velocity has a great influence on the microchannel flows. The lattice Boltzmann simulations of some typical channel flows are conducted with an improved slip boundary condition. Through theoretical analysis, the defects of the existing slip boundary conditions are clarified. To solve these problems, the slip boundary condition of Kuo et al. is ameliorated by considering the tangential momentum change generated by the external force. Besides, the Navier’s slip model is adopted to specify the improved slip boundary condition. With this improved scheme, we study five cases, namely the Couette flow, the force driven Poiseuille flow, the time-dependent force driven Womersley flow, the porous plate flow and the channel flow with a surface-mounted block. Our method is accurate and reliable enough even with a coarse grid. The mass leakage at the wall is avoided and the numerical error generated by the external force is eliminated.

Macroscopic Modeling of Liquid Water Based on Detailed Two-Phase CFD and Application on Full-Cell Scale Multiphysics Simulation

Overview In this work, full-cell scale simulations of PEFC are performed. The simulations deal with the behavior of liquid water in channels and porous structures in GDL and MPL, where the behavior is largely affected by geometry of channels and surface wettability. In order to execute efficient simulations in full-cell scale, all relevant macroscopic transport phenomena in PEFC (mass, chemical species and heat) are considered, where the phenomena are coupled with electrochemical reactions. Computational models for full-cell scale simulations are constructed based on the macroscopic transport models. In order to take into account liquid water flow depending on configurations of channels and porous structures, physical characteristics of liquid flow in channels are extracted from detailed two-phase CFD with resolved single channel with the fine computational mesh whose resolution is order of 10 micrometers. With these simulation techniques, we can estimate effects of liquid water on performances of PEFC. Numerical Modeling and Results A hybrid simulation method of the full-cell scale engineering model simulation and the detailed two-phase CFD is introduced. In order to estimate performance of PEFC cell taking all relevant transport phenomena (mass, chemical species and heat) is solved on the coarse-grained mesh, coupled with electrochemical reactions. The mass transport equation is solved by Darcy’s law, using the distribution of the equivalent hydraulic parameter in flow channels estimated by detailed gas phase CFD. Here mass transport is coupled with heat and chemical species transport equations. Those all transport phenomena are coupled with electrochemical reactions in the MEA. Transport of chemical species and water through the MEA are also considered. Many engineering models are employed in order to consider various transport phenomena, like water up-take into electrolytes and effective transport of oxygen from gas phase to reaction cites in catalyst layers, etc. Electrochemical reactions are modeled by Butler-Volmer equation, in the manner of lumped parameter models. In order to apply this method for conditions in which liquid water is important, two-phase fluid models should be constructed carefully. In our approach, the behavior of liquid water in channels is simulated in advance by the conventional volume-of-fluid model on the fine mesh whose resolution is order of 10 micrometers. Characteristics of gas and liquid transport under the presence of liquid water is extracted as two-phase permeability depending on volume fraction of liquid water and gas flow velocity. Those permeability are imported to the full-cell scale multiphysics simulation on the coarse-grained mesh. Results of calculations show dependence of cell performance on behavior of liquid water, which is effected by geometry of channels and wettability. This method turns out to be efficient to estimate cell performance and all relevant transport phenomena taking liquid water into account.

Performance comparison of four turbulence models for modeling of secondary flow cells in simple trapezoidal channels

Due to the importance of channel flow characteristics in the water conveyance, the study of it is a noteworthy problem for hydraulics experts and much attempts has been accomplished for the modeling of the flow characteristics. One significant problem in this respect is the secondary flow cells and their effect on flow specifications. Widespread experimental and analytical investigations have been accomplished on this phenomenon. However, researchers are trying to replace the expensive and time-consuming experimental approaches and ad hoc analytical models with numerical simulation procedures using computational fluid dynamics (CFD). Selection of the proper turbulence model is one of the most important problems for this type of the numerical modeling. In the present study, after evaluating several turbulence models including k–ε, shear stress transport (SST) and three versions of the Reynolds stress model (RSM) (i.e. LRR-IP, LRR-QI, and SSG models), for the numerical simulation of the secondary flow cells and their effects on trapezoidal channels flow, the more efficient model was selected. Available experimental data and theoretical model was used to validate the selected turbulence model. The results were validated in terms of the free water surface, depth-averaged velocity, and boundary shear stress. The results confirmed the performance and efficiency of SSG version of the Reynolds stress model for the numerical modeling of the secondary flow cells.

Application and Design of Multi-Impingement Cooling Channel in Turbine Blade Trail Edge

Abstract The numerical simulations are used to conduct the comparative study of pin-fins cooling channel and multi-impingement cooling channel on the heat transfer and flow, and to design the multi-impingement channel through the parameters of impinging distance and impingement-jet-plate thickness. The Reynolds number ranges from 1e4 to 6e4. The dimensionless impinging distance is 0.60, 1.68, 2.76, respectively, and the dimensionless impinging-jet-thickness is 0.5, 1.0, 1.5, respectively. The endwall surface, pin-fins surface, impinging-jet-plate surface are the three object surfaces to investigate the channel heat transfer performance. The heat transfer coefficient h $h$ and augmentation factor N u / N u 0 $Nu/N{u_0}$ are selected to measure the surface heat transfer, and the friction coefficient f $f$ is chosen to evaluate the channel flow characteristics. The impinging-jet-plate surface owns higher heat transfer coefficient and larger area than pin-fins surface, which are the main reasons to improve the heat transfer performance of multi-impingement cooling channel. Reducing the impinging distance can improve the endwall surface heat transfer obviously and enhance impingement plate surface heat transfer to some extent, decreasing the thickness of impinging-jet-plate can significantly increase its own heat transfer coefficient, which both all increase the cooling air flow loss.

Experimental investigation of the flow characteristics beneath partial ice covers

In northern climates where water bodies are subjected to sub-freezing temperatures for part of the year it is important to know how the presence of ice impacts the flow characteristics of channels. While many studies have been conducted in channels that are fully open or completely covered with ice, little is known about the hydraulic characteristics of channels with only a partial cover. The goal of this study was to examine the impact of border ice on the discharge and corresponding streamwise velocity distribution in a channel. Using acoustic Doppler velocimetry, detailed three-dimensional velocity measurements were collected in a fully developed cross section of a rectangular flume under steady flow conditions with a symmetrical ice cover extending from both walls. The influence of different extents of partial ice coverages and different Froude numbers was explored. The partial ice cover was found to impact the flow distribution within the channel and a classification system was developed to divide the channel cross-section into open, transitional and covered sections. Partial ice coverage extent was found to have a relative width of transition to be consistent while increasing Froude was found to produce a smaller transition region. Boundary shear stress distributions on the boundaries were examined and a detailed analysis beneath the ice cover is presented. The presented data set represents the most thorough experimental examination of flow characteristics beneath a partial ice cover.

Hydraulics Near Unscreened Diversion Pipes in Open Channels: Large Flume Experiments

AbstractMost of the water diversions on the Sacramento and San Joaquin Rivers (California, United States) and their tributaries are currently unscreened. These unscreened diversions are commonly used for irrigation and are potentially harmful to migrating and resident fishes. A large flume (test section: 18.29 m long, 3.05 m wide and 3.20 m high) was used to investigate the hydraulic fields near an unscreened water diversion under ecologically and hydraulically relevant diversion rates and channel flow characteristics. We investigated all combinations of three diversion rates (0.28, 0.42, and 0.57 m3/s) and three sweeping velocities (0.15, 0.38, and 0.61 m/s), with one additional test at 0.71 m3/s and 0.15 m/s. We measured the three‐dimensional velocity field at seven cross sections near a diversion pipe and constructed regression equations of the observed maximum velocities near the pipe. Because the velocity components in three directions (longitudinal, transverse, and vertical) were significantly greater near the diversion pipe inlet compared with those farther from it, they cannot be neglected in the modeling and design of fish guidance and protection devices for diversion pipes. Our results should be of great value in quantifying the hydraulic fields that are formed around fish guidance devices to design more effective protection for fishes from entrainment into unscreened water‐diversion pipes.