Meandering Behavior Research Articles

PERTURBATION IN MEANDER BEHAVIOUR OF A SUBTROPICAL RIVER IN INDIA UNDER VARIABLE NATURAL AND ANTHROPOGENIC CONTROLS

Meander morphology is perturbed worldwide by human interventions to a certain extent in the Anthropocene. To this end, the present study aims to investigate the meander behaviour, including its deformations, of the Kopai River, a subtropical river in India in the last 50 years (1970-2020). A total of 394 loops (134 loops each in 1970 and 1995 and 126 loops in 2020) were selected for the four river reaches – the upper, middle-upper, middle-lower, and lower reaches. The meandering behaviour was assessed using Mueller’s sinuosity index (SI), meander form index, meander shape index, and radius/wavelength ratio. The study found a higher meandering tendency in the lower and upper reaches controlled by fluvial hydraulics, and topographic factors. However, the middle reaches were comparatively stable. The meander loops along the middle reaches regular meander is the dominant meander type, however, more than 50% of meander loops for the lower reaches were intense meander types. Positive extensions were the predominant type of meander deformation in all the reaches. However, few negative extensions are found in the middle-upper and lower reaches, while irregular changes and cut-offs were only found in the lower reach. Thus, the upper and lower reaches exhibited more natural controls dotted with regular meander progression. The middle reaches characterized by higher Bouguer anomaly and steep basement gradient led to channel confinement. Besides, profuse sand mining and the brick kiln industries are perturbing the meander beahviour through human channel straightening, dwarfing the meander evolution in the middle reaches.

Influence of wake meandering path on floater motions

This study shows how different versions of the dynamic wake meandering (DWM) model will result in different dynamic behaviour of a floating wind turbine. In this study, different softwares as well as different input wind boxes are applied. The effect of these variations were studied using statistics from both the meandering behaviour of the wake and the response of a floating wind turbine. The yaw response is the most sensitive, and this study shows that the standard deviation of the yaw response is between 10% to 60%, depending on the DWM options used when generating the wake wind field.

The Brazil current cyclonic meandering and shelf-slope water exchanges at 27°S–31°S

The Brazil Current (BC) experiences intense mesoscale activity from its origin on the northeastern Brazilian coast till it reaches the Brazil-Malvinas Confluence. The BC meandering behavior near the Cape Santa Marta (27°S – 31°S), has not been well described in the literature. In this study, in situ observations and satellite images are used to describe the BC meanders and cyclonic eddies near the cape. The results obtained from two Lowered Acoustic Doppler Current Profilers (LADCP) sections show that these structures are surface intensified but extend through the whole water column. Conductivity Temperature and Depth (CTD) profiles show evidence of coastal water trapping inside an eddy. Altimeter data show two hot spots of cyclones, one at 28.5°S and the other at 30.5°S, with an average of ∼5 and 7 eddies per year, respectively. The eddies generated on both sites have a mean lifetime of 37 days, and no significant differences were found between their surface properties. Satellite images of chlorophyll show that coastal water horizontal advection is a recurrent phenomenon in these eddies, imprinting an eddy stirring signal into their average composites. Furthermore, using Empirical Orthogonal Functions to analyze the BC flow in a mooring line, we observed that the mesoscales meanders are responsible for explaining at least 1/3 of the flow variability in this area.

Synchrotron X-ray measurements of cellulose in the cell wall of aged wood under uniaxial loading

Abstract The purpose of this study is to examine the effect of wood aging on the mechanical behavior at the cell-wall level. Synchrotron X-ray diffraction (XRD) measurements were performed to investigate the mechanical behavior of cellulose in the S2 layer of aged wood from a 250-year-old attic cabin beam. XRD measurements under uniaxial loading were performed on the cellulose (004) plane; the results were compared with those of recent wood. The cellulose in aged wood exhibited a delayed response to both tensile and compressive loading compared with recent wood. Under compressive loading, cellulose showed an increase in maximum strain and a significant increase in the variation of cellulose orientation angle, indicating that it may exhibit buckling-like meandering behavior. These results could be explained based on the decrease in hemicellulose content in the cell wall due to wood aging. In other words, the results suggested that an amorphous material in the cell wall affected the mechanical behavior of wood at the cell-wall level.

The influence of urbanization on the morphology of the Barak River floodplain in Cachar District, Assam

AbstractIn the present study, the meandering behavior of the Barak River (Cachar), Assam was traced. Flow path length, meander neck length, sinuosity index, and river migration were determined segment-by-segment to demonstrate morphological changes. Decadal land-use land cover (LULC) maps were also prepared at the section scale using Landsat data (1990–2020) validated with the Kappa coefficient to characterize changes along alluvial floodplains. Urban growth and agricultural activities affect river morphology, especially where intensive agriculture was recorded, according to LULC studies. Due to urbanization, forestation constantly decreased, causing river variability. According to the results of the present study, river migration is very slow between Sec 1 and Sec 3. In terms of river stability, the parts are more stable, particularly Sec 1, which features less urbanization and agricultural activity than the other sections. The most vulnerable segments within the study area were considered to be Sec 2 and 4. There is a rather large amount of migration within sections, especially segments CC and GG. The river segments became more vulnerable as a new oxbow lake was formed and LULC changed over a decade. This stretch of Barak is characterized by a broad alluvial floodplain and is shifting since this study applies to various meandering types.

Identification and analysis of very-large-scale turbulent motions using multiscale proper orthogonal decomposition

A multiscale proper orthogonal decomposition (mPOD) has been used to decompose the multiscale features of very long turbulent channel flow. The very-large-scale motions (VLSM) can be clearly visualized, even for flow at relatively low Reynolds number. A new energetic mode, called eVLSM, has been identified, which contains substantial energy. The large-scale structures (apart from eVLSM) are inclined to the streamwise direction and appear to be responsible for the typical meandering behavior or even for the breakup of VLSM.

Dynamics of meandering rivers in finite-length channels: linear theory

Meandering channels are dynamic landforms that arise as a result of fluid mechanic and sedimentary processes. Their evolution has been described by the meander-morphodynamic equations, which dictate that channel curvature and bed topography give rise to local perturbations in streamwise fluid velocity, prompting the preferential erosion and sediment deposition that constitute meander behaviour. Novel mathematical conditions are presented to guarantee unique solutions for the linearized equations in non-periodic domains with finite boundaries. With the boundary condition specification sufficient for the uniqueness proof one finds a well-posed initial-boundary-value problem amenable to standard numerical techniques for partial differential equations. This provides a pathway for improved numerical algorithms for simulations of meandering river dynamics. Previous theoretical analysis for linear stability theory in meandering dynamics has been restricted to spatially periodic systems. The present effort develops new results for linear stability theory in non-periodic systems with temporal driving at system boundaries as well as non-homogeneous initial conditions. Predictions for temporal driving at the inlets for non-periodic finite domains provide clarification for observed behaviour in laboratory flumes where driven conditions at the inlet avoids the long-term decay of all meanders observed in flumes with fixed entry conditions. Linear stability theory for finite domains confirms that a continuous perturbation is required for sustained meandering. Original scaling arguments are presented for the dependence of the meander migration rate on geological parameters, showing that the rate of channel migration increases with increased width, downreach slope and bank erodibility, and decreases with increased volumetric flow rate.

Oil/gas jets in water crossflow: The impact of the droplet size

AbstractDuring undersea oil blowout in crossflow conditions, the oil droplets entrained horizontally which increased the residence time of droplets in the water column. Knowledge of the trajectory of an oil plume is important for predicting the pathways of hydrocarbons and to devise countermeasures. We conducted large-scale experiments in the Ohmsett tank where we released oil from a one-inch vertical orifice that was towed to produce the behavior of a jet in crossflow. The average oil velocity at the orifice was 1.36 m/s and the crossflow velocity was around 0.27 m/s which resulted in a jet-to-crossflow velocity ratio of 5.0. The results were simulated numerically using the Large Eddy Simulation (LES) turbulence model and the mixture multiphase model within the open-source software OpenFOAM. The instruments including ADVs, LISSTs, shadowgraph cameras, holographic camera, and fluorometers were employed. The oil jet released from the nozzle started to meander in the vertical direction most probably due to weak crossflow. The trajectory and meandering behavior of the oil jet, wavy pattern along the leading edge of the jet and column breakup observed in the experiments were captured well with the numerical simulation. The surface breakup just above the orifice created ligaments and droplets downstream of the jet. Larger oil droplets were observed near the upper boundary of the plume due to their higher buoyancy while the smaller droplets were suspended in the water column and they were entrained by water crossflow. This work reveals that different size of droplets determines the overall shape of plumes mostly the upper and lower boundaries of the plume.

An experimental study to characterize the effects of initial ice roughness on the wind-driven water runback over an airfoil surface

In the present study, an experimental study was conducted to characterize the effects of initial ice roughness on the transient behaviors of surface water/ice run-back over a NACA 23012 airfoil model under a glaze icing condition. The experimental study was conducted in the Icing Research Tunnel of Iowa State University (i.e., ISU-IRT). A digital image projection (DIP) technique was applied to provide non-intrusive, temporally-and-spatially-resolved measurements of the thickness distributions of the dynamic water/ice flows over the airfoil surface. Two typical surface morphologies were observed for the surface water runback over the airfoil models: water film flow and water rivulets flow. While the surface water film flow modulated with one primary wave and multiple secondary waves is observed at lower wind speed conditions (i.e., U∞ = 10 m/s), the water rivulets flow is observed at relatively higher wind speed conditions (i.e., U∞ = 15 m/s). The initial ice roughness is found to retard and shorten the primary wave formation in the water film flow. It is also found that the initial ice roughness could trap and decelerate the water flow and decrease the inertia force in the film front, which essentially delays the formation of the rivulets. The water rivulet flow trapped by the initial roughness was found to have a meandering behavior, due to which, the initially formed narrow rivulets merged into wider rivulets as they move downstream. By recognizing the film/rivulets boundary during the dynamic surface water/ice runback process, the quantitative details were extracted, i.e., the formation, transition, and development of the rivulet flows. The initial ice roughness was found to have a significant effect on the rivulet characteristics (e.g., rivulet width, spacing, and height).

Topological Stabilization and Dynamics of Self-Propelling Nematic Shells.

Liquid shells (e.g., double emulsions, vesicles, etc.) are susceptible to interfacial instability and rupturing when driven out of mechanical equilibrium. This poses a significant challenge for the design of liquid-shell-based micromachines, where the goal is to maintain stability and dynamical control in combination with motility. Here, we present our solution to this problem with controllable self-propelling liquid shells, which we have stabilized using the soft topological constraints imposed by a nematogen oil. We demonstrate, through experiments and simulations, that anisotropic elasticity can counterbalance the destabilizing effect of viscous drag induced by shell motility and inhibit rupturing. We analyze their propulsion dynamics and identify a peculiar meandering behavior driven by a combination of topological and chemical spontaneously broken symmetries. Based on our understanding of these symmetry breaking mechanisms, we provide routes to control shell motion via topology, chemical signaling, and hydrodynamic interactions.

Analysis of wake effects on global responses for a floating two-turbine case

While most existing modeling and analysis of floating wind turbines (FWTs) considers isolated systems, interactions among multiple FWTs arranged in an array have received little attention. In this study, two 10 MW semi-submersible FWTs, separated by 8 rotor diameters (D) in the wind direction, are simulated with an ambient wind speed of 10 m/s and in moderate wave conditions using FAST. Farm to investigate the effects of wakes on global responses. Synthetic inflow is generated using three methods: the Kaimal turbulence model, 1) without and 2) with spatial coherence in the lateral and vertical velocity components, and 3) the Mann turbulence model (where spatial coherence in all three dimensions is inherent to the model). The first method results in negligible wake meandering, a relatively uniform wake deficit, while the second and third methods result in meandering of the upstream turbine’s lateral wake center at the downstream turbine’s rotor plane of up to approximately 1D and 1.5D, respectively. The slow meandering behavior of the upstream turbine’s wake resulted in increased low-frequency platform motions for the downstream turbine. Yaw motions were especially susceptible to wake meandering as the standard deviation of the downstream turbine’s yaw motion increased by 28.0 % for the second method and 11.3 % for the third method. Increased low-frequency response in structural loading was also observed. Wake effects led to between 2 % and 30 % greater fatigue damage at the top of the tower for all three methods and at the base of the tower for the second method. However, other results were found to be sensitive to the blade-passing frequency.

The meandering behaviour of large-scale structures in turbulent boundary layers

This paper quantifies the instantaneous form of large-scale turbulent structures in canonical smooth-wall boundary layers, demonstrating that they adhere to a form that is consistent with the self-sustaining streak instability model suggested by Flores & Jiménez (Phys. Fluids, vol. 22, 2010, 071704) and Hwang & Cossu (Phys. Fluids, vol. 23, 2011, 061702). Our motivation for this study stems from previous observations of large-scale streaks that have been spatially locked in position within spanwise-heterogeneous boundary layers. Here, using similar tools, we demonstrate that the randomly occurring large-scale structures in canonical layers show similar behaviour. Statistically, we show that the signature of large-scale coherent structures exhibits increasing meandering behaviour with distance from the wall. At the upper edge of the boundary layer, where these structures are severely misaligned from the main-flow direction, the induced velocities associated with the strongly yawed vortex packets/clusters yield a significant spanwise-velocity component leading to an apparent oblique coherence of spanwise-velocity fluctuations. This pronounced meandering behaviour also gives rise to a dominant streamwise periodicity at a wavelength of approximately $6\unicode[STIX]{x1D6FF}$. We further statistically show that the quasi-streamwise roll-modes formed adjacent to these very large wavy motions are often one-sided (spanwise asymmetric), in stark contrast to the counter-rotating form suggested by conventional conditionally averaged representations. To summarise, we sketch a representative picture of the typical large-scale structures based on the evidence gathered in this study.

Experimental Study and CFD-PBM Simulation of the Unsteady Gas-Liquid Flow in an Airlift External Loop Reactor

The performance of external loop airlift reactor (ELALR) strongly depends on local prevailing hydrodynamics. In this work, the hydrodynamics characterization of the gas-liquid flow is performed in a laboratory scale ELALR. The bubbles plume and the velocity field of the riser are captured using Particle Image Velocimetry (PIV) technique and a three-dimensional CFD-PBM simulation is implemented to provide a full description of the flow structure. The effects of superficial gas velocity on the flow structure and the time-averaged liquid velocity are quantitatively investigated and the numerical results are in reasonable agreement with the PIV measurement. The results show that the gas-liquid two-phases flow in the riser is always unsteady, and the bubble plume meandering behavior is observed clearly. Gas holdup increases with the increase of superficial gas velocity, while the liquid axial velocity shows a trend of first increase and then decrease and the maximum is observed at superficial gas velocity of 30 mm/s. With the increase of superficial gas velocity, the probability density function curve becomes flatter, and changes from unimodal to bimodal. This work helps to gain in-depth understanding of the complex fluid hydrodynamics behavior inside an ELALR.

Meander hydromorphology of ephemeral streams: Similarities and differences with perennial rivers

The geomorphology, hydrology and processes of ephemeral streams are poorly known and studies on the geomorphic characteristics of ephemeral meandering streams (EMS) are even less investigated. We collected geomorphic data (namely, channel width, sinuosity, wavelength and curvature radius) from 107 EMS reaches in different drylands of the world from GoogleEarth Pro® satellite images. Geomorphology features/processes, such as neck cutoffs, point scrolls and chute cutoffs, were also assessed. The main aim of this paper is to investigate similarities and differences between EMS and perennial counterparts of more humid areas (on which almost all hydrogeomorphic models are based) for the purpose of explaining why meandering rivers form in environments where the lack of vegetation, sporadic flows and a high sediment supply should favour the development of braided rivers. EMS showed both geomorphological similarities and differences with perennial rivers. Point bar chute cutoffs and point bar scrolls are the most common geomorphic features of EMS, suggesting a certain stability of meander behaviour, though within a continuous process of changes towards the most probable form, similarly to perennial streams. The bimodal distribution of curvature ratio values departs substantially from that of perennial rivers that are commonly in the 2–3 range. Data from the rivers in this study indicate that their channel pattern is well described by the same sine generated function used for perennial rivers. Wavelength to channel width ratio and bankfull discharge to streambed gradient of EMS unexpectedly plot in different areas of classical diagrams developed for perennial rivers. We interpret such differences in terms of excess energy expenditure for high sediment load transport. We also propose a new function to discriminate the meandering from the braided pattern of ephemeral streams, and present a conceptual model of meander formation in ephemerals streams based on an autogenic process of bank collapse and bar deposition.

Validation of FAST.Farm Against Large-Eddy Simulations

FAST.Farm is a new midfidelity, multiphysics engineering tool for modeling the power performance and structural loads of wind turbines within a wind farm, including wake and array effects. Previous calibration of the tuneable model parameters of FAST.Farm has shown that its prediction of wake dynamics for a single wind turbine across different atmospheric stability conditions and nacelle-yaw errors matches well with high-fidelity large-eddy simulation at a small fraction of the computational expense. This paper presents a validation of FAST.Farm against large-eddy simulation for a series of cases—independent from those used to support the calibration—considering single-turbine and small wind-farm scenarios, which are both subject to variations in inflow and control. The validation has demonstrated that FAST.Farm reasonably accurately predicts: (1) thrust and power for individual turbines both in isolation and down the row of the small wind farm, (2) wake meandering behavior across different atmospheric conditions, and (3) averaged wake-deficit advection, evolution, and merging effects. The validation also highlights potential physics that could be improved in FAST.Farm in the future.

Morphometric Parameters and Neotectorphometric Parameters and Neotectonics of Kalyani River Basin, Ganga Plain: A Remote Sensing and GIS Approach

ABSTRACT The drainage basin of the Kalyani river, a tributary of Gomati river has been mapped and delineated using Survey of India toposheets (1:50,000 scale) and remote sensing satellite data. The digitization, slope map preparation and statistical calculations have been carried out with the help of geographical information system (Arc GIS 10). Kalyani a fifth order river exhibits meandering behavior having 2.45 sinuosity index (SI). The Kalyani river basin has about 1235 km2area with NW-SE sloping trend. The total number of first, second, third, and fourth order streams are 373, 71, 12 and 2 respectively, showing dominance of first order streams in the basin. The mean bifurcation ratio (Rb) of the entire basin is 4.8, which indicates that the drainage is not much influenced by geological structures and exhibits dendritic drainage pattern. Relief ratio (Rr) indicates low to medium surface run-off, and low stream power for erosion. The analysis of river bank height ‘r’ (escarpment) and longitudinal profile of the river closely reveals neotectonic activity at some locations in the basin. To prepare a comprehensive watershed development and management plan, it is important to understand the topography and drainage characteristics of the region.

Morphodynamic evolution and stratal architecture of translating tidal point bars: Inferences from the northern Venice Lagoon (Italy)

AbstractThe widespread distribution of tidal creeks and channels that undertake meandering behaviour in modern coasts contrasts with their limited documentation in the fossil record, where point‐bar elements arising from the interaction between a mix of both fluvial and tidal currents are mainly documented. The sedimentary products of tidal channel‐bend evolution are relatively poorly known, and few studies have focused previously on specific facies models for tidal point bars present in modern settings. This study improves understanding of tidal channel meander bends through a multi‐disciplinary approach that combines analyses of historical aerial photographs, measurements of in‐channel flow velocity, high‐resolution facies analyses of sedimentary cores and three‐dimensional architectural modelling. The studied channel bend (12 to 15 m wide and 2 to 3 m deep) drains a salt marsh area located in the north‐eastern sector of the microtidal Venice Lagoon, Italy. Historical photographs show that, during the past 77 years, the bend has translated seaward ca 15 m. Results show that the channel bend formed on a non‐vegetated mud flat that was progressively colonized by vegetation. Seaward translation occurred under aggradational conditions, with an overall migration rate of 0·2 to 0·3 m year−1, and was promoted by the occurrence of cohesive, poorly erodible outer bank deposits. Ebb currents are dominant, and translation of the channel bend promotes erosion and deposition along the landward and seaward side of the bar, respectively. Tidal currents show a clear asymmetry in terms of velocity distribution, and their offset pattern provides a peculiar grain‐size distribution within the bar. During the flood stage, sand sedimentation occurs in the upper part of the bar, where the maximum flow velocity occurs. During the ebb stage, the bar experiences the secondary helical flow that accumulates sand at the toe of the bar. Lateral stacking of flood and ebb deposits has caused the formation of localized coarsening‐upward and fining‐upward sedimentary packages, respectively.

Hydrodynamic modeling of ionic liquids and conventional amine solvents in bubble column

The green solvent ionic liquids (ILs) are getting attention in variety of industries, because of their unique properties. Few studies explain the hydrodynamics of ILs, but the comparison with conventional amine solvents in CO2 capture process has not so far been reported. The primary target of this research is to develop a state of the art computational fluid dynamics (CFD) model that would be useful to understate hydrodynamic parameters of both ILs and conventional amine solvents in a glance. In present study, three ILs are investigated inside a 3D flat bubble column and their higher viscosities are examined vigilantly by comparison with three amine solvents. The flow pattern, liquid velocity magnitude, CO2 holdup and bubble size distribution are explained. It is reported that current CFD model may be used for higher viscous ILs and amine solvents. Compared with amine solvents, CO2 plume meandering behavior was not observed and very low gas holdup was obtain in ILs. The velocity magnitude of ILs increased with the height in bubble column, but a decreasing trend with column height was followed in the case of amine solvents. CO2 exhibited non-coalescence behavior in ILs, but coalescence and breakup phenomena were seen in amine solvents. Finally, baffled type industrial tray column is also investigated and some useful hydrodynamic aspects in the comparison of pure ionic liquid, 90%-solution of ionic liquid and 30%-solution of MEA are drawn. The experimentally validated CFD model would be useful for switching to ILs from corrosive amine solvents and further investigation of new economical ILs.

Continuous movement behavior of humpback whales during the breeding season in the southwest Indian Ocean: on the road again!

BackgroundHumpback whales are known to undertake long-distance migration between feeding and breeding sites, but their movement behavior within their breeding range is still poorly known. Satellite telemetry was used to investigate movement of humpback whales during the breeding season and provide further understanding of the breeding ecology and sub-population connectivity within the southwest Indian Ocean (SWIO). Implantable Argos satellite tags were deployed on 15 whales (7 males and 6 females) during the peak of the breeding season in Reunion Island. A switching-state-space model was applied to the telemetry data, in order to discriminate between “transiting” and “localized” movements, the latter of which relates to meandering behavior within putative breeding habitats, and a kernel density analysis was used to assess the spatial scale of the main putative breeding sites.ResultsWhales were tracked for up to 71 days from 31/07/2013 to 16/10/2013. The mean transmission duration was 25.7 days and the mean distance travelled was 2125.8 km. The tracks showed consistent movement of whales from Reunion to Madagascar, demonstrating a high level of connectivity between the two sub-regions, and the use of yet unknown breeding sites such as underwater seamounts (La Perouse) and banks (Mascarene Plateau). A localized movement pattern occurred in distinct bouts along the tracks, suggesting that whales were involved in breeding activity for 4.3 consecutive days on average, after which they resume transiting for an average of 6.6 days. Males visited several breeding sites within the SWIO, suggesting for the first time a movement strategy at a basin scale to maximize mating. Unexpectedly, females with calf also showed extensive transiting movement, while they engaged in localized behavior mainly off Reunion and Sainte-Marie (East Madagascar).ConclusionsThe results indicated that whales from Reunion do not represent a discrete population. Discrete breeding sites were identified, thereby highlighting priority areas for conservation. The study is a first attempt to quantify movement of humpback whales within the southwestern Indian Ocean breeding range. We demonstrate a wandering behavior with stopovers at areas that likely represent key breeding habitat, a strategy which may enhance likelihood of individual reproductive success.

Suppression of Spiral Breakup in Excitable Media by Local Periodic Forcing

Lowered excitability leads to unstable meandering of spiral tip, which result in breakup of spiral waves into chaotic states induced by Doppler effects. This phenomenon is responsible for the transition from tachycardia to ventricular fibrillation in cardiac tissues. Numerical simulations show that low-energy local periodic forcing (LPF) applied around spiral tip can efficiently suppress the meandering behavior and consequently prevent spiral breakup. The controllable phase diagrams that describe the amplitude and period of LPF against excitability parameter are presented to illustrate the control region. The underlying mechanism of suppressing spiral meandering behavior is explored by greatly decreasing the radius of the meandering tip. The proposed scheme can potentially contribute to controlling cardiac arrhythmia.