Velocity Redistribution Research Articles

Hydrodynamic and Morphological Adaptation of Two Consecutive Sharp Bends of the Middle Yangtze River to Upstream Damming

AbstractIt remains an open question how the hydrodynamics and morphology in sharp meander bends adapt to the changes in flow and sediment conditions induced by upstream damming. This study reports a comprehensive investigation into the morphological changes, patterns of mean and secondary flows and sediment transport around two consecutive sharp bends in the Middle Yangtze River (MYR), and explains these changes from the viewpoint of flow‐sediment transport‐morphology interaction based on field measurements. With the Three Gorges Project operation, the MYR suffered a remarkable channel degradation due to a sudden decrease of incoming sediment discharge. The point bars were removed, and outer‐bank benches (OBB) formed upstream of the bend apices, which resulted in quite different flow and sediment transport patterns in the bends. The highest velocity zones and center‐region cells of secondary flow were more toward the inner bank due to the lack of point bars, and the outward migration of momentum was delayed. Flow recirculation zones were observed over the OBBs, covering 12%–58% of the channel width. The transport rates of suspended load accounted for 98% of the total sediment load, and the outward shift of the highest transport rate zone was confined by the OBBs, which reduced the effective width of sediment transport. These changes in flow and sediment transport re‐inforced the morphological changes around the sharp bends: the velocity redistribution and evolution of scour factor lagged more behind planform curvature, which further promoted the erosion of point bars and the development of OBB.

Homogeneous weighted motion planning for cooperated manipulator systems under position-torque constrained processing Environment

Real-time motion planning under position and torque constraints is a critical challenge for cooperative manipulator efficiency and safety operation. Real-time motion planning at the velocity level improves computation efficiency, eliminates the complex derivative calculation of the Jacobian matrix, and the velocity planning solution can be used directly for robotic kinematic control. However, little research attention has been attached to handling the position and torque constraints simultaneously at velocity level for cooperative manipulator systems. In this paper, we introduce a novel homogeneous weighted least-norm method (HWLN) for joint velocity redistribution of cooperated manipulators. Within the coupled kinematics-dynamics model of cooperated manipulators, joint position and torque constraints are simultaneously homogenized and taken into account by the constraint performance index. To avoid joint's constraint saturations, two real-time weight updating laws are designed for the joint position and driving torque respectively. The joint velocities of cooperated manipulators are then adaptively redistributed using the pseudo-kinetic-energy minimum optimization criteria. When compared to single manipulator regulation, this strategy takes greater advantage of cooperative redundancy and significantly enhances the position-torque planning performance. Mathematical stability proof is presented. In the meanwhile, numerical experiment results under various joint position and torque constraints demonstrate the effectiveness of the proposed HWLN method. The experimental results for motion planning and control of two 6R ABD-20Kg robotic manipulators are provided.

Free-surface undulation and velocity turbulence in shallow undular hydraulic jumps

Undular hydraulic jumps and bores occur in coastal and estuarial regions at small approach Froude numbers and introduce non-uniform evolution of hydraulic properties in the flow direction, affecting the transport and deposition of sediment and other materials. This paper reports an experimental study of relatively wide undular hydraulic jumps generated in a 1.4-m-wide flume, characterized by inflow aspect ratio less than 0.06 and no lateral shockwave crossing at the first standing wave crest. General free-surface shape observations were conducted for a range of Froude numbers between 1.5 and 2.9 and Reynolds numbers between 2.5 × 104 and 1.2 × 105, while detailed free-surface and velocity measurements were performed at longitudinal and transverse cross-sections using acoustic displacement meters (ADMs) and an acoustic Doppler velocimeter (ADV), at five Froude numbers from 1.5 to 1.9. The results highlight complicated relationship of the three-dimensional free-surface characteristics with the variation of Froude number, which exhibits different features at the first two waves and at other sequent waves. The velocity data highlighted unique features of turbulent velocity redistribution at the first wave, including the distributions of turbulence intensity and Reynolds shear stresses. Particular focus on the formation of a “quiet water” zone below the upward-deflected mainstream flow was reported, with presence of instantaneous reversing velocities above the channel bed. The flow pattern analysis was facilitated with mean vorticity characterization. The results reflect that the evolution of undular hydraulic jump with increasing Froude number is a process of gradual turbulence localization towards the first standing wave, and the transition to a breaking hydraulic jump is achieved by the formation of a surface roller at the steepened jump front overwhelming the “resistance” of the bottom “quiet water” zone and forcing a change in the main flow path.

SIMULATION OF TWO-COMPONENT POWDER MOLDING MELTING UNDER VACUUM SINTERING CONDITIONS

Vacuum sintering of metal powders under controlled heating conditions belongs to the traditional powder technologies employed to obtain dense composite materials. Despite the long history of studies on sintering processes, modeling in this area remains relevant since it provides deeper understanding of the associated physical phenomena. In the present work, we studied a two-component powder pressing system melting model based on the multiphase filtration theory that takes into account the differences in the melting temperatures of the components. The appearance of the liquid phase was modeled by introducing a melting temperature interval. An algorithm for the numerical realization of the model was developed. Employing a titanium-aluminum as an example, we demonstrate that even a temperature field with weak inhomogeneity can cause an inhomogeneous velocity field and redistribution of the fusible component.

LARS XIII: High Angular Resolution 21 cm H i Observations of Lyα Emitting Galaxies

The Lyα emission line is one of the main observables of galaxies at high redshift, but its output depends strongly on the neutral gas distribution and kinematics around the star-forming regions where UV photons are produced. We present observations of Lyα and 21 cm H i emission at comparable scales with the goal to qualitatively investigate how the neutral interstellar medium (ISM) properties impact Lyα transfer in galaxies. We have observed 21 cm H i at the highest possible angular resolution (≈3″ beam) with the Very Large Array in two local galaxies from the Lyman Alpha Reference Sample. We compare these data with Hubble Space Telescope Lyα imaging and spectroscopy, and Multi Unit Spectroscopic Explorer and Potsdam MultiAperture Spectrophotometer ionized gas observations. In LARS08, high-intensity Lyα emission is cospatial with high column density H i where the dust content is the lowest. The Lyα line is strongly redshifted, consistent with a velocity redistribution that allows Lyα escape from a high column density neutral medium with a low dust content. In eLARS01, high-intensity Lyα emission is located in regions of low column density H i, below the H i data sensitivity limit ( < 2 × 1020 cm−2). The perturbed ISM distribution with low column density gas in front of the Lyα emission region plays an important role in the escape. In both galaxies, the faint Lyα emission (∼1×10−16 erg s−1cm−2 arcsec−2) traces intermediate Hα emission regions where H i is found, regardless of the dust content. Dust seems to modulate, but not prevent, the formation of a faint Lyα halo. This study suggests the existence of scaling relations between dust, Hα, H i, and Lyα emission in galaxies.

Temperature, density, and velocity distribution in the mixture prepared by controlled diffusion solidification process

Controlled diffusion solidification (CDS) is a casting process that depends on mixing two precursor alloys to make a required resultant alloy. The CDS process can form a non-dendritic microstructure happening from copious nucleation, resulting in minimizing the tendency toward hot tearing. Pure aluminum was mixed into Al-33wt%Cu through funnels that have 9 and 6 mm diameters to make Al-4.7wt%Cu. Ansys software was employed to simulate the temperature, density, and velocity redistribution in the mixture during the mixing step. The results show that the thermal curves give better indications to predict the periods of nucleation and the heat transferring to the environment. Furthermore, the copious nucleation that occurs in the CDS process can be improved by controlling the agitation during the mixing step by decreasing the velocity of the mixture near the crucible wall and increasing the velocity in the middle of the mixture. The velocity around 1.7 ms−1 and 0.2 ms−1 happening at the middle and near crucible wall, respectively, gives better non-dendritic microstructure.

Experimental and numerical study on the flow field of longitudinally ventilated tunnels with water spray system

Experiments and numerical simulations have been conducted to study the effect of water sprays on the airflow in longitudinally ventilated tunnels. A physical model tunnel containing single and double column spray systems was built at 1:11 scale and tested in a wind tunnel. The experiments captured the velocity redistribution in the upstream area of the sprays, with the most notable feature being flow acceleration close to the zone of the ceiling. The numerical reproduction of the experiments using Fluent 2021R1 was achieved with good agreement, enabling a detailed study of the flow field. The investigation revealed that the water spray induces a water-air jet, causing blockage to the incoming air and deflecting the air stream towards the ceiling. The CFD method was also implemented to study a ceiling spray system and a side-wall spray system, whose spray properties were scaled from a practical design of water a spray system. Dimensional and theoretical analyses showed that the maximum increase in the mean velocity in the near-ceiling acceleration zone is determined by the momentum ratio of the water spray to the ventilation air and the proportions of areas occupied by the water spray in the ceiling zone and the whole tunnel cross section. A correlation model was proposed using the governing parameters, which accurately predicted the velocity increases. The developing trends of the mean ceiling velocities within and downstream of the spray zone were also discussed in detail. The findings are helpful for the coupling design of the tunnel fire fighting system and can provide guidance for the water curtain strategy, which uses water sprays to prevent smoke from spreading.

Large eddy simulation of the heat transfer and unsteady pulsation of supercritical carbon dioxide in a square subchannel

In the present study, a large eddy simulation was conducted to study the heat transfer and turbulent flow pulsation of supercritical carbon dioxide (SCO2) in a square subchannel. Three Reynolds numbers of 5385, 13,028 and 18,934 were selected corresponding to the friction Reynolds number (Reτ) of 180, 395 and 550, respectively. Both of the upward flow, downward flow and pure forced convection (g = 0) were covered. Attention was paid to clarifying the effects of flow direction and Reynolds number on the circumferentially nonuniform heat transfer, the cross-sectional velocity redistribution and the macroscopical flow pulsation. It was found that the wall-temperature distribution along the rod circumference is closely related to buoyancy. The heat transfer of the downward flow is superior to upward flow under identical conditions. Besides, the heat transfer of the upward flow is better than that of the forced convection, indicating buoyancy could enhance the heat transfer. In addition, for the upward flow, the cross-sectional velocity is high near the rod gap, but is low at the subchannel center. A quasi-periodic large-scale velocity pulsation was identified near the rod gap. The magnitude of the velocity pulsation is affected by the flow direction, but is independent of the Reynolds number. On the other hand, the dominant frequency of the velocity pulsation increases with increasing Reynolds number, but is unaffected by the flow direction.

Diffusion-approximation for a kinetic equation with perturbed velocity redistribution process

We derive the hydrodynamic limit of a kinetic equation with a stochastic, short range perturbation of the velocity operator. Under some mixing hypotheses on the stochastic perturbation, we establish a diffusion-approximation result: the limit we obtain is a parabolic stochastic partial differential equation on the macroscopic parameter, the density here.

Backward fluorescence of optically thick Cs vapor: Experiment and Monte-Carlo simulations

Backward fluorescence of thick atomic vapors exhibits a dip in the spectra at line center and sub-Doppler structures. We present backward fluorescence spectra of cesium vapor when excited around D2 line for densities for which atoms undergo a few collisions before emitting a photon. Changes on dip symmetry relative to line center and of sub-Doppler structures are observed. We perform Monte-Carlo simulations to investigate the influence of collisions between atoms and of radiation trapping on the spectra. Sub-Doppler structures are interpreted as avoided quenching for excited atoms flying parallel to the surface of the cell. We found that collisions broaden the sub-Doppler structures but have little influence on the dip symmetry which is highly affected by radiation trapping. Moreover, radiation trapping is the main contribution to velocity redistribution that reduces sub-Doppler structures for the range of densities investigated.

Secondary Flow Effects on Deposition of Cohesive Sediment in a Meandering Reach of Yangtze River

Few researches focus on secondary flow effects on bed deformation caused by cohesive sediment deposition in meandering channels of field mega scale. A 2D depth-averaged model is improved by incorporating three submodels to consider different effects of secondary flow and a module for cohesive sediment transport. These models are applied to a meandering reach of Yangtze River to investigate secondary flow effects on cohesive sediment deposition, and a preferable submodel is selected based on the flow simulation results. Sediment simulation results indicate that the improved model predictions are in better agreement with the measurements in planar distribution of deposition, as the increased sediment deposits caused by secondary current on the convex bank have been well predicted. Secondary flow effects on the predicted amount of deposition become more obvious during the period when the sediment load is low and velocity redistribution induced by the bed topography is evident. Such effects vary with the settling velocity and critical shear stress for deposition of cohesive sediment. The bed topography effects can be reflected by the secondary flow submodels and play an important role in velocity and sediment deposition predictions.

Experimental study of rock burst in coal samples under overstress and true-triaxial unloading through passive velocity tomography

The influence of overstress state on rockbursts in deep underground coal mining was investigated. Laboratory experiments were conducted using a true-triaxial rockburst system. Passive velocity tomography, using acoustic emission (AE), was used to study the internal microfractures and energy evolution of coal samples during true-triaxial compression. AE activities were monitored using eight sensors and every 130 consecutive AE events were used for tomographic calculations. For each coal sample, three phases and between four and six typical tomography results were obtained which reflected significant variations in velocity redistribution. Experimental results show that there was a significant time-delay effect under different loading and unloading stress paths. Compared to triaxial loading, coal samples are more easy to deform and bring to failure under triaxial unloading. Under a constant confining pressure, bursting failure occurs when the axial stress is more than three times the uniaxial compressive strength of the coal sample. The wave velocity inversion results show that the macrofracture positions in coal samples after failure have a positive correlation with the region of wave velocity anomalies, and the low wave velocity zone corresponds to areas of microfracture evolution.

Numerical Study of Change in Shape of Saturated Absorption Spectrum of Argon Arcjet Plasma by Strong Velocity-Changing Collisions

Saturated absorption (SA) spectra of argon arcjet plasma with the effect of strong velocity-changing collisions were numerically investigated. The spectrum was numerically reproduced by simulating the change of the velocity distribution interacting with the pump laser using the rate equation system representing two electronic states in the argon atoms. The SA spectra were classified into five types, which are composed of the linear combination of the Gaussian function, Lorentzian function, and inverted pedestals. We drew the maps of the types of the SA spectrum by varying the collision-induced velocity redistribution rate in the two electronic states and the decay rate among them. The possibility of the determination of the collisional parameters from the experiments was also discussed.

Nonlinear velocity redistribution caused by energetic-particle-driven geodesic acoustic modes, mapped with the beam-plasma system

The nonlinear dynamics of energetic-particle (EP) driven geodesic acoustic modes (EGAM) in tokamaks is investigated, and compared with the beam-plasma system (BPS). The EGAM is studied with the global gyrokinetic (GK) particle-in-cell code ORB5, treating the thermal ions and EP (in this case, fast ions) as GK and neglecting the kinetic effects of the electrons. The wave–particle nonlinearity is only considered in the EGAM nonlinear dynamics. The BPS is studied with a one-dimensional code where the thermal plasma is treated as a linear dielectric, and the EP (in this case, fast electrons) with an N-body Hamiltonian formulation. A one-to-one mapping between the EGAM and the BPS is described. The focus is on understanding and predicting the EP redistribution in phase space. We identify here two distinct regimes for the mapping: in the low-drive regime, the BPS mapping with the EGAM is found to be complete, and in the high-drive regime, the EGAM dynamics and the BPS dynamics are found to differ. The transition is described with the presence of a non-negligible frequency chirping, which affects the EGAM but not the BPS, above the identified drive threshold. The difference can be resolved by adding an ad hoc frequency modification to the BPS model. As a main result, the formula for the prediction of the nonlinear width of the velocity redistribution around the resonance velocity is provided. This article is written as the second of a series of articles (the first being Biancalani et al. (J. Plasma Phys., vol. 83 (6), 2017, 725830602)) on the saturation of EGAMs due to wave–particle nonlinearity.

Relativistic nonlinear whistler waves in cold magnetized plasmas

Starting from the Vlasov-Maxwell equations describing the dynamics of various species in a quasi-neutral plasma immersed in an external solenoidal magnetic field and utilizing a technique known as the hydrodynamic substitution, a relativistic hydrodynamic system of equations governing the dynamics of various species has been obtained. Based on the method of multiple scales, a system comprising three nonlinear Schrodinger equation for the transverse envelopes of the three basic whistler modes, has been derived. Using the method of formal series of Dubois-Violette, a traveling wave solution of the derived set of coupled nonlinear Schrodinger equations in both the relativistic and the non relativistic case has been obtained. An intriguing feature of our description is that whistler waves do not perturb the initial uniform density of plasma electrons. The plasma response to the induced whistler waves consists in transverse velocity redistribution, which follows exactly the behaviour of the electromagnetic whistlers. This property may have an important application for transverse focusing of charged particle beams in future laser plasma accelerators. Yet another interesting peculiarity are the selection rules governing the nonlinear mode coupling. According to these rules self coupling between modes in the non relativistic regime is absent, which is a direct consequence of the vector character of the interaction governed by the Lorentz force.

Vegetation effects on cross-sectional flow in a large amplitude meandering bend

ABSTRACTCross-stream circulation, which develops in meandering bends, exerts an important role in velocity redistribution. This paper investigates how the presence of vegetation could affect the evolution pattern of cross-stream flow along a high-curvature meandering bend. The analysis is conducted with the aid of data collected in a meandering laboratory flume over non-vegetated and vegetated beds. The experiments reveal that, once the vegetation is introduced, the flow pattern determined by the channel’s curvature is interrupted. In the presence of vegetation, the central-region circulation cell seems to be divided into thin circulation cells developing at the top of the vegetated layer and the counter-rotating cell, which develops within the outer-bank region; it includes a fluid area more extended than that observed in the absence of vegetation. As consequence, in the presence of vegetation, the core of high velocity is always found away from the outer bank.

On the Simulation of Floods in a Narrow Bending Valley: The Malpasset Dam Break Case Study

In this paper, we investigate the performance of three-dimensional (3D) hydraulic modeling when dealing with river sinuosity and meander bends. In river bends, the flow is dominated by a secondary current, which has a key role on the flow redistribution. The secondary flow induces transverse components of the bed shear stress and increases the velocity in outward direction, thus generating local erosion and riverbed modifications. When in river bends, the 3D processes prevail, and a 3D computational fluid dynamics (CFD) model is required to correctly predict the flow structure. An accurate description of the different hydrodynamic processes in mildly and sharply curved bends find a relevant application in meanders migration modeling. The mechanisms that drive the velocity redistribution in meandering channels depend on the river’s roughness, the flow depth (H), the radius curvature (R), the width (B) and the bathymetric variations. Here, the hydro-geomorphic characterization of sharp and mild meanders is performed by means of the ratios R/B, B/H, and R/H, and of the sinuosity index. As a case study, we selected the Malpasset dam break on the Reyran River Valley (FR), as it is perfectly suited for investigating performances and issues of a 3D model in simulating the inundation dynamics in a river channel with a varying curvature radius.

Optimization of velocity for energy saving and mitigating fouling in a crude oil preheat train with fixed network structure

Fouling of the inside of tubes in crude oil preheat systems increases pressure drop and pumping costs. In this paper, a method is proposed for optimizing crude oil velocity to reduce fouling of the heat exchanger tubes thereby maintaining heat transfer rates and reducing pressure drop. A simulated annealing algorithm is proposed to describe and optimize the interactions between heat transfer, pressure drop and fouling by adjusting fluid velocity. A specific crude oil heating system is used as a case study to verify the application of the algorithm. Results from case study showed that total annual cost is reduced by 6.4%, indicating that a better performance can be obtained by velocity redistribution using proposed method.

Interaction between free-surface, two-phase flow and total pressure in hydraulic jump

A hydraulic jump is characterised by intense turbulent flow patterns and substantial flow aeration. The flow turbulence, at both macroscopic and microscopic scales, interacts with the air entrainment process and the free-surface. A series of simultaneous measurements of the free-surface fluctuations, jump toe oscillations, void fraction and total pressure variations allowed for an investigation of the interactions between these characteristics. Experiments were conducted for a range of Froude numbers from 3.8 to 8.5. The total pressure measurements were justified for the air–water flow characterisation of the flow region with a positive time-averaged velocity. The interactions between roller surface deformation, air entrainment and diffusion, velocity variation, flow bulking, and the associated total pressure field modulation highlighted different flow regions, hence flow patterns, in the roller. The jump toe oscillation was found closely linked to the air entrapment at the toe and velocity variation in the shear flow. The instable total pressure distribution was primarily associated with the free-surface fluctuation for the bubbly roller region and with the velocity re-distribution for the lower shear region underneath. The present work provides new information on the physical characteristics of hydraulic jumps and a comprehensive insight into the nature of such complex turbulent two-phase flow.

Liquid attachment and momentum transfer by collisions between free-fall solids and liquid spray droplets

Abstract Understanding solid wetting by collisions between flowing solids and spray droplets is essential to optimizing the design and operation of related industrial processes. This paper presents an innovative experimental method to identify the statistical distributions of liquid attachment and collision probability of the impingement of a liquid spray onto free-fall particles. To separate liquid attachment from other contributions to the mass measurements of particles and liquids, individual impingement experiment with hydrophobic or hydrophilic particles of the same otherwise properties is respectively performed under the same conditions. In addition, a series of collection bins are distributed along the spray path to determine the statistical probability of collision properties including coating thickness. Based on these measurements, the collision-induced momentum transfer and velocity redistribution of solids and droplets can be estimated with the aid of a Lagrangian trajectory model. Consequently a general two-parameter of collision probability density function is proposed.