Spatial Velocity Variations Research Articles

Local flow convergence, bed scour, and aquatic habitat formation during floods around wooden training structures placed on sand-gravel bars

Riverine ponds, which are formed and sustained through sediment erosion and deposition, are key habitats for enhancing biodiversity in river reaches. The objective of this study was to understand the roles of traditional river-training wooden structures called “seigyu” on the formation of ponds on nonvegetated bars. Here, the spatial and temporal patterns of the flow and bedform coupled with pond formations for several flood events were assessed. The surface flow patterns were monitored by an unmanned aerial vehicle (UAV) and evaluated by large-scale particle image velocimetry (LSPIV); the maximum flow velocities were 1.3 m s−1 and 1.9 m s−1 during floods when seigyu units were partially and fully submerged, respectively. Although the overall mean flow velocity was greater for the latter events, the spatial variation in flow velocity and dissipation rate of turbulent kinetic energy (TKE) around seigyu was greater for the former events. Such flow patterns affected both bed formation and ecological habitats; ponds were formed at locations beside and behind seigyu, where the flow converged and bed scouring occurred during floods. The frequency and size of ponds around seigyu increased in the early half of the season, and they decreased in the other half when floods were greater in magnitude, which suggests that the bed scouring effect of seigyu was greater in flood stages with partial than fully submergence. Although the bar ponds lack shade to temper the effects of incident light and atmospheric conditions, the ponds displayed smaller daily oscillations in temperature than did the main river, probably due to hyporheic water supply to the ponds. Because many aquatic species cannot tolerate extremely high temperatures in summer, the generation and maintenance of deeper ponds by scouring with sufficient water exchange with the hyporheic zone can be key to enhancing colonization by various aquatic species.

Measuring Temporal and Spatial Variability of Red Blood Cell Velocity in Human Retinal Vessels.

PurposeThe retinal circulation regulates blood flow through various internal and external factors; however, it is unclear how locally these factors act within the retinal microcirculation. We measured the temporal and spatial variability of blood velocity in small retinal vessels using a dual-beam adaptive optics scanning laser ophthalmoscope.MethodsIn young healthy subjects (n = 3), temporal blood velocity variability was measured in a local vascular region consisting of an arteriole, capillary, and venule repeatedly over 2 days. Data consisted of 10 imaging periods separated into two sessions: (1) five 6-minute image acquisition periods with 30-minute breaks, and (2) five 6-minute image acquisition periods with 10-minute breaks. In another group of young healthy subjects (n = 5), spatial distribution of velocity variability was measured by imaging three capillary segments during three 2-minute conditions: (1) baseline imaging condition (no flicker), (2) full-field flicker, and (3) no flicker condition again.ResultsBlood velocities were measurable in all subjects with a reliability of about 2%. The coefficient of variation (CV) was used as an estimate of the physiological variability of each vessel. Over 2 days, the average CV in arterioles was 7% (±2%); in capillaries, it was 19% (±6%); and, in venules, it was 8% (±2%). During flicker stimulation, the average capillary CV was 16% during baseline, 15% during flicker stimulation, and 18% after flicker stimulation.ConclusionsCapillaries in the human retina exhibit spatial and temporal variations in blood velocity. This inherent variation in blood velocity places limits on studying the vascular regulation of individual capillaries, and the study presented here serves as a foundation for future endeavors.

Active faulting controls bedform development on a deep-water fan

Abstract Tectonically controlled topography influences deep-water sedimentary systems. Using 3-D seismic reflection data from the Levant Basin, eastern Mediterranean Sea, we investigate the spatial and temporal evolution of bedforms on a deep-water fan cut by an active normal fault. In the footwall, the fan comprises cyclic steps and antidunes along its axial and external portions, respectively, which we interpret to result from the spatial variation in flow velocity due to the loss of confinement at the canyon mouth. Conversely, in the hanging wall, the seafloor is nearly featureless at seismic scale. Numerical modeling of turbidity currents shows that the fault triggers a hydraulic jump that suppresses the flow velocity downstream, which thus explains the lack of visible bedforms basinward. This study shows that the topography generated by active normal faulting controls the downslope evolution of turbidity currents and the associated bedforms and that seafloor geomorphology can be used to evince syn-tectonic deposition.

Nanoparticle dispersion in porous media: Effects of array geometry and flow orientation.

We investigate the effects of array geometry and flow orientation on transport of finite-sized particles in ordered arrays using Stokesian dynamics simulations. We find that quiescent diffusion is independent of array geometry over the range of volume fraction of the nanoposts examined. Longitudinal dispersion under flow depends on the direction of incident flow relative to the array lattice vectors. Taylor-Aris behavior is recovered for flow along the lattice directions, whereas a nonmonotonic dependence of the dispersion coefficient on the Péclet number is obtained for flow orientations slightly perturbed from certain lattice vectors, owing to a competition between directional locking and spatial velocity variations.

Experimental investigation of gradually-varied unsteady flow passed a circular pile

The flow field around a vertically mounted circular pile exposed to gradually-varied unsteady flow is experimentally investigated. Experiments were conducted in a 30 m-long and 1 m-wide recirculating flume equipped with a variable discharge pump. A circular cylinder with 9 cm diameter was used as the model pile. To understand the influence of accelerating and decelerating flow conditions, three unsteady cases with different unsteadiness degrees were tested as well as a reference steady flow case. The spatial and temporal variations of Reynolds-averaged velocity and turbulence characteristics around the pile, as well as undisturbed flow, were analyzed. Findings show that there are distinct differences between the tested gradually-varied unsteady flow cases and the reference steady flow case. The Reynolds-averaged velocity vs. TKE plots of undisturbed flow indicated a hysteresis effect, such that larger turbulence is generated during the falling stage of the flow compared to the rising stage. This hysteresis was considerably reduced in the pile wake, and even reversed hysteresis was seen at certain cases. The spatial variation of Reynolds-averaged velocity and turbulence in the peak instant of unsteady flow was qualitatively similar to that of steady flow, but quantitatively, turbulence, flow contraction, and velocity deficit in the near-wake region were smaller in the case of unsteady flow. Contrarily, the unsteady flow generated remarkably higher turbulence levels at further downstream in the pile wake. It is concluded that in the case of unsteady flow the pile behaves as if it has a more streamlined shape. The results were also interpreted from structure-bed interactions perspective, explaining the differences between the pile scour induced by steady and unsteady flow conditions.

Tunable lateral spin polarization and spin-dependent collimation in velocity-modulated ferromagnetic-gate graphene structures

The influence of spatial variation of Fermi velocity on the spin-polarized transport properties of massless Dirac fermions in proximity-induced ferromagnetic graphene junction was investigated. We found that the velocity ratio ξ causing the width of the spin conductance dip near the Dirac point is broadened as the Fermi velocity ratio increases. In contrast, the effect of the Fermi velocity mismatch does not affect the shifting of the Dirac point. It leads to the indirect measurement of the Fermi velocity ratio that can be tuned by the spin-dependent conductance. In addition to the presence of both the exchange field and the Fermi velocity modulation, we found high spin filtering occurs when the Fermi velocity ratio ξ ≥ 1. Due to the transmission probability of electron with spin down, T↓ is blocked by the effect of Fermi velocity modulation similar to waves travelling through a different media. Moreover, we also found that the resonant transmission of the electron with spin up can be perfectly transmitted through the Fermi velocity barrier at the limit injected angle θ → π/2 for H/EF ≈ 1. The spin transport properties can be controlled by suitably modifying the strength of the ferromagnetic insulator and varying the Fermi velocity modulation, which leads to the highest spin polarization. By manipulating the behavior of spin-dependent collimation and spin beam splitting in this structure, we proposed a spin transport device for making lateral spin polarization. These interesting features will help make the development of spintronic devices in the graphene-based nanostructure.

Flow and wake characteristics associated with large wood to inform river restoration

Wood is an integral part of a river ecosystem and the number of restoration projects using log placements is increasing. Physical model tests were used to explore how the wood position and submergence level (discharge) affect wake structure, and hence the resulting habitat. We observed a von-Kármán vortex street (VS) for emergent logs placed at the channel center, while no VS formed for submerged logs, because the flow entering the wake from above the log (sweeping flow) inhibited VS formation. As a result, emergent logs placed at the channel center resulted in ten times higher turbulent kinetic energy compared to submerged logs. In addition, both spatial variation in time-mean velocity and turbulence level increased with increasing log length and decreasing submergence level. Submerged logs and logs placed at the channel side created a greater velocity deficit and a longer recirculation zone, both of which can increase the residence time in the wake and deposition of organic matter and nutrients. The results demonstrate that variation in log size and degree of submergence can be used as a tool to vary habitat suitability for different fish preferences. To maximize habitat diversity in rivers, we suggest a diverse large wood placement.

The effect of neglecting spatial variations of the parameters in pollutant transport modeling in rivers

For many environmental projects and plans, it is necessary to model pollutant transport in rivers. Pollutant transport modeling is a complex phenomenon with multiple factors affecting it. The basic governing equation describing pollutant transport is advection–dispersion equation. There are two main parameters in this equation, namely, dispersion coefficient and flow velocity. In non-uniform flow regimes, these parameters are both spatially variable, therefore, solution of the advection–dispersion equation usually accomplished using numerical methods. Spatial variability of these parameters makes it hard to determine them, particularly for dispersion coefficient in which determining it for non-uniform flows using the corresponding formulas or calibration is an uncertain and challenging task. In this study, based on non-dimensionalized transport equation and the Monte Carlo simulation technique, a methodology is presented for evaluating the effects of spatial variability of the parameters on concentration distribution in rivers. The results imply that for many practical cases of pollutant transport modeling in rivers, spatial variations of flow velocity and dispersion coefficient can be neglected with a reasonable error. Consequently, for these cases, constant-coefficient pollutant transport equation can be applied instead of the variable-coefficient one, hence, simple analytical solutions can be employed rather than complicated numerical models. The proposed approach was applied for a real case study, successfully. It is emphasized that the results of the study are fairly general and can provide insight and simplification in different stages of pollutant transport modeling in rivers.

Transparent mirror effect in twist-angle-disordered bilayer graphene

When light is incident on a medium with spatially disordered index of refraction, interference effects lead to near-perfect reflection when the number of dielectric interfaces is large, so that the medium becomes a "transparent mirror." We investigate the analog of this effect for electrons in twisted bilayer graphene (TBG), for which local fluctuations of the twist angle give rise to a spatially random Fermi velocity. In a description that includes only spatial variation of Fermi velocity, we derive the incident-angle-dependent localization length for the case of quasi-one-dimensional disorder by mapping this problem onto one dimensional Anderson localization. The localization length diverges at normal incidence as a consequence of Klein tunneling, leading to a power-law decay of the transmission when averaged over incidence angle. In a minimal model of TBG, the modulation of twist angle also shifts the location of the Dirac cones in momentum space in a way that can be described by a random gauge field, and thus Klein tunneling is inexact. However, when the Dirac electron's incident momentum is large compared to these shifts, the primary effect of twist disorder is only to shift the incident angle associated with perfect transmission away from zero. These results suggest a mechanism for disorder-induced collimation, valley filtration, and energy filtration of Dirac electron beams, so that TBG offers a promising new platform for Dirac fermion optics.

3D numerical study on syngas production in a structured packed bed with connected pellets

Fuel-rich partial combustion in porous media is numerically studied at the pore scale with 3D staggered arrangement of connected pellets. The chemistry is treated with detailed chemical mechanism and the solid-to-solid radiation is taken into account. The numerical results are validated against experimental results and indicates that 3D pore scale numerical modeling gives reasonable syngas components. Considerable heat recirculation in the preheating zone is predicted by 3D pore-scale model. Root mean squares (RMS) is used to quantitatively estimate the local spatial variations of the velocity and gas temperature. It is shown that the distribution of RMS of gas temperature along flow direction is basically wave-like shape and the spatial variation of gas velocity reaches 78% of the average velocity.

Integrated Borehole, Radar, and Seismic Velocity Analysis Reveals Dynamic Spatial Variations Within a Firn Aquifer in Southeast Greenland

AbstractPerennial water storage in firn aquifers has been observed within the lower percolation zone of the southeast Greenland ice sheet. Spatially distributed seismic and radar observations, made ~50 km upstream of the Helheim Glacier terminus, reveal spatial variations of seismic velocity within a firn aquifer. From 1.65 to 1.8 km elevation, shear‐wave velocity (Vs) is 1,290 ± 180 m/s in the unsaturated firn, decreasing below the water table (~15 m depth) to 1,130 ± 250 m/s. Below 1.65 km elevation, Vs in the saturated firn is 1,270 ± 220 m/s. The compressional‐to‐shear velocity ratio decreases in the downstream saturated zone, from 2.30 ± 0.54 to 2.01 ± 0.46, closer to its value for pure ice (2.00). Consistent with colocated firn cores, these results imply an increasing concentration of ice in the downstream sites, reducing the porosity and storage potential of the firn likely caused by episodic melt and freeze during the evolution of the aquifer.

Scanning acoustic microscopy investigation of weld lines in injection-molded parts manufactured from industrial thermoplastic polymer

Scanning acoustic microscopy (SAM) is used to characterize welds in a thermoplastic polymer (ABS) manufactured by injection-molding, particularly at the locations of weld-lines known to form as unavoidable significant defects. Acoustic micrographs obtained at 420 MHz clearly resolve the weld lines with morphological deformations and microelastic heterogenity. This is also where terahertz (THz) measurements, carried out in support of the SAM study, reveal enhanced birefringence corresponding to the location of these lines enabling verification of the SAM results. Rayleigh surface acoustic waves (RSAW), quantified by V(z) curves (with defocusing distance of 85 μm), are found to propagate slower in regions close to the weld lines than in regions distant from these lines. The discrepancy of about 100 m/s in the velocity of RSAW indicates a large variation in the micro-elastic properties between areas close to and distant from the weld lines. The spatial variations in velocity (VR) of RSAWs indicate anisotropic propagation of the differently polarized ultrasonic waves.

Characteristics of spatial variability of shear wave velocity on seismic response of slopes

The soil shear wave velocity has long been recognized as an effective parameter in the assessment of the seismic response of slopes. However, most seismic analyses have been carried out by deterministic models considering just a single realization of soil shear wave velocity. In other words, only a small number of researchers have explored the spatial variability effects of soil shear wave velocity on the seismic response of slopes. This paper investigates this phenomenon stochastically, and the auto-correlation matrix decomposition method based on Monte Carlo Simulation was applied to generate the stationary random fields in order to simulate the spatial variability of soil shear wave velocity. This framework includes interpreting the coefficient of the soil shear modulus variation and auto-correlation length effected on maximum horizontal equivalent acceleration. The results showed that the spatial variation of shear wave velocity might have a significant effect on the dynamic response of the sliding mass if slopes were subjected to weak ground motions. By contrast, it could have a significant effect on maximum horizontal equivalent acceleration induced in slopes subjected to intense ground motions.

Effect of 90\xb0 elbows on pump inlet flow conditions

The use of 90° elbows upstream of a pump inlet can distort the approach flow resulting in spatial and temporal velocity variations and swirling flow that negatively affect pump performance and increase maintenance requirements. In order to attend these flow conditions, pumps have to be installed according to generally accepted standards such as ANSI/HI 9.8. (American national standard for rotodynamic pumps for pump intake design, Hydraulic Institute, Parsippany, 2012). However, in these standards, there is little information about the minimum distance between single and double 90° elbows and a pump. Therefore, this paper presents results for the pipe flow downstream of 90° elbows and its attendance to the standards at the inlet of pumps using Computational Fluid Dynamics (CFD). Prior to its application, the CFD model was validated by comparing the computed velocity profiles with experimental results downstream of a 90° elbow. It is found that it is necessary 3 to 16 pipe diameters from the elbow in order to reduce the swirling flow. The velocity distribution at a cross section was never uniform up to 50 pipe diameters downstream of the elbows. The temporal velocity fluctuation was always low. It is concluded that the specifications of downstream pipe lengths in the current inlet pipe flow standards are not sufficient to achieve the desired flow at the pump inlet.

Distortion matrix approach for ultrasound imaging of random scattering media

Focusing waves inside inhomogeneous media is a fundamental problem for imaging. Spatial variations of wave velocity can strongly distort propagating wave fronts and degrade image quality. Adaptive focusing can compensate for such aberration but is only effective over a restricted field of view. Here, we introduce a full-field approach to wave imaging based on the concept of the distortion matrix. This operator essentially connects any focal point inside the medium with the distortion that a wave front, emitted from that point, experiences due to heterogeneities. A time-reversal analysis of the distortion matrix enables the estimation of the transmission matrix that links each sensor and image voxel. Phase aberrations can then be unscrambled for any point, providing a full-field image of the medium with diffraction-limited resolution. Importantly, this process is particularly efficient in random scattering media, where traditional approaches such as adaptive focusing fail. Here, we first present an experimental proof of concept on a tissue-mimicking phantom and then, apply the method to in vivo imaging of human soft tissues. While introduced here in the context of acoustics, this approach can also be extended to optical microscopy, radar, or seismic imaging.

Investigation on slug flow phenomenon in Alborz morning glory spillway and preventing its destructive effects

Slug flow is a flow pattern which occurs in conveyance systems containing a two phase-fluid flow. Large air bubbles entrapped along these systems interrupt the flow and conduct to undesirable pressures and their fluctuations. Most of the previous concerning studies on slug flow phenomenon were performed in micro-channels with small scales in which the expansion of the air bubbles was negligible. In contrast, we investigated the systems with large pressure drops which conduct to abrupt increases on the volume of air phase. In this research, the verification tests applying CFD techniques were performed in OpenFOAM software by interFoam solver. The performance of morning glory spillways is investigated under steady states. While during the occurrence of flood, by increasing the depth over the spillway crest, the discharge is augmented which conducts to entrap the air pockets with pressure and velocity fluctuations. These fluctuations influence on the life of structures and their performances. This study aimed on unsteady flow in a glory morning spillway and its consequences and proposing the measures for reducing the destructive effects of slug flow. The pressure and velocity fluctuation were considered as the indices for the performance of a hydraulic structure. Spatial and temporal variations of pressure and velocity along the spillway are evaluated. Also, the influences of spillway geometry on slug flow and the measures to attenuate its destructive effects are analyzed. The results showed that the better performances of morning glory spillway are coincided by the diameter of tunnel superior than the height of water above the crest.

Numerical simulation of K-, N- and O-regime boundary-layer transition at early nonlinear stage

Boundary layer transition is usually associated with the amplification and breakdown of three-dimensional waves (3D waves). However, the details of the development of 3D waves in different transition regimes are not well resolved. The present study attempts to examine the transition process at early nonlinear stages of K-, N-, and O-regimes by the solution of nonlinear parabolized stability equations (NPSE). The spatial and temporal variations of streamwise velocity are compared at the one-spike stage. Timelines and material surfaces are presented using Lagrangian tracking method. The development of 3D waves are observed to contribute to the upward lift and downward sweep of flow behaviors, causing inflectional regions in the boundary layer. The evolution of material surfaces reveals that the nonlinear 3D wave manifests as a warped wave front in the near-wall region and as a soliton-like structure at the upper region of the boundary layer. The 3D wave structure is hypothesized as a soliton-like coherent structure. Markedly similar flow behaviors are observed between the regimes, which indicates a similar underlying physical mechanism.

Forensic mapping of seismic velocity heterogeneity in a CO2 layer at the Sleipner CO2 storage operation, North Sea, using time-lapse seismics

CO2 separated from natural gas produced at the Sleipner and Gudrun fields is being injected into the Utsira Sand, with around 18 million tons currently stored. Time-lapse 3D seismics have been deployed to monitor development of the CO2 plume. The 2010 seismic survey resolved, for the first time in 3D, the topmost CO2 layer into distinct reflections from its top and base. Seismic velocity is diagnostic of CO2 layer properties and a forensic interpretative approach is adopted to determine spatial velocity variation in the topmost CO2 layer. Velocity is obtained by equating absolute layer thickness, derived by subtracting a constructed flat CO2 – water contact from the topographical relief of the reservoir top, to the temporal separation of the layer top and base reflections, with appropriate correction for wavelet interference effects. Layer velocities show a systematic and robust spatial variation between a northern area with a mean velocity of 1371 ± 122 ms−1 and a central area with a much higher mean velocity of 1638 ± 103 ms−1. Recent fluid flow simulations of the topmost CO2 layer have shown that incorporating a high permeability channel in the model reservoir significantly improves the history-match. This high permeability channel corresponds remarkably closely to the low seismic velocities mapped in the northern area, with higher layer velocities of the central area interpreted as more argillaceous, less permeable overbank deposits. The new velocity analysis therefore provides independent support for including deterministic permeability heterogeneity in predictive fluid flow modelling of Sleipner.

Prediction of critical heat flux on surface under boiling in case of IVR-ERVC by using CFD simulation

The efficacy of in-vessel through external reactor vessel cooling (IVR-ERVC) is strongly correlated with the critical heat flux (CHF) on the lower head of reactor pressure vessel (RPV). The high limit of CHF prevents the RPV from failure by extracting much heat out of the vessel through the circulation of working fluid in external flow channel. In present study, the deionized water is allowed to flow through curved flow channel having radius of curvature of 368mm and hydraulic diameter of 40mm with square cross-section. The heat flux of 1500kWm−2 is applied perpendicular to the convex wall of the channel. The CHF on the wall of flow channel has been predicted by incorporating CFD based two-phase (liquid-vapor) boiling model. The investigations on spatial variation of pressure, temperature, and velocity and heat flux acquired as a result of numerical simulations have been discussed in detail. The CHF and average HTC is predicted as 1.798e+04kWm−2 and 38.981kWm−2K−1 respectively.

Computational investigation on the effects of rib on the slug flow phenomenon; using OpenFOAM

Slug flow is a two-phase fluid flow pattern, characterized by a series of liquid slugs dispersed by relatively large air bubbles. Air bubbles produced and trapped during the slug flow phenomenon conduct to interruption in flow which induces pressure and velocity fluctuations. These effects have destructive circumstances in conduits and conveyance structures. This paper deals with studying numerically two-phase flows using computational fluid dynamic (CFD) techniques performed in OpenFOAM (an open source software) by interFoam solver. Most of the previous concerning studies on slug flow were performed in micro-channels with small scales in which the expansion of the air bubbles was negligible. By contrast, we investigated the systems with large pressure drops which conduct to abrupt increases on the volume of air phase. The slug flow phenomenon could be created by introducing air and water with different velocities at inlet of a culvert with air to water velocity ratio varied from 1.1 to 34. First, this study focused on temporal and spatial variations of pressure and velocity along the culvert. After that, by dimensional analysis, the nondimensional parameters influencing the slug flow phenomenon are extracted and analyzed. Finally, different strategies for reducing the destructive effects of slug flow, including the shape and location of ribs, are evaluated and the best strategy is proposed.