Increased Flow Shear Research Articles

Varied root effects on soil detachment with growth time and hydrodynamic characteristics

AbstractPlant roots significantly reduce soil detachment by overland flow. However, there have been few studies on how plant growth time and hydrodynamic characteristics affect the erosion‐reducing potential of plant roots. This study cultivated ryegrass (Lolium perenne L.) and alfalfa (Medicago sativa L.) as representative of plants possessing fibrous roots and taproots, respectively, in silty soil from the Loess Plateau, China. Root‐soil composites were collected monthly from March to September 2021 and subjected to flow scouring in a hydraulic flume at a 15° slope using five different flow discharges (0.05, 0.1, 0.15, 0.2 and 0.3 L s−1). The results of the study indicated that root length density, root surface area density and root mass density exhibited a significant increase during the early growth phase, followed by a slight decrease. All parameters of fibrous roots revealed greater values than those of taproots, except for root mass density. The reduction in soil detachment under varying flow shear stresses showed a significant increase during the early phases of growth, followed by a gradual decline. Notably, fibrous roots demonstrated a greater impact on soil detachment than taproots, and this discrepancy fluctuated over time. Moreover, the contributions of fibrous and taproots to reducing soil detachment decreased from 55.81% to 39.66% and from 38.21% to 20.99% with increasing flow shear stress, respectively. It indicated that the erosion‐reducing potential of plant roots was greater when subjected to low‐flow shear stress compared to high‐flow shear stress. This study can provide a scientific basis for understanding the erosion‐reducing potential of plant roots at different growing stages and under varying hydrodynamic characteristics.

Flow Rate Affects Nanoparticle Uptake into Endothelial Cells.

Nanoparticles are commonly administered through systemic injection, which exposes them to the dynamic environment of the bloodstream. Injected nanoparticles travel within the blood and experience a wide range of flow velocities that induce varying shear rates to the blood vessels. Endothelial cells line these vessels, and have been shown to uptake nanoparticles during circulation, but it is difficult to characterize the flow-dependence of this interaction in vivo. Here, a microfluidic system is developed to control the flow rates of nanoparticles as they interact with endothelial cells. Gold nanoparticle uptake into endothelial cells is quantified at varying flow rates, and it is found that increased flow rates lead to decreased nanoparticle uptake. Endothelial cells respond to increased flow shear with decreased ability to uptake the nanoparticles. If cells are sheared the same way, nanoparticle uptake decreases as their flow velocity increases. Modifying nanoparticle surfaces with endothelial-cell-binding ligands partially restores uptake to nonflow levels, suggesting that functionalizing nanoparticles to bind to endothelial cells enables nanoparticles to resist flow effects. In the future, this microfluidic system can be used to test other nanoparticle-endothelial cell interactions under flow. The results of these studies can guide the engineering of nanoparticles for in vivo medical applications.

E × B flow shear mitigates ballooning-driven edge-localized modes at high collisionality: experiment and simulation

By using the specific co-neutral beam injection (co-NBI) and counter-NBI systems on EAST, an alternating flow shear discharge has been obtained to study the impact of the flow shear on ballooning-driven edge localized modes (ELMs) at a fixed high collisionality (). The results reveal that the increased flow shear can significantly mitigate ELMs, or even totally suppress ELMs when the shear is large enough. Our simulations with BOUT support the observations on EAST, and further indicate that the increased can both reduce the linear growth rate of the ballooning mode and shorten its growth time (phase coherence time, PCT). The enhanced nonlinear interactions shorten the PCT of the ballooning mode, as validated by the bispectrum study on EAST. All those studies suggest a new way to control ELMs.

Turbulence and sheared flow structures behind the isotopic dependence of the L-H power threshold on DIII-D

Measurements of long wavelength ( < 1) density fluctuation characteristics in the edge of both Deuterium (D) and Hydrogen (H) plasmas across the L-H transition on DIII-D demonstrate the existence of single or double bands of low-wavenumber turbulence observed near the edge of H and D plasmas. These are strongly correlated with the L to H-mode transition power threshold (PLH) and can help explain the isotopic and density dependence of PLH, and how the PLH difference is reduced at higher density. Understanding and accurately predicting the L-H power threshold is critical to accessing to H-mode, and operating and achieving high confinement in burning plasmas such as ITER. Above about ne ~ 4 × 1019 m−3, PLH is seen to converge for H and D, and increases for both with higher density. Surprisingly, the PLH increases significantly at low density in H but not in D plasmas. Two distinct frequency bands of density fluctuations are observed in the D plasmas at low density, ne ~ 1.2–1.5 × 1019 m−3, but not in H plasmas with similar density, which appears to be correlated to the much lower power threshold in D at low density. Consistently, E × B shear in the region of r/a ~ 0.95–1.0 is larger in D plasmas than in H plasmas at low density; as the PLH increases with increasing density, the dual mode structure disappears while E × B shear becomes similar and small for both D and H plasmas at higher density, ne ~ 5 × 1019 m−3, where PLH is similar for both D and H plasmas. The increased edge fluctuations, increased flow shear, and the dual-band nature of edge turbulence correlating with lower PLH may account for the strong isotope and density dependencies of PLH and support current L-H transition theories but suggest a complex behavior that can inform a more complete model of the L-H transition threshold.

Tidal power extraction on a streamwise bed slope

Three-dimensional Reynolds-averaged Navier–Stokes computations are presented for an actuator disc representation of an ideal tidal stream energy extracting device, operating within a channel that slopes in the streamwise direction. Downwards facing, horizontal and upwards facing slopes are considered at the same mass flow rate and depth at the device position, for both high and low channel blockage ratios. The downwards facing slope is shown to present a greater available kinetic power to the disc than the horizontal and upwards facing slopes. This is due to the velocity profile being more strongly sheared as a result of the adverse pressure gradient, which results in greater thrust variation over the disc area. Conversely, during uphill flows devices experience reduced thrust variation, as the velocity profile is more uniform due to the favourable pressure gradient. Although downhill flows can deliver greater overall power due to increased flow shear, uphill flows are more efficient at converting the power presented to the disc into power removed and this is shown to be due to the downstream flow constriction and resulting increased pressure coefficient drop through the bypass flow.

Asymmetric magnetic reconnection with a flow shear and applications to the magnetopause

AbstractWe perform a systematic theoretical and numerical study of antiparallel two‐dimensional magnetic reconnection with asymmetries in the plasma density and reconnecting magnetic field strength in addition to a bulk flow shear across the reconnection site in the plane of the reconnecting fields, which commonly occurs at planetary magnetospheres. We analytically predict the speed at which an isolated X line is convected by the flow, the reconnection rate, and the critical flow speed at which reconnection no longer takes place for arbitrary reconnecting magnetic field strengths, densities, and upstream flow speeds, and we confirm the results with two‐fluid numerical simulations. The predictions and simulation results counter the prevailing model of reconnection at Earth's dayside magnetopause which says reconnection occurs with a stationary X line for sub‐Alfvénic magnetosheath flow, reconnection occurs but the X line convects for magnetosheath flows between the Alfvén speed and double the Alfvén speed, and reconnection does not occur for magnetosheath flows greater than double the Alfvén speed. In particular, we find that X line motion is governed by momentum conservation from the upstream flows, which are weighted differently in asymmetric systems, so the X line convects for generic conditions including sub‐Alfvénic upstream speeds. For the reconnection rate, as with symmetric reconnection, it drops with increasing flow shear and there is a cutoff speed above which reconnection is not predominant. However, while the cutoff condition for symmetric reconnection is that the difference in flows on the two sides of the reconnection site is twice the Alfvén speed, we find asymmetries cause the cutoff speed for asymmetric reconnection to be higher than twice the asymmetric form of the Alfvén speed. The stronger the asymmetries, the more the cutoff exceeds double the asymmetric Alfvén speed. This is due to the fact that in asymmetric reconnection, the plasma with the smaller mass flux into the dissipation region contributes a smaller mass to the dissipation region, so the effect of its flow on opposing the release of energy by the reconnected magnetic fields is diminished and the reconnection is not suppressed to the extent previously thought. The results compare favorably with an observation of reconnection at Earth's polar cusps during a period of northward interplanetary magnetic field, where reconnection occurs despite the magnetosheath flow speed being more than twice the magnetosheath Alfvén speed, the previously proposed suppression condition. These results are expected to be of broad importance for magnetospheric physics of Earth and other planets; particular applications are discussed.

Modeling Particle Shape-Dependent Dynamics in Nanomedicine

One of the major challenges in nanomedicine is to improve nanoparticle cell selectivity and adhesion efficiency through designing functionalized nanoparticles of controlled sizes, shapes, and material compositions. Recent data on cylindrically shaped filomicelles are beginning to show that non-spherical particles remarkably improved the biological properties over spherical counterpart. Despite these exciting advances, non-spherical particles have not been widely used in nanomedicine applications due to the lack of fundamental understanding of shape effect on targeting efficiency. This paper intends to investigate the shape-dependent adhesion kinetics of non-spherical nanoparticles through computational modeling. The ligand-receptor binding kinetics is coupled with Brownian dynamics to study the dynamic delivery process of nanorods under various vascular flow conditions. The influences of nanoparticle shape, ligand density, and shear rate on adhesion probability are studied. Nanorods are observed to contact and adhere to the wall much easier than their spherical counterparts under the same configuration due to their tumbling motion. The binding probability of a nanorod under a shear rate of 8 s(-1) is found to be three times higher than that of a nanosphere with the same volume. The particle binding probability decreases with increased flow shear rate and channel height. The Brownian motion is found to largely enhance nanoparticle binding. Results from this study contribute to the fundamental understanding and knowledge on how particle shape affects the transport and targeting efficiency of nanocarriers, which will provide mechanistic insights on the design of shape-specific nanomedicine for targeted drug delivery applications.

Effects of Flow Shear Stress and Mass Transport on the Construction of a Large-Scale Tissue-Engineered Bone in a Perfusion Bioreactor

Currently, a tissue-engineered bone is usually constructed using a perfusion bioreactor in vitro. In the perfusion culture, fluid flow can exert shear stress on the cells seeded on scaffold, improving the mass transport of the cells. This experiment studied the effects of flow shear stress and mass transport, respectively, on the construction of a large-scale tissue-engineered bone using the critical-sized beta-tricalcium phosphate scaffold seeded with human bone marrow-derived mesenchymal stem cells (hBMSCs). This was done by changing flow rate and adding dextran into the media, thus changing the media's viscosity. The cells were seeded onto the scaffolds and were cultured in a perfusion bioreactor for up to 28 days with different fluid flow shear stress or different mass transport. When the mass transport was 3 mL/min, the flow shear stress was, respectively, 0.005 Pa (0.004-0.007 Pa), 0.011 Pa (0.009-0.013 Pa), or 0.015 Pa (0.013-0.018 Pa) in different experiment group obtained by simulation and calculation using fluid dynamics. When the flow shear stress was 0.015 Pa (0.013-0.018 Pa), the mass transport was, respectively, 3, 6, or 9 mL/min. After 28 days of culture, the construction of the tissue-engineered bone was assessed by osteogenic differentiation of hBMSCs and histological assay of the constructs. Extracellular matrix (ECM) was distributed throughout the entire scaffold and was mineralized in the perfusion culture after 28 days. Increasing flow shear stress accelerated the osteogenic differentiation of hBMSCs and improved the mineralization of ECM. However, increasing mass transport inhibited the formation of mineralized ECM. So, both flow shear stress and transport affected the construction of the large-scale tissue-engineered bone. Moreover, the large-scale tissue-engineered bone could be better produced in the perfusion bioreactor with 0.015 Pa (0.013-0.018 Pa) of fluid flow shear stress and 3 mL/min of mass transport.

Anomalous parallel momentum transport due to E×B flow shear in a tokamak plasma

Nondiffusive anomalous momentum transport in toroidal plasmas occurs through symmetry breaking mechanisms. In this paper the contribution of sheared E×B flows to parallel momentum transport [R. R. Dominguez and G. M. Staebler, Phys Fluids B 5, 3876 (1993)] is investigated with nonlinear gyrokinetic simulations in toroidal geometry. The background perpendicular shear is treated independently from the parallel velocity shear to isolate a nondiffusive, nonpinch contribution to the parallel momentum flux. It is found that the size of the term depends strongly on the magnetic shear, with the sign reversing for negative magnetic shear. Perpendicular shear flows are responsible for both symmetry breaking and suppression of turbulence, resulting in a shearing rate at which there is a maximum contribution to the momentum transport. The E×B momentum transport is shown to be quenched by increasing flow shear more strongly than the standard linear quench rule for turbulent heat diffusivity.

Localized Turbulence Suppression and Increased Flow Shear near theq=2Surface during Internal-Transport-Barrier Formation

Broadband turbulent fluctuations in the plasma density are transiently suppressed when low-order rational q surfaces first appear in negative central magnetic shear plasmas on the DIII-D tokamak, which can lead to the formation of internal transport barriers. Increased localized flow shear is simultaneously observed. It transiently exceeds the measured turbulence decorrelation rate, providing a mechanism to trigger the formation of the transport barrier. This increased flow shear and turbulence suppression propagates radially outward, following the q=2 surface.

Impact of root architecture on the erosion‐reducing potential of roots during concentrated flow

AbstractMany studies focus on the effects of vegetation cover on water erosion rates, whereas little attention has been paid to the effects of the below ground biomass. Recent research indicates that roots can reduce concentrated flow erosion rates significantly. In order to predict this root effect more accurately, this experimental study aims at gaining more insight into the importance of root architecture, soil and flow characteristics to the erosion‐reducing potential of roots during concentrated flow. Treatments were (1) bare, (2) grass (representing a fine‐branched root system), (3) carrots (representing a tap root system) and (4) carrots and fine‐branched weeds (representing both tap and fine‐branched roots). The soil types tested were a sandy loam and a silt loam. For each treatment, root density, root length density and mean root diameter (D) were assessed. Relative soil detachment rates and mean bottom flow shear stress were calculated. The results indicate that tap roots reduce the erosion rates to a lesser extent compared with fine‐branched roots. Different relationships linking relative soil detachment rate with root density could be established for different root diameter classes. Carrots with very fine roots (D &lt; 5 mm) show a similar negative exponential relationship between root density and relative soil detachment rate to grass roots. With increasing root diameter (5 &lt; D &lt; 15 mm) the erosion‐reducing effect of carrot type roots becomes less pronounced. Additionally, an equation estimating the erosion‐reducing potential of root systems containing both tap roots and fine‐branched roots could be established. Moreover, the erosion‐reducing potential of grass roots is less pronounced for a sandy loam soil compared with a silt loam soil and a larger erosion‐reducing potential for both grass and carrot roots was found for initially wet soils. For carrots grown on a sandy loam soil, the erosion‐reducing effect of roots decreases with increasing flow shear stress. For grasses, grown on both soil types, no significant differences could be found according to flow shear stress. The erosion‐reducing effect of roots during concentrated flow is much more pronounced than suggested in previous studies dealing with interrill and rill erosion. Root density and root diameter explain the observed erosion rates during concentrated flow well for the different soil types tested. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Use of an In Situ Erosion Flume for Measuring Stability of Sediment Deposits in Hamilton Harbour, Canada

An in situ erosion flume was used to measure the stability of sediment deposits in Hamilton Harbour, Ontario, Canada. The flume consists of a rectangular duct with an opening at the bottom. A submerged pump attached to the downstream end of the flume circulates the ambient water through the flume, thereby generating turbulent shear flows inside the flume. When the flume rests on a sediment deposit, the exposed part of the sediment deposit is subjected to the flow shear stress. By applying a continually increasing flow shear stress on the sediment deposit and by measuring the amount of sediment erosion, it is possible to assess the erosional stability of the sediment deposits. An under water video camera was mounted on the flume to get visual images of the sediment erosion process. The flume was used at two sites in the Harbour. The erosional resistances measured by the flume for the two sites were different. Measurement of dry density of the sediment deposits using an ultrasonic device was carried out to explain the differences in the stability of sediment deposits from the two sites.

Simulation Studies of High-Latitude Magnetospheric Boundary Dynamics

Magnetic reconnection at the high-latitude magnetopause is studied using 2.5-dimensional Hall-MHD simulation. Concentric flow vortices and magnetic islands appear when both Hall effect and sheared flow are considered. Plasma mixing across the magnetopause occurs in the presence of the flow vortices. Reconnected structure generated in the vicinity of the subsolar point changes its geometry with increasing flow shear while moving to high latitudes. In the presence of flow shear, with the Hall-MHD reconnection a higher reconnection rate than with the traditional MHD is obtained. The out-of-plane components of flow and magnetic field produced by the Hall current are redistributed under the action of the flow shear, which makes the plasma transport across the boundaries more complicated. The simulation results provide some help in understanding the dynamic processes at the high latitude magnetopause.

Simulation studies of high-latitude magnetospheric boundary dynamics

Magnetic reconnection at the high-latitude magnetopause is studied using 2.5-dimensional Hall-MHD simulation. Concentric flow vortices and magnetic islands appear when both Hall effect and sheared flow are considered. Plasma mixing across the magnetopause occurs in the presence of the flow vortices. Reconnected structure generated in the vicinity of the subsolar point changes its geometry with increasing flow shear while moving to high latitudes. In the presence of flow shear, with the Hall-MHD reconnection a higher reconnection rate than with the traditional MHD is obtained. The out-of-plane components of flow and magnetic field produced by the Hall current are redistributed under the action of the flow shear, which makes the plasma transport across the boundaries more complicated. The simulation results provide some help in understanding the dynamic processes at the high latitude magnetopause.