Increasing Step Height Research Articles

Geometry effect on flow fluctuation and heat transfer in unsteady forced convection over backward and forward facing steps

This study presents a direct numerical simulation of the geometry effect on fluid flow and heat transfer characteristics in the transitional flow over backward and forward facing steps. A computer program of FORTRAN code is used to solve the governing equations according to finite volume method. The effects of the bottom wall length and the step height are investigated. With the increase of bottom wall length, a fundamental frequency of flow fluctuation appears at Lb = 10 h and then disappears at Lb = 14 h. This fundamental fluctuating frequency decreases with the increase of step height. The fact that the heat transfer performance is better in the reattachment region at Lb = 12 h and S = h indicates that the periodic flow fluctuation with a larger fluctuating velocity has positive effects on heat transfer enhancement. The results of the relationship between flow fluctuation and heat transfer enhancement are significant for certain practical applications of energy conversion.

Effect of interlayer bonding strength and bending stiffness on 2-dimensional materials’ frictional properties at atomic-scale steps

Atomic-scale steps generally presented in 2-dimensional materials have important influence on the overall nanotribological properties of surface. Frictional properties at atomic-scale steps of two types of 2-dimensional materials are studied using calibrated atomic force microscopy (AFM) tip sliding against the steps. The lateral force at uncovered step is larger than covered step due to the bending of step edge. The lateral force at monolayer uncovered step edge of h-BN is lower than graphene because h-BN possesses higher interlayer bonding strength than graphene and the bending of h-BN step edge is suppressed to some extent. The high uncovered step exhibits much larger lateral force than low uncovered step, which could be mainly induced by increased bending stiffness of step edge rather than increased step height. The results revealed that interlayer bonding strength and bending stiffness have great influence on the lateral force at atomic-scale steps. The studies can provide a further understanding of frictional properties at atomic scale steps and could be helpful for the applications of 2-dimensional materials as lubricant coating.

Variable Damping Force Tunnel for Gait Training Using ALEX III.

Haptic feedback affects not only the quality of training but can also influence the physical design of robotic gait trainers by determining how much force needs to be applied to the user and the nature of the force. This paper presents the design of a variable damping force tunnel and explores the effect of the shape and strength of the damping field using ALEX III, a treadmill-based exoskeleton developed at Columbia University. The study consists of 32 healthy subjects who were trained for 40 minutes in the device. The subjects were trained to follow a footpath with a 50% increase in step height, so the foot would have 1.5 times the ground clearance. Subjects were assigned to one of four groups: linear high, linear low, parabolic high, and parabolic low. Linear or parabolic denotes the shape of the damping field, and high or low denotes the rate of change (strength) of the field based on error. It is shown that the new controller is capable of inducing gait adaptations in healthy individuals while walking in the device. All groups showed adaptations in step height, while only the high strength groups showed changes in normalized error area, a measure of how closely the desired path was followed.

Morphological dynamics of an englacial channel

Abstract. Despite an interest in the hydraulic functioning of supraglacial and englacial channels over the last 4 decades, the processes and forms of such ice-bounded streams have remained poorly documented. Recent glaciological research has demonstrated the potential significance of so-called "cut-and-closure" streams, where englacial or subglacial flow paths are created from the long-term incision of supraglacial channels. These flow paths are reported to exhibit step-pool morphology, comprising knickpoints and/or knickzones, exaggerated in dimensions in comparison to supraglacial channels. However, little is known of the development of such channels' morphology. Here, we examine the spatial organisation of step pools and the upstream migration of steps, many of which form knickzones, with repeated surveys over a 10-year period in an englacial conduit in cold-based Austre Brøggerbreen, Svalbard. The observations show upstream step recession to be the dominant process for channel evolution. This is paralleled by an increase in average step height and conduit gradient over time. Characteristic channel-reach types and step-riser forms are consistently observed in each of the morphological surveys reported. We suggest that the formation of steps has a hydrodynamic origin, where step-pool geometry is more efficient for energy dissipation than meanders. The englacial channel system is one in rapid transition towards a quasi-equilibrium form within a decadal timescale. The evolution and recession of knickzones reported here result in the formation of a 37 m deep moulin shaft, suggesting that over time an incising supraglacial channel may evolve towards an englacial channel form exhibiting a stable end-point characterised by a singular vertical descent, which potentially can reach the glacier bed. This challenges the prevailing notions that crevasses or hydrofractures are needed to form deep moulins. Our observations highlight the need to further examine the adjustment processes in cut-and-closure channels to better understand their coupling to supraglacial meltwater sources and potential significance in cold-based glacier hydrology and ice dynamics.

Step Crowding Effects Dampen the Stochasticity of Crystal Growth Kinetics.

Crystals grow by laying down new layers of material which can either correspond in size to the height of one unit cell (elementary steps) or multiple unit cells (macrosteps). Surprisingly, experiments have shown that macrosteps can grow under conditions of low supersaturation and high impurity density such that elementary step growth is completely arrested. We use atomistic simulations to show that this is due to two effects: the fact that the additional layers bias fluctuations in the position of the bottom layer towards growth and by a transition, as step height increases, from a 2D to a 3D nucleation mechanism.

Sequence of stannous and sodium fluoride solutions to prevent enamel erosion

ObjectivesInvestigate the timing of stannous (SnF2) and sodium fluoride (NaF) application with and without salivary pellicle to prevent enamel erosion. MethodsHuman buccal molar enamel samples (n=120, REC ref 12/LO/1836) were randomly assigned to three groups testing SnF2 and NaF basic fluoride formulation and commercial mouthrinses with and without the presence of human saliva. Samples were randomly allocated to 2 subgroups: immersion in either fluoride for 1min either before or after citric acid immersion (0.3%, pH 3.2, 10min), and the cycle repeated 5 times. For human saliva group, samples were immersed in 80ml of natural saliva for 24h prior to the experiment. Analysis was done using non-contacting profilometry and microhardness change. Data were not normal and were log transformed. A linear model tested statistical differences between the groups. ResultsSnF2 application before erosion statistically reduced step height compared to application after erosion for all groups (solutions: 6.5μm (±1.2), 7.5μm (±0.8); p=0.01, mouthrinses: 3.2μm (±0.6), 4.2μm (±0.7); p<0.0001, mouthrinses with saliva: 2.5μm (±0.4), 3.1μm (±0.6); p=0.002, before and after respectively). In contrast, application of NaF before erosion increased step height compared to application after, but this was only statistically significant for the saliva group (before: 5.6μm (±0.3) and after: 4.9μm (±0.3); p=0.023). Presence of saliva increased microhardness change (p<0.0001). Within this group, greatest microhardness change was observed when SnF2 was applied before erosion and when NaF was applied after erosion (SnF2: 156.6KHN (±32.8), 123KHN (±20.1); p=0.02. NaF: 119.5KHN (±33.5), 218KHN (±24.9), before, and after respectively). ConclusionSnF2 reduced step height formation overall when compared to NaF, but particularly when applied before citric acid immersion. In contrast, NaF reduced step height when applied after citric acid immersion, but only in the presence of saliva. Clinical significanceStannous fluoride can be recommended over sodium fluoride to patients at risk of dental erosion and is optimally applied before erosion occurs. If sodium fluoride is to be used in the presence of saliva it is optimally applied after erosion has occurred.

Squeeze film lubrication between circular stepped plates: Rabinowitsch fluid model

In this paper, a theoretical analysis on the squeeze film characteristics between circular stepped plates lubricated with Rabinowitsch fluid is presented. By using Rabinowitsch fluid model, the modified Reynolds type equation is derived to study the dilatant and pseudoplastic nature of the fluid in comparison with Newtonian fluid. The closed form solution is obtained by using perturbation method. According to the results obtained, the load-carrying capacity and squeeze film time increases for dilatant fluids as compared to the corresponding Newtonian fluids whereas the reverse trend is observed for pseudoplastic fluids. Further, it is observed that the response time decreases as the step height increases.

S11-2. The effects of walking exercises on spinal cord injury

The aim of this study was to compare the functional effects of early and delayed interventions involving walking exercises with treadmills, using animal subjects. Contusive spinal cord injury (SCI) was made at the Th10 level in adult Sprague–Dawley rats, which were then assigned to one of the following conditions: control, early training (days 7 to 20), and delayed training (days 21 to 34). A significant increase in step height and step length was observed in the early-training group, while the delayed-training group showed no significant improvement. In addition, a reduction in synergic contraction was observed in electromyographic activity of the hind limb muscles, as well as a reduction in spastic muscle activities, but this was observed only in the early-training group. Immunohistochemical analysis of the lumbar spinal cord using synaptic markers showed a significant decrease in the inhibitory synapses and an increase in the excitatory synapses which were caused by an earlier start to training. Further studies are necessary to determine underlying mechanisms involved the above process.

Non-intrusive detection of air–water surface roughness in self-aerated chute flows

Stepped spillways are known to enhance the energy dissipation potential when compared with common smooth invert chutes. In the skimming flow regime, the self-aerated flow becomes more chaotic and surface waves (referred to as air–water surface roughness) are enhanced when the discharge decreases. In this study, experiments on smooth invert and stepped chutes models with a slope of 1: 2 were conducted to characterize this surface roughness by the use of a high-speed camera and ultrasonic sensor. It was found that the amount of entrapped air – at the water level where the air concentration is 90% – is reduced when a stepped spillway is considered. With decrease in step height (and on smooth invert chutes), entrapped air became more relevant. Wave heights increased with increase in step height and exceeded significantly the characteristic water level with 90% air concentration which was detected by a conductivity probe. A wide range of wave frequencies indicated the turbulent structure of stepped spillway flows.

Squeeze Film Lubrication between Porous Parallel Stepped Plates with Couple Stress Fluids

This paper presents the theoretical study of squeeze film lubrication between porous parallel stepped plates with couple stress fluids. The lubricant with additives in the film region and also in the porous region in modeled as Stokes couple stress fluid. The modified Reynolds equation is derived for porous parallel stepped plates with couple stress fluids. The closed form expressions are obtained. According to the results obtained, the influence of couple stresses enhances the squeeze film pressure, load carrying capacity and decreases the response time as compared to the classical Newtonian-lubricant case. The load carrying capacity decreases as the step height increases.

Progressive limb ataxia following inferior olive lesions

Cerebellar climbing fibres originate in the inferior olive (IO). Temporary IO inactivation produces movement deficits. Does permanent inactivation produce similar deficits and, if so, do they recover? The excitotoxin, kainic acid, was injected into the rostral IO of three cats. Behaviour was measured during reaching and locomotion. Two cats were injected during the reaching task. Within minutes, grasping became difficult and the trajectories of the reaches showed higher arcing than normally seen. During locomotion, both cats showed head and trunk deviation to the injected side, walking paths curved to the injected side, and the paws were lifted higher than normal. Limbs contralateral to the injections became rigid. Within 1 day, posture had normalized, locomotion was unsteady and high lifting of the paws had reversed to a tendency to drag the dorsum of the paws. Passive body movement produced vestibular signs. Over a few days, locomotion normalized and vestibular signs disappeared. Reach trajectories were normal but grasping deficits persisted. Over the first week, the amplitude of limb lift during reaching and locomotion began to increase. The increase continued over time and, after several months, limb movements became severely ataxic. The effects followed the somatotopy of the rostral IO: a loss of cells in medial rostral IO only affected the forelimb, whereas a loss of cells in medial and lateral IO affected both forelimb and hindlimb. Deficits produced by IO lesions involve multiple mechanisms; some recover rapidly, some appear stable, and some worsen over time. The nature of the progressive deficit suggests a gradual loss of Purkinje cell inhibition on cerebellar nuclear cells.

Effect of Step Geometry on Local Scour Downstream of Stepped Chutes

A chute is characterized by a steep bed slope associated with torrential flow. The chute flow may be either smooth or stepped. Stepped spillways (or chutes) have become a popular method for handling flood releases. Scour hole formed downstream of a chute may affect the safety and stability of the hydraulic structure. In this study, the effect of step geometry on the dynamics of local scour processes is analyzed in the context of the scour that takes place downstream from a stepped chute. Three different step heights have been used to study the scour process for various chute angles, stilling basin sill heights, tailwater depths and flow rate conditions. A total of 216 laboratory tests have been conducted and the findings have been analyzed to determine the influence of step geometry on the scouring at the downstream of the stepped chute. The results show that the equilibrium depth of scour is highly dependent on the step geometry. In addition, the equilibrium depth of scour decreases while the step height increases. Also, the equilibrium depth of scour also increases with an increase in the discharge and the chute angle.

Effect of robotic performance-based error-augmentation versus error-reduction training on the gait of healthy individuals

Effective locomotion training with robotic exoskeletons requires identification of optimal control algorithms to better facilitate motor learning. Two commonly employed training protocols emphasize use of training stimuli that either augment or reduce performance errors. The current study sought to identify which of these training strategies promote better short-term modification of a typical gait pattern in healthy individuals as a framework for future application to neurologically impaired individuals. Ten subjects were assigned to each of a performance-based error-augmentation or error-reduction training group. All subjects completed a 45-min session of treadmill walking at their preferred speed with a robotic exoskeleton. Target templates prescribed an ankle path for training that corresponded to an increased step height. When subjects’ instantaneous ankle positions fell below the inferior virtual wall of the target ankle path, robotic forces were applied that either decreased (error-reduction) or increased (error-augmentation) the deviation from the target path. When the force field was turned on, both groups walked with ankle paths better approximating the target template compared to baseline. When the force field was removed unexpectedly during catch and post-training trials, only the error-augmentation group maintained an ankle path close to the target ankle path. Further investigation is required to determine if a similar training advantage is provided for neurologically impaired individuals.

Effect of surface step on nanoindentation of thin films by multiscale analysis

Nanoindentation simulations on flat and stepped surfaces are respectively investigated using the quasicontinuum method based on the embedded-atom method potential. Effect of surface step considering indenter size and step height is studied. Results show that the critical load for the first dislocation emission will be decreased with the increase of step height. However, the effect of surface step will be weakened if the indenter size continues to increase. Initial atomistic structures after dislocation nucleation and emission are discussed systematically. The initial dislocations are not quite identically nucleated under the stepped surface. Stress distribution analysis reveals that the shear stress in the slip planes close to the step is much larger than the shear stress in the slip planes far from the step for nanoindentation on the stepped surface. The multiscale simulation results are consistent with experimental results and analytic solutions. The conclusions about step effect considering indenter size and step height are helpful for understanding the microscopic mechanism of nanoindentation tests on thin films with surface step.

Direct MRI-guided stereotaxic viral mediated gene transfer of alpha-synuclein in the Göttingen minipig CNS.

The aim was to establish a non-primate large animal PD model by lentiviral vector mediated mutant alpha-synuclein overexpression in the substantia nigra. Lentivirus encoding A53T alpha-synuclein (6 x 2.5 μl) was stereotaxically injected into the substantia nigra of six adult female Göttingen minipigs. Contralateral control injections encoding enhanced green fluorescent protein (EGFP) were performed. Gait-analysis was performed pre- and postoperatively. PCR of the transgenes and immunohistochemical staining against alpha-synuclein, EGFP, GFAP and TH was performed after 20 weeks. Gait analysis revealed a significant increase in step length and height, and a decrease in the double stand phase. PCR verified the mesencephalic presence of transgenes. IHC analysis showed alpha-synuclein expression in nigral neurons, around the injection tract and in related nigrostriatal projections. The alpha-synuclein positive neurons appeared swollen and vacuolated, in contrast to the EGFP-injected control side. To transduct all nigrostriatal cells with few microinjections, wider dissemination of the transgene must be achieved.

Perturbations of ground support alter posture and locomotion coupling during step initiation in Parkinson’s disease

During the initiation of stepping, anticipatory postural adjustments (APAs) for lateral weight transfer and propulsion normally precede the onset of locomotion. In Parkinson's disease (PD), impaired step initiation typically involves altered APA ground force production with delayed step onset and deficits in stepping performance. If, as in stance and gait, sensory information about lower limb load is important for the control of stepping, then perturbations influencing loading conditions could affect the step initiation process. This study investigated the influence of changes in lower limb loading during step initiation in patients with PD and healthy control subjects. Participants performed rapid self-triggered step initiation with the impending single stance limb positioned over a pneumatically actuated platform. In perturbation trials, the stance limb ground support surface was either moved vertically downward (DROP) or upward (ELEVATE) by 1.5 cm shortly after the onset of the APA phase. Overall, PD patients demonstrated a longer APA duration, longer time to first step onset, and slower step speed than controls. In both groups, the DROP perturbation reinforced the intended APA kinetic changes for lateral weight transfer and resulted in a significant reduction in APA duration, increase in peak amplitude, and earlier time to first step onset compared with other conditions. During ELEVATE trials that opposed the intended weight transfer forces both groups rapidly adapted their stepping to preserve standing stability by decreasing step length and duration, and increasing step height and foot placement laterally. The findings suggested that sensory information associated with limb load and/or foot pressure modulates the spatial and temporal parameters of posture and locomotion components of step initiation in interaction with a centrally generated feedforward mode of neural control. Moreover, impaired step initiation in PD may at least acutely be enhanced by augmenting the coupling between posture and locomotion.

Assessments of critical flux in a pilot-scale membrane bioreactor

In this study, the influence of various parameters such as determination methods and step height and length of incremental flux on critical flux values were investigated. Experiments were carried out on a pilot-scale membrane bioreactor (MBR) treated municipal wastewater. Three of the five critical flux determination methods, such as flux linearity, 90% permeability and flux cycling conducted in this study, indicated a decline in critical flux values as the step height of incremental flux increased. However, the hysteresis method and the two-third (2/3) flux limitation method showed an increase and independence of critical flux to the step height of incremental flux, respectively. On the other hand, the step length of incremental flux had no obvious effect on critical flux values evaluated by all critical flux determination methods. Like critical flux, sustainable flux has negative relationship with the increase of step height but no influence of step length was found in this study.

Self-Patterned Mixed Phospholipid Monolayers for the Spatially Selective Deposition of Metals

Metal-reactive organosulfur groups were patterned onto mica and silicon surfaces by dewetting instabilities during the Langmuir-Blodgett (LB) deposition of phase-separated mixed phospholipid monolayers. Monolayers were formed from binary mixtures of dipalmitoylphosphatidylcholine (DPPC), dilauroylphosphatidylcholine (DLPC), and their ω-methyldisulfide-dialkylphosphatidylcholine analogues, DSDPPC and DSDLPC. Patterns of highly parallel stripes of condensed DPPC or DSDPPC, protruding by 0.7-0.9 nm from a fluid matrix of DLPC or DSDLPC, were observed over areas extending at least 30 × 30 μm(2) in the LB films. The average stripe width varied from ca. 150 to 500 nm, depending on the lipid composition and deposition pressure. X-ray photoelectron spectroscopy confirmed that the phospholipid-monolayer-bound methydisulfides react with Au vapor to form a gold-thiolate species. The adsorption of thermally evaporated Au, Ag, and Cu onto DSDPPC/DLPC and DPPC/DSDLPC patterns was investigated by field emission gun scanning electron microscopy (FEGSEM) and atomic force microscopy (AFM). A change in phase contrast is observed in FEGSEM and AFM over the methyldisulfide-functionalized areas following metal deposition due to metal-thiolate bond formation. An increase in step height between the DSDPPC stripes and nonfunctionalized DLPC background following metal deposition, as well as the resistance of the metal-coated DSDPPC or DSDLPC regions to detergent extraction from the surface, attest to a selective metallization of the pattern. Our results indicate that the preferential adsorption of vapor-deposited metal onto the ω-methyldisulfide-terminated phase occurs at submonolayer coverages. The chemical reactivity exhibited by the organosulfur-modified phospholipid LB films make these templates potentially interesting for the fabrication of solid-supported patterns of metal nanostructures.

Hydraulics of discharge tunnel service gate: the flow regimes

During experiments which were conducted on a discharge tunnel laboratory model, bottom cavity beneath the free-falling nappe immediate downstream of the service gate was focused on. Three different flow regimes were detected each of which with particular features. The results provide a fair characterization of each flow regime. They highlight some differences between the limits of flow regimes in terms of dimensionless bottom cavity length and Froude number values under various step heights. The results also demonstrate that increase in step height at service gate provides more aeration in lower range of flow rates.

Crystal growth mechanisms of the (0 1 0) face of α-lactose monohydrate crystals

The growth rates of the (0 1 0) face of α-lactose monohydrate crystals were measured at 30, 40 and 50 °C in the relative supersaturation range 0.55–2.33 in aqueous solutions. The mechanisms of growth were investigated. Spiral growth was found to be the mechanism of growth up to a critical relative supersaturation ( s−1) crit=1.9 at 30 °C. Above the critical relative supersaturation, the crystal growth mechanisms were predicted to change. All growth models fit equally well to the growth rates. No two-dimensional nucleation was observed above critical supersaturation by AFM. On the other hand increased step height and roughness on the edges of steps were observed. It was concluded that the growth mechanism of the (0 1 0) face of α-lactose monohydrate crystal is spiral growth. A parabolic relationship was obtained below critical supersaturation followed by a linear relationship with relative supersaturation.