Channel Roughness Research Articles

Capillary rise in superhydrophilic rough channels

Surface tension forces enable a liquid to rise against gravity when wettable tubes or porous media are in contact with the pool of liquid. While the rise dynamics in the media of homogeneous porosity are well known, those in heterogeneous porous media still remain poorly understood. Here, we employ a vertical channel formed by two parallel plates decorated with micropillars, as a simple model of bidisperse porous media, and observe the rise dynamics of various viscous liquids. We find the bulk rise speed to be higher than that in dry smooth channels but equal to that between prewetted smooth channels. As the bulk approaches its equilibrium height, a film emerges ahead of the bulk meniscus, which is driven by the high surface energy of the microdecorated surface. The film extension grows initially like t but later like t1/2, with t being time. We construct theoretical models to predict the critical height where the film emerges and to rationalize the power laws of the film extension. In particular, we show that the dominant viscous resistance to the film extension is provided by the flow from the reservoir through the bulk in the early stages and by the film itself in the late stages. Our study opens a pathway to scrutinize the complicated flow dynamics arising within and across voids of heterogeneous porous media with an easily observable experimental setup of a well-defined geometry.

FLOW CHARACTERISTICS OF OPEN CHANNELS WITH FLOATING VEGETATION

Floating vegetation (Water hyacinth) causes many problems in waterways where it retards flow movement and changes flow structure. In this study the impact of water hyacinth on flow behavior in open channels was investigated in a laboratory flume. Water hyacinth was harvested from natural streams. Four different vegetation densities, three different vegetation root depths and three different vegetation zone lengths were used for smooth and rough channel bed. The case without water hyacinth was considered as comparative case. The water surface profiles for all cases were investigated. Results show that in the presence of water hyacinth water surface profile rises before vegetation zone then goes down until reaches to normal water surface. Results also show that water surface profile influenced by vegetation density, vegetation root depth, and vegetation zone length and bed roughness. Also the effect of vegetation parameters (density, root depth, zone length) on heading up, discharge and equivalent Manning's roughness coefficient was investigated and it is found that these parameters have a great effect on it. Multiple regression equations based on the dimensional analysis theory were deduced to calculate heading up, discharge and equivalent Manning's roughness coefficient.

Use of high-water marks and effective discharge calculation to optimize the height of bank revetments in an incised river channel

In deeply incised rivers, bankfull discharge (i.e. the flow filling the channel to the top of the banks) does not represent channel forming flow and increasingly large flows are associated with increasingly large boundary shear stress. In such rivers, solid bank revetments (rip-rap, gabions, retaining wall) are usually constructed to the top of the banks—similarly as in vertically stable rivers—despite the fact that the upper parts of the banks may never be flooded. To optimize the height of solid bank revetments in deeply incised channels, it is thus important to determine whether a flood magnitude can be identified, for which the combination of flow duration and bedload transport rate results in the highest river efficiency to transport bedload and perform geomorphic work. This question was explored in the Morávka River, Czech Carpathians, which deeply incised into non-resistant flysch bedrock over the past few decades. Observations of high-water marks (e.g. trash lines, wash lines) after a flood in 2014 enabled reconstruction of the peak flood stage in the deeply incised reach and the adjacent, vertically stable reach. These observations, together with post-flood measurements of cross-sectional channel geometry, distances between consecutive cross sections and estimates of channel roughness, were used in one-dimensional hydraulic modelling aimed to determine a peak discharge of the flood in a number of cross sections in both reaches. Despite the close proximity of both reaches, markedly higher discharge values were obtained for the incised reach and the discrepancy was used to calibrate roughness coefficients for the incised reach. A flow-duration curve determined on the basis of a 25-year series of daily discharges in the upstream gauging station together with data about channel geometry and roughness in the incised cross sections were used to simulate bedload transport at successive discharges with the BAGS sediment transport model. The calculations indicated the effective discharge for bedload transport in the incised reach to have the recurrence interval of 7–12 years. The upper limit of the reach-average effective discharge is associated with the stage slightly higher than a half of the bankfull depth. This highlights the disparity between effective and bankfull discharges and indicates that in incised channels, channel forming flow should be linked with the former. The vertical extent of the reach-average effective discharge is proposed as an indicator of the optimal, cost-effective height of solid bank-protection structures in the deeply incised channel and the suitability of this approach is discussed.

Exploitation of Velocity Gradients by Sympatric Stream Salmonids: Basic Insights and Implications for Instream Flow Management

AbstractHydraulic heterogeneity can strongly influence habitat selection by stream fishes. Velocity gradients created by channel roughness and flow obstructions may be particularly important for species that feed on drifting invertebrates, where maintaining focal points in low‐velocity microhabitats adjacent to faster water allows fish to scan a larger water volume for prey while minimizing swimming costs. However, these velocity gradients are rarely integrated into habitat suitability criteria used for defining instream flow requirements, which are generally based on mean column velocity measurements at the focal point location. It is also unclear how velocity gradient exploitation differs among sympatric drift‐feeding species. We measured the use of velocity gradients by two sympatric juvenile salmonids, Coho Salmon Oncorhynchus kisutch and steelhead O. mykiss, in a midorder cobble–boulder‐dominated river. We compared focal point velocities of fish to adjacent velocities within their foraging area and compared the magnitude of velocity and kinetic energy gradients between species. We then explored how lateral velocity gradients may bias instream flow assessments by deriving two sets of velocity habitat suitability curves (HSCs): conventional HSCs using average water column velocities measured at focal point locations and spatially averaged HSCs incorporating adjacent velocities (four body lengths from focal points). These contrasting HSCs were then used as input into the physical habitat simulation model to predict the influence of flow on habitat availability. Both species often used focal velocities that were lower than adjacent points, but the magnitude of these velocity gradients was higher for steelhead, consistent with known differences in foraging behavior, habitat selection, and physiology. Incorporating adjacent velocities into HSCs resulted in a ~40% (steelhead) and ~10% (Coho Salmon) increase in flows predicted to optimize habitat availability. Thus, small‐scale heterogeneity in velocity used by drift‐feeding fish can lead to large biases in flow assessments.

Theoretical foundation for calculating hydrodynamic characteristics of turbulent fluid flow in tubular mixing devices to intensify reagent wastewater treatment

The results of theoretical studies of the hydrodynamic laws of fluid flow and changes in the characteristics of the wall layer of the flow in tubular mixing devices, used in technological processes of reagent-based wastewater treatment, are presented. A mathematical relation, which allows determining the critical value of the Reynolds number Re, at which the regime of fluid movement in a tubular mixing device passes into the region of the quadratic resistance law of rough channels with the maximum degree of turbulence of flows, is obtained. It is shown that the main technical characteristics of tubular mixing devices are: the magnitude of the pulsation component of the local velocity ∆υl (m/s) and the value of the turbulent diffusion coefficient DT (m2/s) in the wall region of the turbulent fluid flow. Mathematical relations, which allow calculating the magnitude of the pulsation component of the local velocity and the turbulent diffusion coefficient in the wall region of the fluid flow in the pressure channels of circular cross section, are obtained. Using the proposed calculation technique will allow to optimize the operation regime of tubular mixing devices.

Seasonal macrophyte growth constrains extent, but improves quality, of cold‐water habitat in a spring‐fed river

AbstractRooted aquatic macrophytes affect abiotic conditions in low‐gradient rivers by altering channel hydraulics, consuming biologically available nutrients, controlling sediment transport and deposition, and shading the water surface. Due to seasonal macrophyte growth and senescence, the magnitude of these effects may vary temporally. Seasonal changes in aquatic macrophyte biomass, channel roughness and flow velocity, were quantified and trends were related to spatiotemporal patterns in water temperature in a low‐gradient, spring‐fed river downstream from high‐volume, constant‐temperature groundwater springs. Between spring and summer, a nearly threefold increase in macrophyte biomass was positively correlated with channel roughness and inversely related to flow velocity. On average, flow velocity declined by 34% during the study period, and channel roughness increased 63% (from 0.064 to 0.104). During the spring and fall period, the location of a minimum water temperature variability “node” migrated upstream more than 4 km, whereas daily maximum water temperature cooled by 2–3°C. Water temperature modelling shows that the longitudinal extent of cold‐water habitat was shortened due to increased channel roughness independent of seasonal surface water diversions. These results suggest that macrophyte growth mediates spatiotemporal patterns of water temperature, constraining available cold‐water habitat while simultaneously improving its quality. Understanding complex spatial and temporal dynamics between macrophyte growth and water temperature is critical to developing regulatory standards reflective of naturally occurring variability and has important implications for the management and conservation of cold‐water biota.

Effect of axial conduction in integral rough friction stir channels: experimental thermo-hydraulic characteristics analyses

Integral mini-channels fabricated in the metal substrate by friction stir channeling process are different from conventional mini-channels due to irregular shape, and large surface roughness. The effect of axial wall conduction on heat transfer in such mini-channels is significant, which could limit their utilisation as cooling channels in heat transfer based applications. Hence, in this study, we present an experimental analysis of the thermo-hydraulic performance of a friction stir channel to identify the axial wall conduction effect using de-ionized water as working fluid. The thermo-hydraulic performance parameters are analysed in the range of Reynolds number of 500 to 2100. The local wall temperature distribution along the length of the mini-channel is found to be non-uniform due to the significant axial wall conduction effect. The distinct pattern of numerical heat flux distribution is used as evidence to confirm the presence of axial wall conduction effect which is implied by the local wall temperature distribution. The higher average heat transfer and flow characteristics compared to the theoretical predictions for conventional mini-channels are observed as a result of simultaneously developing flow and the presence of surface roughness.

An Anti-Eavesdropping Strategy for Precoding-Aided Spatial Modulation With Rough CSI of Eve

In this paper, an anti-eavesdropping strategy is proposed for secure precoding-aided spatial modulation networks, under the assumption that the rough channel state information of eavesdropper can be obtained at the transmitter. Traditionally, artificial noise (AN) can be always projected into the null-space of the legitimate channel, however it may lead to some security loss since this strategy dispenses with a holistic consideration for secure transmissions. To reduce the computational complexity of our optimization problem, we derive a closed-form expression that is a loose bound of the approximate rate over the illegitimate channel. Then a concave maximization problem is formulated for optimizing the covariance matrix of AN. Simulation results show that our proposed low-complexity scheme performs closely to the method which directly maximizes the approximate secrecy rate expression, and harvests significant secrecy rate gains compared with the traditional null-space projection benchmark.

The effect of channel roughness to the flow velocity (Case Study: secondary channel of the Saddang irigation area, Pinrang Regency)

The channel roughness phenomenon can result in changes in flow velocity. The research is aimed to analyze the magnitude of the speed of the flow that is made out of some roughness channels are different. Research is carried out in line Secondary Regional Irrigation Alitta village Bentenge Subdistrict Paleteang district Pinrang. Channels that are reviewed are the channel secondary which is made of some roughness channel are different by opening the doors as deep as 20 cm, 30 cm, and 40 cm. The results of the study indicate that happens changes the speed of each roughness channel. It is caused by the difference in the roughness of each channel and the sedimentation of each channel.

Surface integrity in abrasive flow machining (AFM) of internal channels created by selective laser melting (SLM) in different building directions

Selective laser melting (SLM) can produce a conformal cooling channel that is widely used in the mold industry. However, SLM inherent rough surface can cause stress concentration, reducing the fatigue life of the mold. In addition to the surface roughness, residual stress can affect fatigue life. Therefore, it is necessary to improve surface integrity, such as surface roughness and residual stress, of the internal channels created by SLM. The internal channels (ø3 mm) are created by SLM in horizontal and vertical directions. As a counterpart of the SLM process, the internal channel (ø3 mm) is also made by the conventional process, such as electrical discharge machining (EDM) in a wrought bar. After internal channels are finished by abrasive flow machining (AFM), the evolution of the internal channel’s surface integrity, such as surface roughness and residual stress, is investigated. The surface roughness of the horizontally and vertically SLM built as well as EDM generated channels are measured with a stylus. The initial surface topography of the SLM built channels differs significantly depending on the SLM building direction. The stylus measurement reveals significant improvement (< 1 µm Ra) in surface roughness of all internal channels after AFM. The residual stress on the internal surface channel is measured with X-ray diffraction (XRD). After AFM finish, the compressive residual stress in the AFM flow direction is found to be induced on the SLM built and EDM generated channels. Thus, AFM finish can be considered to be beneficial to the surface integrity of the internal channel created by SLM and EDM.

Effect of roughness coefficient on discharge and flow depth by using hydraulic model for Nethravathi River Basin, India

The river stage and discharge are dynamic due to various factors affecting the flow characteristics in a natural channel. The channel roughness plays an important role since it is not a constant parameter and varies along the length of the river. The objective of the present study is to assess the variation of Manning's roughness coefficient on flow characteristics of Nethravathi River. In the study, 1D Saint-Venant equation-based HEC-RAS hydraulic model was used to simulate the effect of roughness coefficient (Manning's coefficient n) on discharge and stage of river flow. The model result showed good consensus between model computed flow discharge values and observed flow discharge measured at downstream gauging station. The study found that the computed stage values increased whereas the associated peak discharge decreased with the increase in Manning's roughness coefficient. The stage-discharge rating curves revealed that Manning's n value is relatively more sensitive at higher discharge values.

Hydrodynamic and thermal flow in nanochannel to study effects of roughness by estimation the atoms positions via MD method

PurposeThis paper aims to investigate atoms type and channel roughness effects on fluid behavior in nanochannel.Design/methodology/approachThe results of mechanical properties of these structures are reported in this work by using molecular dynamics method.FindingsThe results show that nanochannel roughness is a limiting factor in flowing fluid in nanochannel. Moreover, fluids with less atomic weight have more free movement in ideal and non-ideal nanochannels.Originality/valueFor the study of mechanical properties of fluid/nanochannel system, the authors calculated parameters such as potential energy, density, temperature and velocity profiles of simulated fluids.

Reynolds number effects in rib-roughened turbulent channel flow

A study involving the direct numerical simulation (DNS) of turbulent rough channel flow at Reynolds number $$\hbox {Re}_{\tau }=180$$ is undertaken, to investigate the effects of Reynolds number variation on the various statistical quantities and the near-wall dynamical structures. The definition of Reynolds number predominantly considered in this study, $$\hbox {Re}_{\tau }$$, is based on the wall friction velocity $$u_{\tau }$$ and the half-height of the channel h. The channel walls are roughened by square ribs elongated along the span of the channel. The ribs are arranged at a constant pitch width and in a manner such that the crests face the ones on the opposite wall, i.e., the roughness on the channel is symmetric about the channel centerline. The results obtained from the current DNS are compared with those from two DNSs at a higher Reynolds number of 400, one of them with roughness height $$k^+$$ comparable to that in the current DNS and the other with a higher value of $$k^+$$. The mean streamline patterns are contrasting in the presence or absence of the secondary vortex; the effects on the location of a seemingly persistent saddle point are also observed. Variation of the skin friction coefficient over a pitch width has apparently lent support to the observations. A measure of the roughness function is presented. The near-wall structures in terms of the contours of the fluctuating velocity reveal finer scale structures at the higher Reynolds number, when the value of $$k^+$$ is also high.

Experimental study of channel roughness effect in diffusion bonded pulsating heat pipes

This study investigates the influence of surface finishing on thermal performance of copper flat pulsating heat pipes (PHP). Two copper flat PHPs with overall dimensions of 208 × 150 × 5 mm3 were fabricated using the diffusion bonding technique. In both devices, the internal surfaces were modified. In the first one, the channel inner surface was sanded with standard sandpaper Grit N1200. In the second PHP (called hybrid), the evaporator zone was sanded with standard sandpaper Grit N100 and the condenser section with Grit N1200. Both devices counted ten parallel channels with hydraulic diameter of 2.5 mm. Distillated water was used as working fluid. The transient thermal performance and thermal resistance were analyzed, for filling ratios of 10, 15, 25, 37.5, 50 and 75% and for tilt angles of 0° and 90° bottom heat mode. Start-up, thermal stability and hysteresis were experimentally tested. Three regimes were observed: conduction, unstable, stable. In general, the hybrid PHP showed lower thermal resistances, except for the filling ratio of 75% at vertical position. The hybrid device shows stop-over instabilities at power inputs between 140 and 180 W, at vertical position and low filling ratios. Higher surface roughness increases the number of vapor nucleation sites, which, in turn, improves the phase change at evaporator zone and facilitates the start-up conditions at low filling ratios. However, increasing roughness also rises pressure drops, mainly at high filling ratios.

Influence of structured surface roughness peaks on flow and heat transfer performances of micro- and mini-channels

The effect of surface roughness peaks (SRP) on laminar air-flow and heat transfer performances in rough micro- and mini-channels is analyzed by 3D computational fluid dynamics (CFD) simulation method. The variations of channel height, SRP height, pitch and width, aligned and off-set SRP, and hybrid SRP in rectangular channels are studied. The parametric ranges of channel height, SRP height and constricted diameter of the channel are 150 μm to 400 μm, 15 μm to 45 μm, and 179 μm to 663 μm, respectively. The results showed that heat transfer performance of smooth micro- and mini-channels with low aspect ratio is higher than the conventional predictions. The maximum convective heat transfer depends on the optimum combination of channel size and roughness height. Furthermore, increasing SRP pitch has less impact on the heat transfer while increasing roughness width improves the performance index. Hybrid SRP in a channel is more effective than constant SRP at lower Reynolds number with the performance index improved up to 12%. Aligned peaks are more efficient than the off-set arrangement and the average rise of performance is 9% under present conditions. Relative roughness with varying parameters can be configured for practical purposes without compromising the compactness.

Bed Shear Stress Influence on Local Scour Geometry Properties in Various Flume Development Conditions

Numerous approaches in sediment mobility studies highlighted the key meaning of channel roughness, which results not only from bed material granulation but also from various bed forms presence, caused by continuous sediment transport. Those forms are strictly connected with the intensity of particle transport, and they eventuate from bed shear stress. The present paper comprised of local scours geometric dimensions research in three variants of lengthwise development of laboratory flume in various hydraulic properties, both in “clear-water” and “live-bed” conditions of sediment movement. Lots of measurements of the bed conformation were executed using the LiDAR device, marked by a very precise three-dimensional shape description. The influence of the bed shear stress downstream model on scours hole dimensions of water structure was investigated as one of the key factors that impact the sediment transport intensity. A significant database of 39 experimental series, lasting averagely 8 h, was a foundation for delineating functional correlations between bed shear stress-and-critical shear stress ratio and geometry properties of local scours in various flume development cases. In the scope of mutual influence of bed shear stress and water depth, high correlation coefficients were attained, indicating very good and good functional correlations. Also, the influence of bed shear stress and the total length of the scour demonstrated a high correlation coefficient.

Representing natural and artificial in‐channel large wood in numerical hydraulic and hydrological models

AbstractThe influence of naturally occurring in‐channel large wood (LW) on the hydraulics, hydrology and geomorphology of rivers is well documented. To inform management and better understand naturally occurring or artificially placed LW, hydraulic and hydrological models are applied to predict the possible benefits and drawbacks for habitat, sediment management and flood risk mitigation. However, knowledge and guidance on appropriate representation in models, needed to underpin realistic predictions, is lacking. This could lead to unrealistic expectations of the effectiveness of LW for different river management goals. To date, seven types of LW representation in hydraulic and hydrological models have been applied, the range partly reflecting the variety of LW, model types, scales and purposes. The most common approach is by altering channel roughness to represent flow resistance. Although qualitatively the effects of LW have been captured using models, to date quantitative validation, as well as transferable knowledge to help a priori parameterization of LW representations, remain limited. Therefore, additional empirical investigations and robust model validation are required to inform defensible LW representations for specific purposes and scales in numerical models coupled with better accounting of input uncertainty to improve confidence in predictions. Future studies should also consider a greater range of artificial and natural LW features, settings, larger spatial scales and better account for temporal variability of flow, morphology and LW configuration.This article is categorized under: Water and Life &gt; Methods Science of Water &gt; Methods Water and Life &gt; Nature of Freshwater Ecosystems

Impacts of atomistic surface roughness on electronic transport in n-type and p-type MoS2 field-effect transistors

We investigate the impacts of channel curvature induced by atomistic surface roughness on electronic-transport properties in monolayer transition-metal dichalcogenide field-effect transistors (FETs) through ab initio calculations. By focusing on the channel current, we demonstrate that the Ion current and Ion/Ioff ratio are both significantly suppressed in n-type and p-type MoS2 FETs when increasing the channel curvatures. We also evaluate the performance of CMOS inverters and demonstrate that the channel curvatures can dramatically reduce the inverter performance. These findings point to the fact underlining that surface engineering for channel roughness optimization at the atomic level is strongly required in MoS2-based devices.

Shear force estimation in rough boundaries using SVR method

The accuracy of support vector regression (SVR) procedure in modeling the percentage of shear force carried by walls in a rectangular channel with rough boundaries was investigated. The SVR model is extended, and the more appropriate kernel function and input combination are studied. Finally, the SVR model with an exponential kernel function and three influence parameters was selected as the best SVR model with the lowest error. The output of this more appropriate SVR model is presented as a program. Then, this most appropriate SVR model is compared with three equations presented by other researchers for rough and smooth channels. The SVR model with the highest accuracy and lowest statistical values (RMSE of 0.565) performed the best compared with the other equations.

Turbulence characteristics in a rough open channel under unsteady flow conditions

ABSTRACT Most open channel flows are generally unsteady in nature. Very few studies have been carried out in the past to investigate the turbulence characteristics in unsteady open channel flows over a rough bed at different test reaches. In the present study, experiments are conducted for two different flow depths over a rough bed and hydrographs are passed at the inlet of a rectangular flume. With consideration of laboratory constraints, the present study characterizes the unsteady flow as high catchment with long duration flood. The behavior of velocities in both the rising limb and falling limb for same flow depth is observed. Classical hysteresis effect of stage-discharge (h ~ Q) rating curve is illustrated. The present study investigates the turbulent intensities and Reynolds stresses under unsteady flow conditions. The turbulence intensities and Reynolds stresses in the rising limb are higher than that of the falling limb at same flow depth. Turbulence characteristics, i.e. turbulent kinetic energy (TKE), mean kinetic energy (MKE) and turbulent kinetic dissipation rate are also analyzed. It is found that the turbulence intensities, Reynolds stresses, and Turbulence characteristics are gradually decreased towards the downstream reaches.