Channel Roughness Research Articles

Dynamic evolution of powder stream convergence with powder feeding durations in direct energy deposition

During direct energy deposition (DED), changes in the powder feeding behavior significantly affect the deposition process. In this study, a high-speed photography method was developed to observe the dynamic evolution of the powder stream for various powder feeding durations, and the synchronous evolution of the inner channel characteristics of the nozzle were also measured. Combined with powder stream simulations, the contribution of the evolution of each channel characteristic on the powder divergence was discussed. The results show that as the powder feeding duration increased, the inner channel characteristics of the nozzle changed dynamically, which led to the powder stream divergence. For a powder feeding duration of 60 h, the nominal powder spot size increased by 41.4% at the plane 15 mm below the nozzle exit. As the powder feeding time increased, the inner wall roughness, inner channel diameter, and the exit size of the nozzle also increased owing to the scouring erosion from the high-speed powder particles. Subsequently, changes in the inner wall roughness and inner channel diameter of the nozzle tended to be smaller owing to the formation of a work hardening layer. It was also demonstrated that the changes in the inner wall roughness of the nozzle had a negligible effect on the powder stream divergence, an increase in the inner channel diameter of the nozzle extended the maximum divergence range of the particle trajectory at the nozzle exit, and the expansion of the nozzle exit size reduced the constraint causing the allowed trajectory range of the ejected powder particles to increase. The deposited height gradually decreased as the powder feeding duration increased, resulting in a 38.7% decrease after 60 h.

Thermohydraulic Behavior of Minichannel Surface Simulated With Gaussian Function and Actual Roughness Data Generated Using Three-Dimensional Optical Surface Profilometer

Abstract The effect of surface roughness on the thermohydraulics in minichannels has been studied numerically. Fluid flow (at low Reynolds number) through a typical three-dimensional (3D) channel subjected to constant heat flux (at the bottom) is analyzed incorporating surface roughness on the solid–fluid interfaces characterized by its true random and nonperiodic nature. Two different approaches are adopted to model the rough channel surfaces. Topographic measurements have been performed on a stainless steel minichannel using an optical surface profilometer (OSP) to generate digital replica of the rough surface. Alternatively, the Gaussian function defined by two statistical parameters, namely average roughness (Ra) and correlation length (Cl), are employed to imitate the random nature of rough interface. At the outset, conjugate heat transfer simulations have been performed on the rough channel models and the results are validated against the experimental data. Finally, the effect of surface roughness on both local and global nondimensional performance parameters is analyzed and compared with findings from simulations performed on a similar smooth channel. The outcomes reveal an enhanced friction factor for flow over a rough surface, attributable to the near wall shear rate fluctuations experienced by the flow. Unlike smooth channels, the local Nusselt number (Nuy) exhibits continuous fluctuations along the channel axial length. The fully developed (Nufd) and the average (Nu¯) counterparts of the Nusselt number show enhanced magnitudes when compared to the theoretical predictions of the same in a smooth surface channel. This amplification can be attributed to two simultaneously acting factors: augmentation in heat transfer area and chaotic mixing due to flow perturbation. The magnitude of enhancement in terms of fully developed Nusselt number (Nufd) is roughly 1.3 times of its corresponding value in a smooth channel and the factor remains invariant of the supplied heat.

Effect of Surface Roughness during Peristaltic Movement in a Nonuniform Channel

In this study, the effect of the roughness parameter during the peristaltic transport of a Newtonian fluid in a nonuniform channel has been explored. The motivation of this study comes from various research studies in the area of life sciences and engineering, which reveal that the wall of living beings’ arteries and all other surfaces have roughness to some extent. As peristalsis is a major mode of transporting biological fluids in various organs, the effect of surface roughness during peristaltic flow becomes very significant. The problem of peristaltic motion of a Newtonian fluid through a rough nonuniform channel having sinusoidal-shaped roughness has been investigated in the current work. To analyze the flow, analytic formulation of pressure rise, friction force, velocity, and pressure gradient has been carried out under the low Reynolds number and long-wavelength approximation. Results obtained for zero surface roughness from the current model are in complete agreement with previous studies available in the literature that have been carried out without considering the surface roughness of the wall. Numerical outcomes for the properties mentioned above have been plotted for analyzing the impact of roughness on the physical and flow parameters.

Model of surface roughness and material removal using abrasive flow machining of selective laser melted channels

PurposeInternal channels produced by selective laser melting (SLM) have rough surfaces that require post-processing. The purpose of this paper is to develop an empirical model for predicting the material removal and surface roughness (SR) of SLM-manufactured channels owing to abrasive flow machining (AFM).Design/methodology/approachA rheological model was developed to simulate the viscosity and power-law index of an AFM medium. To simulate the pressure distribution and velocity in the SLM channels, the fluid behavior and SR in the channels were simulated by using computational fluid dynamics. The results of this simulation were then applied to create an empirical model that can be used to predict the SR and material removal thickness. To verify this empirical model, it was applied to an actual part fabricated by SLM. The results were compared with the measurements of the SR and channel diameter subsequent to AFM.FindingsThe proposed model exhibits maximum deviation between the model and the measurement of −1.1% for the down-skin SR, −0.2% for the up-skin SR and −0.1% for material removal thickness.Practical implicationsThe results of this study show that the proposed model can avoid expensive iterative tests to determine whether a given channel design leads to the desired SR after smoothing by AFM. Therefore, this model helps to design an AFM-ready channel geometry.Originality/valueIn this paper, a quantitatively validated AFM model was proposed for complex SLM channels with varying orientation angles.

Feedback Mechanism in Bifurcating River Systems: the Effect on Water-Level Sensitivity

Accurate and reliable estimates of water levels are essential to assess flood risk in river systems. In current practice, uncertainties involved and the sensitivity of water levels to these uncertainties are studied in single-branch rivers, while many rivers in deltas consist of multiple distributaries. In a bifurcating river, a feedback mechanism exists between the downstream water levels and the discharge distribution at the bifurcation. This paper aims to quantify the sensitivity of water levels to main channel roughness in a bifurcating river system. Water levels are modelled for various roughness scenarios under a wide range of discharge conditions using a one-dimensional hydraulic model. The results show that the feedback mechanism reduces the sensitivity of water levels to local changes of roughness in comparison to the single-branch river. However, in the smaller branches of the system, water-level variations induced by the changes in discharge distribution can exceed the water-level variations of the single-branch river. Therefore, water levels throughout the entire system are dominated by the conditions in the largest branch. As the feedback mechanism is important, the river system should be considered as one interconnected system in river maintenance of rivers, flood-risk analyses, and future planning of river engineering works.

Large eddy simulation of inertial particles dispersion in a turbulent gas-particle channel flow bounded by rough walls

The purpose of this paper is to understand the capability and consistency of large eddy simulation (LES) in Eulerian–Lagrangian studies aimed at predicting inertial particle dispersion in turbulent wall-bounded flows, in the absence of ad hoc closure models in the Lagrangian equations of particle motion. The degree of improvement granted by LES models is object of debate, in terms of both accurate prediction of particle accumulation and local particle segregation; therefore, we assessed the accuracy in the prediction of the particle velocity statistics by comparison against direct numerical simulation (DNS) of a finer computational mesh, under both one-way and two-way coupling regimes. We performed DNS and LES at friction Reynolds number $$\hbox {Re}_{\tau }=180$$ in smooth and rough channels, tracking particles with different inertia, with the aim to conduct a parametric study that examines the accuracy of particle statistics obtained from LES computations. The issue has been widely analysed in turbulent flow bounded by smooth walls, whereas the effect of rough boundaries on momentum coupled two-phase flows has been much less investigated until now. The action of the roughness of the wall is studied in terms of both turbulence modification and particle interaction with the wall surface due to particle rebounding off the solid boundary, without the introduction of a virtual rebound model. Results show that resolved LES adequately predicts particle-induced changes in both fluid and particle statistics in rough channels, at least for the range of parameters considered here.

Analysis of the different sources of stress acting in fully rough turbulent flows over geometrical roughness elements

The discrete element roughness method is considered in this article for the prediction of turbulent flows over rough walls. This approach is derived by ensemble- and volume-averaging the Navier–Stokes equations, providing double-averaged Navier–Stokes equations, and yielding three unknown terms in the momentum equation: the Reynolds stress and dispersive stress tensors and the average drag force acting on the roughness elements. This work aims at analyzing these different terms, quantifying their respective contributions to the near-wall momentum budget, and providing guidance for their modeling. For this purpose, relevant data of turbulent flows are required. Roughness-resolved Reynolds-averaged Navier–Stokes simulations of transitionally and fully rough channel flows over academic roughness configurations are performed at high friction Reynolds numbers ranging from 3500 to 8000. Comparisons with existing and new velocity measurements performed in rough-wall turbulent boundary layers provide support to the simulation results, a particular emphasis being given on the validity of the numerical results in the wake of the roughness elements. These numerical results highlight the influence of roughness elements geometry and density on the roughness drag coefficient and the dispersive stress. It is particularly suggested that the standard roughness drag closure model should be revisited for double-averaged flows. Furthermore, the dispersive stress is shown to mainly originate from the wake of the roughness elements, an observation that could be leveraged for its modeling. However, since this stress contributes only marginally to the global stress and to the skin friction coefficient, such a modeling may not be critical at first order.

A Note on the Effects of Local Blockage and Dynamic Tuning on Tidal Turbine Performance

Abstract Numerical simulations are used to explore the potential for local blockage effects and dynamic tuning strategies to enhance the performance of turbines in tidal channels. Full- and partial-width arrays of turbines, modeled using the volume-flux-constrained actuator disc and blade element momentum theories, are embedded within a two-dimensional channel with a naturally low ratio of drag to inertial forces. For steady flow, the local blockage effect observed by varying the cross-stream spacing between the turbines is found to agree very well with the predictions of the two-scale actuator disc theory of Nishino and Willden (2012, “The Efficiency of an Array of Tidal Turbines Partially Blocking a Wide Channel,” J. Fluid Mech., 708, pp. 596–606). For oscillatory flow, however, results show that, consistent with the findings of Bonar et al. (2019, “On the Arrangement of Tidal Turbines in Rough and Oscillatory Channel Flow,” J. Fluid Mech., 865, pp. 790–810), the shorter and more highly blocked arrays produce considerably more power than predicted by two-scale theory. Results also show that, consistent with the findings of Vennell (2016, “An Optimal Tuning Strategy for Tidal Turbines,” Proc. R. Soc. A, 472(2195), p. 20160047), the “dynamic” tuning strategy, in which the tuning of the turbines is varied over the tidal cycle, can only produce significantly more power than a temporally fixed turbine tuning if the array has a large number of turbine rows or a large local blockage ratio. For all cases considered, trends are consistent between the two turbine representations but the effects of local blockage and dynamic tuning are found to be much less significant for the more realistic tidal rotor than for the idealized actuator disc.

Flow resistance law under suspended sediment laden conditions

The uniform flow resistance equation, in the form due to Manning or Darcy-Weisbach, is widely applied to establish the stage-discharge relationship of a river cross-section. The application of this equation, namely the slope-area method, allows to indirectly measure the corresponding river discharge by measurements of bed slope, water level, cross-section area, wetted perimeter and an estimate of channel roughness. In this paper, a recently deduced flow resistance equation for open channel flow was tested during conditions of suspended sediment-laden flow. First, the flow resistance equation was determined by dimensional analysis and by applying the condition of incomplete self-similarity for the flow velocity profile. Then the analysis was developed by the following steps: (i) for sediment-laden flows characterized by known values of mean diameter and concentration of suspended sediments, a relationship (Eq. (28)) between the Γ function of the velocity profile, the channel slope and the Froude number was calibrated by the available measurements; and (ii) a relationship for estimating the Γ function (Eq. (29)) which also takes into account the mean concentration of suspended particles was also established. The theoretical flow resistance law (Eq. (26)) coupled with the relationship for estimating the Γ function (Eq. (28) or Eq. (29)), which is characterized by the applicability of a wide range of flow conditions, allowed to estimate the Darcy-Weisbach friction factor for flows with suspended-load. The analysis showed that for large-size mixtures the Darcy-Weisbach friction factor can be accurately estimated neglecting the effect of mean concentration of suspended sediments while for small-size mixtures the friction factor decreases when the mean sediment concentration increases.

Zonal and Overall Discharge Prediction Using Momentum Exchange in Smooth and Rough Asymmetric Compound Channel Flows

AbstractThe apparent shear stress acting on the vertical interface between floodplain(s) and main channel has been shown to be very influential for the estimation of the zonal and overall discharge...

Effects of aspect ratio on higher-order moments, conditional statistics, TKE budget and anisotropy in narrow open channel flow

The purpose of the present study is to investigate the influence of aspect ratio on the higher- order statistics of velocity fluctuations, conditional statistics, turbulent kinetic energy (TKE) fluxes, TKE budget, and Reynolds stress anisotropy in hydraulically rough narrow open channel flow (OCF). In the experiments, the aspect ratios were maintained low and varied between 2.5 and 4. The velocities were measured with an acoustic Doppler velocimeter (ADV). The higher-order moments, quadrant analysis, and TKE budget from the present experiments were compared with available literature data in wide and narrow OCFs to elucidate the effect of the aspect ratio. In this analysis, the third-order moments of velocity fluctuations were found to be sensitive to the aspect ratio in the outer region. The fractional contributions of all quadrant events are approximately equal in magnitude in lower aspect ratio flows, whereas ejections and sweeps are the dominant events as the aspect ratio increases. The upward transfer of TKE flux increases in the outer layer with increase in aspect ratio. In the inner layer, the TKE production and rate of dissipation are found to be increasing with decreasing aspect ratio. Therefore, the production and dissipation of TKE are dependent on the aspect ratio of the flow. The analysis of Reynolds stress AIM reveals that for low aspect ratio flows turbulence tends to attain rod-like axisymmetric turbulence only in the intermediate layer whereas for higher aspect ratio, turbulence attains rod-like axisymmetric turbulence throughout the depth.

River Channel Forms in Relation to Bank Steepness: A Theoretical Investigation Using a Variational Analytical Method

Riverbanks vary considerably in anti-scourability and consequently take various profiles. By using an isosceles trapezoid as the generalized form of river channel cross-sections and then incorporating the effects of bank angle into the variational analytical approach developed by Huang and Nanson (2000), this study presents a detailed theoretical investigation of the self-adjustment of alluvial channel forms. It is demonstrated that when alluvial channel flow achieves stable equilibrium, a significant decrease in riverbank steepness leads to a slight decrease in maximum sediment (bedload) discharge, and yet results in a significant increase in optimal channel width and a considerable decrease in optimal channel depth. The hydraulic geometry relations, theoretically derived for bank steepness to vary across a wide range, show that among the multivariant controls, the roles of bed sediment size, channel roughness, flow discharge and sediment (bedload) discharge are independent of bank steepness. While the effects of bank steepness illustrated in the theoretically derived hydraulic geometry relations are highly consistent with the results of threshold theory and previous empirical studies, limitations on using bank angle to reflect the anti-scourability of natural riverbanks are also highlighted.

Correction to: Rarefied gas flow through a rough channel into a vacuum

Correction to: Rarefied gas flow through a rough channel into a vacuum

A novel method reveals how channel retentiveness and stocks of detritus (CPOM) vary among streams differing in bed roughness

Abstract Coarse particulate organic matter (CPOM) is a fundamental resource in freshwater streams, providing food, shelter and habitat for diverse invertebrate taxa and playing a key role in metabolism in low‐order streams. Benthic CPOM stocks are determined by rates of supply and breakdown of detritus and by channel retentiveness (i.e. the capacity for the channel to trap and retain CPOM). We focussed on factors affecting the retentiveness of channels, which theoretically differs among streams with different sediment sizes and concomitant channel morphology. We developed a new, rapid method to measure retentiveness using line‐intercept surveys along transects. With this rapid approach, we surveyed 32 sites from three types of streams (smooth sandy channels, n = 10; gravel channels of intermediate roughness, n = 12; rough cobble channels, n = 10) in Victoria, south‐eastern Australia, and tested the simple hypotheses that: (1) retentiveness increases in channels with increasing channel roughness (i.e. sandy versus gravel versus cobble‐bed streams); (2) different types of channel features (e.g. log jams, cobbles, depositional areas) differ in the efficiency with which they retain CPOM. The line‐intercept survey method was readily adapted to measure retentiveness as m of retentive structure per m of transect (i.e. the Linear Coverage Index) and trapping efficiency as m of CPOM per m of retentive element, for 10 different types of retentive elements. Unexpectedly, the retentiveness of channels did not increase with channel roughness. This occurred because channels with different roughness were dominated by different types of retentive structure. Retentive structure in cobble sites was dominated by cobbles themselves, which were highly retentive in other studies but poorly retentive in our system. Gravel and sand sites had more log jams and depositional areas, such as pools and backwaters, and these features were more effective at trapping CPOM. Thus, retention of CPOM was highest in gravel and sand sites. Our method provides a new tool for investigators testing hypotheses about CPOM retention in streams. The method is rapid, requires a minimum of equipment and personnel, and may be applied in any wadeable stream. Retentiveness is calculated in intuitive units that are directly comparable among sites and may have utility as variables in models of CPOM dynamics. We hope this method will open up new avenues for research that may shed light on how CPOM stocks vary among streams, with implications for diversity of aquatic fauna and ecosystem functions such as decomposition.

Poiseuille Flow of Soft Polycrystals in 2D Rough Channels.

Polycrystals are partially ordered solids where crystalline order extends over mesoscopic length scales, namely, the grain size. We study the Poisuielle flow of such materials in a rough channel. In general, similar to yield stress fluids, three distinct dynamical states, namely, flowing, stick-slip, and jammed can be observed, with a yield threshold dependent on channel width. Importantly, the interplay between the finite channel width, and the intrinsic ordering scale (the grain size) leads to a new type of spatiotemporal heterogeneity. In wide channels, although the average flow profile remains pluglike, at the underlying granular level, there is vigorous grain remodeling activity resulting from the velocity heterogeneity among the grains. As the channel width approaches typical grain size, the flowing polycrystalline state breaks up into a spatially heterogeneous mixture of flowing liquid like patches and chunks of nearly static grains. Despite these static grains, the average velocity still shows a parabolic profile, dominated by the moving liquidlike patches. However, the solid-liquid front moves at nearly constant speed in the opposite direction of the external drive.

Semi-blind joint channel estimation and data detection on sphere manifold for MIMO with high-order QAM signaling

A low-complexity semi-blind scheme is proposed for joint channel estimation and data detection on sphere manifold for multiple-input multiple-output (MIMO) systems with high-order quadrature amplitude modulation signaling. Specifically, the optimal channel estimator is expressed in the least squares form in terms of the received signals and unknown transmitted data, and by splitting the channel and transmitted data into their real parts and imaginary parts, the data detection becomes a problem defined on a scaled sphere manifold in the real domain. Our semi-blind algorithm consists of three stages: (i) a few training symbols are employed to provide a rough initial MIMO channel estimate which in turn yields the initial zero-forcing (ZF) estimate of data samples; (ii) the Riemannian conjugate gradient algorithm is used to estimate the data samples in real domain, and the detected data samples are used to estimate the final MIMO channel matrix; and (iii) the final ZF data detection is carried out based on the final MIMO channel estimate. In particular, we present the first order Riemannian geometry of the sphere manifold which is utilized in the Riemannian conjugate gradient algorithm for solving (ii). Simulation results are employed to demonstrate the effectiveness of the proposed approach.

Roughness effect in an initially laminar channel flow

The possibility of generating and maintaining turbulence in an initially laminar channel flow is investigated for two Reynolds numbers and 2100 (based on bulk velocity, and half-height of the channel, ). The study is carried out through a direct numerical simulation based on the lattice Boltzmann method (LBM). The channel consists of two parallel walls separated by a distance where the roughness elements are mounted on both walls. It was observed that when the transverse square bars span half the width of the channel and are mounted in a ‘staggered’ formation, the flow becomes fully turbulent with strong similarities to fully rough wall turbulent channel flows at much higher Reynolds number, as reported in the literature. For example, the rough wall mean velocity profile exhibits a significant downward shift when compared to the smooth wall one. Also, the turbulent kinetic energy budget is similar to its counterpart in rough wall turbulent channel flows at much higher Reynolds numbers than the present ones. It is further shown that the present velocity spectra compared very well with that obtained in a rough wall turbulent boundary layer. Finally, some elements of the possible physical mechanism allowing the generation, growth and sustainability of turbulence are proposed.

The Value of Distributed High-Resolution UAV-Borne Observations of Water Surface Elevation for River Management and Hydrodynamic Modeling

Water level or water surface elevation (WSE) is an important state variable of rivers, lakes, and wetlands. Hydrodynamic models of rivers and streams simulate WSE and can benefit from spatially distributed WSE observations, to increase model reliability and predictive skill. This has been partially addressed by satellite radar altimetry, but satellite altimetry is unable to deliver useful data for small rivers. To overcome such limitations, we deployed a radar altimetry system on an unmanned aerial vehicle (UAV), to map spatially distributed WSE. We showed that UAV altimetry can provide observations of WSE with a very high spatial resolution (ca. 0.5 m) and accuracy (ca. 3 cm), in a time-saving and cost-effective way. Furthermore, we investigated the value of this dataset for the calibration and validation of hydrodynamic models. Specifically, we introduced spatially distributed roughness parameters in a hydrodynamic model and estimated these parameters, using the observed WSE profiles along the stream as input. A case study was conducted in the Åmose stream, Denmark. The results showed that UAV-borne WSE can identify significant variations of the Manning–Strickler coefficients, along this small and highly vegetated stream and over time. Moreover, the model performed extremely well using distributed roughness coefficients, but it could not reproduce WSE satisfactorily using uniform roughness. We concluded that distributed roughness coefficients should be considered, especially for small vegetated rivers, to improve model performance, both locally and globally. Spatially distributed parameterizations of the effective channel roughness could be constrained with UAV-borne WSE. This study demonstrated for the first time that UAV-borne WSE can help to understand the variations of hydraulic roughness, and can support efficient river management and maintenance.

Rarefied gas flow through a rough channel into a vacuum

Rarefied gas flow through a rough channel of finite length into a vacuum was investigated using the direct simulation Monte Carlo method. The non-equilibrium effects at the input and output of the channel were considered by including certain pre-and post-channel regions into the geometry under consideration. The mass flow rate through a short and long channel was computed in a wide range of gas rarefaction from the free molecular regime to the hydrodynamic near. It is shown that the noticeable effect of surface roughness on flow rate is manifested in the free molecular and transition regimes. The analysis is provided for the flow field inside the channel as well as in the upstream and downstream regions. The results obtained are consistent with the theoretical, numerical, and experimental data available in the open literature.

Discharge Estimates for Ungauged Rivers Flowing over Complex High-Mountainous Regions based Solely on Remote Sensing-Derived Datasets

Reliable information about river discharge plays a key role in sustainably managing water resources and better understanding of hydrological systems. Therefore, river discharge estimation using remote sensing techniques is an ongoing research goal, especially in small, headwater catchments which are mostly ungauged due to environmental or financial limitations. Here, a novel method for river discharge estimation based entirely on remote sensing-derived parameters is presented. The model inputs include average river width, estimated from Landsat imagery by using the modified normalized difference water index (MNDWI) approach; average depth and velocity, based on empirical equations with inputs from remote sensing; channel slope from a high resolution shuttle radar topography mission digital elevation model (SRTM DEM); and channel roughness coefficient via further analysis and classification of Landsat images with support of previously published values. The discharge of the Lhasa River was then estimated based on these derived parameters and by using either the Manning equation (Model 1) or Bjerklie equation (Model 2). In general, both of the two models tend to overestimate discharge at moderate and high flows, and underestimate discharge at low flows. The overall performances of both models at the Lhasa gauge were satisfactory: comparisons with the observations yielded Nash–Sutcliffe efficiency coefficient (NSE) and R2 values ≥ 0.886. Both models also performed well at the upper gauge (Tanggya) of the Lhasa River (NSE ≥ 0.950) indicating the transferability of the methodology to river cross-sections with different morphologies, thus demonstrating the potential to quantify streamflow entirely from remote sensing data in poorly-gauged or ungauged rivers on the Tibetan Plateau.