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The present paper focuses on the investigation of the turbulent characteristics of an open-channel flow by employing the micropolar model. The underlying model has already been proven to correctly describe the secondary phase of turbulent wall-bounded flows. The open-channel case comprises an ideal candidate to further test the micropolar model as many environmental flows carry a secondary phase, the behavior of which is of great interest for applications such as sedimentation transport and debris flow. Direct Numerical Simulations (DNSs) have been carried out on an open channel for R e b = 11,200 based on mean crossectional velocity, channel height, and the fluid kinematic viscosity. The simulated results are compared against previous experimental as well as Langrangian DNS data of similar flows, with excellent agreement. The micropolar model is capable of describing the same problem but in an Eulerian frame, thus significantly simplifying the computational cost and complexity. [ABSTRACT FROM AUTHOR]

Flow resistance estimate is a challenging topic for establishing flooding propensity of streams, designing river restoration works, and evaluating the use of soil bioengineering practices. In this paper, flume measurements with rigid cylinders set in two arrangements (aligned, staggered) were used to evaluate the effect of rigid emergent vegetation on flow resistance. A well‐known theoretical flow resistance equation was firstly reviewed. Then, it was calibrated and tested by measurements performed for these arrangements with six concentrations (0.53–11.62 stems dm−2). The analysis was conducted using three approaches: (i) distinguishing the experimental runs corresponding to different arrangements and stem concentrations; (ii) using only a scale factor representing the effect of the stem concentration; and (iii) joining all available data. The results demonstrated that the flow resistance equation gives an accurate estimate of the Darcy–Weisbach friction factor f, characterized, for the best approach among the tested ones, by errors less than or equal to ±5% for 95.9% of the examined cases for the aligned arrangement and for the staggered arrangement less than or equal to ±5% for 94.2% of the examined cases. For both arrangements, the measurements demonstrated that, for a given longitudinal distance between vegetation elements, flow resistance increases for decreasing values of the transverse distance and, for a given transverse distance, f decreases for increasing values of the longitudinal distance between elements. Finally, in the range of the investigated stem concentrations, the influence of the arrangement on flow resistance resulted negligible. [ABSTRACT FROM AUTHOR]

The present paper focuses on the investigation of the turbulent characteristics of an open-channel flow by employing the micropolar model. The underlying model has already been proven to correctly describe the secondary phase of turbulent wall-bounded flows. The open-channel case comprises an ideal candidate to further test the micropolar model as many environmental flows carry a secondary phase, the behavior of which is of great interest for applications such as sedimentation transport and debris flow. Direct Numerical Simulations (DNSs) have been carried out on an open channel for Reb = 11,200 based on mean crossectional velocity, channel height, and the fluid kinematic viscosity. The simulated results are compared against previous experimental as well as Langrangian DNS data of similar flows, with excellent agreement. The micropolar model is capable of describing the same problem but in an Eulerian frame, thus significantly simplifying the computational cost and complexity.

Sedimentary bedforms such as ripples and dunes are generated both by river flows and sediment‐laden gravity currents. Gravity current deposits are usually parameterized using existing bedform phase diagrams which are based on data from laboratory experiments and field observations of open‐channel flows. Yet, it is not evident that open‐channel flow bedform phase diagrams are applicable to gravity current deposits. Gravity current hydrodynamics are dependent on vertical density variation, that is density stratification, and therefore are fundamentally different from open‐channel flows. New experiments to produce gravity current deposits are conducted and compared to existing open‐channel flow data. It is shown that a parameter phase‐space based on the lower layer of stratified gravity currents (i.e. that part below the velocity maximum) significantly improves the prediction of bedform type compared to bedform phase diagrams derived from layer‐averaged parameters. These results confirm that bedforms produced by gravity currents can only be predicted accurately using the characteristics of the lower layer of stratified flow. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a well‐balanced and positive‐preserving nonstaggered central scheme, which has second‐order accuracy on both time and spatial scales, for open‐channel flows with the variable channel width and the nonflat bottom. We perform piecewise linear reconstructions of the conserved variables and energy as well as discretize the source term using the property that the energy remains constant, so that the complex source term and the flux can be precisely balanced so as to maintain the steady state. The scheme also ensures that the cross‐sectional wet area is positive by introducing a draining time‐step technique. Numerical experiments demonstrate that the scheme is capable of accurately maintaining both the still steady‐state solutions and the moving steady‐state solutions, simultaneously. Moreover, the scheme has the ability to accurately capture small perturbations of the moving steady‐state solution and avoids generating spurious oscillations. It is also capable of showing that the scheme is positive‐preserving and robust in solving the dam‐break problem. [ABSTRACT FROM AUTHOR]

Summary: In this paper, we construct and study an exactly well‐balanced positivity‐preserving nonstaggered central scheme for shallow water flows in open channels with irregular geometry and nonflat bottom topography. We introduce a novel discretization of the source term based on hydrostatic reconstruction to obtain the exactly well‐balanced property for the still water steady‐state solution even in the presence of wetting and drying transitions. The positivity‐preserving property of the cross‐sectional wet area is obtained by using a modified "draining" time‐step technique. The current scheme is also Riemann‐solver‐free. Several classical problems of open‐channel flows are used to test these properties. Numerical results confirm that the current scheme is robust, exactly well‐balanced and positivity‐preserving. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical model for predicting the vertical distribution of suspended sediment concentrations in open-channel flows. The model developed uses a new sediment diffusivity coefficient obtained by the application of the Itakura and Kishi correction method to the Kerssens sediment diffusivity profile. The new coefficient is calculated using composite operators applied to the Rouse sediment diffusivity profile. The diffusivity profile proposed increases as a parabolic profile from zero at the bottom to a maximum value in the lower part of the flow, and then decreases slowly towards the water surface without reaching zero. The maximal value is calculated at a level between 20 and 30% of the total water depth as explained previously by Coleman. Based on this diffusivity profile, a theoretical formulation for computing sediment concentration is derived analytically, using the well-known convection–diffusion equation. We compare results from the proposed model with experimental measurements and existing theoretical models, obtained by previous researchers, in order to evaluate the applicability of the present model to predict particle concentration profiles in open-channel flows. The computed values of sediment concentrations are in good agreement with the experimental data reported in the literature. [ABSTRACT FROM AUTHOR]

Sediment mobility in stream corroborates many significant mechanisms in terms of interactions of transported particles in the carrier fluid flows. In general, the sediment entrainment in stream refers to temporal and continual motion of non-cohesive grains. This phenomenon significantly influences the near-bed turbulent flow characteristics in streams. Laboratory experiments were conducted in a rectangular open-channel flume of 15 m long, 0.90 m wide and 0.75 m deep fitted with side glass walls and the study is concerned with the determination of von Kármán coefficient with non-cohesive bed-load to strive the eddy traversing length, sizes and TKE dissipation rate close to the bed. Two modes of experimental conditions are considered: (1) immobile-bed with no sediment transport known as clear-water flows; and (2) continuous weak bed-load sediment transport, without bed-forms development refer to a mobile-bed flows. In both cases, the sediment beds should remain flat and hydraulically rough. The experimental result reveals that the von Kármán coefficient goes down in the traditional log-law of wall in mobile-bed streams with respect to the flow over an immobile bed. A close observation in sediment mobility due to bed load transport shows the reduction in traversing length of an eddy and increment in near-bed eddy size. The increment in near-bed eddy sizes are attributed to the reduction in turbulent kinetic energy dissipation rate close to the stream bed. [ABSTRACT FROM AUTHOR]

The present work investigates the effect of the flow profile induced by an inlet condition on the roll-wave evolution in turbulent clear-water flows. The study employs theoretical and numerical analyses. Firstly, the influence of the inlet condition on the spatial evolution of a single perturbation in a hypercritical flow is examined through the expansion near a wavefront analysis. The results show that an accelerated unperturbed profile reduces the disturbance spatial growth. A decelerated profile causes an increase. The effect of the flow profile on the spatial evolution of roll-wave trains is then numerically investigated solving the Saint Venant equations with a second-order Runge-Kutta Total Variation Diminishing (TVD) Finite Volume scheme. The numerical simulations comply with the analytical results for the initial and transition phases of the roll-wave development. The unperturbed profile influences even the roll-waves statistical characteristics in the final stage, with a more evident effect in case of accelerated profiles. The influence of the flow profile should be therefore accounted for in the formulation of predictive criteria for roll-waves appearance based on the estimation of the disturbance spatial growth rate.

Summary: In this study, a depth‐integrated nonhydrostatic flow model is developed using the method of weighted residuals. Using a unit weighting function, depth‐integrated Reynolds‐averaged Navier‐Stokes equations are obtained. Prescribing polynomial variations for the field variables in the vertical direction, a set of perturbation parameters remains undetermined. The model is closed generating a set of weighted‐averaged equations using a suitable weighting function. The resulting depth‐integrated nonhydrostatic model is solved with a semi‐implicit finite‐volume finite‐difference scheme. The explicit part of the model is a Godunov‐type finite‐volume scheme that uses the Harten‐Lax‐van Leer‐contact wave approximate Riemann solver to determine the nonhydrostatic depth‐averaged velocity field. The implicit part of the model is solved using a Newton‐Raphson algorithm to incorporate the effects of the pressure field in the solution. The model is applied with good results to a set of problems of coastal and river engineering, including steady flow over fixed bedforms, solitary wave propagation, solitary wave run‐up, linear frequency dispersion, propagation of sinusoidal waves over a submerged bar, and dam‐break flood waves. [ABSTRACT FROM AUTHOR]

In hydraulic engineering, free-surface aeration is a natural phenomenon occurring in smooth channel flows. In self-aerated flows, a key aspect that has not yet been well understood is the formation mechanism of free-surface air entrainment. In this research, the process of free-surface entrapped deformation is analyzed theoretically and the critical radius of curvature for air entrainment is obtained, affected by flow mean velocity and depth. When the severity of local free-surface deformation exceeds the critical condition, the entrapped free surface encounters closure in the unstable deformation movement process, resulting in air entrainment. This inference agrees well with observed experimental results that are obtained from the processes of surface entrapped deformation and air entrainment captured by a high-speed camera-based data acquisition system. This agreement indicates that self-aeration occurs in low-velocity open-channel flows. It is also confirmed that free-surface turbulent deformation provides a mechanism for air entrainment. [ABSTRACT FROM AUTHOR]

Volumetric water metering is a critical need for rationalizing abiana (water tariff) rates in the Indus Basin Irrigation System (IBIS). Measuring water flows in a mogha (small irrigation outlets) is extremely challenging due to various problems of scale. Due to the extremely large number of such outlets in IBIS (numbering tens of thousands), a manual measurement and reporting system is completely infeasible. An ICT powered automation is an obvious approach towards solving this scalability problem. However, the challenges for an electronic solution are enormously high. While such an automation has been successfully demonstrated for distributary irrigation canals (10s-100s of cusecs) in the IBIS by our group using a smart water metering approach, measuring such flows at the outlet level (<3 cusecs) is hampered by challenges in hydraulics, frequent siltation, unavailability of grid energy to power electronics, unreliable telecommunications backbone and institutional support to service the meters. In this paper, we report our development and deployment of a test unit for discharge measurement on an outlet using a Low Power Wireless Smart Water Meter, and a long throated flume to generate a modular flow of water to measure discharge in small rectangular channels and to analyse the accuracy of measurement of unit. The deployed solution can be used to remotely transmit depth of water, discharge, state of channel, therefore, free flow or submerged using GSM/GPRS module. Two ultrasonic sensors take reading of depth of water in channel, one on the approach section h 1 and another on tail water section h 2 for estimation of state of flume. Rating curve of the discharge measurement unit is generated by WinFlume and results of real system discharge measurements are compared for accuracy. [ABSTRACT FROM AUTHOR]

Abstract: A double-averaged Navier–Stokes equations (DANS) model has been developed for depth-limited open channel flows over gravels. Three test cases are used to validate the model: an open-channel flow over a densely packed gravel bed with small-scale uniform roughness (D/d 50 ∼13, d 50 =median diameter of roughness elements, D =water depth), open-channel flows over large-scale sparsely distributed roughness elements (D/Δ ∼2.3–8.7, Δ =roughness height) and steep slope gravel-bed river flows with D/d 50 ∼7–25. Various methods of treatment of the gravel-induced resistance effect have been investigated. The results show that the wall function approach (WFA) is successful in simulating flows over small gravels but is not appropriate for large gravels since the vertical profile of the longitudinal velocity does not follow the logarithmic–linear relationship. The drag force method (DFM) performs better but the non-logarithmic velocity distribution generated by sparsely distributed gravels cannot be simulated accurately. Noting that the turbulence length scale within the gravel layer is governed by the gravel size, the DANS model incorporating the DFM and a modified Spalart–Allmaras (S–A) turbulence closure is proposed. The turbulence length scale parameter in the S–A model is modified to address the change in the turbulence structure within the gravel layer. The computed velocity profiles agree well with the corresponding measured profiles in all cases. Particularly, the model reproduces the S-shape velocity profile for sparsely distributed large size roughness elements. The modeling methodology is robust and can be easily integrated into the existing numerical models. [Copyright &y& Elsevier]

Abstract: The present work deals with the development and application of a 2D Smoothed Particle Hydrodynamics (SPH) model to simulate a broad range of open-channel flows. Although in the last decades the SPH modelling has been widely used to simulate free-surface flows, few applications have been performed for free-surface channels. For this reason, an appropriate algorithm is developed to enforce different upstream and downstream flow conditions and simulate uniform, non-uniform and unsteady flows. First, the proposed algorithm is validated for a viscous laminar flow in open channel characterized by Reynolds numbers of order . The second test case deals with a hydraulic jump for which different upstream and downstream conditions are needed. Varying the Froude number, several types of jumps are investigated with specific focus on the velocity field, pressure forces, water depths and location of the jump. Comparisons between numerical results, theory and experimental data are provided. Finally, the interaction between a flash flood generated by an unsteady inflow condition and a bridge is shown as an example of an engineering application. [Copyright &y& Elsevier]

The main focus of this Special Issue of Water is the state-of-the-art and recent research on turbulence and flow–sediment interactions in open-channel flows. Our knowledge of river hydraulics is becoming deeper and deeper, thanks to both laboratory/field experiments related to the characteristics of turbulence and their link to the erosion, transport, deposition, and local scouring phenomena. Collaboration among engineers, physicists, and other experts is increasing and furnishing new inter/multidisciplinary perspectives to the research in river hydraulics and fluid mechanics. At the same time, the development of both sophisticated laboratory instrumentation and computing skills is giving rise to excellent experimental–numerical comparative studies. Thus, this Special Issue, with ten papers by researchers from many institutions around the world, aims at offering a modern panoramic view on all the above aspects to the vast audience of river researchers. [ABSTRACT FROM AUTHOR]