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This paper presents a numerical model for the evaluation of the actual torque in Gerotor units. The model consists of two major modules: the pre-processor module and the HYGESim module. The preprocessor module consists of the geometric and the mechanical module. The geometric pre-processor module considers the CAD geometry of Gerotor with tolerances as input and it provides as output the geometric features needed to evaluate the rotor loading and the flow features. The mechanical preprocessor module evaluates the forces of interaction at the contact points between the rotors. The flow displaced by the unit is evaluated using a lumped parameter model whereas the lubricating gaps are evaluated by solving the Reynolds Equation. The main novel aspects consist of the evaluation of the frictional losses at various interfaces. An Elasto-Hydrodynamic Lubrication (EHL) approach is used to evaluate the frictional losses at the contact points between the rotors. Tests on a prototype Gerotor unit are performed for the model validation, particularly as pertains to the features of the shaft torque. Additionally, the paper comments on the distribution of the different torque loss contributions associated with the operation of the unit taken as reference.

The study of the interaction of mud-flows with obstacles is important to define inundation zones in urban areas and to design the possible structural countermeasures. The paper numerically investigates the impact of a mud-flow on rigid obstacles to evaluate the force acting on them using two different depth-integrated theoretical models, Single-Phase Model (SPM) and Two-Phase Model (TPM), to compare their performance and limits. In the first one the water-sediment mixture is represented as a homogeneous continuum described by a shear-thinning power-law rheology. Alternatively, the two-phase model proposed by Di Cristo et al in 2016 is used, which separately accounts for the liquid and solid phases. The considered test cases are represented by a 1D landslide flowing on a steep slope impacting on a rigid wall and a 2D mud dam-break flowing on a horizontal bottom in presence of single and multiple rigid obstacles. In the 1D test case, characterized by a very steep slope, the Two-Phase Model predicts the separation between the two phases with a significant longitudinal variation of the solid concentration. In this case the results indicate appreciable differences between the two models in the estimation of both the wave celerity and the magnitude of the impact, with an overestimation of the peak force when using the Single-Phase Model. In the 2D test-cases, where the liquid and solid phases remain mixed, even if the flow fields predicted by the two models present some differences, the essential features of the interaction with the obstacles, along with the maximum impact force, are comparable.

The evaluation of the impact force on structures due to a flood wave is of utmost importance for estimating physical damage and designing adequate countermeasures. The present study investigates, using 2D shallow-water approximation, the morphodynamics and forces caused by a dam-break wave against a rigid obstacle in the presence of an erodible bed. A widely used coupled equilibrium model, based on the two-dimensional Saint&ndash, Venant hydrodynamic equations combined with the sediment continuity Exner equation (SVEM), is compared with a more complex two-phase model (TPM). Considering an experimental set-up presented in the literature with a single rigid obstacle in a channel, two series of tests were performed, assuming sand or light sediments on the bottom. The former test is representative of a typical laboratory experiment, and the latter may be scaled up to a field case. For each test, two different particle diameters were considered. Independently from the particle size, it was found that in the sand tests, SVEM performs similarly to TPM. In the case of light sediment, larger differences are observed, and the SVEM predicts a higher force of about 26% for both considered diameters. The analysis of the flow fields and the morphodynamics shows these differences can be essentially ascribed to the role of inertia of the solid particles.

A two–sided model is proposed to investigate the stability problem of the steady uniform flow of a power–law fluid layer down an inclined porous layer. The governing equations for the clear fluid flow are coupled with the ones for the seepage in the porous layer, for which the generalized Darcy’s law is employed. A linear stability analysis of both two–dimensional and depth–integrated formulations is carried out in order to individuate the neutral stability conditions. The influence of the dimensionless governing parameters on the stability properties is deeply discussed. The nonlinear effects on the stability of the flow are finally investigated through the numerical solution of the depth–integrated model.

The stability of the steady uniform flow of a power–law fluid down an inclined porous layer heated from below is investigated by means of a two–layer model. Under the the long-wave approximation, the flow model is deduced starting from the continuity and momentum equations in the fluid film, the continuity and the generalized Darcy’s law in the porous layer, and the energy balances in both the film and the porous layer. The associated boundary conditions account for the surface tension and the Beavers-Joseph velocity slip at the film bottom. A linear stability analysis of both two–dimensional and one–dimensional, i.e. depth–integrated, formulations is performed to define the neutral stability conditions, separately analyzing the S- and H-mode contributions. The influence of all the dimensionless governing parameters on the stability is discussed. Finally, the numerical solution of the depth–integrated model allows for an investigation of the nonlinear growth of H-mode unstable perturbations up to the formation of permanent waves and a discussion of the role of the magnitude of thermocapillary effects on the wave shape. Present results, taken collectively, indicate that, similarly to the isothermal case, the presence of a porous medium strongly modifies the stability bounds even in non-isothermal conditions, suggesting its potentiality as a passive control for the themocapillary instability for both H- and S-modes. Moreover, it is found that the shear-thinning behaviour leads to flows which are less stable but also less susceptible to thermocapillary effects.

The Kinematic Wave Model (KWM), representing a simplification of the Full Wave Model (FWM) under the shallow-water approximation, is still used for predicting flood propagation requiring a reduced computational effort and for being more easily coupled with high-resolution topographic data. It is as suitable approach also for predicting debris and mud-flows. In this work the applicability of the KWM for reproducing the propagation of mud-flows characterized by a power-law shear-thinning rheology is investigated. The power-law model is suitable for fluids with negligible yield stress, named mud-flows, such as fine sediment-water mixtures which are often encountered in natural estuaries or in landsides. For this reason, it is very important to individuate the hydraulic conditions in which the KWM can be safely applied for predicting mud flow propagation. In this study the power-law shallow-layer model of Ng and Mei (1994) is considered for the analysing the applicability conditions of the KWM through an unsteady analysis. Comparing the flow depth profiles predicted by KWM with those of the Full Wave Model the applicability conditions are characterized by evaluating the error on the dimensionless maximum flow depth along the channel. The preliminary results presented herein confirm a strong dependency from the rheology and indicate that criteria for clear-water flows do not apply to shearthinning fluids. Moreover, the error increases moving in the downstream direction and higher errors pertain to lower wave durations of the discharge hydrograph. Further investigations will provide specific criteria for the applicability of the KWM for predicting mud-flows propagation.

The Kinematic Wave Model (KWM), representing a simplification of the Full Wave Model (FWM) under the shallow-water approximation, is often used in the one-dimensional formulation for describing the dynamics of debris/mud flood. The present work investigates the applicability conditions of the KWM to analyze mud floods, characterized by a rheological behaviour expressed through a power-law model. The study is carried out through the numerical solution of both the Full and Kinematic Wave models changing the characteristic time of hydrograph, i.e. the wave duration, imposed at the channel inlet. The predictions of the two models are compared in terms of maximum flow depth, maximum discharge and peak discharge at the downstream channel end. The study is performed for several values of the Froude (F) and the Kinematic Wave (K) numbers. Similarly to the clear-water case, present results show that higher errors in applying KWM pertain to shorter flood wave durations, but the performance of KWM appears to strongly depend on the value of the rheological index, becoming worse as the fluid rheology becomes more shear-thinning. The study furnishes applicability criteria representing a guideline for practical applications in terms of minimum wave duration above which the KWM error in reproducing the considered flood characteristics (the maximum flow depth, the maximum discharge and the peak discharge at the downstream channel end) is less or equal than 5%. Finally, it has been shown that the wave period criterion, obtained considering linearized wave dynamics, may be safely applied at least for fluid with moderate shear-thinning behavior and for moderate values of the dimensionless number KF2.

The flow between two parallel plates driven by a pulsatile pressure gradient was studied analytically with a second-order velocity expansion. The resulting velocity distribution was compared with a numerical solution of the momentum equation to validate the analytical solution, with excellent agreement between the two approaches. From the velocity distribution, the analytical computation of the discharge, wall shear stress, discharge, and dispersion enhancements were also computed. The influence on the solution of the dimensionless governing parameters and of the value of the rheological index was discussed.

The adoption of the Diffusive Wave (DW) instead of the Full Dynamic (FD) model in the analysis of mud flood routing within the shallow-water framework may provide a significant reduction of the computational effort, and the knowledge of the conditions in which this approximation may be employed is therefore important. In this paper, the applicability of the DW approximation of a depth-integrated Herschel–Bulkley model is investigated through linear analysis. Assuming as the initial condition a steady hypocritical decelerated flow, induced by downstream backwater, the propagation characteristics of a small perturbation predicted by the DW and FD models are compared. The results show that the spatial variation on the initial profile may preclude the application of DW model with a prescribed accuracy. Whenever the method is applicable, the rising time of the mud flood must satisfy additional constraints, whose dependence on the flow depth, along with the Froude number and the rheological parameters, is deeply analyzed and discussed.

A depth-averaged model with a power-law rheology, usually adopted for debris flow of fine sediment-water mixtures, is analytically studied, considering the complete (Full Dynamic Model) and the simplified momentum equations, known as Kinematic, Diffusion, and Quasi-steady Models. Applying the Laplace transform to the linearized equations, the analytical first-order expressions of the upstream and downstream spatio-temporal channel response functions are derived for the full and the approximate models. Both shear-thinning and shear-thickening fluids have been considered in hypocritical and linearly stable conditions, to investigate the influence of the rheology on the wave propagation. The effect of the upstream and downstream boundary conditions is considered assuming a finite channel length. The analytic description of the various linearized approximations allows a multi-faceted comparison of the wave propagation. About the basic wave characteristics, in the entire investigated range of Froude numbers and for all the considered rheologies, the Kinematic and Diffusive models underestimate the primary wave celerity and the Quasi-steady model underestimates the wave round-trip time. However, the examination of the response function reveals that the approximate models may provide relatively accurate reproduction of the full dynamic response. Moreover, the analytical solutions in the Laplace variables permit a systematic analysis of some relevant shape factors of the response functions. Independently on the models and the rheology, the damping of the downstream response function is several orders of magnitude larger than that of the upstream one. The results of the shape factor analysis of the upstream response function indicate that the diffusive approximation is the most accurate one. The performed comparison among the wave characteristics of the different approximations provides indications on the applicability of the simplified models for predicting debris or mud flows.

The paper explores the possibility of predicting the occurrence of antidunes in gradually-varied flows through non-equilibrium morphological models natively derived, under the long-wave approximation, for fast geomorphic transients. Starting from a recent two-phase gradually-varied flow model, an improved version, which partially accounts for the bottom curvature effects, is proposed and analyzed. The normal mode analysis carried out on the linearized gradually-varied flow model shows the existence of unstable conditions at moderate Froude numbers characterized by bed perturbations migrating in the upstream direction. However, a proper wavelength selection may be obtained only when the bottom curvature is accounted for. Direct numerical simulations in linear regime of the improved model indicate the existence of unstable dominant wavelengths, which comply with experimental evidences. Full non-linear simulations confirm the ability of the improved two-phase model in predicting the antidunes formation.

Simplified wave models - such as kinematic, diffusion and quasi-steady - are widely employed as a convenient replacement of the full dynamic one in the analysis of unsteady open-channel flows, and especially for flood routing. While their use may guarantee a significant reduction of the computational effort, it is mandatory to define the conditions in which they may be confidently applied. The present paper investigates the applicability conditions of the kinematic, diffusion and quasisteady dynamic shallow wave models for mud flows of power-law fluids. The power-law model describes in an adequate and convenient way fluids that at low shear rates fluids do not posses yield stress, such as clay or kaolin suspensions, which are frequently encountered in Chinese rivers. In the framework of a linear analysis, the propagation characteristics of a periodic perturbation of an initial steady uniform flow predicted by the simplified models are compared with those of the full dynamic one. Based on this comparison, applicability criteria for the different wave approximations for mud flood of power-law fluids are derived. The presented results provide guidelines for selecting the appropriate approximation for a given flow problem, and therefore they may represent a useful tool for engineering predictions.

"The occurrence of roll-waves in mud-flows is commonly investi-gated starting from the formulation of the marginal stability threshold of a linearized one-dimensional viscoplastic (shear-thinning) flow model. Since for this non-newtonian rheological model such a threshold may oc-cur in hypocritical flow, the downstream boundary condition may have a non-negligible effect on the spatial growth\/decay of the perturbation. The paper presents the solution of the 1D linearized flow of a Herschel & Bulkley fluid in a channel of finite length, in the neighbourhood of a hypocritical base uniform flow. Both linearly stable and unstable condi-tions are considered. The analytical solution is found applying the Laplace transform method and obtaining the first-order analytical expressions of the upstream and downstream channel response functions in the time domain. The effects of both the yield stress and the rheological law exponent are discussed, recovering as particular cases both power-law and Bingham fluids. The theoretical achievements may be used to extend semi-empirical criteria commonly employed for predicting roll waves occurrence in clear-water even to mud-flows."

In this paper, a lumped parameter approach for predicting cavitation in fluid power systems is described. The modeling approach is based on the homogeneous mixture one fluid model for cavitation and takes into account the effects of compressibility of both the liquid phase and gaseous phase, related to aeration and fluid vaporization. By using the homogeneous mixture theory, equations suitable for lumped parameter models are derived, including the orifice equation to describe flow through hydraulic connections, the constitutive relations, and the pressure built up equations for hydraulic chambers. The proposed equations are then used in the paper to simulate a Gerotor pump under cavitating conditions. Results are validated experimentally for different operating conditions.

Summary This paper analyses conditions for roll-waves formation in homogeneous granular flows with the aim of predicting their occurrence in finite length channels. With reference to a wide class of constitutive equations of the flowing medium, the response of the linearized one-dimensional flow model to a pointwise instantaneous disturbance is studied and a closed form of the Green’s function, in both stable and unstable conditions, is provided. The exponential growth of the disturbance peak is used to predict the spatial evolution of roll-waves in dry flows of dense granular materials and a criterion for the minimum channel length necessary to appreciate roll-wave development is proposed. The comparison among theoretical and experimental data available in the literature appears promising.

This paper investigates the spatial evolution of a disturbance in an open-channel flow of a power-law fluid at non-uniform accelerated and decelerated initial profiles, up to the occurrence of roll-waves in mild and steep slope channels. Both theoretical and numerical analyses are applied to the depth-averaged continuity and momentum conservation equations, deduced from the von Kármán's integral method. For the theoretical investigation, the nonlinear near-front expansion technique was applied. Then, the full nonlinear problem in its conservative formulation was numerically solved. Independently of the rheology of the flowing medium, non-uniform initial conditions strongly influence the perturbation celerity, the disturbance evolution and the roll-wave development. For mild slope channels, an initially decelerated profile of shear-thinning fluids has a stabilizing effect, while the opposite is found for accelerated profiles. For shear-thickening fluids, only the stabilizing effect caused by a decelerated profile is observed. In steep slope channels, independently of the fluid rheology, decelerated initial conditions promote roll-wave occurrence, while accelerated conditions inhibit the perturbation growth. Although experimental verifications are needed, the present results have to be properly accounted for in defining roll-wave prediction methods and in assigning appropriate boundary conditions to enhance or to reduce their formation.

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.

Fast geomorphic transients may involve complex scenarios of sediment transport, occurring near the bottom as bed load (i.e., saltating, sliding, and rolling) or as suspended load in the upper portion of the flow. The two sediment transport modalities may even coexist or alternate each other during the same event, especially when the shear stress varies considerably. Modeling these processes is therefore a challenging task, for which the usual representation of the flow as a mixture may result in being unsatisfactory. In the present paper, a new two-phase depth-averaged model is presented that accounts for variable sediment concentration in both bed and suspended loads. Distinct phase velocities are considered for bed load, whereas the slip velocity between the two phases is neglected in the suspended load. It is shown that the resulting two-phase model is hyperbolic, and the analytical expression of the eigenvalues is provided. The entrainment/deposition of sediment between the bottom and the bed load layer is based on a modified van Rijn transport parameter, whereas for the suspended sediment a first-order exchange law is considered. A numerical finite-volume method is used for the simulation of three dam break experiments found in the literature, which are effectively reproduced in terms of both free surface elevation and bottom deformation, confirming the key role played by the solid concentration variability even for two-phase models.

A theoretical analysis of the Saint-Venant one-dimensional flow model is performed in order to define the nature of its instability. Following the Brigg criterion, the investigation is carried out by examining the branch points singularities of dispersion relation in the complexωandkplanes, whereωandkare the complex pulsation and wave number of the disturbance, respectively. The nature of the linearly unstable conditions of flow is shown to be of convective type, independently of the Froude number value. Starting from this result a linear spatial stability analysis of the one-dimensional flow model is performed, in terms of time asymptotic response to a pointwise time periodic disturbance. The study reveals an influence of the disturbance frequency on the perturbation spatial growth rate, which constitutes the theoretical foundation of semiempirical criteria commonly employed for predicting roll waves occurrence.

In the present paper the stability of gradually varying mud-flows in wide open channel is analyzed. The governing equations derive from a Saint-Venant-like approach with a Herschel & Bulkley model for the fluid rheology. In order to define the stability of a given initial accelerating or decelerating flow depth profile, the spatial evolution of a wavefront is analyzed. A stability criterion based on the linearized flow model is introduced, showing that the streamwise non-uniformity of the flow substantially influences the stability limits. A positive flow depth slope induces a stabilization while a negative one determines a destabilizing effect. The dependency of the linear stability on the values of rheological parameters is deeply discussed. A non-linear analysis of the wavefront propagation, along with the full non-linear solution of the problem, has been carried out to confirm the role played by the initial profile on the flow stability. The results of the presented study may be useful for engineering applications in order to prevent or to inhibit roll-waves formation in subcritical flows of a Herschel & Bulkley fluid.

The fully developed turbulent flow in an annular pipe with radius ratio of 1/2 is investigated for Reynolds numbers based on hydraulic diameter and bulk velocity ranging from 12000 to 46000. The study is carried out processing data obtained from wall resolved large eddy simulations employing the dynamic subgrid scale eddy viscosity model of Germano et al. (1991) as modified by Lilly (1992). Scaling in wall variables shows that the turbulence intensities in the inner radius region are systematically less than the corresponding plane channel data, while the differences in the outer radius region reduce considerably. Because of the transverse curvature the total shear stress deviates from the linear distribution by as much as 12% in the inner wall region at y^+ =100. The curvature effects decrease as the Reynolds number is increased. Thus in the low Reynolds number range similarity arguments have to be considered with augmented caution with respect to the plane channel case.

Unsteady shallow-layer flows may be described through full dynamic models or using simplified momentum equations, based on kinematic, diffusion, and quasi-steady approximations, which guarantee a reduction of the computational effort. This paper aims to investigate through linear analysis the applicability range of simplified shallow-wave models with special concern to unsteady flows of mud. Considering a three-equation depth-integrated Herschel-Bulkley model, the applicability of the approximated wave models is discussed comparing the propagation characteristics of a small perturbation of an initial steady uniform flow as predicted by the simplified models with those of the full dynamic model. Based on this comparison, applicability criteria for the different wave approximations for mud flows of Herschel-Bulkley fluids, which account for the effects of the rheological parameters, are derived. The results show that accounting for the fluid rheology is mandatory for the choice of an appropriate simplified model. DOI: 10.1061/(ASCE)HE.1943-5584.0000881. © 2014 American Society of Civil Engineers.

The paper addresses the prediction of roll-waves occurrence in mud-flows. The spatial growth of a point-wise disturbance is analytically described, based on the linearized flow model of a Herschel and Bulkley fluid, in the neighborhood of an initial uniform base condition. The theoretical achievements allow to generalize to mud-flows the minimum channel criterion commonly used for the prediction of roll-waves in clear-water. The applicability of the criterion is discussed through the comparison with literature laboratory data concerning unstable flows without rollwaves.

"The study of formation and evolution of waves in mud-flows is strongly . motivated by their destructive power. The rheology of these flows, which involve . massive solid transport, is often described using linear or non-linear viscoplastic . models. The paper analyzes the wave dynamics of mud-flows, presenting the . linearized response of the 1D uniform flow of a viscoplastic fluid with yield . stress, i.e. Herschel & Bulkley fluid, to a pointwise impulsive forcing term. The . solution in either linearly stable or unstable conditions is found, for both . subcritical and supercritical flows, through the bilateral inverse Laplace . transform. The analysis offers a unified description of the behaviour of different . non-newtonian fluids, recovering as particular cases both the Bingham and the . power-law models. The influence of the dimensionless governing numbers and of . the rheological parameters on the shape and peak of waves is investigated. "

The flow of a thin layer of power-law fluid on a porous inclined plane is considered. The unsteady equations of motion are depth-integrated according to the von Karman momentum integral method. The variation of the velocity distribution with the depth is accounted for, and it is furthermore assumed that the flow through the porous medium is governed by the modified Darcy’s law. The stability condition is deduced considering the hierarchy of kinematic and gravity waves. The response of the linearized model to a Dirac-delta disturbance in unbounded domain is analytically deduced, in both stable and unstable conditions 1624 C. Di Cristo, M. Iervolino and A. Vacca of flow. The influence of the effect of power-law exponent and bottom permeability on the disturbance propagation is finally analyzed, suggesting indications about the choice of the bottom permeability in order to improve the performance of industrial processes.

In the present paper, an analytical expression of the Green’s function. of linearized Saint–Venant equations (LSVEs) for shallow water waves is. provided and applied to analyse the propagation of a perturbation superposed. to a uniform flow. Independently of the kinematic character of the base flow,. i.e., subcritical or supercritical uniform flow, the effects of a non-uniform. vertical velocity profile and a non-constant resistance coefficient are. accounted for. The use of the Darcy–Weisbach friction law allows a unified. treatment of both laminar and turbulent conditions. The influence on the. wave evolution of the wall roughness and the fluid viscosity are finally. discussed, showing that in turbulent regime the assumption of constant. friction coefficient may lead to an underestimation of both amplification and. damping factors on the wave fronts, especially at low Reynolds numbers.. This conclusion has to be accounted for, particularly in describing hyper-. concentrated suspensions or other kinds of Newtonian mixtures, for which. the high values of the kinematic viscosity may lead to relatively low. Reynolds numbers.

The paper introduces a 2D shallow water model based on a two-phase formulation for the analysis of fast geomorphic transients occurring in the context of river morphodynamics. Mass and momentum conservation principles are separately imposed for both phases. The model naturally accounts for non-equilibrium solid transport, since neither instantaneous adaptation hypothesis nor any lag equation is employed to represent sediment dynamics. The hyperbolic character of the proposed model is shown to be preserved independently on the flow conditions. Results from numerical simulations of both 1D and 2D test-cases are compared with literature experimental data and with available numerical solutions.

The present paper investigates the response of a porous media to an unsteady forcing resulting from the superposition of an harmonic component to a mean one. The analysis is carried out both in terms of global parameters and local fields obtained processing data from numerical solution of the Navier-Stokes equations at pore level performed with a spectrally accurate multi-domain algorithm.

The paper investigates the phenomena occurring in a Taylor–Couette flow system subject to a steady axial pressure gradient in a small envelope of the Taylor– Reynolds state space under transitional regimes. A remarkable net power reduction necessary to simultaneously drive the two flows compared to that required to drive the Taylor–Couette flow alone is documented under non-trivial conditions. The energy transfer process characterizing the large-scale coherent structures is investigated by processing a set of statistically independent realizations obtained from direct numerical simulation. The analysis is conducted with an incompressible three-dimensional Navier–Stokes flow solver employing a spectral representation of the unknowns.

The initial phase of roll-wave development is investigated by means of spatial linear stability analysis using the St. Venant equations, subject to a pointwise time-varying oscillating disturbance. The predicted spatial growth is compared with both Vedernikov’s results and those computed with a fully non-linear model. It is shown that for large values of the channel slope Vedernikov’s theory systematically overpredicts the roll waves spatial growth rate, whereas the present analysis yields significant improvements. A modification of Montuori’s criterion for the minimum channel length prediction is finally proposed, which agrees with available experimental data independently of the channel slope.

The present work investigates the effects of a forced harmonic oscillations of fixed frequency on the characteristics of a turbulent pipe flow with a mean Reynolds number based on the cross sectional bulk velocity and pipe diameter of 5900. The setup refers to an extremely thin Stokes layer so that the vorticity associated to the oscillating motion is generated in a small near wall region. When the ratio of the amplitudes of the oscillating and bulk velocities is increased from 1 to 11, the turbulence is affected by the harmonic forcing so much that the near wall coherent structures are substantially modified, although not suppressed through a reverse transition process. This results in an overall space and time averaged resistance reduction which, for the largest amplitude case, adds up to 27% of the non-pulsating flow at the same bulk Reynolds number. The present contribution is focused on the analysis of the phase locked averaged statistics of the Reynolds stress tensor components and velocity spectra. The analysis is carried out processing a set of statistically independent samples obtained from wall resolved large Eddy simulations with a spectrally accurate Navier-Stokes code. (c) 2007 Elsevier Ltd. All rights reserved.

In the paper a linear stability analysis of a one–dimensional morphodynamical model is presented. The local equilibrium assumption on the solid discharge is removed and a dynamical description of the bed–load transport is employed. Both unsteady and quasi–steady flow models are analysed and discussed. The results reveal the existence, at moderate Froude number values, of perturbations related to bed elevation instability, migrating in the upstream direction both in quasi–steady and full unsteady models.

The paper describes the effects of a forced harmonic oscillations of fixed frequency and amplitudes in the range Λ = Um /Ub = 1 ÷ 11 on the characteristics of a turbulent pipe flow with a bulk Reynolds number of 5900. The resulting Stokes layer δ is a fraction of the pipe radius (χ = R/δ = 53) so that the vorticity associated to the oscillating motion is generated in a small near wall region. The analysis is carried out processing a set of statistically independent samples obtained from wall resolved Large Eddy Simulations; time and space averaged global quantities, extracted for the sake of comparison with recent experimental data, confirm the presence of a non negligible drag reduction phenomenon. Phase averaged profiles of the Reynolds stress tensor components provide valuable material for the comprehension of the effects of the time varying mean shear upon the near wall turbulent flow structures. The large scale of motion are directly computed through numerical integration of the space filtered three dimensional Navier-Stokes equations with a spectrally accurate code; the subgrid scale terms are parametrized with a dynamic procedure.Copyright © 2003 by ASME

The study of unsteady shallow-layer flows, as those occurring during floods, may be performed through either fully dynamic models or using simplified momentum equations (e.g. kinematic, diffusion and quasi-steady approximations). Among the latter, kinematic approximation may provide a significant reduction of the computational effort, which represents a key advantage in time-critical applications. The proper application of this simplified model is however generally subjected to some limitations, which the modeler has to know and accurately verify to avoid errors. The present paper aims to investigate the applicability range of the kinematic approximation to shallow flows, with special concern to unsteady mud-flows, such as the hyper-concentrated floods. In the present analysis the rheological model proposed by O'Brien et al. (1993), broadly appreciated and widespread in the technical community, is considered. This model relies on a quadratic relationship linking the shear rate to the total shear stress, accounting for turbulent, dispersive and cohesive yield stresses. To provide relatively simple applicability criteria, linear analysis is applied to compare the celerity of a small perturbation of an initial steady uniform flow as predicted by the simplified model with those of the full dynamic model. Based on this comparison, applicability criteria for the kinematic approximation for mud-flows are derived. In particular, the proposed criteria rely on the comparison between the flood rising-time and a threshold time-scale: whenever the former is larger than the latter, the estimation of the propagation celerity of the simplified model is guaranteed with a prescribed accuracy. These criteria are presented considering the effect of all model parameters, based on the particular case of a pyroclastic mud, typically encountered in volcanic areas of southern Italy. The achieved results are discussed, aiming to guide the choice of the appropriate simplified model for mud flood analysis