Direct numerical simulation of open-channel flow over a fully-rough wall at moderate relative submergence

Direct numerical simulation of open-channel flow over a bed of spheres arranged in a regular pattern has been carried out at bulk Reynolds number and roughness Reynolds number (based on sphere diameter) of approximately 6900 and 120, respectively, for which the flow regime is fully-rough. The open-channel height was approximately 5.5 times the diameter of the spheres. Extending the results obtained by Chan-Braun et al. (J. Fluid Mech., vol. 684, 2011, 441) for an open-channel flow in the transitionally-rough regime, the present purpose is to show how the flow structure changes as the fully-rough regime is attained and, for the first time, to enable a direct comparison with experimental observations. The results indicate that, in the vicinity of the roughness elements, the average flow field is affected both by Reynolds number effects and by the geometrical features of the roughness, while at larger wall-distances this is not the case, and roughness concepts can be applied. The flow-roughness interaction occurs mostly in the region above the virtual origin of the velocity profile, and the effect of form-induced velocity fluctuations is maximum at the level of sphere crests. The spanwise length scale of turbulent velocity fluctuations in the vicinity of the sphere crests shows the same dependence on the distance from the wall as that observed over a smooth wall, and both vary with Reynolds number in a similar fashion. Moreover, the hydrodynamic force and torque experienced by the roughness elements are investigated. Finally, the possibility either to adopt an analogy between the hydrodynamic forces associated with the interaction of turbulent structures with a flat smooth wall or with the surface of the spheres is also discussed, distinguishing the skin-friction from the form-drag contributions both in the transitionally-rough and in the fully-rough regimes.Comment: 46 pages, 26 figure

Direct numerical simulation of the oscillatory flow around a sphere resting on a rough bottom

The oscillatory flow around a spherical object lying on a rough bottom is investigated by means of direct numerical simulations of continuity and Navier-Stokes equations. The rough bottom is simulated by a layer/multiple layers of spherical particles, the size of which is much smaller that the size of the object. The period and amplitude of the velocity oscillations of the free stream are chosen to mimic the flow at the bottom of sea waves and the size of the small spherical particles falls in the range of coarse sand/very fine gravel. Even though the computational costs allow only the simulation of moderate values of the Reynolds number characterizing the bottom boundary layer, the results show that the coherent vortex structures, shed by the spherical object, can break-up and generate turbulence, if the Reynolds number of the object is sufficiently large. The knowledge of the velocity field allows the dynamics of the large scale coherent vortices shed by the object to be determined and turbulence characteristics to be evaluated. Moreover, the forces and torques acting on both the large spherical object and the small particles, simulating sediment grains, can be determined and analysed, thus laying the groundwork for the investigation of sediment dynamics and scour developments.Comment: 35 pages, 21 figure

Direct numerical simulations of ripples in an oscillatory flow

Sea ripples are small-scale bedforms which originate from the interaction of an oscillatory flow with an erodible sand bed. The phenomenon of sea ripple formation is investigated by means of direct numerical simulation in which the sediment bed is represented by a large number of fully-resolved spherical grains (i.e, the flow around each individual particle is accounted for). Two sets of parameter values (differing in the amplitude and frequency of fluid oscillations, among other quantities) are adopted which are motivated by laboratory experiments on the formation of laminar rolling-grain ripples. The knowledge on the origin of ripples is presently enriched by insights and by providing fluid- and sediment-related quantities that are difficult to obtain in the laboratory (e.g. particle forces, statistics of particle motion, bed shear stress). In particular, detailed analysis of flow and sediment bed evolution has confirmed that ripple wavelength is determined by the action of steady recirculating cells which tend to accumulate sediment grains into ripple crests. The ripple amplitude is observed to grow exponentially consistent with established linear stability analysis theories. Particles at the bed surface exhibit two kinds of motion depending on their position with respect to the recirculating cells: particles at ripple crests are significantly faster and show larger excursions than those lying on ripple troughs. In analogy with segregation phenomenon of polydisperse sediments the non-uniform distribution of the velocity field promotes the formation of ripples. The wider the gap between the excursion of fast and slow particles, the the larger the resulting growth rate of ripples. Finally, it is revealed that, in the absence of turbulence, the sediment flow rate is driven by both the bed shear stress and the wave-induced pressure gradient, the dominance of each depending on the phase of the oscillation period. In phases of maximum bed shear stress, the sediment flow rate correlates more with the Shields number while the pressure gradient tends to drive sediment bed motion during phases of minimum bed shear stress

Direct Numerical Simulation of Oscillatory Flow Over a Wavy, Rough, and Permeable Bottom

The results of a direct numerical simulation of oscillatory flow over a wavy bottom composed of different layers of spherical particles are described. The amplitude of wavy bottom is much smaller in scale than typical bed forms such as sand ripples. The spherical particles are packed in such a way to reproduce a bottom profile observed during an experiment conducted in a laboratory flow tunnel with well-sorted coarse sand. The amplitude and period of the external forcing flow as well as the size of the particles are set equal to the experimental values and the computed velocity field is compared with the measured velocity profiles. The direct numerical simulation allows for the evaluation of quantities, which are difficult to measure in a laboratory experiment (e.g., vorticity, seepage flow velocity, and hydrodynamic force acting on sediment particles). In particular, attention is focused on the coherent vortex structures generated by the vorticity shed by both the spherical particles and the bottom waviness. Results show that the wavy bottom triggers transition to turbulence. Moreover, the forces acting on the spherical particles are computed to investigate the mechanisms through which they are possibly mobilized by the oscillatory flow. It was found that forces capable of mobilizing surface particles are strongly correlated with the particle position above the mean bed elevation and the passage of coherent vortices above them

Turbulent spots in an oscillatory flow over a rough wall

We describe the results of a direct numerical simulation inspired by laboratory experiments (Carstensen et al., 2012), which showed the formation of turbulent spots in an oscillatory boundary layer over a rough wall. Even though, differently from the experiments, the wall we consider is characterized by a regular roughness, turbulent spots with similar characteristics are observed. The numerical results provide information on transition to turbulence and on the early stages of formation of the turbulent spots. In particular, the formation of low-speed streaks and the subsequent generation of turbulent spots are described. The speed of the extreme points of the spots is obtained from the numerical results. Moreover, the effects of the wall roughness on the speed of the turbulent spots are discussed

Transition to turbulence in an oscillatory flow over a rough wall

A study of the oscillatory incompressible flow close to a wall covered with fixed rigid spheres is carried out by numerical means to provide information on unsteady flows over a rough wall. The simulations are carried out for two bottom configurations, characterized by different values of the diameter of the spheres and different values of the Reynolds number for a total of 10 cases. Three different flow regimes are identified as functions of both the Reynolds number and the diameter of the spheres. The force exerted by the flow on the spheres is discussed also in relation to the different flow regimes. 2016 Cambridge University Press

Soil bio-engineering techniques to protect slopes and prevent shallow landslides

Soil bio-engineering techniques consist in the use of plants, often combined with wooden structures, for the protection and reinforcement of soil and contextually for the re-naturalization of anthropic areas. They are often possibly adopted in place of traditional geo-engineering works because of ecological and economic benefits although a deeper knowledge of their actual mechanical action is requested by practitioners. The present contribution was developed in the framework of a research project focused on the mitigation of landslide risk and is divided into two parts. After a brief overview of the most common soil bio-engineering practices, the results of experiments that were performed on plant roots and on rooted soil samples are shown and discussed in the first part of the paper. Indeed, the role of roots, which is fundamental in almost all soil bio-engineering techniques, is emphasized by the practice of re-vegetation. Presently, the mechanical action of roots on a saturated sandy soil undergoing shear strain, is identified at the scale of volume element. Advantages and limits of Wu and Waldron\u2019s model, which is frequently used to estimate the contribution of roots to the soil shear strength, are also investigated. In the second part of the paper, a general description of live wooden cribwalls is given which is aimed at making practitioner aware of the effectiveness of this technique to retain cuts a few meters high and guarantee the stability of slopes. The mechanical contribution of plants, that progressively enhance the performance of cribwalls, is shown in terms of increments of the safety factor of the structure

Experimental Investigation on the Mechanical Contribution of Roots to the Shear Strength of a Sandy Soil

A common soil-bio-engineering practice to prevent the erosion and the movement of soil along natural hill-slopes is the hydroseeding of herbs and shrubs, whose roots can contribute to the stability of the surficial layers of soil. So far a clear picture of the soil-roots interaction is still missing. In the present paper, the results of triaxial tests carried out both on soil-specimens naturally rooted by plants that are typical of the Mediterranean scrub (i.e. spartium-junceum, ligustrum and arbutus-unedo) and on samples reconstituted using the same material as the rooted ones, are reported. A discussion arises from the comparison between the results obtained on rooted and on reconstituted specimens. Moreover, high-precision measurements of the tensile stress of individual root fibers were also made. Finally, an interpretation of the mechanical contribution of roots to the soil strength under conditions mimicking the effect of an intense storm on the hill slopes is also provided

Pattern formation in a thin layer of sediment

A stability analysis is performed to investigate the process which leads to the formation of bottom forms, when a thin layer of sediment covers a rigid substratum subject to the oscillatory flow induced by a surface gravity wave. The amplitude of the bottom perturbation is assumed to be so small to linearize the hydrodynamic problem and the flow field is determined by analytical means. However, nonlinear effects are significant and play a fundamental role into the morphodynamic problem, when the rigid substratum is bared by the growth of the most unstable mode. Hence, the development of the bottom profile is evaluated by a numerical approach. The model results show that the wavelength of the morphological patterns, which are generated when the rigid substratum is bared, is longer than the wavelength of the bottom forms which appear when the rigid substratum is always covered by the sediment. The results of the theoretical model are supported by some experimental observations made in our laboratory