1,721,144 research outputs found
An improved MAC Method (SIMAC) for free surface unsteady high Reynolds Flows
No full textINT. JOURNAL FOR NUMERICAL METHODS IN FLUID
Large eddy simulation in hydraulic engineering: Examples of laboratory-scale numerical experiments
No full textOver the years, large eddy simulation (LES) has emerged as a tool to study problems in fluid mechanics characterized by complex
physics and geometry. Among these, attention has been paid to studying problems of relevance in hydraulics and environmental fluid
mechanics. For many years, LES has been used as an underresolved, or coarse, direct numerical simulation (DNS) where the scales unrepresented
by the grid are modeled by means of a sub-grid-scale model, designed to drain energy from the resolved scales of motion. This
method, although limited in applicability because of its computational cost, has allowed exploitation of the physics of a class of idealized flow
fields of importance in hydraulic engineering. This study reports on investigations into processes of interest to hydraulic engineering. Some
significant examples of such studies, together with relevant research from the literature, are given. Specifically, a description of literature
related to turbulence in presence of longitudinal bars and local scours, studies of irregular roughness present in hydraulic applications, studies
of Lagrangian and Eulerian dispersion processes, and studies of gravity currents is given. Although unable to give an answer to real-scale
problems in hydraulic engineering, such studies allow unveiling of the physics behind phenomena present in hydraulics and, on the other
hand, allow improved parametrization to be used in reduced-order models. The study is concluded with the author’s point of view on the
importance of LES in hydraulic engineering in the upcoming future
Numerical analysis of performance of wavebreakers exposed to regular waves in static and floating configuration
Full text linkIn the present paper we investigate, through numerical analysis, the hydrodynamic behavior of wavebreakers both in static and in floating configuration. The aim is to evaluate and compare the performance of wavebreakers in regular waves in the range of intermediate depth waters. The analysis is performed through evaluation of the waves transmitted downward and reflected back and the dissipative behavior of the wavebreaker. We simulate numerically the fluid dynamic field using the Unsteady Reynolds Averaged Navier Stokes equations (URANS) with the k − ε turbulence model, both for the water and the air phases, using the Volume of Fluid (VOF) method to detect the interface. We simulate a numerical wave tank, generating the waves at a lateral boundary of the domain and allowing its own propagation into the domain. First we study the static configuration of the wavebreaker, so it is considered fixed in space. Afterward, we consider the wavebreaker as a rigid body with a Single Degree of Freedom (SDOF) in the vertical direction and we analyze the interaction between the wave system and the structure. With this purpose we use the URANS equations over a dynamic mesh in conjunction with a Fluid–Structure-Interaction (FSI) algorithm, where the mesh displacement is associated to the body’s motion through a diffusive Laplace equation; the motion of the solid body is evaluated using the momentum equation of a rigid body subject to hydrodynamic loading. We study two different wavebreakers, the rectangular one and the Π shape one, and evaluate the differences in terms of transmitted, reflected and dissipated energy. First we assess the algorithm of generation and propagation of the regular waves comparing numerical results with analytical data. Afterward, we evaluate the performance of the two wavebreakers in terms of coefficients of transmission, reflection and dissipation and we compare our numerical results with data from the standard Wiegel Theory, 1960 and successive modifications. Finally, we study the performance of the wave system in presence of the floating body. This is done in two steps: we initially validate the results with those of the analytical solution of the governing equation of a SDOF rigid body forced by regular wave trains; successively we calculate the transmission coefficients for a number of waves with different length and height and compare the results with literature empirical formulas
Large eddy simulation of two-way coupling sediment transport
No full textIn the present paper numerical simulations are used to investigate suspended sediment transport and its effect on the dynamics of the turbulent boundary layer. We use an Euler–Euler methodology based on single-phase approach. Large eddy simulation is employed to resolve the large scales of motion, whereas
the contribution of the small scales is parametrized by the use of a dynamic Smagorinsky model. In order to account for sediment-induced buoyancy on momentum, a buoyancy term is considered in the threedimensional Navier–Stokes equations through the use of the Boussinesq approximation. We consider
four sediment sizes and the simulations are performed for both one-way and two-way coupling approaches to gain a better description of sediment–turbulence interaction. The level of stratification for each particle size is qualified by the bulk Richardson number which increases by decreasing the grain
size. The analysis reveals that the reduction of sediment size produces a larger suspension and sediment concentration in the flow field, due to the concurrence of increased available concentration at the wall and reduced deposition velocity. Comparison of concentration profiles between one-way and two-way
coupling clearly shows the remarkable effect of stratification on the velocity and concentration mean profiles. This is particularly true for small sediments which are more likely suspended in the fluid column. In agreement with experimental literature results, our study shows that suspended sediment concentration
reduces the von Kàrmàn constant of the velocity profile. The analysis of second order statistics and energy power spectra show turbulent suppression due to stratification effects, in agreement with previous studies. The gradient Richardson number distribution along the channel height demonstrates the
increased level of stratification along the fluid column by decreasing the grain size. Momentum and concentration diffusivity are also discussed. The non-dimensional concentration diffusivity compares very well with the Coleman’s experimental data in the range of parameters shared by the two studies. Overall,
the results of our study confirm that a single-phase mathematical model is a good candidate to simulate suspended sediment transport. Our study also shows that differences between one-way and two-way coupling approaches are negligible for relatively large sediments, that, on the other hand, are more likely
transported according to the bed-load mode. For smaller particles, transported according to the suspension-load mode, the two-way coupling approach reproduces the reduction of turbulence activity already observed in physical experiments
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