Rui M. L. Ferreira

Dam-break flows over mobile beds: Experiments and benchmark tests for numerical models

In this paper, the results of a benchmark test launched within the framework of the NSF–PIRE project “Modelling of Flood Hazards and Geomorphic Impacts of Levee Breach and Dam Failure” are presented. Experiments of two-dimensional dam-break flows over a sand bed were conducted at Université catholique de Louvain, Belgium. The water level evolution at eight gauging points was measured as well as the final bed topography. Intense scour occurred close to the failed dam, while significant deposition was observed further downstream. From these experiments, a benchmark was proposed to the scientific community, consisting of blind test simulations, that is, without any prior knowledge of the measurements. Twelve different teams of modellers from eight countries participated in the study. Here, the numerical models used in this test are briefly presented. The results are commented upon, in view of evaluating the modelling capabilities and identifying the challenges that may open pathways for further research.

SPH–DCDEM model for arbitrary geometries in free surface solid–fluid flows

Abstract A unified discretization of rigid solids and fluids is introduced, allowing for resolved simulations of fluid–solid phases within a meshless framework. The numerical solution, attained by Smoothed Particle Hydrodynamics (SPH) and a variation of Discrete Element Method (DEM), the Distributed Contact Discrete Element Method (DCDEM) discretization, is achieved by directly considering solid–solid and solid–fluid interactions. The novelty of the work is centred on the generalization of the coupling of the DEM and SPH methodologies for resolved simulations, allowing for state-of-the-art contact mechanics theories to be used in arbitrary geometries, while fluid to solid and vice versa momentum transfers are accurately described. The methods are introduced, analysed and discussed. Initial validations on the DCDEM and the fluid coupling are presented, drawing from test cases in the literature. An experimental campaign serves as a validation point for complex, large scale solid–fluid flows, where a set of blocks in several configurations is subjected to a dam-break wave. Blocks are tracked and positions are then compared between experimental data and the numerical solutions. A Particle Image Velocimetry (PIV) technique allows for the quantification of the flow field and direct comparison with numerical data. The results show that the model is accurate and is capable of treating highly complex interactions, such as transport of debris or hydrodynamic actions on structures, if relevant scales are reproduced.

The terms of turbulent kinetic energy budget within random arrays of emergent cylinders

This article is aimed at quantifying and discussing the relative magnitude of key terms of the equation of conservation of turbulent kinetic energy (TKE) in the inter-stem space of a flow within arrays of vertical cylinders simulating plant stems of emergent and rigid vegetation. The spatial distribution of turbulent quantities and mean flow variables are influenced by two fundamental space scales, the diameter of the stems and the local stem areal number-density. Both may vary considerably since the areal distribution of plant stems in natural systems is generally not homogeneous; they are often arranged in alternating sparse and dense patches. The magnitude of the terms of the budget of TKE in the inter-stem space has seldom been quantified experimentally and is currently not well known. This work addresses this research need. New databases, consisting of three-component LDA velocity series and two-component PIV velocity maps, obtained in carefully controlled laboratory conditions, were used to calculate the terms of the TKE budget. The physical system comprises random arrays of rigid and emergent cylinders with longitudinally varying areal number-density. It is verified that the main source of TKE is vortex shedding from individual cylinders. The rates of production and dissipation are not in equilibrium. Regions with negative production, a previously unreported feature, are identified. Turbulent transport is particularly important along the von Karman vortex street. Convective rate of change of TKE and pressure diffusion are most relevant in the vicinity of the cylinders.

Two-dimensional depth-averaged modelling of dam-break flows over mobile beds

The paper is aimed at the description and validation of a novel two-dimensional depth-averaged simulation tool for highly unsteady discontinuous flows over complex time-evolving geometries. The conceptual model is developed within the shallow-flow framework and features non-equilibrium sediment transport. A fully conservative finite-volume discretization scheme is employed. The treatment of bed-slope source terms ensures that the scheme is well balanced and leads to correct energy losses in discontinuities. Existing laboratory data are used to validate the model and to discuss some of its embedded formulations. Blind-test comparison shows that the model is capable of describing the main dam-break flow and bed morphology features. The solution is shown to be sensitive to the formulations for sediment transport capacity and for adaptation length (Λ). A better functional relation of Λ with the Shields parameter is investigated. The quality of the solution is shown to be only marginally improved by the inclusion of turbulent stresses.

Mathematical modelling of shallow flows: Closure models drawn from grain-scale mechanics of sediment transport and flow hydrodynamicsThis paper is one of a selection of papers in this Special Issue in honour of Professor M. Selim Yalin (1925–2007).

Mathematical modelling of river processes is, nowadays, a key element in river engineering and planning. River modelling tools should rest on conceptual models drawn from mechanics of sediment transport, river mechanics, and river hydrodynamics. The objectives of the present work are (i) to describe conceptual models of sediment transport, deduced from grain-scale mechanics of sediment transport and turbulent flow hydrodynamics, and (ii) to present solutions to specific river morphology problems. The conceptual models described are applicable to the morphologic evolution of rivers subjected to the transport of poorly sorted sediment mixtures at low shear stresses and to geomorphic flows featuring intense sediment transport at high shear stresses. In common, these applications share the fact that sediment transport and flow resistance depend, essentially, on grain-scale phenomena. The idealized flow structures are presented and discussed. Numerical solutions for equilibrium and nonequilibrium sediment tran...

Maximum level and time to peak of dam-break waves on mobile horizontal bed

This experimental study focuses the influence of bed material mobility and initial downstream water level on maximum water level and time to peak of dam-break waves. It covers horizontal bed conditions on fixed bed, sand bed, and pumice bed. Results include water surface level time evolution, maxima wave levels and time to peak. The influence of bed material mobility and downstream water level was identified and characterized, stressing the importance of using mathematical models with appropriate sediment transport formulations instead of purely hydrodynamic models to simulate dam-break waves on mobile bed channels.

Comparison Between Two Hydrodynamic Models for Flooding Simulations at River Lima Basin

According to EU flood risks directive, flood hazard maps should include information on hydraulic characteristics of vulnerable locations, i.e. the inundated areas, water depths and velocities. These features can be assessed by the use of advanced hydraulic modelling tools which are presented in this paper based on a case study in the river Lima basin, Portugal. This river includes several flood-prone areas. Ponte Lima town is one of the places of higher flood risk. The upstream dams can lower the flood risks if part of its storage capacity is allocated for mitigating flood events. However, proper management of dam releases and the evaluation of downstream river flows should be considered for preventing flood damages. A hydrological and a one-dimensional hydrodynamic model were implemented, and at a particular flood-prone town, inundation was assessed using a two-dimensional model. The hydrological model is based on the well known Sacramento model. For this purpose, two different modelling implementations were analysed: a model based on a finite element mesh and a model based on rectangular grids. The computational performance of the two modelling implementations is evaluated. Historical flood events were used for model calibration serving as a basis for the establishment of different potential flood scenarios. Intense precipitation events in the river’s basin and operational dam releases are determinant for the occurrence of floods at vulnerable downstream locations. The inundation model based on the unstructured mesh reveals to be more computationally efficient if high spatial resolution is required. A new combination of software tools for floods simulation is presented including an efficient alternative for simulation of 2-D inundation using a finite element mesh instead of a grid.

The Effect of Latex and Chitosan Biopolymer on Concrete Properties and Performance

this paper presents the results of a study in which the combination of two polymeric additives in concrete with the intention of improving its mechanical and durability performance is analysed. The additives are a synthetic latex and a biopolymer – chitosan. An evaluation of the mechanical properties as well as the phases formed based on scanning electron microscopy (SEM) and X-ray diffraction (XRD) was performed. The concretes were prepared with each of the polymers separately, and the results were ordinary. However, when combined, the results show an interesting interaction improving the mechanical strengths of the concrete. Several concrete samples were prepared with 0 – 4 % of each polymer with 1 % increments. The mechanical properties were shown to be sensitive to the incorporation of polymers. The desired effect of the interaction between the biopolymer and the latex was observed, because the strengths increased when both additives were present, namely for the combination of 2 % of each polymer. SEM images revealed a heterogeneous distribution in the polymer cementitious matrix, mainly with regards to latex. The presence of well defined polymer fibers on a fracture surface of composites prepared with biopolymer (4 %) was observed, indicating that the fibre pullout and not fracture was the cause of failure, resulting from the poor adherence of the fibers in matrix. Composites prepared with both polymers revealed abundant formation of C-S-H and the absence of ettringite, explaining the improvement of mechanical properties. The presence of reticulated structures of C-S-H dispersed in the microstructure and involving the calcium hydroxide corroborates the results of mechanical properties, mainly for the percentages of 3 % of biopolymer and 1 % of latex.

Turbulent Flows within Random Arrays of Rigid and Emergent Cylinders with Varying Distribution

AbstractThe spatial distribution of emergent vegetation in rivers is frequently nonuniform. Here, spatial nonuniformity is simulated in the laboratory by varying longitudinally the stem areal number density (m). Time- and space-averaged flow variables, corresponding to different values and gradients of m and different stem Reynolds numbers, are calculated from instantaneous velocity maps obtained with particle image velocimetry (PIV). Results show that Reynolds stresses are completely determined by the local spatially averaged number of stems per unit area; they are not sensitive to local spatial gradients of m. These variables seem to adjust locally to the spatial variations of the stem distribution. Contrarily, the spatial distribution of the time-averaged flow is influenced not only by the local value of m but also by its spatial gradients. Form-induced stresses express this influence. They reveal a subsistence of flow complexity generated by the upstream stem distribution as a form of spatial memory. ...

Turbulent Flow Hydrodynamics and Sediment Transport: Laboratory Research with LDA and PIV

Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) are indirect techniques to measure flow velocities. Both require small particles in the moving fluid, both measure specific statistics of the velocity of such particles, and both are among the least intrusive techniques to measure flow velocities. LDA techniques have been used in the scope of river hydraulics since the early 1980s, while PIV techniques have been growingly used in the last 15 years. The main objective of the present chapter is to show applications of LDA and PIV techniques to fluvial hydraulics, in the scope of research work conducted at Instituto Superior Tecnico, Technical University of Lisbon, Portugal. The work is divided in three parts. The first is dedicated to the presentation of basic concepts of LDA and PIV measurements. The second part addresses the characterization of turbulent open-channel flows over flat rough mobile beds and over flat rough fixed beds with LDA. The near-bed turbulent flow structure and the bursting cycle in mobile and immobile beds are discussed. The third part is aimed at describing the flow within the scour hole of a wall-mounted vertical cylinder on a mobile bed, using PIV. The structure of the flow, namely the downflow and the vortex system inside the hole, is presented and discussed, and the main scour mechanism is identified. In both cases, the relevance of LDA and PIV techniques for the understanding of sediment-flow interaction is highlighted.