Jonathan Lambrechts | UCLouvain (University of Louvain) (original) (raw)

Papers by Jonathan Lambrechts

An unstructured-mesh, finite element, depth-integrated model of the hydrodynamics of the whole Gr... more An unstructured-mesh, finite element, depth-integrated model of the hydrodynamics of the whole Great Barrier Barrier Reef (GBR), Australia, has been developed and implemented on a parallel computer. Far away from reefs, islands and important bathymetric features, the mesh size may be as large as a few kilometres, whereas, in the vicinity of reefs and islands, the grid is drastically refined, leading to meshes that can be 100 metres in size. This enables our model to simulate motions characterized by a wide range of space and time scales. Large scale currents, i.e. the tides, the wind-induced circulation and the bifurcation of the East Australian Current, are reproduced with an accuracy that is comparable to that achieved by today’s largescale models of the GBR. The model is also successful at representing small-scale processes, such as tidal jets, their instabilities, as well as the eddies developing in the wake of islands and headlands. The influence of the small scales on the large scales has been investigated. The model will be used for studying the connectivity between subdomains of the GBR. Once validated, an ecological module will be coupled with the present hydrodynamic code.

The development of suitable and fast time integration methods for ocean modeling con- stitutes an... more The development of suitable and fast time integration methods for ocean modeling con- stitutes an important challenge. No single time-discretisation works well for all physical processes in a complex marine model, as dierent subsystems have widely dierent charac- teristics in terms of time scales, dynamic behaviour, and accuracy requirements. We believe that building appropriate time stepping strategies for multi-scale computations will enable us to gain an order of magnitude. Indeed, unstructured-mesh generation processes are complex and, even though it is possible to control average element sizes in specic regions of the domain, it is not the case for each element size. The smallest element is usually much more smaller than the criterion that was prescribed a priori and it determines the stable time step for the entire model. Therefore, the computational eciency of explicit time-stepping methods may be drastically low. Multirate schemes represent a class of methods that use variou...

Gmsh is an open-source three-dimensional finite element grid generator with a build-in CAD engine... more Gmsh is an open-source three-dimensional finite element grid generator with a build-in CAD engine and post-processor. Its design goal is to provide a fast, light and user- friendly meshing tool with parametric input and advanced visualization capabilities. This paper presents new developments that are available in GMSH in different topics of mesh generation. We will focus mainly on curvilinear meshing, surface reparametrization and quad/hex meshing.

The development of suitable and fast time integration methods for ocean modeling constitutes an i... more The development of suitable and fast time integration methods for ocean modeling constitutes an important challenge. No single time-discretisation works well for all physical processes in a complex marine model, as different subsystems have widely different characteristics in terms of time scales, dynamic behaviour, and accuracy requirements. The primitive equations for ocean flows allow for the existence of phenomena exhibiting a wide spectrum of propagation speeds. Typically, external gravity waves propagate at 101 − 102 ms−1 and internal waves at a few meters per second, whereas advection is characterized by speeds ranging from 10−3 to 1 ms−1. We believe that building appropriate time stepping strategies for multi-scale computations will enable us to gain an order of magnitude. For instance, consider the case of a typical mesh of the Great Barrier Reef (GBR) made up of about 1 million triangles. Element sizes were determined in order to capture the relevant bathymetric and topogr...

The EGU General Assembly, 2012

The hydrosphere is made up of a number media, such as the oceans, the shelf seas, the estuaries, ... more The hydrosphere is made up of a number media, such as the oceans, the shelf seas, the estuaries, the rivers, the land surface and ground water as well as the sea ice - which, for the sake of simplicity, is considered herein to be part of the hydrosphere. The processes taking place in these domains are vastly different in nature and are characterized by a wide range of space- and time-scales. The components of the hydrosphere interact with each other. For instance, the shallow marine and estuarine regions, though accounting for less than 1% of the volume of the oceans, have a biomass far from negligible as compared to that of the oceans, implying that they play a significant role in global biogeochemical cycles. This is one of the reasons why models are now needed that deal with most, if not all, of the components of the hydrospheric system. Numerical models of each of the components of the hydrosphere already exist. However, an integrated model of the whole hydrosphere has yet to be...

We present a model to solve grains/ uid ows. Mixtures of grains and uids occur in a large number ... more We present a model to solve grains/ uid ows. Mixtures of grains and uids occur in a large number of industrial and geophysical applications like uidised beds, mixing, mud ows, landslides, submarine avalanches, etc. The granular phase is solved by a contact dynamics method at the particle scales. For the hydrodynamic part, we solve the incompressible Navier-Stokes-Brinkman equations in a porous media by a nite element method. Those equations represent in a single model a continuous transition between the Darcy regime at high particle concentration and a classical Navier-Stokes ow at low concentration. At high Reynolds number, the term of Forchheimer is used to take the particle shapes into account. This approach allows the simulation of a granular phase with a large number of particles with dierent size, shape, and compactness. The interactions between the grains and the uid mixture occurs at a mesoscopic scale. The mesoscopic scale is chosen independently of the mesh resolution of t...

In the recent years, high-order numerical methods have been shown to yield superior computational... more In the recent years, high-order numerical methods have been shown to yield superior computational efficiency in problems with high resolution requirements, compared to standard second-order solvers based on Finite Volume technology. However, as the maturity of high-order schemes such as Discontinuous Galerkin methods is being brought to the level of practical application, many contributions show that their accuracy strongly depends on the accuracy of the geometrical discretization (see for instance [1]). It is thus necessary to develop the matching curvilinear mesh generation technology in order to fully benefit from the efficiency of high-order schemes. In this talk, we will present the methods that have been developed in the framework of the IDIHOM project for the generation of high-order unstructured meshes. We adopt an indirect approach that consists in creating first a linear unstructured (or hybrid mesh), and then curving its boundaries to match the geometry. This often result...

The Tonle Sap Lake, which is located in the Cambodian floodplain of the Mekong River System, play... more The Tonle Sap Lake, which is located in the Cambodian floodplain of the Mekong River System, plays an important role for storage and water supply in Cambodia as well as in the low land region of the Mekong River System. Every year the lake stores water in the flood season from May to October while it acts as a spillway providing water to the Mekong Delta in the dry season from November to April. In other words, the lake depth can rise by approximately 10 m and the water volume can increase 5 times during the year, showing various drying and wetting areas in the lake and its adjacent floodplains. In terms of hydrodynamic modelling, these wetting/drying areas are still challenging because of several difficulties, e.g. large CPU time, slow transition of wet-dry interface and, possibly lack of mass conservation. In this paper, we present a wetting drying algorithm with a limitation for the fluid depth and a blending parameter in order to ensure local mass conservation and rapid transiti...

The EGU General Assembly, 2008

An unstructured-mesh, finite element, depth-integrated model o f the hydrodynamics o f the whole ... more An unstructured-mesh, finite element, depth-integrated model o f the hydrodynamics o f the whole G reat Barrier Barrier Reef (GBR), Australia, has been developed and implemented on a parallel computer. Far away from reefs, islands and im portant bathymetric features, the mesh size may be as large as a few kilometres, whereas, in the vicinity o f reefs and islands, the grid is drastically refined, leading to meshes that can be 100 metres in size. This enables our model to simulate motions characterized by a wide range o f space and time scales. Large scale currents, i.e. the tides, the w ind-induced circulation and the bifurcation o f the East Australian Current, are reproduced with an accuracy that is com parable to that achieved by today's large-scale models o f the GBR. The model is also successful at representing small-scale processes, such as tidal jets, the ir instabilities, as well as the eddies developing in the wake o f islands and headlands. Both large and small scales ...

While a consensus is forming in the computational physics community on the superior efficiency of... more While a consensus is forming in the computational physics community on the superior efficiency of high-order numerical schemes, it has been shown that linear geometrical discretizations can deprive such schemes from their benefits. The need for curvilinear mesh generation tools is becoming more acute as high-order methods are being brought to the level of practical applications. In this talk, we report on the development of a high-order unstructured mesh generation method based on an unconstrained optimization procedure [1], occasionally supplemented with an analytical technique. As invalid elements often appear when creating high-order meshes, emphasis is put on robustness: the optimization procedure is specifically designed to enforce mesh validity, that is evaluated through the estimation of provably correct bounds for the Jacobian of the elements [2]. The objective function can also account for the distance between the real geometry and the mesh boundary, in order to optimize th...

Suspended particulate matter has a great influence on most of the biological and chemical organis... more Suspended particulate matter has a great influence on most of the biological and chemical organisms in aquatic environments. A good representation of the sediment dynamics is therefore essential for ecological modeling. To this purpose, a fine sediment module is developed and coupled to the two-dimensional component of the finite-element model SLIM. In this module, two tracers are calculated: the concentration of suspended sediments and the concentration of sediments on the bottom. The first one is transported according to the hydrodynamics and diffused, while the second one does not move. Classical parameterizations of erosion and deposition allow exchange between tracers. The talk focusses on two studies using SLIM and implying sediments. The first case studied is the Scheldt Estuary (Belgium, the Netherlands). The model, covering the whole continental shelf, the estuary, the tidal river and the main tributaries of the Scheldt, is thoroughly multi-scale (de Brye et al, 2010). In t...

An unstructured-mesh, finite element, depth-integrated model of the hydrodynamics of the whole Gr... more An unstructured-mesh, finite element, depth-integrated model of the hydrodynamics of the whole Great Barrier Barrier Reef (GBR), Australia, has been developed and implemented on a parallel computer. Far away from reefs, islands and important bathymetric features, the mesh size may be as large as a few kilometres, whereas, in the vicinity of reefs and islands, the grid is drastically refined, leading to meshes that can be 100 metres in size. This enables our model to simulate motions characterized by a wide range of space and time scales. Large scale currents, i.e. the tides, the wind-induced circulation and the bifurcation of the East Australian Current, are reproduced with an accuracy that is comparable to that achieved by today’s largescale models of the GBR. The model is also successful at representing small-scale processes, such as tidal jets, their instabilities, as well as the eddies developing in the wake of islands and headlands. The influence of the small scales on the large scales has been investigated. The model will be used for studying the connectivity between subdomains of the GBR. Once validated, an ecological module will be coupled with the present hydrodynamic code.

The development of suitable and fast time integration methods for ocean modeling con- stitutes an... more The development of suitable and fast time integration methods for ocean modeling con- stitutes an important challenge. No single time-discretisation works well for all physical processes in a complex marine model, as dierent subsystems have widely dierent charac- teristics in terms of time scales, dynamic behaviour, and accuracy requirements. We believe that building appropriate time stepping strategies for multi-scale computations will enable us to gain an order of magnitude. Indeed, unstructured-mesh generation processes are complex and, even though it is possible to control average element sizes in specic regions of the domain, it is not the case for each element size. The smallest element is usually much more smaller than the criterion that was prescribed a priori and it determines the stable time step for the entire model. Therefore, the computational eciency of explicit time-stepping methods may be drastically low. Multirate schemes represent a class of methods that use variou...

Gmsh is an open-source three-dimensional finite element grid generator with a build-in CAD engine... more Gmsh is an open-source three-dimensional finite element grid generator with a build-in CAD engine and post-processor. Its design goal is to provide a fast, light and user- friendly meshing tool with parametric input and advanced visualization capabilities. This paper presents new developments that are available in GMSH in different topics of mesh generation. We will focus mainly on curvilinear meshing, surface reparametrization and quad/hex meshing.

The development of suitable and fast time integration methods for ocean modeling constitutes an i... more The development of suitable and fast time integration methods for ocean modeling constitutes an important challenge. No single time-discretisation works well for all physical processes in a complex marine model, as different subsystems have widely different characteristics in terms of time scales, dynamic behaviour, and accuracy requirements. The primitive equations for ocean flows allow for the existence of phenomena exhibiting a wide spectrum of propagation speeds. Typically, external gravity waves propagate at 101 − 102 ms−1 and internal waves at a few meters per second, whereas advection is characterized by speeds ranging from 10−3 to 1 ms−1. We believe that building appropriate time stepping strategies for multi-scale computations will enable us to gain an order of magnitude. For instance, consider the case of a typical mesh of the Great Barrier Reef (GBR) made up of about 1 million triangles. Element sizes were determined in order to capture the relevant bathymetric and topogr...

The EGU General Assembly, 2012

The hydrosphere is made up of a number media, such as the oceans, the shelf seas, the estuaries, ... more The hydrosphere is made up of a number media, such as the oceans, the shelf seas, the estuaries, the rivers, the land surface and ground water as well as the sea ice - which, for the sake of simplicity, is considered herein to be part of the hydrosphere. The processes taking place in these domains are vastly different in nature and are characterized by a wide range of space- and time-scales. The components of the hydrosphere interact with each other. For instance, the shallow marine and estuarine regions, though accounting for less than 1% of the volume of the oceans, have a biomass far from negligible as compared to that of the oceans, implying that they play a significant role in global biogeochemical cycles. This is one of the reasons why models are now needed that deal with most, if not all, of the components of the hydrospheric system. Numerical models of each of the components of the hydrosphere already exist. However, an integrated model of the whole hydrosphere has yet to be...

We present a model to solve grains/ uid ows. Mixtures of grains and uids occur in a large number ... more We present a model to solve grains/ uid ows. Mixtures of grains and uids occur in a large number of industrial and geophysical applications like uidised beds, mixing, mud ows, landslides, submarine avalanches, etc. The granular phase is solved by a contact dynamics method at the particle scales. For the hydrodynamic part, we solve the incompressible Navier-Stokes-Brinkman equations in a porous media by a nite element method. Those equations represent in a single model a continuous transition between the Darcy regime at high particle concentration and a classical Navier-Stokes ow at low concentration. At high Reynolds number, the term of Forchheimer is used to take the particle shapes into account. This approach allows the simulation of a granular phase with a large number of particles with dierent size, shape, and compactness. The interactions between the grains and the uid mixture occurs at a mesoscopic scale. The mesoscopic scale is chosen independently of the mesh resolution of t...

In the recent years, high-order numerical methods have been shown to yield superior computational... more In the recent years, high-order numerical methods have been shown to yield superior computational efficiency in problems with high resolution requirements, compared to standard second-order solvers based on Finite Volume technology. However, as the maturity of high-order schemes such as Discontinuous Galerkin methods is being brought to the level of practical application, many contributions show that their accuracy strongly depends on the accuracy of the geometrical discretization (see for instance [1]). It is thus necessary to develop the matching curvilinear mesh generation technology in order to fully benefit from the efficiency of high-order schemes. In this talk, we will present the methods that have been developed in the framework of the IDIHOM project for the generation of high-order unstructured meshes. We adopt an indirect approach that consists in creating first a linear unstructured (or hybrid mesh), and then curving its boundaries to match the geometry. This often result...

The Tonle Sap Lake, which is located in the Cambodian floodplain of the Mekong River System, play... more The Tonle Sap Lake, which is located in the Cambodian floodplain of the Mekong River System, plays an important role for storage and water supply in Cambodia as well as in the low land region of the Mekong River System. Every year the lake stores water in the flood season from May to October while it acts as a spillway providing water to the Mekong Delta in the dry season from November to April. In other words, the lake depth can rise by approximately 10 m and the water volume can increase 5 times during the year, showing various drying and wetting areas in the lake and its adjacent floodplains. In terms of hydrodynamic modelling, these wetting/drying areas are still challenging because of several difficulties, e.g. large CPU time, slow transition of wet-dry interface and, possibly lack of mass conservation. In this paper, we present a wetting drying algorithm with a limitation for the fluid depth and a blending parameter in order to ensure local mass conservation and rapid transiti...

The EGU General Assembly, 2008

An unstructured-mesh, finite element, depth-integrated model o f the hydrodynamics o f the whole ... more An unstructured-mesh, finite element, depth-integrated model o f the hydrodynamics o f the whole G reat Barrier Barrier Reef (GBR), Australia, has been developed and implemented on a parallel computer. Far away from reefs, islands and im portant bathymetric features, the mesh size may be as large as a few kilometres, whereas, in the vicinity o f reefs and islands, the grid is drastically refined, leading to meshes that can be 100 metres in size. This enables our model to simulate motions characterized by a wide range o f space and time scales. Large scale currents, i.e. the tides, the w ind-induced circulation and the bifurcation o f the East Australian Current, are reproduced with an accuracy that is com parable to that achieved by today's large-scale models o f the GBR. The model is also successful at representing small-scale processes, such as tidal jets, the ir instabilities, as well as the eddies developing in the wake o f islands and headlands. Both large and small scales ...

While a consensus is forming in the computational physics community on the superior efficiency of... more While a consensus is forming in the computational physics community on the superior efficiency of high-order numerical schemes, it has been shown that linear geometrical discretizations can deprive such schemes from their benefits. The need for curvilinear mesh generation tools is becoming more acute as high-order methods are being brought to the level of practical applications. In this talk, we report on the development of a high-order unstructured mesh generation method based on an unconstrained optimization procedure [1], occasionally supplemented with an analytical technique. As invalid elements often appear when creating high-order meshes, emphasis is put on robustness: the optimization procedure is specifically designed to enforce mesh validity, that is evaluated through the estimation of provably correct bounds for the Jacobian of the elements [2]. The objective function can also account for the distance between the real geometry and the mesh boundary, in order to optimize th...

Suspended particulate matter has a great influence on most of the biological and chemical organis... more Suspended particulate matter has a great influence on most of the biological and chemical organisms in aquatic environments. A good representation of the sediment dynamics is therefore essential for ecological modeling. To this purpose, a fine sediment module is developed and coupled to the two-dimensional component of the finite-element model SLIM. In this module, two tracers are calculated: the concentration of suspended sediments and the concentration of sediments on the bottom. The first one is transported according to the hydrodynamics and diffused, while the second one does not move. Classical parameterizations of erosion and deposition allow exchange between tracers. The talk focusses on two studies using SLIM and implying sediments. The first case studied is the Scheldt Estuary (Belgium, the Netherlands). The model, covering the whole continental shelf, the estuary, the tidal river and the main tributaries of the Scheldt, is thoroughly multi-scale (de Brye et al, 2010). In t...