Mesoscopic Methods in Engineering and Science

Matter, conceptually classified into fluids and solids, can be completely described by the microscopic physics of its constituent atoms or molecules. However, for most engineering applications a macroscopic or continuum description has usually been sufficient, because of the large disparity between the spatial and temporal scales relevant to these applications and the scales of the underlying molecular dynamics. In this case, the microscopic physics merely determines material properties such as the viscosity of a fluid or the elastic constants of a solid. These material properties cannot be derived within the macroscopic framework, but the qualitative nature of the macroscopic dynamics is usually insensitive to the details of the underlying microscopic interactions

Lattice Boltzmann method for computational aeroacoustics on non-uniform meshes: a direct grid coupling approach

The present study proposes a highly accurate lattice Boltzmann direct coupling cell-vertex algorithm, well suited for industrial purposes, making it highly valuable for aeroacoustic applications. It is indeed known that the convection of vortical structures across a grid refinement interface, where cell size is abruptly doubled, is likely to generate spurious noise that may corrupt the solution over the whole computational domain. This issue becomes critical in the case of aeroacoustic simulations, where accurate pressure estimations are of paramount importance. Consequently, any interfering noise that may pollute the acoustic predictions must be reduced. The proposed grid refinement algorithm differs from conventionally used ones, in which an overlapping mesh layer is considered. Instead, it provides a direct connection allowing a tighter link between fine and coarse grids, especially with the use of a coherent equilibrium function shared by both grids. Moreover, the direct coupling makes the algorithm more local and prevents the duplication of points, which might be detrimental for massive parallelization. This work follows our first study (Astoul~\textit{et al. 2020}) on the deleterious effect of non-hydrodynamic modes crossing mesh transitions, which can be addressed using an appropriate collision model. The Hybrid Recursive Regularized model is then used for this study. The grid coupling algorithm is assessed and compared to a widely-used cell-vertex algorithm on an acoustic pulse test case, a convected vortex and a turbulent circular cylinder wake flow at high Reynolds number.Comment: also submitted to Journal of Computational Physic

The Numerical Simulation of Fluid Flow

This book collects the accepted contributions to the Special Issue "The Numerical Simulation of Fluid Flow" in the Energies journal of MDPI. It is focused more on practical applications of numerical codes than in its development. It covers a wide variety of topics, from aeroacoustics to aerodynamics and flow-particles interaction

An Investigation of the Lattice Boltzmann Method for Large Eddy Simulation of Complex Turbulent Separated Flow

Lattice Boltzmann method (LBM) is a relatively recent computational technique for fluid dynamics that derives its basis from a mesoscopic physics involving particle motion. While the approach has been studied for different types of fluid flow problems, its application to eddy-capturing simulations of building block complex turbulent flows of engineering interest has not yet received sufficient attention. In particular, there is a need to investigate its ability to compute turbulent flow involving separation and reattachment. Thus, in this work, large eddy simulation (LES) of turbulent flow over a backward facing step, a canonical benchmark problem which is characterized by complex flow features, is performed using the LBM. Multiple relaxation time formulation of the LBM is considered to maintain enhanced numerical stability in a locally refined, conservative multiblock gridding strategy, which allows efficient implementation. Dynamic procedure is used to adapt the proportionality constant in the Smagorinsky eddy viscosity subgrid scale model with the local features of the flow. With a suitable reconstruction procedure to represent inflow turbulence, computation is carried out for a Reynolds number of 5100 based on the maximum inlet velocity and step height and an expansion ratio of 1.2. It is found that various turbulence statistics, among other flow features, in both the recirculation and reattachment regions are in good agreement with direct numerical simulation and experimental data

Lattice Boltzmann simulations of environmental flow problems in shallow water flows

The lattice Boltzmann method (LBM) proposed about decades ago has been developed and applied to simulate various complex fluids. It has become an alternative powerful method for computational fluid dynamics (CFD). Although most research on the LBM focuses on the Navier-Stokes equations, the method has also been developed to solve other flow equations such as the shallow water equations. In this thesis, the lattice Boltzmann models for the shallow water equations and solute transport equation have been improved and applied to different flows and environmental problems, including solute transport and morphological evolution. In this work, both the single-relaxation-time and multiple-relaxation-time models are used for shallow water equations (named LABSWE and LABSWEMRT, respectively), and the large eddy simulation is incorporated into the LABSWE (named LABSWETM) for turbulent flow. The capability of the LABSWETM was firstly tested by applying it to simulate free surface flows in rectangular basins with different length -width ratios, in which the characteristics of the asymmetrical flows were studied in details. The LABSWEMRT was then used to simulate the one- and two-dimensional shallow water flows over discontinuous beds. The weighted centred scheme for force term, together with the bed height for a bed slope, was incorporated into the model to improve the simulation of water flows over a discontinuous bed. The resistance stress was also included to investigate the effect of the local head loss caused by flows over a step. Thirdly, the LABSWEMRT was extended to simulate a moving body in shallow water. In order to deal with the moving boundaries, three different schemes with second-order accuracy were tested and compared for treating curved boundaries. An additional momentum term was added to reflect the interaction between the following fluid and the solid, and a refilled method was proposed to treat the wetted nodes moving out from the solid nodes. Fourthly, both LABSWE and LABSWEMRT were used to investigate solute transport in shallow water. The flows are solved using LABSWE and LABSWEMRT, and the advection-diffusion equation for solute transport was solved with a LBM-BGK model based on the D2Q5 lattice. Three cases: open channel flow with a side discharge, shallow recirculation flow and flow in a harbour, were simulated to verify the methods. In addition, the performance of LABSWEMRT and LABSWE were compared, and the results showed that the LABSWMRT has better stability and can be used for flow with high Reynolds number. Finally, the lattice Boltzmann method was used with the Euler-WENO scheme to simulate morphological evolution in shallow water. The flow fields were solved by the LABSWEMRT with the improved scheme for the force term, and the fifth order Euler-WENO scheme was used to solve the morphological equation to predict the morphological evolution caused by the bed-load transport

Efficient algorithms for the realistic simulation of fluids

Nowadays there is great demand for realistic simulations in the computer graphics field. Physically-based animations are commonly used, and one of the more complex problems in this field is fluid simulation, more so if real-time applications are the goal. Videogames, in particular, resort to different techniques that, in order to represent fluids, just simulate the consequence and not the cause, using procedural or parametric methods and often discriminating the physical solution. This need motivates the present thesis, the interactive simulation of free-surface flows, usually liquids, which are the feature of interest in most common applications. Due to the complexity of fluid simulation, in order to achieve real-time framerates, we have resorted to use the high parallelism provided by actual consumer-level GPUs. The simulation algorithm, the Lattice Boltzmann Method, has been chosen accordingly due to its efficiency and the direct mapping to the hardware architecture because of its local operations. We have created two free-surface simulations in the GPU: one fully in 3D and another restricted only to the upper surface of a big bulk of fluid, limiting the simulation domain to 2D. We have extended the latter to track dry regions and is also coupled with obstacles in a geometry-independent fashion. As it is restricted to 2D, the simulation loses some features due to the impossibility of simulating vertical separation of the fluid. To account for this we have coupled the surface simulation to a generic particle system with breaking wave conditions; the simulations are totally independent and only the coupling binds the LBM with the chosen particle system. Furthermore, the visualization of both systems is also done in a realistic way within the interactive framerates; raycasting techniques are used to provide the expected light-related effects as refractions, reflections and caustics. Other techniques that improve the overall detail are also applied as low-level detail ripples and surface foam