Schillaci, E.; Favre, F.; Antepara, O.; Balcazar, N.; Oliva, A. International journal of computational methods and experimental measurements Vol. 6, num. 1, p. 98-109 DOI: 10.2495/CMEM-V6-N1-98-109 Data de publicació: 2018 Article en revista
In this work, a numerical framework for the direct numerical simulation of tsunami waves generated by landslide events is proposed. The method, implemented on the TermoFluids numerical platform, adopts a free surface model for the simulation of momentum equations; thus, considering the effect of air on the flow physics negligible. The effect of the solid motion on the flow is taken into account by means of a direct forcing immersed boundary method (IBM).
The method is available for 3-D unstructured meshes; however, it can be integrated with an adaptive mesh refinement (AMR) tool to dynamically increase the local definition of the mesh in the vicinity of the interfaces, which separate the phases or in the presence of vortical structures.
The method is firstly validated by simulating the entrance of objects into still water surfaces for 2-D and 3-D configurations. Next, the case of tsunami generation from a subaerial landslide is studied and the results are validated by comparison to experimental and numerical measurements. Overall, the model demonstrates its efficiency in the simulation of this type of physics, and a wide versatility in the choice of the domain discretization.
This paper analyzes a low-dissipation discretization for the resolution of immiscible, incompressible multiphase flow by means of interface-capturing schemes. The discretization is built on a three-dimensional, unstructured finite-volume framework and aims at minimizing the differences in kinetic energy preservation with respect to the continuous governing equations. This property plays a fundamental role in the case of flows presenting significant levels of turbulence. At the same time, the hybrid form of the convective operator proposed in this work incorporates localized low-dispersion characteristics to limit the growth of spurious flow solutions. The low-dissipation discrete framework is presented in detail and, in order to expose the advantages with respect to commonly used methodologies, its conservation properties and accuracy are extensively studied, both theoretically and numerically. Numerical tests are performed by considering a three-dimensional vortex, an exact sinusoidal function, and a spherical drop subjected to surface tension forces in equilibrium and immersed in a swirling velocity field. Finally, the turbulent atomization of a liquid-gas jet is numerically analyzed to further assess the capabilities of the method.
Schillaci, E.; Lehmkuhl, O.; Antepara, O.; Oliva, A. Journal of physics: conference series Vol. 745, num. 3, p. 1-8 DOI: 10.1088/1742-6596/745/3/032114 Data de publicació: 2016-09-01 Article en revista
This paper presents a numerical model that intends to simulate efficiently the surface instability that arise in multiphase flows, typically liquid-gas, both for laminar or turbulent regimes. The model is developed on the in-house computing platform TermoFluids , and operates the finite-volume, direct numerical simulation (DNS) of multiphase flows by means of a conservative level-set method for the interface-capturing. The mesh size is optimized by means of an adaptive mesh refinement (AMR) strategy, that allows the dynamic re-concentration of the mesh in the vicinity of the interfaces between fluids, in order to correctly represent the diverse structures (as ligaments and droplets) that may rise from unstable phenomena. In addition, special attention is given to the discretization of the various terms of the momentum equations, to ensure stability of the flow and correct representation of turbulent vortices. As shown, the method is capable of truthfully simulate the interface phenomena as the Kelvin-Helmholtz instability and the Plateau-Rayleigh instability, both in the case of 2-D and 3-D configurations. Therefore it is suitable for the simulation of complex phenomena such as simulation of air-blast atomization, with several important application in the field of automotive and aerospace engines. A prove is given by our preliminary study of the 3-D coaxial liquid-gas jet.
Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd.
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In this work, a finite-volume Adaptive Mesh Refinement (AMR) method for two phase flows able to dynamically achive the required mesh size in specific parts of the domain has been developed. The Navier-Stokes
equations for incompressible flows are coupled to a robust conservative level-set method for the capture of the interface between the fluids. The effectiveness of the method has been firstly validated by simulating some known test-cases, as the vortex flow and the rising bubbles. Finally, it has been applied to a possible industrial application, the simulation of a jet injector.
In this paper, we report the development of a parallel mesh multiplication code to subdivide a base mesh (containing tetrahedral, pyramids, prisms or hexahedral) into a finer mesh without user intervention, taking care of the quality of refined meshes generated by the algorithm. First, a coarse mesh is generated with an unstructured mesh generator and subdivided to the level of resolution needed for the simulation. On the refinement process of tetrahedral elements, geometrical properties are taking into account in order to preserve the shape quality of the subdivided elements. Then, the mesh is conformed on the solid surfaces to the original geometry, since linear subdivision ignores surface curvatures, and interior points of the mesh are adapted using Radial Basis functions, due to the surface correction. Moreover, a smoothing algorithm for tetrahedral mesh is applied to improve the mesh quality according to element geometry metric or based in minimize numerical errors in the CFD solution computed with unstructured meshes. Finally, the applicability of this method is shown on the numerical simulation of the turbulent flow over an Ahmed car at Reh = 7.68 × 10e5.
In this paper a parallel adaptive mesh refinement (AMR) strategy for large eddy simulations (LES) of turbulent flows is presented. The underlying discretization of the Navier-Stokes equations is based on a finite-volume symmetry-preserving formulation, with the aim of preserving the symmetry properties of the continuous differential operators and ensure both, stability and conservation of kinetic-energy balance. The conservation properties are tested for the meshes resulting from the AMR process, which typically contain transitions between zones with different level of refinement. Our AMR scheme applies a cell-based refinement technique, with a physics-based refinement criteria based on the variational multi-scale (VMS) decomposition theory. The overall AMR process, from the selection of the cells to be refined/coarsened till the pre-processing of the resulting mesh, has been implemented in a parallel code, for which the parallel performance has been attested on an AMD Opteron based supercomputer. Finally, the robustness and accuracy of our methodology is shown on the numerical simulation of the turbulent flow around a square cylinder at Re = 22,000 and the turbulent flow around two side-by-side square cylinders at Re = 21,000.
Due to the high impact in final cost reduction, the development of active systems of load control for wind turbines has recently gained renewed interest. The use of these systems results in a faster and more detailed action over the blade’s load than modern pitch-control systems, using aerodynamic control surfaces to locally modify the flux. Numerical simulations of these fluxes are presented in this work as a powerful tool for its understanding. A combination of large-eddy simulations (LES) techniques and the immersed-boundary method was employed in order to meet the conditions imposed by the mobile parts of the control systems embedded in the computational mesh. Use was also made of the Adaptive Mesh Refinement (AMR) method presented in  to minimize the amount of computational cells and therefore obtain an adequate resolution of the different length scales. A simulation of the experiment presented by Yeung et al.  was performed as a validation of the proposed model. The experiment consists of an aerodynamic profile deploying a control-surface with a
In the present work, a parallel adaptive mesh refinement (AMR) strategy for large-eddy simulations (LES) is proposed and tested for a fully 3D geometry.
The underlying discretization of the Navier-Stokes equations is based on a finite-volume symmetry-preserving formulation, with the aim of preserving the symmetry properties of the continuous differential operators to ensure stability and conservation of kinetic-energy balance. The proposed AMR scheme applies a cell-based refinement technique, with a physics-based refinement criteria based on the variational multiscale(VMS) decomposition theory and an equalized histogram of the vorticity field. This strategy has been
tested in other turbulent problems around bluff bodies in 2D and 3D. To carry out the simulation of turbulent flow around complex geometries with AMR, an immerse boundary method is implemented based on a finite volume approach. Finally, the robustness and accuracy of our methodology is shown on the numerical simulation of the turbulent flow over an Ahmed car at Re = 7.68x10^5, which reproduces the basic fluid dynamics features of real cars, i.e. vortex shedding, flow reattachment and recirculation bubbles.
Adaptive mesh refinement (AMR) methods focus on the refinement/coarsening of certain zones of the mesh according to the dynamic characteristics of the flow, in order to get a suitable grid resolution at any part of the domain and time step of a numerical simulation.
The benefit from this method is an automatic and dynamic mesh adaptation to accurately solve flow problems, otherwise the construction of a fixed (static) mesh needs a maximum grid resolution to be established, from the beginning of the simulation, in zones that will not be required in other time step of the simulation.
Apart from reducing the computing requirements for the simulation, it is also important that the algorithm achieves a good parallel performance in current supercomputers, to take advantage of the increasingly available computing power. In order to accomplish these objectives, the development of a parallel adaptive mesh refinement code for three-dimensional structured meshes on distributed-memory machines is presented. Our AMR scheme applies a cell-based refinement technique, where an octree data structure is used keeping track of the cells connectivity through the different levels of refinement and a physics-based refinement criteria is developed based on the variational multi-scale (VMS) decomposition theory. This approach has been validated in turbulent problems around bluff bodies in 2D and 3D domains.
The proposed work focuses on the parallelization strategy and includes a performance study of the algorithm. The overall AMR process, from the selection of the cells to be refined/coarsened to the partitioning and pre-processing of the resulting mesh has been implemented in parallel and tested on a AMD Opteron based supercomputer.
Finally, the applicability, robustness and accuracy of our algorithm is shown on the numerical simulation of the turbulent flow around a wall mounted cube at Re=7235. which reproduces some of the turbulent flows features around bluff bodies, i.e. flow separation, vortex shedding and appearance of vortex at the upstream face and in the wake of the cube.
In the present work two LES models for predicting turbulent flow and an Adaptive Mesh Refinement (AMR) technique are proposed and tested for a fully 3D geometry: turbulent flow around a wall-mounted cube at Reh=7235. The wall-adapting eddy viscosity model within a variational multiscale method (VMS-WALE) and the QR model are tested to predict the flow around the body. The numerical algorithm used to solve the governing equations preserves the symmetry and conservation properties. AMR algorithm is applied to get enough grid-resolution to solve the vortical structures near the body, adapting the mesh according to physics-based refinement criteria. High order conservative schemes are applied in the connection between coarse and fine regions. The numerical results obtained are assessed and compared to the results of the direct numerical simulations (DNS) on the basis of first and second order statistics.
In this paper a parallel adaptive mesh refinement for LES simulation of turbulent flows is presented. The AMR scheme applies a cell- based refinement technique to get enough grid-resolution to solve the small scales structures, adapting the mesh according to physics- based refinement criteria. A flexible tree data structure is used to keeping track of the mesh adaptation and an edge-based data structure to save and search cell connectivity. The AMR framework is combined with parallel algorithms for partitioning and balancing of the computational mesh. Numerical results for turbulent flow around a square cylinder at Re = 22000 are compared to experimental data.