Cartesian Mesh Generation with Local Refinement for Immersed Boundary Approaches

In this paper, an efficient and robust Cartesian Mesh Generation with Local Refinement for an Immersed Boundary Approach is proposed, whose key feature is the capability of high Reynolds number simulations by the use of wall function models, bypassing the need for accurate boundary layer discretization. Starting from the discrete manifold model of the object to be analyzed, the proposed model generates Cartesian adaptive grids for a CFD simulation, with minimal user interactions; the most innovative aspect of this approach is that the automatic generation is based on the segmentation of the surfaces enveloping the object to be analyzed. The aim of this paper is to show that this automatic workflow is robust and enables to get quantitative results on geometrically complex configurations such as marine vehicles. To this purpose, the proposed methodology has been applied to the simulation of the flow past a BB2 submarine, discretized by non-uniform grid density. The obtained results are comparable with those obtained by classical body-fitted approaches but with a significant reduction of the time required for the mesh generation.

2D tracer transport on Cartesian and icosahedral grids: scheme comparisons for idealized test cases

<p>The distribution of tracers in the atmosphere results from the presence and emission of gaseous and particulate matter, as well as their transport, sedimentation and (photo-)chemical transformations. Understanding and quantifying these processes in the atmosphere can be addressed through the use of global-scale or regional-scale chemistry-transport numerical models such as CHIMERE (Mailler et al., 2016).</p><p> While possible in principle, it is impractical to use this model to represent long-range transport of dense plumes of gas and aerosols, resulting for instance from massive emissions by volcanic eruptions, forest fires and desertic aerosol tempests. Indeed such studies requiring both large domains and high resolution have a prohibitive numerical cost due to the formulation of CHIMERE on a regular Cartesian mesh. This limitation is shared by all currently operational chemistry-transport models. Additionally, traditional Cartesian meshes pose a numerical singularity at the poles, where the longitude lines converge.</p><p> These limitations may be lifted by replacing CHIMERE’s Cartesian mesh by a fully unstructured mesh. This would allow modelers to vary resolution in space, and hence to focus computational resources in key regions with sharp variations (e.g. volcanic eruptions) where high spatial and temporal resolution is required.</p><p> As a first step in this direction, we compare the numerical performance of transport schemes formulated on Cartesian meshes and schemes formulated on unstructured meshes (Dubey et al., 2015). To focus on differences due to numerics, the unstructured mesh is a quasi-uniform icosahedral mesh such as the one used by global dynamical core DYNAMICO (Dubos et al., 2015). Spatial and temporal coupled and de-coupled schemes of various order are implemented in each mesh framework. A suite of test cases is used to evaluate different properties of the mesh-scheme pairings.To avoid the Cartesian pole singularity, the Cartesian mesh covers a limited domain excluding the poles. Analytical wind fields adapted to this limited domain are used. Metrics are evaluated using the quantities obtained in the simulations, such as convergence using root mean square errors, shape preservation using non-linear tracer relations, and diffusion using total entropy. The stability and monotonicity of the used schemes are also numerically validated.</p><p> We find that a scheme of the Van Leer family on the unstructured mesh has a performance slightly inferior to a similar scheme on a Cartesian mesh. However, since this loss in quality remains moderate, it should be possible to more than compensate for it with a variable resolution. We are currently investigating this question and will present variable-resolution results if this ongoing work is timely completed. If successful, fully unstructured meshes would be a significant step forward in the modeling of scale interactions in atmospheric chemistry, and would potentially allow breakthrough for the understanding of such interactions.</p>

Large-Eddy Simulations of Rim Seal Flow in a One-Stage Axial Turbine

The flow field in a one-stage axial flow turbine with 30 stator and 62 rotor blades including the wheel space is investigated by large-eddy simulation (LES). The Navier-Stokes equations are solved using a massively parallel finite-volume solver based on a Cartesian mesh with immersed boundaries. The strict conservation of mass, momentum, and energy is ensured by an efficient cut-cell/level-set ansatz, where a separate level-set solver describes the motion of the rotor. Both solvers use individual subsets of a shared Cartesian mesh, which they can adapt independently. The focus of the analysis is on the flow field inside the rotor stator cavity between the stator and rotor disks. Two cooling gas mass flow rates are investigated for the same rim seal geometry. First, the time averaged flow field for both simulations is compared, followed by a detailed investigation of the unsteady flow field. The results for the cooling effectiveness are compared to experimental data. Both cases show good agreement with experimental data. It is shown that for the lower cooling gas mass flux several of the wheel space’s acoustic waves are excited. This is not observed for the higher cooling gas mass flux. The excited waves lead to stable, i.e., bounded, fluctuations inside the wheel space and result in a significantly higher hot gas ingestion.