Un mailleur hybride structuré/non structuré en trois dimensions

La génération de maillage -- Les maillages structurés -- Les maillages non structurés -- Les maillages hybrides -- Modélisation géométrique 3D -- Représentation des modèles géométriques -- Algorithmes géométriques -- Génération de maillages hybrides -- Zonage automatique -- Les opérations de discrétisation -- Validation -- Applications industrielles

Efficient high-order discrete geometric model from CAD

International audienceIn the area of geometric modeling, major challenges are linked to the efficient visualization of CAD surfaces and to the generation of meshes well suited for numerical simulations. In this context, the elaboration and implementation of a discrete geometric model provide a simple and universal representation model, without the need for CAD. Such a model is a geometrically accurate representation of the model by means of a triangulation composed of polynomial elements. A first study aiming to build a discrete universal model has been carried for a model using degree 1 (one) elements, a "triangulation" composed of quadrilaterals and triangles. The advantage of this model of degree 1 lies in its geometric simplicity. However, in the case of complex surfaces, it may require a very large number of elements, in particular to represent some essential model characteristics such as high geometric curvatures. The discrete model is essentially used for visualization purpose and can also be considered for meshing. In this latter context, this discrete model is a universal representation regardless of the original analytical model which enables geometric queries (position, first and second derivatives, …) to be formulated without reference to the CAD model. In addition, another benefit of the discrete model is to isolate meshing operations from curve and surface parameterization. This paper presents an extension of the degree 1 approach allowing to efficiently reconstruct a high order discrete geometric model (using quadric, cubic, …, elements). The reconstruction is made up of several steps. It first proceeds by discretizing and approximating patch boundary curves using polynomials of a user-specified order. Next, for each parametric domain, a grid conforming to this discretization is built. This grid is geometrically adapted through refinement in order to obtain an accurate geometric model. The resulting discrete network of curve and surface elements is then dynamically refined so that it conforms to the original CAD data within a user-specified tolerance, while minimizing the overall model size. By construction, the model is essentially composed of quadrilateral curved elements, possibly with some triangular elements in the vicinity of the boundary. Edges of quadrilaterals are aligned with parametric directions of the original surfaces and are geometrically conformal. Several numerical operations required by this reconstruction process are computationally intensive. These operations have been carefully designed to minimize computational and memory resources. Some illustrative examples will demonstrate the efficiency of the proposed approach

Simulation of the postoperative trunk appearance in scoliosis surgery

Persistence of external trunk asymmetry after scoliosis surgical treatment is frequent and difficult to predict by clinicians. This is a significant problem considering that correction of the apparent deformity is a major factor of satisfaction for the patients. A simulation of the correction on the external appearance would allow the clinician to illustrate to the patient the potential result of the surgery and would help in deciding on a surgical strategy that could most improve his/her appearance. We describe a method to predict the scoliotic trunk shape after a spine surgical intervention. The capability of our method was evaluated using real data of scoliotic patients. Results of the qualitative evaluation were very promising and a quantitative evaluation based on the comparison of the simulated and the actual postoperative trunk surface showed an adequate accuracy for clinical assessment. The required short simulation time also makes our approach an eligible candidate for a clinical environment demanding interactive simulations.CIHR / IRS

On the evaluation of mesh resolution for large-eddy simulation of internal flows using openfoam

ABSTRACT: The central aim of this paper is to use OpenFOAM for the assessment of mesh resolution requirements for large-eddy simulation (LES) of flows similar to the ones which occur inside the draft-tube of hydraulic turbines at off-design operating conditions. The importance of this study is related to the fact that hydraulic turbines often need to be operated over an extended range of operating conditions, which makes the investigation of fluctuating stresses crucial. Scale-resolving simulation (SRS) approaches, such as LES and detached-eddy simulation (DES), have received more interests in the recent decade for understanding and mitigating unsteady operational behavior of hydro turbines. This interest is due to their ability to resolve a larger part of turbulent flows. However, verification studies in LES are very challenging, since errors in numerical discretization, but also subgrid-scale (SGS) models, are both influenced by grid resolution. A comprehensive examination of the literature shows that SRS for different operating conditions of hydraulic turbines is still quite limited and that there is no consensus on mesh resolution requirement for SRS studies. Therefore, the goal of this research is to develop a reliable framework for the validation and verification of SRS, especially LES, so that it can be applied for the investigation of flow phenomena inside hydraulic turbine draft-tube and runner at their off-design operating conditions. Two academic test cases are considered in this research, a turbulent channel flow and a case of sudden expansion. The sudden expansion test case resembles the flow inside the draft-tube of hydraulic turbines at part load. In this study, we concentrate on these academic test cases, but it is expected that hydraulic turbine flow simulations will eventually benefit from the results of the current research. The results show that two-point autocorrelation is more sensitive to mesh resolution than energy spectra. In addition, for the case of sudden expansion, the mesh resolution has a tremendous effect on the results, and, so far, we have not capture an asymptotic converging behavior in the results of Root Mean Square (RMS) of velocity fluctuations and two-point autocorrelation. This case, which represents complex flow behavior, needs further mesh resolution studies

Simulation-based investigation of unsteady flow in near-hub region of a Kaplan turbine with experimental comparison

ABSTRACT: This paper presents a detailed comparison of steady and unsteady turbulent flow simulation results in the U9 Kaplan turbine draft tube with experimental velocity and pressure measurements. The computational flow domain includes the guide vanes, the runner and the draft tube. A number of turbulence models were studied, including the standard k - epsilon, RNG k - epsilon, SST and SST-SAS models. Prediction of the flow behavior in the conical section of the draft tube directly below the runner cone is very sensitive to the prediction of the separation point on the runner cone. The results demonstrate a significant increase in precision of the flow modeling in the runner cone region by using unsteady flow simulations compare to stage simulation. The prediction of the flow in the runner cone region, however, remains delicate, and no turbulence model could accurately predict the complex phenomena observed experimentally