Koen Hillewaert

IDIHOM: Industrialization of High-Order Methods - A Top-Down Approach: Results of a Collaborative Research Project Funded by the European Union, 2010 - 2014

The IDIHOM Project.- Research Activities.- Assessment of High-Order Methods.- Conclusion and Recommendations.

Assessment of a High-Order Discontinuous Galerkin Method for the Direct Numerical Simulation of Transition at Low-Reynolds Number in the T106C High-Lift Low Pressure Turbine Cascade

An implicit time integration, high-order discontinuous Galerkin method is assessed on the DNS of the flow in the T106C cascade at low Reynolds number. This code, aimed at providing high orders of accuracy on unstructured meshes for DNS and LES simulations on industrial geometries, was previously successfully assessed on fundamental, academic test cases. The computational results are compared to the experimental values and literature, and the obtained flow field characteristics are discussed. Although adequate resolution is supposed to be attained, discrepancies with respect to the experiment are found. These differences were furthermore consistently found by all authors in the workshop on high-order methods for CFD. The origins are therefore conjectured to result from insufficient adequation between computational setup and experiments, as no modeling is assumed. A plan for further investigation is proposed.Copyright

Development and Validation of a Massively Parallel High-Order Solver for DNS and LES of Industrial Flows

This work is part of the development of a new generation CFD solver, Argo, based on the discontinuous Galerkin Method (DGM), specifically targeted towards accurate, adaptive, reliable and fast DNS and LES of industrial aerodynamic flows. Several aspects were investigated in IDIHOM. A first activity was the optimisation of the parallellisation strategy, resulting in highly efficient scaling, demonstrated on some of the largest computers in Europe. A second activity concerned the assessment and validation on several academic benchmark problems of the capability of DGM to perform direct numerical simulation (DNS) and (implicit) Large Eddy Simulation (iLES). Two moderately complex flows are treated, namely the ILES of the transitional flow in the low pressure turbine cascade T106C and the isothermal jet issueing from the JEAN nozzle.

Application of wall-models to discontinuous Galerkin LES

Wall-resolved Large-Eddy Simulations (LES) are still limited to moderate Reynolds number flows due to the high computational cost required to capture the inner part of the boundary layer. Wall-modeled LES (WMLES) provide more affordable LES by modeling the near-wall layer. Wall function-based WMLES solve LES equations up to the wall, where the coarse mesh resolution essentially renders the calculation under-resolved. This makes the accuracy of WMLES very sensitive to the behavior of the numerical method. Therefore, best practice rules regarding the use and implementation of WMLES cannot be directly transferred from one methodology to another regardless of the type of discretization approach. Whilst numerous studies present guidelines on the use of WMLES, there is a lack of knowledge for discontinuous finite-element-like high-order methods. Incidentally, these methods are increasingly used on the account of their high accuracy on unstructured meshes and their strong computational efficiency. The present pa...

Development of a Discontinuous Galerkin Solver for High Quality Wall-Resolved/Modelled DNS and LES of Practical Turbomachinery Flows on Fully Unstructured Meshes

The development of a high-order CFD solver for LES of turbomachinery is discussed. It is integrated in a flexible multiphysics platform Argo based on the discontinuous Galerkin Method. The DGM bridges the gap between the flexibility of the industrial solvers and the accuracy of the academic methods, as it is able to reach high order of accuracy on fully unstructured and hybrid meshes. Due to its inherent data locality, it also features high serial and parallel efficiency. The method provides a natural framework for adaptation of mesh size and interpolation order, which can be used later to further reduce computational cost and at the same time increase reliability of industrial DNS and LES. The paper mainly focuses on the physical modelling aspects and their interaction with the discretisation. In particular implicit LES and wall modelling is discussed. The approaches are tested on the wall-resolved and modelled LES of the turbulent channel flow. Finally the approach is applied to resolved LES of the near-transonic transitional flows in a low-pressure turbine cascade at Re = 9.4 × 104 and a compressor cascade at Re = 6.0 × 105. Either cases feature the full span and include end wall effects.Copyright

Cross-Validation of Numerical and Experimental Studies of Transitional Airfoil Performance

The aerodynamic performance characteristic of airfoils are the main input for estimating wind turbine blade loading as well as annual energy production of wind farms. For transitional flow regimes these data are difficult to obtain, both experimentally as well as numerically, due to the very high sensitivity of the flow to perturbations, large scale separation and performance hysteresis. The objective of this work is to improve the understanding of the transitional airfoil flow performance by studying the S826 NREL airfoil at low Reynolds numbers (Re = 4.10 and 1.10) with two inherently different CFD methodologies, in combination with wind tunnel experiments. Large-Eddy Simulations (LES) performed with a novel high order code based on the Discontinuous Galerkin Method are compared to LES from the well established wind turbine CFD code EllipSys3D. Both codes are considering natural transition. The similarity of the results obtained by these two very different simulation methods seems to demonstrate the validity of the computations. Differences are however observed with the experimental results. To understand these discrepancies, further analyses have been performed on both the numerical and the experimental sides. On the numerical side, the span sensitivity study showed that span lengths of 10 and 40% of the chord were leading to similar results. On the experimental side, the flow visualizations using oil streaks indicated strong 3D effects under the form of stall cells on a significant part of the span, as well as walls effects for Re = 1.10. Considering the sensitivity of the measurements to the tunnel environment, the strong similarity of the LES results inspires confidence in the validity of the computations. .

Improving an Unstructured CFD Solver for Aeroacoustics Applications

This paper reports on the development of a computational methodology for the simulation of aeroacoustics problems. An acoustic analogy is adopted and a in-house threedimensional unstructured flow solver is coupled to the Actran/LA commercial finite element solver that uses a variational formulation of the Lighthill analogy. Numerical investigations are performed in order to assess the appropriate level of accuracy required for the computational fluid dynamics part of this methodology.

Results and Conclusions of the European Project IDIHOM on High-Order Methods for Industrial Aerodynamic Applications

In aeronautical industry numerical flow simulation has become a key element in the aerodynamic design process. However, in order to meet the ambitious goals for air traffic of the next decades, significant investment in enhancing the capabilities and tools of numerical simulations in various aspects is required. Within the 7th European Research Framework Programme, the collaborative target research project IDIHOM was initiated. The overall objective of this project was to enhance and mature adaptive high-order simulation methods for large-scale applications. Compared to its low-order counterparts, high-order methods have shown large potential to either increase the predictive accuracy related to the descretization error at given costs or to significantly reduce computational expenses for a prescribed accuracy. The IDIHOM project was driven by a top-down approach, in which dedicated enhancements and improvements of the complete high-order simulation framework, including grid generation, flow solver and visualization, were led by a suite of underlying and challenging test cases. The project gathered 21 partners from industry, research organizations and universities with well-proven expertise in high-order methods. It started end October 2010 and finished March 2014. This paper presents the achievements of the project and highlights strenghts, weaknesses and perspectives of higher-order methods for aerodynamic applications.

Discontinuous Galerkin discretization coupled with sharp interface method for ablative materials

The interaction between the mechanisms of ablation and boundary layer transition is one of the phenomena that leads to difficulties in heat-flux prediction during atmospheric entries. The objective of this work is to develop a computational tool based on a high- order discontinuous Galerkin method (DGM), that simulates and models the flow and wall phenomena in a unified approach. As a first step, a one-dimensional code has been developed for the in-depth thermal response of ablative materials. This paper demonstrates the capability of DGM for discretizing the highly-nonlinear problem equations, discusses specificities of the method to ensure stability and illustrates the subsequent validation. A sharp interface method is proposed to face the challenges of recession front modeling.

Development and Study of Discretization Schemes on Sample Unstructured Grid Topologies for Large Eddy Simulation

Large eddy simulation (LES) on complex geometries by way of unstructured grids can be a tricky problem. As far as spatial discretization is concerned, it is well-known that standard Euler or Reynolds averaged Navier-Stokes (RANS) based schemes are too dissipative to perform LES since their numerical stabilization interacts strongly with the subgrid scale model. As, in the present approach, this spurious interaction is avoided, a low dissipation scheme has to be implemented. This scheme is built on a non-dissipative central scheme that conserves the discrete kinetic energy to reach stability. To prevent the generation of spurious numerical noise in underresolved non-turbulent parts of the simulation domain, a controlled amount of high order numerical dissipation is supplemented. As tetrahedra are reported to be suboptimal in regions where the grid stretching is large, discretization on hybrid meshes is also discussed. Finally, the present methodology is validated with DNS & LES applications.