A matrix-free high-order discontinuous Galerkin compressible Navier-Stokes solver: A performance comparison of compressible and incompressible formulations for turbulent incompressible flows

Both compressible and incompressible Navier-Stokes solvers can be used and are used to solve incompressible turbulent flow problems. In the compressible case, the Mach number is then considered as a solver parameter that is set to a small value, $\mathrm{M}\approx 0.1$, in order to mimic incompressible flows. This strategy is widely used for high-order discontinuous Galerkin discretizations of the compressible Navier-Stokes equations. The present work raises the question regarding the computational efficiency of compressible DG solvers as compared to a genuinely incompressible formulation. Our contributions to the state-of-the-art are twofold: Firstly, we present a high-performance discontinuous Galerkin solver for the compressible Navier-Stokes equations based on a highly efficient matrix-free implementation that targets modern cache-based multicore architectures. The performance results presented in this work focus on the node-level performance and our results suggest that there is great potential for further performance improvements for current state-of-the-art discontinuous Galerkin implementations of the compressible Navier-Stokes equations. Secondly, this compressible Navier-Stokes solver is put into perspective by comparing it to an incompressible DG solver that uses the same matrix-free implementation. We discuss algorithmic differences between both solution strategies and present an in-depth numerical investigation of the performance. The considered benchmark test cases are the three-dimensional Taylor-Green vortex problem as a representative of transitional flows and the turbulent channel flow problem as a representative of wall-bounded turbulent flows

Design of a Modular Monolithic Implicit Solver for Multi-Physics Applications

The design of a modular multi-physics high-order space-time finite-element framework is presented together with its extension to allow monolithic coupling of different physics. One of the main objectives of the framework is to perform efficient high- fidelity simulations of capsule/parachute systems. This problem requires simulating multiple physics including, but not limited to, the compressible Navier-Stokes equations, the dynamics of a moving body with mesh deformations and adaptation, the linear shell equations, non-re effective boundary conditions and wall modeling. The solver is based on high-order space-time - finite element methods. Continuous, discontinuous and C1-discontinuous Galerkin methods are implemented, allowing one to discretize various physical models. Tangent and adjoint sensitivity analysis are also targeted in order to conduct gradient-based optimization, error estimation, mesh adaptation, and flow control, adding another layer of complexity to the framework. The decisions made to tackle these challenges are presented. The discussion focuses first on the "single-physics" solver and later on its extension to the monolithic coupling of different physics. The implementation of different physics modules, relevant to the capsule/parachute system, are also presented. Finally, examples of coupled computations are presented, paving the way to the simulation of the full capsule/parachute system

Recent Developments for the Eddy Solver

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A discontinuous Galerkin method for implicit LES of moderate Reynolds number flows

peer reviewedThis work is part of the development of a new generation of CFD solvers on the basis of the discontinuous Galerkin Method (DGM), specifically targeted towards accurate, adaptive, reliable and fast DNS and LES of industrial aerodynamic flows. In this study, the ability of the method to perform accurate implicit LES is investigated. The method is firstly assessed on the well-known turbulent channel flow. Several Reynolds number (up to Reτ = 950) are studied. The results show a fair agreement with the reference DNS, showing the ability of the method to perform accurate ILES on regular grids. Then, the method is applied on several advanced benchmarks (studied in the European project IDIHOM), performed at moderate Reynolds number. The 2D periodic hill flow, the low pressure turbine blade T106C and the JEAN nozzle benchmarks are considered. Encouraging results have been obtained, paving the way to the use of the method for industrial applications

Towards a Discontinuous Galerkin solver for scale-resolving simulations of moderate Reynolds number flows, and application to industrial cases

Due to the continuously increasing economical and environmental constraints, the standard industrial CFD methods (mostly Reynolds Averaged Navier-Stokes equations, RANS) are no longer sufficient to answer the design requirements of the industry, in particular when off-design performance and noise need to be predicted. Therefore, scale-resolving simulations, where the full (Direct Numerical Simulation, DNS) or at least a significant portion (Large-Eddy Simulation, LES) of the turbulence spectrum is resolved, are required. However, as these simulations require a nearly flawless representation of very small turbulent structures, current industrial solvers require huge computational resources in order to provide sufficient accuracy. The discontinuous Galerkin method (DGM) could alleviate this to a large extent as it seems to bridge the gap between the flexibility of industrial codes and the accuracy of academic solvers. During this thesis, the flexibility and the parallel efficiency of a DGM solver has been improved to tackle the large requirements of DNS and LES. The method was subsequently assessed for DNS and LES based on canonical benchmarks. Due to its interesting dissipation and dispersion properties, DGM seems to offer an accuracy similar to pseudo-spectral (PS) solvers for DNS. As the dissipation targets only the smallest scales, the method seems well suited to use an implicit LES approach. This approach has been validated on the simulation of homogeneous isotropic turbulence and on the channel flow at several Reynolds numbers. Finally, the method has been successfully applied on industrial cases, including a low pressure turbine blade, airfoil profiles and a high Mach number jet flow, thereby showing the maturity of the method.(FSA - Sciences de l) -- UCL, 201

Brand Whitlock

info:eu-repo/semantics/publishe

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

peer reviewedThe 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 4 and 1.10 5) 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 5. 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.

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

The Discontinuous Galerkin Method as an Enabling Technology for DNS and LES of Industrial Aeronautical Applications

To enhance prediction capacities and therefore allow more advanced aeronautic and aero-propulsive design, new CFD tools are required. State of the art codes are based on second order accurate finite volume methods and are primarily developed for statistical turbulence modeling approaches. Given the limitations of these models, more direct approaches such as DNS or LES are required for the prediction of off-design aerodynamic performance, noise generation, transitional flows ..