Koen Hillewaert

• Doctor of Engineering
• Lecturer at University of Liège

Fluid models represent a valid alternative to kinetic approaches in simulating low-temperature discharges: a well-designed strategy must be able to combine the ability to predict a smooth transition from the quasineutral bulk to the sheath, where a space charge is built at a reasonable computational cost. These approaches belong to two families: mu...

Quantifying and predicting free-stream turbulence effects on the aerodynamic performance of turbomachinery flows continue to be of primary interest to advance the physical understanding and the modeling of the loss mechanisms. This work is motivated by the need to expose turbine flows to a wide class of inlet turbulence fields, characterized by the...

The transonic aerodynamics of a low pressure turbine (LPT) cascade, mimicking geared turbofan conditions during cruise, is currently under study using direct numerical simulation. In the first step of this endeavour, transition, separation and loss generation were examined at Mach 0.7, 0.9, and 0.95, with a constant Reynolds number of based on the...

Quantifying and predicting free-stream turbulence effects on the aerodynamic performance of turbomachinery flows continue to be of primary interest to advance the physical understanding and the modeling of the loss mechanisms. This work is motivated by the need to expose turbine flows to a wide class of inlet turbulence fields, characterized by the...

Outline of PhD project concerning the aerodynamics of low-pressure turbines when subject to high incidences. Gives objectives and methods for research, including boundary layer physics related questions.

In the context of fundamental flow studies, experimental databases are expected to provide uncertainty margins on the measured quantities. With the rapid increase in available computational power and the development of high-resolution fluid simulation techniques, Direct Numerical Simulation and Large Eddy Simulation are increasingly used in synergy...

This paper presents the numerical characterization of a highly loaded compressor by means of 3D unsteady RANS simulations. The focus is on critical flow structures and their evolution at different operating points of the machine. First, the numerical setup and mesh quality are presented to support the reliability of the provided results. The compar...

Due to modern evolutions in propulsion, in view of obtaining higher propulsive efficiency, turbofan engines are subjected to ever higher inlet flow distortions. In these systems, structural damage can be induced by distortion patterns with excitation close to the blade natural frequency. To fill the gap currently present in literature about the imp...

An important question for turbomachine designers is how to deal with blade and flowpath geometric variabilities stemming from the manufacturing process. The challenge consists in identifying where stringent manufacturing tolerances are absolutely necessary and where looser tolerances can be used, as some geometric variations have little or no effec...

RANS turbulence models still represent a weak point of industrial simulations of turbomachinery flows. In presence of a large blade loading the turbulence model can impact severely on the prediction of the end-wall flow and secondary structures in the blade passage, with an important effect on the computed global performance and flow stability. Thi...

Wall models reduce the computational cost of large eddy simulations (LES) by modeling the near-wall energetic scales and enable the application of LES to complex flow configurations of engineering interest. However, most wall models assume that the boundary layer is fully turbulent, at equilibrium, and attached. Such models have also been successfu...

This chapter describes space adaptive approaches developed by six TILDA partners for the application in scale-resolving simulations. They are designed to provide sufficient spatial resolution in regions where required and to allow a lower resolution elsewhere for efficiency reasons. Adaptation techniques considered include mesh (h-refinement), orde...

While the exascale computing era is approaching, the growing gap between computing resources and IO bandwidth for massively parallel simulations has already become a major bottleneck for the scientific discovery process. In this context, various strategies to enable and accelerate the analysis of data produced by massively parallel high-order metho...

This chapter describes space adaptive approaches developed by six TILDA partners for the application in scale-resolving simulations. They are designed to provide sufficient spatial resolution in regions where required and to allow a lower resolution elsewhere for efficiency reasons. Adaptation techniques considered include mesh (h-refinement), orde...

This paper describes the evaluation of a newly developed viscous time-marching through-flow solver to two test cases to assess the applicability of the method using correlations from the literature to modern blade designs. The test cases are the classic axial compressor stage CME2 and a modern highly loaded multi-stage axial low-pressure compressor...

Preliminary high-fidelity simulations of the MTU T161 low pressure turbine cascade with diverging end walls have been performed on massively parallel computational resources with four different high-order methods at outlet isentropic Mach number M2s=0.601 and two outlet isentropic Reynolds numbers, namely Re2s=90K and Re2s=200K. First the flow regi...

Microscopic defects may occur during powder-bed fusion processes which originate from molten pool instabilities. To investigate this issue, a numerical model focusing on the interaction between laser and powder, melt pool formation and thermodynamics effects is on development. The approach used in the model assumes an incompressible Newtonian flow...

This book offers detailed insights into new methods for high-fidelity CFD, and their industrially relevant applications in aeronautics. It reports on the H2020 TILDA project, funded by the European Union in 2015-2018. The respective chapters demonstrate the potential of high-order methods for enabling more accurate predictions of non-linear, unstea...

This book offers detailed insights into new methods for high-fidelity CFD, and their industrially relevant applications in aeronautics. It reports on the H2020 TILDA project, funded by the European Union in 2015-2018. The respective chapters demonstrate the potential of high-order methods for enabling more accurate predictions of non-linear, unstea...

The ultra-high bypass ratio geared turbofan (GTF) has been identified as a key engine concept to reduce emissions and noise footprint of the next generation of civil aircrafts. The increase in rotational speed of the low-pressure turbine (LPT) enabled by the geared architecture offers potential benefits in efficiency and considerable savings in eng...

At high Reynolds numbers the use of explicit in time compressible flow simulations with spectral/hp element discretization can become significantly limited by time step. To alleviate this limitation we extend the capability of the spectral/hp element open-source software framework, Nektar++, to include an implicit discontinuous Galerkin compressibl...

At high Reynolds numbers, the use of explicit in time compressible flow simulations with spectral/$hp$ element discretization can become significantly limited by time step. To alleviate this limitation we extend the capability of the spectral/$hp$ element open-source software framework, Nektar++, to include an implicit discontinuous Galerkin compre...

High-fidelity models have been developed in the recent years to predict the response of light-weight ablative thermal pro- tection materials used to build heat shields of re-entry cap- sules. Because the main components of these ablators are typically also present in some spacecraft/satellite compo- nents, there is a growing interest in exploring t...

The aim is to cater for the large diversity of high order methods by including a specification of the interpolation spaces for solutions in the file, fixing the bare essentials: the coordinate system. The choice of Lagrangian interpolation spaces is applicable to the mesh as well as the solution description. This generalization will: • allow to acc...

This paper presents two recent numerical tools developed respectively to perform traditional post-processing and more advanced in situ processing of high-order polynomial data generated by massively parallel finite element codes. For post-processing and visualisation of high-order solutions, we present a new ParaView plugin that integrates Gmsh use...

Wall-Modeled Large-Eddy Simulation (WMLES) alleviates the near-wall grid requirement by employing a wall-model to reconstruct the wall shear-stress. In this way, WMLES simultaneously reduces the computational cost associated with Wall-Resolved LES (WRLES) and opens the door towards higher Reynolds numbers (Piomelli, Wall-modeled large-eddy simulati...

This paper concerns implicit large eddy simulation (ILES) of turbulent flows of industrial interest using high order discontinuous Galerkin method (DGM). DGM has a high potential for industrial applications using ILES. As dissipation is only active on very small scale features, the method mimics a subgrid scale model, while its high accuracy ensure...

Ablative materials are extensively used in aerospace applications to protect the integrity of the spacecraft during atmospheric entry. Both thermal and mechanical stresses have to be withstood in the severe operating conditions typical of space missions. An accurate modeling of the phenomena taking place when these materials are exposed to such a h...

This paper presents an in-depth study, using wall-resolved Large-Eddy Simulation (wrLES), of a high Reynolds number airfoil in a near-stall condition. The flow around the NACA4412 airfoil, a widely used test case for turbulence modeling validation, is computed at Reynolds number Re = 1.64 × 10⁶ and angle of attack α = 12∘. In the first part of the...

Under realistic flight conditions, the turbulent flow physics over an airfoil is characterized by a wide range of time and length scales. The specific flow phenomenon, that is the subject of this presentation, is that of flow separation at high Reynolds number and high angles of attack (AoA). Due to its importance for airfoil/wing design, the accur...

A highly scalable stabilized finite element flow solver is employed in the current study to simulate the flow around the NACA-4412 wing at a moderate chord Reynolds number (Rec = 400,000) with an angle of attack of 5 degrees. The flow under investigation involves a wide range of scales and complicated flow physics induced by the geometry, such as w...

Some possible future High Fidelity CFD codes for LES simulation of turbomachinery are compared on several test cases increasing in complexity, starting from a very simple inviscid Vortex Convection to a multistage axial experimental compressor. Simulations were performed between 2013 and 2016 by major Safran partners (Cenaero, Cerfacs, CORIA and On...

High pressure turbine (HPT) blades are cooled partly by guiding air from the compressor through internal ducts.

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 coar...

This paper describes validation work for high-order discontinuous Galerkin Methods on Large Eddy Simulation (LES) of canonical test cases including shocks, namely compressible homogeneous isotropic turbulence and homogeneous turbulence passing a shock. Three different DG methods are considered, and the impact of their shock-stabilization mechanisms...

The interaction between a chemically reactive boundary layer and an ablative material is one of the most challenging phenomena to model when predicting the heat load during atmospheric entries of spacecraft. The gas-material interactions are usually assumed to occur at the surface while the ablation of new low density materials in strong diffusion...

This paper aims at evaluating the potential of the Discontinuous Galerkin (DG) methodology for Large-Eddy Simulation (LES) of wind turbine airfoils. The DG method has shown high accuracy, excellent scalability and capacity to handle unstructured meshes. It is however not used in the wind energy sector yet. The present study aims at evaluating this...

Implementation of WMLES approach on a Discontinuous Galerkin Methodology. Sensitivity analysis on the plane channel flow at Re_tau=5200. Demonstration of the importance of the input position and the polynomial order. Work presented in the session: MS409 Current trends in modelling and simulation of turbulent flows.

In future aviation and particularly in hypersonic systems new propellants will be used, such as liquid hydrogen, liquid methane and possibly liquid oxygen. Previous FP7 funded studies in Europe such as FAST20XX, ATLLAS or LAPCAT investigated advanced vehicles with these fuels for passenger transport like the SpaceLiner or Lapcat A2 and some of thei...

This study reports the results of a numerical investigation and optimization of the hydrodynamic and thermal performance of two new types of pin-fin and plate-fin heat sinks. The first type consists of inclined cones and is inspired by the larger heat transfer extents in impinging flow conditions, whilst the second type concerns wavy form plate-fin...

This paper examines the effect of non-equilibrium solidification and melting in fusion welding processes. A novel macroscopic non-equilibrium phase-transition model is proposed. The model is implemented in a high-order discontinuous Galerkin finite-element solver, providing a very accurate and flexible discretization of the governing equations. We...

The development of a high-order CFD solver for LES of turbomachinery is discusssed. It is integrated in a flexible multi-physics platform ArgoDG based on the discontinuous Galerkin Method. The DGM bridges the gap between the flexibility of the industrial solver and the accuracy of the academic methods, as it is able to reach high order of accuracy...

This paper presents the second validation step of a compressible discontinuous Galerkin solver with symmetric interior penalty (DGM/SIP) for the direct numerical simulation (DNS) and the large eddy simulation (LES) of complex flows. The method has already been successfully validated for DNS of an academic flow and has been applied to flows around c...

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 sca...

This 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 fir...

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 Fra...

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 strat...

This section gathers the descriptions of all test cases concerning internal flows, as well as the most important results obtained by the participants in the IDIHOM project. The test cases comprise a transonic compressor (NASA Rotor 37), a subsonic nozzle (JEAN), low pressure turbine cascades in turbulent (T106A) and transitional conditions (T106C)...

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 approache...

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 c...

This paper describes the development of a multiphysics welding simulation model based on the discontinuous Galerkin (DG) finite-element method. Our numerical model implements a classical enthalpy-porosity constitutive law accounting for hydrodynamic and thermal effects occurring during the phase transition from solid to liquid metal. The objective...

This paper focuses on the assessment of a discontinuous Galerkin method for the simulation of vortical flows at high Reynolds number. The Taylor–Green vortex at Re = 1600 is considered. The results are compared with those obtained using a pseudo-spectral solver, converged on a 512³ grid and taken as the reference. The temporal evolution of the diss...

This work focuses on the development of a strongly coupled discontinuous Galerkin scheme for the modeling of unsteady incompressible inertial two-phase flows using the level set method. The new approach uses a discontinuous Galerkin discretization for both the incompressible Navier-Stokes and the level set equations and the whole system is solved i...

The stability and convergence of the interior penalty (IP) discontinuous Galerkin method are closely related to the penalty coefficient σfσf. Using sharp trace inequalities adapted to the functional space, Shahbazi (2005) [7] has derived optimal values of σfσf for constant diffusivity problems on triangular meshes. We propose a generalisation of hi...

The interaction between the mechanisms of ablation and boundary layer transition is one of the phenomena that leads to diffculties 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 p...

The use of a high-order discontinuous Galerkin/interior penalty method for the Reynolds’ averaged Navier-Stokes computation, based on the Spalart-Allmaras model, of compressible turbulent flows is investigated. In particular different stabilizations of the iterative process are compared and discussed, including a new definition for the penalty para...

This paper presents the second step of the validation of a compressible discontinuous Galerkin method (DGM) for the direct numerical simulation (DNS) and the large eddy simulation (LES) of turbomachinery flows. The method has already been successfully validated for the DNS of academic flows as well as for the flow around complex geometries. During...

After several years of planning, the 1st International Workshop on High-Order CFD Methods was successfully held in Nashville, Tennessee, on January 7 and 8, 2012, just before the 50th Aerospace Sciences Meeting. AIAA, AFOSR and DLR provided much needed support, financial and moral. Over 70 participants from all over the world across the research sp...

This paper discusses the use of a high-resolution code based on the discontinuous Galerkin method (DGM) for the direct numerical simulation (DNS) of natural convection flows in buildings. The high order of accuracy, combined to geometric flexibility and parallel scalability of DGM, provide - on paper - An ideal method for this type of simulation. I...

This paper is concerned with the ability of a flow solver using a high-order discontinuous Galerkin method (DGM) to perform large eddy simulation (LES) of turbulent flows. Several modeling approach are considered: the Smagorinsky model (Smag), the regularized variational multiscale model (RVMs) and the implicit LES (ILES). Results on decaying homog...

The direct numerical simulation of a turbine cascade at Reis = 85000 and Mis = 0.6 has been undertaken using a fourth-order accurate discontinuous Galerkin / symmetric interior penalty method. This method combines the high accuracy, typical of the dedicated academic DNS and LES codes, to the geometrical flexibility of industrial finite volume codes...

An innovating approach is proposed to solve vectorial conservation laws on curved manifolds using the discontinuous Galerkin method. This new approach combines the advantages of the usual approaches described in the literature. The vectorial fields are expressed in a unit non-orthogonal local tangent basis derived from the polynomial mapping of cur...

It is well known that small-scale effects near contact lines have a crucial importance on the radial heat transfer within a grooved heat pipe evaporator. This paper studies this problem using a multi-scale model which is composed of two parts, macroscopic and microscopic. At the macroscopic scale, we solve the heat conduction problem for the solid...

This works reports on unsteady turbulent flow simulations around complex geometries. In this domain, Cenaero has developed a parallel implicit compress-ible flow solver for unstructured tetrahedral meshes, called Argo. In order to catpure the flow physics properly , a special care has been devoted to the spatial discretization, here using a kinetic...

The electron beam welding (EBW) process is extensively used for assembling titanium and other high strength components in the aircraft engine industry. For such applications, it is important to predict distortions and residual stresses after the welding process. In welding simulation, identifying the main physical phenomena is important to formulat...

This paper describes a multi-scale model for evaluating the radial heat transfer within a grooved heat pipe evaporator. The model is composed of two parts, macroscopic and microscopic, which cannot be decoupled from each other. In the macroscopic part, we solve the heat conduction problem in the solid and in the liquid phases, thanks to a finite-el...

This paper deals with the high-order discontinuous Galerkin (DG) method for solving wave propagation problems. First, we develop a one-dimensional DG scheme and numerically compute dissipation and dispersion errors for various polynomial orders. An optimal combination of time stepping scheme together with the high-order DG spatial scheme is present...

We study the efficient use of the discontinuous Galerkin finite element method for the computation of steady solutions of the Euler equations. In particular, we look into a few methods to enhance computational efficiency. In this context we discuss the applicability of two algorithmical simplifications that decrease the computation time associated...

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...

An elegant yet practical framework for the application of p-and h-Multigrid for DGFEM is first presented and analysed theoretically. After that a hybrid implicit-explicit p-Multigrid iteration strategy for the discretisation of the steady Euler equations with the discontinuous Galerkin finite element method is presented and investigated experimenta...

This paper presents the experimental and numerical investigation of an outward volute of rectangular cross section. The investigation is carried out at the level of - stage performance, - volute performance and - detailed flow field study at selected peripheral positions for various operating points. The objective of the investigation was to gain f...

A numerical procedure to predict the impeller/volute interaction in a single-stage centrifugal compressor is presented. The method couples a three-dimensional unsteady flow calculation in the impeller with a three-dimensional time-averaged flow calculation in the volute through an iterative updating of the boundary conditions on the interface of bo...

Near-optimal CPU efficiency for the basic operations of the Newton-Krylov-BILU iterations for the discontinuous Galerkin method (DGM) can be obtained by exploiting its data locality, as it allows to rewrite these operations in terms of BLAS/LAPACK operations on contiguous vectors and matrices of considerable size. Further significant enhancements a...